Chemical decontamination method and chemical decontamination equipment

Abstract

To provide a chemical decontamination method capable of confirming in a short time that a reductive decontamination aqueous solution after ultraviolet irradiation does not contain an oxidizer.SOLUTION: A pipe to be decontaminated is subjected to oxidative decontamination, decomposition of an oxidative decontamination agent, reductive decontamination using an oxalic acid aqueous solution, irradiation with ultraviolet rays, and decomposition of oxalic acid (S6). An oxalic acid aqueous solution containing hydrogen peroxide is irradiated with ultraviolet rays (S7) to decompose the oxalic acid, and Fe3+ in the aqueous solution is reduced to Fe2+. The logarithm of Fe3+ / Fe2+ (concentration ratio) is determined based on the oxidation reduction potential of the oxalic acid aqueous solution measured after ultraviolet irradiation (S12). When the logarithm of Fe3+ / Fe2+ is greater than 1 (S13), the supply of the aqueous oxalic acid solution to the cation exchange resin column is stopped (S14), and the amount of hydrogen peroxide supplied is reduced (S15). When the logarithm of Fe3+ / Fe2+ is less than 1, an aqueous oxalic acid solution is supplied to a cation exchange resin tower (S16), and the oxalic acid is decomposed by a catalyst and hydrogen peroxide in a catalyst tower (S17).SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a chemical decontamination method and a chemical decontamination apparatus, and more particularly to a chemical decontamination method and a chemical decontamination apparatus suitable for application to the decontamination of structural members contaminated with radioactive nuclides in a boiling water nuclear power plant. [Background technology]

[0002] In nuclear power plants, in order to reduce the radiation exposure dose of workers during periodic inspections and dismantling of the nuclear power plant for plant decommissioning, and also to reduce the amount of radioactive waste generated during dismantling of the nuclear power plant, work has traditionally been carried out to remove radionuclides from the surfaces of the components (equipment, piping, etc.) of the nuclear power plant that are contaminated with radionuclides using chemical agents (hereinafter referred to as chemical decontamination).

[0003] Chemical decontamination is generally performed by alternating multiple times between decontamination using an oxidizing agent (oxidative decontamination) and decontamination using a reducing agent (reductive decontamination). Oxidative decontamination mainly removes the chromium (Cr) oxide film on the surface of the components of the nuclear plant using an oxidizing agent (e.g. potassium permanganate). Reductive decontamination mainly removes the iron (Fe) oxide film on the surface of the components using a reducing agent (e.g. organic acid containing oxalic acid). At this time, the pH of the reducing agent may be adjusted by adding hydrazine, taking into account the effect on the components.

[0004] Metal ions and metal cations such as radionuclides dissolved from the surfaces of component parts by chemical decontamination agents (oxidative decontamination agents and reductive decontamination agents) are adsorbed and removed by the cation exchange resin by supplying the chemical decontamination solution to a cation exchange resin tower filled with cation exchange resin in reductive decontamination. Potassium permanganate used in oxidative decontamination reacts with oxalic acid, a reducing agent, after oxidative decontamination and is reductively decomposed to manganese ions, which are adsorbed and removed by the ion exchange resin tower. Oxalic acid and hydrazine used in reductive decontamination are decomposed in a catalyst tower (decomposition device) located downstream of the ion exchange resin tower.

[0005] As a technology relating to a conventional chemical decontamination method, for example, there is JP 2000-105295 A. JP 2000-105295 A discloses a chemical decontamination method for chemically removing radionuclides attached to the surface of a component member contaminated with radionuclides and in contact with reactor water, the chemical decontamination method including a step of performing reduction decontamination using a reduction decontamination agent containing at least two or more components, and a step of decomposing the reduction decontamination agent using a catalytic tower that decomposes at least two or more chemical substances contained in the reduction decontamination agent after the step.

[0006] The Fe ions dissolved in the reducing agent by the reductive decontamination become Fe oxides and adhere to the catalyst in the catalyst tower, shortening the catalyst's life. It has also been reported that the decomposition rate of oxalic acid and hydrazine decreases when Fe oxides adhere to the catalyst (K.Ishida et al., "Low Corrosive Chemical Decontamination Method Using pH Control, (II) Decomposition of Reducing Agent by Using catalyst with Hydrogen Peroxide", Journal of Nuclear Science and Technology, Vol.39, No.9, p.941-949 (September 2002)).

[0007] Therefore, it has been desired to further improve the removal efficiency of Fe ions dissolved in the reduction decontamination aqueous solution by the cation exchange resin after reduction decontamination.

[0008] A chemical decontamination method in which a reducing aqueous solution is irradiated with ultraviolet light is disclosed in Japanese Patent Application Laid-Open No. 2000-105295. and described in JP 2018-159647 A and the like.

[0009] Japanese Patent Application Laid-Open Publication No. 2000-105295 describes performing reduction decontamination on the surface of an object to be chemically decontaminated (e.g., piping) in a nuclear power plant using a reduction decontamination liquid containing oxalic acid (reducing decontamination agent) and hydrazine, decomposing the reduction decontamination agent contained in the reduction decontamination liquid used in the reduction decontamination, and performing oxidative decontamination on the surface using an oxidative decontamination agent, for example, an oxidative decontamination liquid containing potassium permanganate. In the decomposition process of the reduction decontamination agent, oxalic acid, which is a reduction decontamination agent, is decomposed in a decomposition device having a catalyst to which hydrogen peroxide is supplied, and is further decomposed by irradiating the reduction decontamination liquid with ultraviolet light in an ultraviolet light irradiation device arranged in parallel with the decomposition device. The irradiation of ultraviolet light to the reduction decontamination liquid not only decomposes oxalic acid, but also converts trivalent iron complexes generated in the reduction decontamination using the reduction decontamination liquid into Fe 2+ The resulting Fe 2+ is removed in the cation exchange resin tower, preventing the cationic trivalent iron complex that reaches the decomposition equipment from precipitating on the surface of the catalyst in the decomposition equipment and shortening the catalyst's life.

[0010] In the chemical decontamination method for a nuclear power plant described in JP 2018-159647 A, Fe contained in an oxalic acid solution is 3+ Fe 2+ In order to reduce the oxalic acid solution to Fe, the oxalic acid solution is irradiated with ultraviolet light in an ultraviolet irradiation device, and the oxalic acid solution irradiated with ultraviolet light is supplied to a cation exchange resin tower, whereby the Fe contained in the oxalic acid solution is reduced to Fe. 2+ is removed in the cation exchange resin tower. 2+ The oxalic acid solution from which Fe has been removed is fed to a decomposition device that also supplies hydrogen peroxide, where the oxalic acid is decomposed. 3+ Fe 2+ The iron ions (Fe 2+ ) increases. 3+ The oxalic acid aqueous solution that has been irradiated with ultraviolet light and passed through the cation exchange resin tower is supplied to a decomposition device that has a catalyst inside and is supplied with hydrogen peroxide, and the oxalic acid contained in the oxalic acid aqueous solution is decomposed in the decomposition device by the action of the catalyst and hydrogen peroxide.

[0011] In the method for decomposing organic acids in waste liquid described in JP 2008-36591 A, a decomposition device having a hydrogen peroxide supply unit and an ultraviolet irradiation device is used. Hydrogen peroxide is supplied from the hydrogen peroxide supply unit to an aqueous solution containing oxalic acid, which is the waste liquid, and ultraviolet light is irradiated to the aqueous solution containing oxalic acid and hydrogen peroxide by the ultraviolet irradiation device. 2+ When the solution is fed into an oxalic acid solution containing Fe 3+ and ·OH (hydroxyl radical) are generated in the oxalic acid solution. Oxalic acid contained in the oxalic acid solution is decomposed by ·OH and also by irradiation with ultraviolet light. Furthermore, by irradiating the oxalic acid solution with ultraviolet light, Fe 3+ Fe 2+ It is also stated that the

[0012] JP 2009-109427 A describes the same thing as JP 2008-36591 A. JP 2009-109427 A further describes adjusting the amount of hydrogen peroxide supplied upstream of the ultraviolet irradiation device so that the hydrogen peroxide concentration at the outlet side of the ultraviolet irradiation device becomes zero.

[0013] JP 2019-191075 A shows in FIG. 8 that one end of the circulation pipe of the chemical decontamination apparatus is connected to the bottom of the decontamination container, and the other end of the circulation pipe is connected to the top of the decontamination container. For example, multiple pieces of a piping system (objects to be chemically decontaminated) generated by dismantling a nuclear power plant that is the subject of decommissioning measures are stored in the decontamination container, and the decontamination container is then sealed. A chemical decontamination aqueous solution is circulated in a closed loop including the decontamination container and the circulation pipe, and chemical decontamination is performed on the multiple pieces in the decontamination container. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] JP 2000-105295 A [Patent Document 2] JP 2018-159647 A [Patent Document 3] JP 2008-36591 A [Patent Document 4] JP 2009-109427 A [Patent Document 5] JP 2019-191075 A [Non-patent literature]

[0015] [Non-Patent Document 1] K. Ishida et al, "Low Corrosive Chemial Decontamination Method Using pH Control, (II) Decomposition of Reducing Agent by Using Catalyst with Hydrogen Peroxide", Journal of Nuclear Science and Technology, Vol. 39, No. 9, p. 941-949 (September 2002) Summary of the Invention [Problem to be solved by the invention]

[0016] The purpose of removing Fe ions is to suppress the decrease in catalyst activity and to rapidly decompose oxalic acid. On the other hand, UV irradiation, which adjusts the valence of Fe ions contained in an oxalic acid solution, has the ability to cause the Fenton reaction and decompose oxalic acid when used in combination with an oxidizing agent such as hydrogen peroxide. Therefore, if this reaction can be utilized, it can contribute to the original purpose of rapidly decomposing oxalic acid. However, if hydrogen peroxide is simply added before UV irradiation, unreacted hydrogen peroxide may reach the cation exchange resin tower, oxidize and decompose the cation exchange resin, contaminating the reduction decontamination waste liquid with the decomposition components of the ion exchange resin, and making it impossible to discharge it. For this reason, it is desirable to establish a method that can quickly confirm that the reduction decontamination solution after UV irradiation does not contain the oxidizing agent when the reduction decontamination solution containing the oxidizing agent is irradiated with UV rays.

[0017] An object of the present invention is to provide a chemical decontamination method and a chemical decontamination apparatus that can quickly confirm that a reduction decontamination aqueous solution after ultraviolet irradiation does not contain an oxidizing agent. [Means for solving the problem]

[0018] The chemical decontamination method of the present invention for achieving the above-mentioned object is characterized in that an aqueous solution of a reducing decontamination agent is brought into contact with a surface of a component of a nuclear power plant that is in contact with reactor water to perform reducing decontamination of the component; In the process of decomposing the reducing decontamination agent contained in the aqueous solution, The aqueous solution is irradiated with ultraviolet light, and either the oxidation-reduction potential or the corrosion potential of the aqueous solution irradiated with the ultraviolet light is measured. Based on either the oxidation-reduction potential or the corrosion potential measured, the amount of oxidizing agent supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device that irradiates the ultraviolet light and the aqueous solution in the ultraviolet irradiation device is adjusted.

[0019] According to the chemical decontamination method having the above-mentioned characteristics, either the oxidation-reduction potential or the corrosion potential of the reducing decontamination aqueous solution irradiated with ultraviolet light is measured, and based on either the measured oxidation-reduction potential or the corrosion potential, it is possible to quickly confirm that the reducing decontamination aqueous solution irradiated with ultraviolet light does not contain an oxidizing agent.

[0020] The chemical decontamination apparatus of the present invention for achieving the above-mentioned object is characterized by a circulation pipe for supplying an aqueous solution containing a reducing decontamination agent to a piping system to be chemically decontaminated, which is a component of a nuclear power plant; either an oxidation-reduction potential measuring device for measuring the oxidation-reduction potential of the aqueous solution in the circulation pipe to which ultraviolet light is irradiated, or a corrosion potential measuring device for measuring the corrosion potential of the aqueous solution in the circulation pipe to which ultraviolet light is irradiated; and a control device that controls the amount of the oxidant supplied to the aqueous solution based on either the oxidation-reduction potential measured by the oxidation-reduction potential measuring device or the corrosion potential measured by the corrosion potential measuring device.

[0021] (A1) bringing an aqueous solution of a reduction decontamination agent into contact with a surface of a component of a nuclear power plant that comes into contact with reactor water to perform reduction decontamination of the component; In the process of decomposing the reducing decontamination agent contained in the aqueous solution, A more preferred configuration of the chemical decontamination method is described below, in which the aqueous solution is irradiated with ultraviolet light, either the oxidation-reduction potential or the corrosion potential of the aqueous solution irradiated with the ultraviolet light is measured, and the amount of oxidant supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device that irradiates the ultraviolet light and the aqueous solution in the ultraviolet irradiation device is adjusted based on the measured oxidation-reduction potential or the measured corrosion potential.

[0022] In addition, the description in the above (A1) from "contacting an aqueous solution of a reducing decontamination agent with a surface of a component of a nuclear plant that comes into contact with reactor water to perform reducing decontamination of the component" to "adjusting the amount of oxidizing agent supplied to at least one of the aqueous solutions" corresponds to the configuration described in claim 1.

[0023] (A2) Preferably, in the above (A1), it is desirable that the control device adjusts the amount of oxidant supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device and the aqueous solution in the ultraviolet irradiation device based on the measured oxidation-reduction potential or the measured corrosion potential.

[0024] (A3) Preferably, in the above (A1) or (A2), the amount of the oxidizing agent supplied to the aqueous solution is adjusted based on the measured oxidation-reduction potential and corrosion potential of the aqueous solution irradiated with ultraviolet light. 2+ Fe 3+and the control device adjusts the amount of the oxidant supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device and the aqueous solution in the ultraviolet irradiation device based on the logarithm of the concentration ratio or the concentration ratio.

[0025] (A4) Preferably, in any one of (A1) to (A3) above, the control device determines whether the aqueous solution irradiated with ultraviolet light contains the oxidant based on either the logarithm of the determined concentration ratio or the determined concentration ratio.

[0026] (A5) Preferably, in the above (A4), the control device determines that the aqueous solution irradiated with ultraviolet light contains the oxidant when either the logarithm of the determined concentration ratio is greater than 1 or the determined concentration ratio is greater than 10.

[0027] (A6) Preferably, in the above (A4) or (A5), when the aqueous solution irradiated with ultraviolet light contains the oxidant, the control device stops the supply of the aqueous solution to the cation exchange resin tower and reduces the amount of the oxidant supplied to the aqueous solution.

[0028] (A7) Preferably, in the above (A4) or (A5), when the control device determines that the aqueous solution does not contain the oxidant and that the logarithm of the determined concentration ratio is smaller than a first set value that is 1 or less, the amount of the oxidant supplied to the aqueous solution is increased so that the logarithm of the concentration ratio becomes the first set value, or when the control device determines that the determined concentration ratio is smaller than a second set value that is 10 or less, the concentration ratio becomes the second set value.

[0029] (A8) Preferably, in any one of the above (A1) to (A7), the reducing decontamination agent contained in the aqueous solution is decomposed by irradiating the aqueous solution with ultraviolet light, and the reducing decontamination agent is decomposed in a catalytic tower in which a catalyst is present by the catalyst and the oxidizing agent supplied to the catalytic tower. (B1) A second pipe separate from the first pipe, which is connected to the reactor pressure vessel and is a component of a nuclear power plant and is to be chemically decontaminated, is connected to the first pipe to form a closed loop including the first pipe and the second pipe; An aqueous solution containing a reduction decontamination agent is supplied from the second pipe to the first pipe to perform reduction decontamination on the inner surface of the second pipe; In the step of decomposing the reducing decontamination agent contained in the aqueous solution, the aqueous solution connected to the second pipe and returned from the first pipe to the second pipe is irradiated with ultraviolet light, and either an oxidation-reduction potential or a corrosion potential of the aqueous solution irradiated with the ultraviolet light is measured; A chemical decontamination method for adjusting a supply amount of the oxidizing agent to the aqueous solution based on either the measured oxidation-reduction potential or the measured corrosion potential, comprising: The supply amount of the oxidizing agent to the aqueous solution is adjusted based on the measured oxidation-reduction potential and the measured corrosion potential of Fe in the aqueous solution irradiated with the ultraviolet light. 2+ Fe 3+ A more preferred configuration of the chemical decontamination method in which either the logarithm of the concentration ratio of the oxidant or the concentration ratio is calculated, and the amount of the oxidant supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device irradiated with ultraviolet light and the aqueous solution in the ultraviolet irradiation device is adjusted based on either the logarithm of the concentration ratio or the concentration ratio calculated.

[0030] In the above (B1) configuration, the description from "a second pipe, which is separate from a first pipe, is connected to a first pipe that is a component of a nuclear power plant and is a target of chemical decontamination, to form a closed loop including the first pipe and the second pipe" to "a chemical decontamination method in which the amount of the oxidant supplied to the aqueous solution is adjusted based on the measured oxidation-reduction potential" corresponds to the configuration described in claim 10. In addition, in the above (B1) configuration, the description from "the amount of the oxidant supplied to the aqueous solution is adjusted based on either the measured oxidation-reduction potential or the measured corrosion potential" to "a chemical decontamination method in which the amount of the oxidant supplied to at least one of the aqueous solution in the upstream and the aqueous solution in the ultraviolet irradiation device is adjusted" corresponds to the configuration described in claim 11.

[0031] (B2) Preferably, in the above (B1), it is desirable to determine whether the oxidant is contained in the aqueous solution irradiated with ultraviolet light based on either the logarithm of the determined concentration ratio or the determined concentration ratio.

[0032] (B3) Preferably, in the above (B2), when the logarithm of the determined concentration ratio is greater than 1 or when the determined concentration ratio is greater than 10, it is desirable to determine that the aqueous solution irradiated with ultraviolet light contains the oxidizing agent.

[0033] (B4) Preferably, in the above (B2) or (B3), when the aqueous solution irradiated with ultraviolet light contains the oxidizing agent, it is desirable to stop supplying the aqueous solution to the cation exchange resin tower and reduce the amount of the oxidizing agent supplied to the aqueous solution.

[0034] (B5) Preferably, in the above (B2) or (B3), when it is determined that the aqueous solution does not contain the oxidant and when it is determined that the logarithm of the determined concentration ratio is smaller than a first set value that is 1 or less, the amount of the oxidant supplied to the aqueous solution is increased by either setting the logarithm of the concentration ratio to the first set value, or setting the concentration ratio to the second set value when it is determined that the determined concentration ratio is smaller than a second set value that is 10 or less. (B6) Preferably, in any one of the above (B2) to (B5), the reducing decontamination agent contained in the aqueous solution is decomposed by irradiating the aqueous solution with ultraviolet light, and the reducing decontamination agent is decomposed in a catalytic tower in which a catalyst is present by the catalyst and the oxidizing agent supplied to the catalytic tower. (B7) Preferably, in the above (B6), when it is determined that the aqueous solution does not contain the oxidizing agent, the aqueous solution is supplied to a cation exchange resin tower, metal cations contained in the aqueous solution are removed in the cation exchange resin tower, the oxidizing agent is supplied to the aqueous solution from which the metal cations have been removed, and the aqueous solution containing the oxidizing agent and the reduction decontamination agent is supplied to the catalyst tower.

[0035] (C1) bringing an aqueous solution of a reduction decontamination agent into contact with a surface of a component of a nuclear power plant that comes into contact with reactor water to perform reduction decontamination of the component; In the process of decomposing the reducing decontamination agent contained in the aqueous solution, A more preferred configuration of the chemical decontamination method is described below, in which the aqueous solution is irradiated with ultraviolet light, either the oxidation-reduction potential or the corrosion potential of the aqueous solution irradiated with the ultraviolet light is measured, and the amount of oxidant supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device that irradiates the ultraviolet light and the aqueous solution in the ultraviolet irradiation device is adjusted based on the measured oxidation-reduction potential or the measured corrosion potential.

[0036] In addition, the above configuration of (C1), from "contacting an aqueous solution of a reducing decontamination agent with a surface of a component of a nuclear plant that comes into contact with reactor water to perform reducing decontamination of the component" to "adjusting the amount of oxidizing agent supplied to at least one of the aqueous solutions," corresponds to the configuration described in claim 1. (C2) Preferably, in the above (C1), it is desirable to determine whether the aqueous solution irradiated with ultraviolet light contains the oxidizing agent based on either the measured oxidation-reduction potential or the measured corrosion potential. (C3) Preferably, in the above (C2), it is desirable to determine that the aqueous solution irradiated with ultraviolet light contains the oxidizing agent when the measured oxidation-reduction potential is greater than 400 mV vs. SHE or when the measured corrosion potential is greater than 400 mV vs. SHE. (C4) Preferably, in the above (C2) or (C3), when the aqueous solution irradiated with ultraviolet light contains the oxidizing agent, it is desirable to stop supplying the aqueous solution to the cation exchange resin tower and reduce the amount of the oxidizing agent supplied to the aqueous solution. (C5) Preferably, in the above (C2) or (C3), when it is determined that the aqueous solution does not contain the oxidant and the measured oxidation-reduction potential is smaller than a third set value of the oxidation-reduction potential, the amount of the oxidant supplied to the aqueous solution is increased so that the oxidation-reduction potential becomes the third set value, or when it is determined that the measured corrosion potential is smaller than a fourth set value of the corrosion potential, the corrosion potential becomes the fourth set value. (C6) Preferably, in any one of the above (C2) to (C4), the reducing decontamination agent contained in the aqueous solution is decomposed by irradiating the aqueous solution with ultraviolet light, and the reducing decontamination agent is decomposed in a catalytic tower in which a catalyst is present by the catalyst and the oxidizing agent supplied to the catalytic tower. (C7) Preferably, in the above (C6), when it is determined that the aqueous solution does not contain the oxidizing agent, the aqueous solution is supplied to a cation exchange resin tower, metal cations contained in the aqueous solution are removed in the cation exchange resin tower, the oxidizing agent is supplied to the aqueous solution from which the metal cations have been removed, and the aqueous solution containing the oxidizing agent and the reduction decontamination agent is supplied to the catalyst tower. (D1) A second pipe separate from the first pipe is connected to the first pipe, which is a component of a nuclear power plant and is to be chemically decontaminated and is connected to the reactor pressure vessel, to form a closed loop including the first pipe and the second pipe; An aqueous solution containing a reduction decontamination agent is supplied from the second pipe to the first pipe to perform reduction decontamination on the inner surface of the second pipe; In the step of decomposing the reducing decontamination agent contained in the aqueous solution, the aqueous solution connected to the second pipe and returned from the first pipe to the second pipe is irradiated with ultraviolet light, and either an oxidation-reduction potential or a corrosion potential of the aqueous solution irradiated with the ultraviolet light is measured; A more preferred configuration of the chemical decontamination method in which the amount of the oxidant supplied to the aqueous solution is adjusted based on either the measured oxidation-reduction potential or the measured corrosion potential will be described below.

[0037] In addition, among the configuration of (D1) above, the description from "connecting a second pipe, which is separate from a first pipe, to a first pipe to be chemically decontaminated, which is a component of a nuclear power plant and is connected to a reactor pressure vessel, to form a closed loop including the first pipe and the second pipe" to "a chemical decontamination method for adjusting the amount of the oxidant supplied to the aqueous solution based on either the measured oxidation-reduction potential or the measured corrosion potential" corresponds to the configuration described in claim 10.

[0038] (D2) Preferably, in the above (D1), it is desirable to determine whether the aqueous solution irradiated with ultraviolet light contains the oxidizing agent based on either the measured oxidation-reduction potential or the measured corrosion potential.

[0039] (D3) Preferably, in the above (D2), it is desirable to determine that the aqueous solution irradiated with ultraviolet light contains the oxidizing agent when the measured oxidation-reduction potential is greater than 400 mV vs. SHE or when the measured corrosion potential is greater than the corrosion potential of 400 mV vs. SHE.

[0040] (D4) Preferably, in the above (D2) or (D3), when the aqueous solution irradiated with ultraviolet light contains the oxidizing agent, it is desirable to stop supplying the aqueous solution to the cation exchange resin tower and reduce the amount of the oxidizing agent supplied to the aqueous solution.

[0041] (D5) Preferably, in the above (D2) or (D3), when it is determined that the aqueous solution does not contain the oxidant and the measured oxidation-reduction potential is smaller than a third set value of the oxidation-reduction potential, the amount of the oxidant supplied to the aqueous solution is increased so that the oxidation-reduction potential becomes the third set value, or when it is determined that the measured corrosion potential is smaller than a fourth set value of the corrosion potential, the corrosion potential becomes the fourth set value. (D6) Preferably, in any one of the above (D2) to (D4), the reducing decontamination agent contained in the aqueous solution is decomposed by irradiating the aqueous solution with ultraviolet light, and the reducing decontamination agent is decomposed in a catalytic tower in which a catalyst is present by the catalyst and the oxidizing agent supplied to the catalytic tower. (D7) Preferably, in the above (D6), when it is determined that the aqueous solution does not contain the oxidizing agent, the aqueous solution is supplied to a cation exchange resin tower, metal cations contained in the aqueous solution are removed in the cation exchange resin tower, the oxidizing agent is supplied to the aqueous solution from which the metal cations have been removed, and the aqueous solution containing the oxidizing agent and the reduction decontamination agent is supplied to the catalyst tower. Effect of the Invention

[0042] According to the present invention, the time required for decomposing the reduction decontamination agent can be further shortened. [Brief description of the drawings]

[0043] [Figure 1] 1 is a flowchart showing the procedure of a chemical decontamination method according to a first embodiment of the present invention, which is applied to a boiling water nuclear power plant S13. [Diagram 2] 2 is a detailed flowchart of a decomposition process of a reduction decontamination agent included in the procedure of the chemical decontamination method shown in FIG. 1. [Diagram 3] FIG. 3 is an explanatory diagram showing a state in which a chemical decontamination apparatus used in carrying out the chemical decontamination method of the first embodiment shown in FIGS. 1 and 2 is connected to a recirculation piping of a boiling water nuclear power plant. [Figure 4] FIG. 4 is a detailed configuration diagram of the chemical decontamination apparatus shown in FIG. 3. [Diagram 5] FIG. 5 is a detailed configuration diagram of the oxidizing agent supplying device shown in FIG. [Figure 6] This is a characteristic diagram showing the relationship between the logarithm of the ratio of the Fe3+ concentration to the Fe2+ concentration in a reduction decontamination aqueous solution and the oxidation-reduction potential of the reduction decontamination aqueous solution, and is an explanatory diagram showing the non-detection area and the detection area of ​​hydrogen peroxide, respectively. [Figure 7] This is a characteristic diagram showing the relationship between the logarithm of the ratio of the Fe3+ concentration to the Fe2+ concentration in the reduction decontamination solution and the corrosion potential of the reduction decontamination solution, and is an explanatory diagram showing the non-detection region of hydrogen peroxide and the detection region of hydrogen peroxide, respectively. [Figure 8] FIG. 2 is a detailed configuration diagram of a chemical decontamination apparatus used in a chemical decontamination method according to a second embodiment of the present invention, which is applied to a boiling water nuclear power plant. [Figure 9] FIG. 3 is an explanatory diagram showing a state in which a chemical decontamination apparatus used in a chemical decontamination method according to a third embodiment, which is applied to a boiling water nuclear power plant, is connected to a purification system piping of the boiling water nuclear power plant, which is another preferred embodiment of the present invention. [Figure 10] 13 is a detailed flowchart of a decomposition step of a reduction decontamination agent included in the procedure of a chemical decontamination method according to a fourth embodiment of the present invention, which is applied to a boiling water nuclear power plant. [Figure 11]FIG. 11 is a detailed configuration diagram of a chemical decontamination apparatus used in carrying out the chemical decontamination method of Example 4. [Figure 12] FIG. 5 is a detailed configuration diagram of a chemical decontamination apparatus used in carrying out a chemical decontamination method according to a fifth embodiment of the present invention, which is applied to a boiling water nuclear power plant. [Figure 13] 13 is a detailed flowchart of a decomposition step of a reduction decontamination agent included in the procedure of a chemical decontamination method according to a sixth embodiment of the present invention, which is applied to a boiling water nuclear power plant. [Figure 14] FIG. 13 is a detailed configuration diagram of a chemical decontamination apparatus used in carrying out the chemical decontamination method of Example 6. [Figure 15] FIG. 13 is a detailed configuration diagram of a chemical decontamination apparatus used in carrying out a chemical decontamination method according to a seventh embodiment of the present invention, which is applied to a boiling water nuclear power plant. [Figure 16] 13 is a detailed flowchart of a decomposition step of a reduction decontamination agent included in the procedure of a chemical decontamination method according to an eighth embodiment of the present invention, which is applied to a boiling water nuclear power plant. [Figure 17] FIG. 13 is a detailed configuration diagram of a chemical decontamination apparatus used in carrying out the chemical decontamination method of Example 8. [Figure 18] FIG. 13 is a detailed configuration diagram of a chemical decontamination apparatus used in carrying out a chemical decontamination method according to a ninth embodiment, which is applied to a boiling water nuclear power plant, which is another preferred embodiment of the present invention. [Figure 19] 10 is a flowchart showing the procedure of a chemical decontamination method according to a tenth embodiment, which is another preferred embodiment of the present invention and is applied to a boiling water nuclear power plant. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] The inventors have carried out various studies on a chemical decontamination method applicable to a nuclear power plant, which can further shorten the time required for the decomposition step of a reduction decontamination agent in the chemical decontamination method applicable to a nuclear power plant.

[0045] In these studies, the inventors have investigated the effect of removing Fe ions contained in the reduction decontamination solution by irradiating the solution with ultraviolet light. 3+ ) divalent Fe ions (Fe 2+ ), an oxidizing agent (e.g., hydrogen peroxide) is injected into the reducing decontamination aqueous solution, and the reducing decontamination aqueous solution containing the oxidizing agent is irradiated with ultraviolet light to decompose the reducing decontamination agent (e.g., oxalic acid), and further, the reducing decontamination agent is decomposed by using a catalyst and an oxidizing agent downstream of the ultraviolet light irradiation. That is, the inventors considered that the time required for decomposition of the reducing decontamination agent can be further shortened by combining the decomposition of the reducing decontamination agent by irradiating the reducing decontamination aqueous solution containing the oxidizing agent with ultraviolet light and the decomposition of the reducing decontamination agent by using a catalyst and an oxidizing agent.

[0046] In this case, if an oxidizing agent (e.g., hydrogen peroxide) is contained in the reduction decontamination aqueous solution flowing into the cation exchange resin tower, which is arranged upstream of the catalyst tower and removes divalent Fe ions and cations of radioactive nuclides, the cation exchange resin in the cation exchange resin tower will be oxidized and deteriorated by the oxidizing agent. This will shorten the life of the cation exchange resin. Furthermore, decomposition products of the cation exchange resin generated by the deterioration of the cation exchange resin will be mixed into the reduction decontamination aqueous solution. Therefore, it is necessary to prevent the inflow of the oxidizing agent into the cation exchange resin tower.

[0047] The inventors have conducted extensive research into this issue. As a result of their research, the inventors have found that there is a correlation between the oxidation-reduction potential of the reducing decontamination solution downstream of the position of ultraviolet irradiation of the reducing decontamination solution and the presence or absence of hydrogen peroxide in the reducing decontamination solution. The reducing decontamination solution contains Fe ions, and the oxidation-reduction potential (E) of the reducing decontamination solution is determined by the divalent Fe ion concentration [Fe 2+ ] versus trivalent Fe ion concentration [Fe 3+ This can be expressed by the Nernst equation, which includes the ratio of

[0048] E=E 0 +(RT / nF) log([Fe 3+ ] / [Fe 2+]) …(1) Here, E 0 is the standard electrode potential, R is the gas constant, T is the temperature of the reduction decontamination aqueous solution, n is the valence, and F is the Faraday constant.

[0049] In addition, the [Fe 3+ ] / [Fe 2+ ] is Fe 2+ Fe 3+ Concentration ratio of (hereinafter, Fe 3+ / Fe 2+ Concentration ratio), log([Fe 3+ ] / [Fe 2+ ]) is the logarithm of the concentration ratio. 3+ ] / [Fe 2+ ]) and their concentration ratios, although the values ​​are different, are both [Fe 3+ ] / [Fe 2+ ] is essentially the same concept.

[0050] By using a reduction decontamination solution, such as an oxalic acid solution, on the surface of a chemically decontaminated object in a nuclear power plant, the oxide film containing iron and radioactive nuclides formed on the surface of the object (constituent material of the nuclear power plant) is dissolved. 3+ and radioactive nuclide ions are dissolved in an aqueous solution of oxalic acid. Oxalic acid ((COOH) 2 ) and Fe 3+ As described above, when ultraviolet light is irradiated to an oxalic acid solution containing Fe, the reaction shown in the following formula (2) occurs, and Fe 3+ Fe 2+ Furthermore, hydrogen peroxide, an oxidizing agent injected into the oxalic acid solution, reacts sharply with divalent Fe ions to generate hydroxyl radicals (·OH) according to the following formula (3):

[0051] Fe 3+ + (COOH) 2 → Fe 2+ + CO 2 + H 2O (ultraviolet irradiation environment) …(2) Fe 2+ +H 2 O 2 → Fe 3+ +OH - +·OH …(3) As can be seen from the reaction formula (3), if divalent Fe ions are present in the oxalic acid solution, these divalent Fe ions react with the hydrogen peroxide in the reduction decontamination solution, consuming the hydrogen peroxide. Therefore, it is believed that there is almost no hydrogen peroxide in the reduction decontamination solution, which contains a large amount of divalent Fe ions. In this state, the aforementioned Fe 3+ / Fe 2+ Conversely, when there are almost no divalent Fe ions, hydrogen peroxide can exist in the reduction decontamination solution. In this case, Fe 3+ / Fe 2+ The concentration ratio becomes larger.

[0052] With this in mind, the inventors discovered that Fe in an oxalic acid solution 3+ / Fe 2+ The relationship between the concentration ratio, the redox potential of the oxalic acid solution, and the presence or absence of hydrogen peroxide in the oxalic acid solution was investigated through experiments.

[0053] In this experiment, a simulated oxalic acid aqueous solution was used, which was generated by adding 2.7 g of oxalic acid dihydrate and 0.16 g of iron (II) oxalate dihydrate to 1000 g of water at 90°C, which is the same condition as the aqueous solution used for reduction decontamination in a nuclear power plant, for example, a boiling water nuclear power plant (BWR plant). This simulated oxalic acid aqueous solution was filled in a container, for example, a beaker. Iron (II) oxalate dihydrate was generated by mixing 2.7 g of oxalic acid dihydrate and 0.16 g of iron (II) oxalate dihydrate in the simulated oxalic acid aqueous solution. 3+ and Fe 2+ The source of the formation of iron(II) oxalate dihydrate is dissolved in the simulated oxalic acid solution, and the Fe 3+ and Fe 2+A redox potential meter using a standard hydrogen electrode was immersed in the simulated oxalic acid solution to measure the redox potential of the simulated oxalic acid solution. A portion of the simulated oxalic acid solution was taken from the beaker and filtered through a filter. The Fe in the filtered simulated oxalic acid solution was 2+ and Fe 3+ The concentrations of each of the above were measured. Next, hydrogen peroxide was injected into the simulated oxalic acid solution so that the hydrogen peroxide concentration in the simulated oxalic acid solution became 12 ppm. The oxidation-reduction potential of the simulated oxalic acid solution containing hydrogen peroxide and the Fe concentration of the simulated oxalic acid solution containing hydrogen peroxide were measured. 2+ and Fe 3+ The concentrations of each of the three compounds were measured, and the hydrogen peroxide concentration of the simulated oxalic acid solution was analyzed. 2+ Concentration, Fe 3+ The measurements of the concentration and hydrogen peroxide concentration were repeated.

[0054] The measured redox potentials obtained from the above experiments were 2+ Fe 3+ The logarithm of the concentration ratio (log([Fe 3+ ] / [Fe 2+ ]), i.e., Fe 2+ Fe 3+ The characteristic diagram in Figure 6 was created based on the logarithm of the concentration ratio of Fe and hydrogen peroxide and the presence or absence of detection. 2+ Fe 3+ The logarithm of the concentration ratio of Fe and FeO is shown on the horizontal axis, and the redox potential is shown on the horizontal axis. The redox potential on the horizontal axis is a value measured with a redox potentiometer using a standard hydrogen electrode. 2+ Fe 3+ Concentration ratio of (Fe 3+ / Fe 2+ Concentration ratio) logarithmic area, no hydrogen peroxide detected, Fe 3+ / Fe 2+ The logarithmic range of the concentration ratio is shown. In FIG. 6, hydrogen peroxide is detected, Fe 3+ / Fe2+ The area of ​​the logarithm of the concentration ratio is Fe 3+ / Fe 2+ The logarithm of the concentration ratio exceeds "1" and hydrogen peroxide is not detected. 3+ / Fe 2+ The area of ​​the logarithm of the concentration ratio is Fe 3+ / Fe 2+ This is the region where the logarithm of the concentration ratio is less than "1".

[0055] Hydrogen peroxide detected, Fe 3+ / Fe 2+ The area of ​​the logarithm of the concentration ratio is the [Fe 3+ ] / [Fe 2+ ], i.e., Fe 3+ / Fe 2+ Expressed as a concentration ratio, Fe 3+ / Fe 2+ The concentration ratio exceeds "10". In addition, the Fe 3+ / Fe 2+ The area of ​​the logarithm of the concentration ratio is Fe 3+ / Fe 2+ Expressed as a concentration ratio, Fe 3+ / Fe 2+ This is the region where the concentration ratio is "10" or less.

[0056] Based on the characteristics shown in Figure 6, the Fe concentration was calculated based on the measured redox potential of the oxalic acid solution. 3+ / Fe 2+ When the logarithm of the concentration ratio exceeds "1", the inventors considered that it can be determined that hydrogen peroxide remains in the oxalic acid solution irradiated with ultraviolet light. 3+ / Fe 2+ The inventors considered that even when the concentration ratio exceeds "10", it can be determined that hydrogen peroxide remains in the aqueous oxalic acid solution irradiated with ultraviolet light.

[0057] As described above, the presence or absence of hydrogen peroxide in the reducing decontamination solution can be determined by measuring the oxidation-reduction potential of the reducing decontamination solution after ultraviolet irradiation. As a result, when hydrogen peroxide, which is an oxidizing agent, does not exist in the reducing decontamination solution irradiated with ultraviolet rays, i.e., when Fe 3+ / Fe 2+ When the logarithm of the concentration ratio is "1" or less, and Fe 3+ / Fe 2+ When the concentration ratio is equal to or less than "10", the reducing decontamination aqueous solution can be supplied to a cation exchange resin tower. Then, hydrogen peroxide is injected into the reducing decontamination aqueous solution discharged from the cation exchange resin tower, and the reducing decontamination aqueous solution containing hydrogen peroxide can be supplied to a catalyst tower having a catalyst present therein.

[0058] Based on the measured redox potential, Fe 2+ Fe 3+ and the amount of oxidizing agent supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device that irradiates the aqueous solution for reduction decontamination with ultraviolet light and the aqueous solution in the ultraviolet irradiation device can be adjusted based on the logarithm of the concentration ratio or the concentration ratio obtained. 2+ Fe 3+ Adjusting the amount of oxidizing agent supplied to the reduction decontamination aqueous solution based on either the logarithm of the concentration ratio or the determined concentration ratio means adjusting the amount of oxidizing agent supplied to the reduction decontamination aqueous solution based on the measured oxidation-reduction potential.

[0059] Based on the measured redox potential, Fe 2+ Fe 3+ When the logarithm of the concentration ratio exceeds "1" or the calculated concentration ratio exceeds "10", the amount of oxidizing agent, for example, hydrogen peroxide, injected into the reduction decontamination solution upstream of the ultraviolet irradiation device is reduced to reduce the amount of Fe. 2+ Fe 3+The logarithm of the concentration ratio can be set to 1 or less, or the calculated concentration ratio can be set to 10 or less, and the hydrogen peroxide concentration of the reduction decontamination aqueous solution discharged from the ultraviolet irradiation device can be set to "0." Therefore, the reduction decontamination aqueous solution discharged from the ultraviolet irradiation device can be supplied to the cation exchange resin tower.

[0060] UV-irradiated Fe 3+ / Fe 2+ The reduction decontamination aqueous solution having a logarithm of the concentration ratio of "1" or less, or the reduction decontamination aqueous solution having a logarithm of the concentration ratio of "10" or less is supplied to the cation exchange resin tower, so that the deterioration of the cation exchange resin in the cation exchange resin tower can be suppressed. In addition, the Fe 3+ / Fe 2+ By injecting an oxidizing agent (e.g., hydrogen peroxide) into either a reducing decontamination aqueous solution having a logarithm of the concentration ratio of "1" or less or a reducing decontamination aqueous solution having a logarithm of the concentration ratio of "10" or less, and supplying this reducing decontamination aqueous solution to a catalytic tower, the amount of Fe ions flowing into the catalytic tower is reduced, and the reducing decontamination agent (e.g., oxalic acid) can be decomposed in the catalytic tower while suppressing the decrease in activity of the catalyst in the catalytic tower. With this method, the reducing decontamination agent is decomposed by the irradiation of the reducing decontamination aqueous solution with ultraviolet light and the action of the catalyst and the oxidizing agent, so the decomposition efficiency of the reducing decontamination agent can be improved.

[0061] More than that, Fe 3+ / Fe 2+ The region where an oxidizing agent (e.g., hydrogen peroxide) is present and the region where an oxidizing agent is not present have been described based on either the logarithm of the concentration ratio or the concentration ratio. However, as shown in the characteristic diagram of FIG. 6, the presence or absence of an oxidizing agent in a reduction decontamination aqueous solution (e.g., an oxalic acid aqueous solution) can also be identified by the oxidation-reduction potential. Identification of the presence or absence of an oxidizing agent in a reduction decontamination aqueous solution by the oxidation-reduction potential will be described with reference to FIG. 6. In FIG. 6, the region of the oxidation-reduction potential where hydrogen peroxide is detected is the region where the oxidation-reduction potential exceeds "400 mV vs. SHE", and the region of the oxidation-reduction potential where hydrogen peroxide is not detected is the region where the oxidation-reduction potential is "400 mV vs. SHE" or less.

[0062] When the redox potential exceeds the set value of the redox potential, "400 mV vs. SHE," the amount of hydrogen peroxide supplied to at least one of the reduction decontamination aqueous solution (e.g., an oxalic acid aqueous solution) upstream of the ultraviolet irradiation device and the oxalic acid aqueous solution in the ultraviolet irradiation device can be reduced to reduce the redox potential of the oxalic acid aqueous solution to "400 mV vs. SHE" or less. Therefore, the oxalic acid aqueous solution discharged from the ultraviolet irradiation device does not contain hydrogen peroxide, and this oxalic acid aqueous solution can be supplied to the cation exchange resin tower.

[0063] In order to clarify the decomposition process of oxalic acid in the examples described later, the reactions of Fe ions, hydrogen peroxide, and oxalic acid in the ultraviolet irradiation, cation exchange resin tower, and catalyst tower, respectively, will be described.

[0064] In a reduction decontamination solution, for example, an oxalic acid solution, trivalent Fe ions react with oxalic acid to form Fe(III) oxalate complexes (Fe III (C 2 O 4 ) 3 3- ) is formed.

[0065] Fe 3+ +3(COOH) 2 → Fe III (C 2 O 4 ) 3 3- +6H + …(4) Fe(III) oxalate complex (Fe III (C 2 O 4 ) 3 3- ) is irradiated with ultraviolet light, and the valence of the Fe ion is reduced from trivalent to divalent, forming an Fe(II) oxalate complex (Fe II (C 2 O 4 ) 2 2-) and the oxalate radical (·C 2 O 4 - ) is produced (see reaction in formula (5) below). Furthermore, as shown in formula (6), this oxalate anion radical reacts with an Fe(III) oxalate complex. The reaction in formula (6) is publicly known. In the reaction in formula (6), an electron from the oxalate anion radical is extracted by the Fe(III) of the Fe(III) oxalate complex, and one oxalate ion molecule is decomposed into carbon dioxide. Then, the Fe(III) that extracted the electron is reduced to Fe(II), and as shown on the right side of formula (6), the Fe(III) oxalate complex becomes an Fe(II) oxalate complex, and one extra oxalic acid is removed to produce an oxalate ion (C 2 O 4 2- ) is generated.

[0066] Fe III (C 2 O 4 ) 3 3- → Fe II (C 2 O 4 ) 2 2- +·C 2 O 4 - …(5) C 2 O 4 - +Fe III (C 2 O 4 ) 3 3- → 2CO 2 +Fe II (C 2 O 4 ) 2 2- +C 2 O 4 2- …(6) The oxalic acid complex with divalent iron ions supplies divalent Fe ions to the oxalic acid aqueous solution up to the saturation solubility, and these ions undergo the Fenton reaction shown in the above equation (2) with hydrogen peroxide added to the oxalic acid aqueous solution.

[0067] The hydroxyl radical (·OH) generated by the reaction shown in formula (2) reacts with the oxalate ion to produce the oxalate anion radical (·C 2 O 4 2- ) is formed, and oxalic acid is decomposed into carbon dioxide via reaction (6).

[0068] OH+C 2 O 4 2- → ·C 2 O 4 2- +OH - …(7) Through these reactions, the decomposition of oxalic acid progresses due to UV irradiation, and when hydrogen peroxide is completely consumed, the formation of divalent iron ions progresses, so the redox potential at the outlet of the UV irradiation device becomes +400mV or less, and even if water is passed through the ion exchange resin, there is no hydrogen peroxide, so the resin does not deteriorate, and the divalent Fe ions are adsorbed onto the cation exchange resin by ion exchange according to the reaction shown in formula (8). In return, protons or hydrazine are released from the cation exchange resin.

[0069] Fe 2+ +(R-SO 3 - H + ) n → (R-SO 3 - H + ) n-2 (SO 3 - )2Fe 2+ +2H + …(8) This reduces the iron concentration in the oxalic acid solution, suppressing the deposition of iron oxide on the catalyst surface in the catalytic tower located downstream of the cation exchange resin tower, and inhibiting the decrease in the decomposition activity of the catalyst. Hydrogen peroxide injected into the oxalic acid solution just before the catalytic tower is decomposed by the action of the catalyst in the catalytic tower. The hydroxyl radical (·OH) generated at that time is converted into oxalate anion radical (·C2 O 4 2- Then, the oxalic acid in the aqueous solution is decomposed into carbon dioxide by the reaction shown in formula (6).

[0070] As described above, the oxalic acid contained in the aqueous oxalic acid solution can be decomposed by irradiating the aqueous oxalic acid solution with ultraviolet light and by the action of a catalyst and an oxidizing agent (e.g., hydrogen peroxide), so that the efficiency of decomposing the oxalic acid can be improved.

[0071] Furthermore, instead of the oxidation-reduction potential, the corrosion potential of the reduction decontamination aqueous solution (oxalic acid aqueous solution) may be measured, and the amount of the oxidizing agent supplied to the oxalic acid aqueous solution may be adjusted based on the measured corrosion potential. This will be specifically described with reference to FIG.

[0072] Hydrogen peroxide (oxidizer) is detected, and Fe is determined based on the corrosion potential. 3+ / Fe 2+ The area of ​​the logarithm of the concentration ratio is calculated based on the redox potential shown in Figure 6. 2+ Fe 3+ Concentration ratio of (Fe 3+ / Fe 2+ As shown in Fig. 7, the logarithm of the Fe concentration ratio 3+ / Fe 2+ The logarithm of the concentration ratio exceeds "1" and hydrogen peroxide is not detected. 3+ / Fe 2+ The area of ​​the logarithm of the concentration ratio is Fe 3+ / Fe 2+ The logarithm of the concentration ratio is less than 1. The Fe concentration is calculated based on the corrosion potential of the oxalic acid solution irradiated with ultraviolet light in the ultraviolet irradiation device. 2+ Fe 3+ When the logarithm of the concentration ratio exceeds "1", the amount of hydrogen peroxide supplied to at least one of the oxalic acid aqueous solutions upstream of the ultraviolet irradiation device and the oxalic acid aqueous solution in the ultraviolet ray collection device is determined as follows: 2+ Fe 3+ The concentration ratio is adjusted based on the logarithm of the concentration ratio of

[0073] In addition, the Fe calculated based on the corrosion potential at which hydrogen peroxide (oxidizer) was detected 3+ / Fe 2+ The concentration ratio region is Fe 3+ / Fe 2+ This is the region where the concentration ratio exceeds "10" and hydrogen peroxide is not detected. 3+ / Fe 2+ The concentration ratio region is Fe 3+ / Fe 2+ This is the region where the concentration ratio is "1" or less.

[0074] From the above, the Fe 2+ Fe 3+ Adjusting the amount of oxidant supplied to the reducing decontamination aqueous solution based on either the logarithm of the concentration ratio or the concentration ratio means adjusting the amount of oxidant supplied to the reducing decontamination aqueous solution based on the measured corrosion potential.

[0075] Based on the measured corrosion potential, Fe 2+ Fe 3+ When the logarithm of the concentration ratio of Fe exceeds "1", or when the Fe concentration ratio is calculated based on the measured corrosion potential, 2+ Fe 3+ When the concentration ratio exceeds "10", for example, the amount of oxidizing agent, for example, hydrogen peroxide, injected into the reduction decontamination solution upstream of the ultraviolet irradiation device can be reduced to reduce the amount of Fe. 2+ Fe 3+ The logarithm of the concentration ratio can be set to 1 or less, or the concentration ratio can be set to "10" or less. This makes it possible to set the hydrogen peroxide concentration in the reduction decontamination aqueous solution discharged from the ultraviolet irradiation device to "0", and the reduction decontamination aqueous solution discharged from the ultraviolet irradiation device can be supplied to the cation exchange resin tower.

[0076] Fe calculated based on corrosion potential 3+ / Fe 2+The region where an oxidizing agent (e.g., hydrogen peroxide) is present and the region where an oxidizing agent is not present have been described based on either the logarithm of the concentration ratio or the concentration ratio. However, as shown in the characteristic diagram of FIG. 7, the presence or absence of an oxidizing agent in a reduction decontamination aqueous solution (e.g., an oxalic acid aqueous solution) can also be identified by the corrosion potential. Identification of the presence or absence of an oxidizing agent in a reduction decontamination aqueous solution by the corrosion potential will be described with reference to FIG. 7. In FIG. 7, the corrosion potential region where hydrogen peroxide is detected is the region where the corrosion potential exceeds "400 mV vs. SHE", and the corrosion potential region where hydrogen peroxide is not detected is the region where the corrosion potential is equal to or less than "400 mV vs. SHE".

[0077] An embodiment of the present invention that reflects the above-mentioned study results will be described in detail below. EXAMPLES

[0078] A chemical decontamination method according to a first embodiment, which is a preferred embodiment of the present invention, will be described with reference to Figures 1, 2, 3, 4, and 5. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a boiling water reactor plant (BWR plant).

[0079] The schematic configuration of this BWR plant will be described with reference to Fig. 3. The BWR plant 1, which is a nuclear power plant, includes a nuclear reactor 2, a turbine 9, a condenser 10, a recirculation system, a reactor cleanup system, and a feedwater system. The nuclear reactor 2 is a steam generating device, and has a reactor pressure vessel (hereinafter referred to as RPV) 3 incorporating a nuclear core 4, and a plurality of jet pumps 5 are installed in an annular downcomer formed between the outer surface of a core shroud (not shown) surrounding the nuclear core 4 in the RPV 3 and the inner surface of the RPV 3. A large number of fuel assemblies (not shown) are loaded in the nuclear core 4. The fuel assembly includes a plurality of fuel rods filled with a plurality of fuel pellets made of nuclear fuel material.

[0080] The recirculation system has a recirculation system pipe 6 made of stainless steel, and a recirculation pump 7 installed in the recirculation system pipe 6. The feedwater system is configured by installing a condensate pump 12, a condensate purification device (e.g., a condensate demineralizer) 13, a low-pressure feedwater heater 14, a feedwater pump 15, and a high-pressure feedwater heater 16 in this order from the condenser 10 toward the RPV 3 in a feedwater pipe 11 connecting the condenser 10 and the RPV 3. The reactor cleanup system is installed a cleanup system pump 21, a regenerative heat exchanger 22, a non-regenerative heat exchanger 23, and a reactor water purification device 24 in this order in a cleanup system pipe 20 connecting the recirculation system pipe 6 and the feedwater pipe 11. The cleanup system pipe 20 is connected to the recirculation system pipe 6 upstream of the recirculation pump 7. The reactor 2 is installed in a reactor containment vessel 19 arranged in a reactor building (not shown).

[0081] Cooling water (hereinafter referred to as reactor water) in the RPV 3 is pressurized by a recirculation pump 7 and is injected into a jet pump 5 through a recirculation system piping 6. The reactor water present around the nozzle of the jet pump 5 in the downcomer is also sucked into the jet pump 5 and is supplied to the reactor core 4 together with the reactor water injected into the jet pump 5. The reactor water supplied to the reactor core 4 is heated by heat generated by nuclear fission of the nuclear fuel material in the fuel rods in the fuel assemblies, and a part of it becomes steam. This steam is guided to a turbine 9 through a main steam piping 8 to rotate the turbine 9 after moisture is removed by a steam separator (not shown) and a steam dryer (not shown) installed in the RPV 3. A generator (not shown) connected to the turbine 9 rotates, generating electricity.

[0082] Steam discharged from the turbine 9 is condensed in a condenser 10 to become water. This water is supplied as feed water through a feed water piping 11 into the RPV 3. The feed water flowing through the feed water piping 11 is pressurized by a condensate pump 12, impurities are removed in a condensate purification device 13, and the pressure is further increased by a feed water pump 15. This feed water is heated by extracted steam extracted from the turbine 9 through an extraction piping 17 in a low pressure feed water heater 14 and a high pressure feed water heater 16, and is introduced into the RPV 3. A drain water recovery piping 18 connected to the high pressure feed water heater 16 and the low pressure feed water heater 14 is connected to the condenser 10.

[0083] A part of the reactor water flowing in the recirculation system piping 6 flows into the purification system piping 20 by driving the purification system pump 21, and is cooled in the regenerative heat exchanger 22 and the non-regenerative heat exchanger 23, and then purified in the reactor water purification device 24. The purified reactor water is heated in the regenerative heat exchanger 22 and returned to the RPV 3 via the purification system piping 20 and the feedwater piping 11.

[0084] In the chemical decontamination method of this embodiment, a chemical decontamination apparatus 28 is used, and this chemical decontamination apparatus 28 is connected to the recirculation piping 6 as shown in FIG.

[0085] The detailed configuration of the chemical decontamination apparatus 28 will be described with reference to Fig. 4. The chemical decontamination apparatus 28 used in this embodiment includes a circulation pipe 29, circulation pumps 30 and 38, a heater 31, a cation exchange resin tower 34, a mixed bed resin tower 35, a water level adjustment tank 37, a chemical decontamination agent supply device 47, a pH adjuster injection device 57, a cooler 32, an ultraviolet ray irradiation device 33, a catalyst tower 36, an oxidizing agent supply device 39, an oxidation-reduction potentiometer 61, a concentration ratio grasping device 62, and a display device 63.

[0086] An on-off valve 64, a circulation pump 30, a valve 65, a heater 31, a valve 66, a flow rate control valve 67, an ultraviolet ray irradiation device 33, a flow rate control valve 68, a cation exchange resin tower 34, a flow rate control valve 70, a catalyst tower 36, a water level adjustment tank 37, a circulation pump 38, and an on-off valve 71 are provided in the circulation pipe 29 in this order from the upstream. For example, a low pressure mercury lamp is used as an ultraviolet ray source for the ultraviolet ray irradiation device 33. The catalyst tower 36 is filled with an activated carbon catalyst having, for example, 0.5% ruthenium impregnated on the surface of the activated carbon. One end of a pipe 72 bypassing the valve 65, the heater 31, and the valve 66 is connected to the circulation pipe 29 between the circulation pump 30 and the valve 65. The other end of the pipe 72 is connected to the circulation pipe 29 between the valve 66 and the flow rate control valve 67. A valve 73, a cooler 32, and a valve 74 are provided in the pipe 72. One end of a pipe 69A bypassing the cation exchange resin tower 34 is connected to the circulation pipe 29 between the connection point of the pipe 78 and the circulation pipe 29 and the flow control valve 68. The other end of the pipe 69A is connected to the circulation pipe 29 between the connection point of the pipe 80 and the circulation pipe 29 and the cation exchange resin tower 34. A flow control valve 69 and a mixed bed resin tower 35 are installed in the pipe 69A. The cation exchange resin tower 34 is filled with a cation exchange resin. This cation exchange resin is an H-type cation exchange resin. The mixed bed resin tower 35 is filled with a mixture of an H-type cation exchange resin, which is a cation exchange resin, and an OH-type anion exchange resin, which is an anion exchange resin.

[0087] As shown in FIG. 5, the oxidizing agent supply device 39 has a chemical tank 40, supply pumps 41 and 44, and supply pipes 42 and 45. One end of the supply pipe 42 is connected to the chemical tank 40, and the other end of the supply pipe 42 is connected to the circulation pipe 29 between the flow rate control valve 67 and the ultraviolet ray irradiation device 33. The supply pump 41 and the valve 43 are installed in the supply pipe 42. One end of the supply pipe 45 is connected to the chemical tank 40, and the other end of the supply pipe 45 is connected to the circulation pipe 29 between the flow rate control valve 70 and the catalyst tower 36. The other end of the supply pipe 45 may be directly connected to the ultraviolet ray irradiation device 33, instead of being connected to the circulation pipe 29 between the flow rate control valve 70 and the catalyst tower 36. The chemical tank 40 is filled with hydrogen peroxide as an oxidizing agent. As the oxidizing agent, ozone or water having oxygen dissolved therein may be used in addition to hydrogen peroxide. The supply pump 44 and the valve 46 are installed in the supply pipe 42. An oxidation-reduction potentiometer 61 is connected to the circulation pipe 29 between the connection point of the pipe 78 and the circulation pipe 29 and the ultraviolet irradiation device 33 .

[0088] One end of a pipe 75 that bypasses the flow rate control valve 67, the ultraviolet irradiation device 33, the flow rate control valve 68, the cation exchange resin tower 34, the flow rate control valve 70, and the catalyst tower 36 is connected to the circulation pipe 29 between the connection point between the one end of the pipe 72 on the valve 74 side and the circulation pipe 29, and the flow rate control valve 67. The other end of the pipe 75 is connected to the circulation pipe 29 between the connection point between the circulation pipe 29 and the supply pipe 55, and the catalyst tower 36. The flow rate control valves 76, 77A, and 77 are installed in the pipe 75 in this order from the upstream. One end of a pipe 78 is connected to the pipe 75 between the flow rate control valve 76 and the flow rate control valve 77A. The other end of the pipe 78 is connected to the circulation pipe 29 between the ultraviolet irradiation device 33 and the flow rate control valve 68. The other end of the pipe 78 is connected to the pipe 75 between the flow rate control valve 76 and the flow rate control valve 77A. One end of a pipe 80 having a flow rate control valve 81 is connected to the circulation pipe 29 between the cation exchange resin tower 34 and the flow rate control valve 70. The other end of the pipe 80 is connected to the pipe 75 between the flow rate control valve 77A and the flow rate control valve 77.

[0089] The chemical decontamination agent supply device 47 has an oxidative decontamination agent tank 48, an oxidative decontamination agent supply pipe 49, a reductive decontamination agent tank 51, a reductive decontamination agent supply pipe 52, a tank 54, and a supply pipe 55. The oxidative decontamination agent supply pipe 49 connected to the oxidative decontamination agent tank 48 is connected to the tank 54. A valve 50 is provided on the oxidative decontamination agent supply pipe 49. The reductive decontamination agent supply pipe 52 connected to the reductive decontamination agent tank 51 is connected to the tank 54. A valve 53 is provided on the reductive decontamination agent supply pipe 52. A supply pipe 55 having a valve 56 and connected to the tank 54 is connected to the circulation pipe 29 between the catalyst tower 36 and the water level adjustment tank 37.

[0090] The oxidative decontamination agent is permanganic acid or potassium permanganate. The reductive decontamination agent is at least one of oxalic acid, malonic acid, formic acid, and ascorbic acid. The oxidative decontamination agent tank 48 is filled with an aqueous solution of an oxidative decontamination agent, such as potassium permanganate. The reductive decontamination agent tank 51 is filled with an aqueous solution of a reductive decontamination agent, such as oxalic acid.

[0091] A pH adjuster injection device 57 has a chemical tank 58 and an injection pipe 59. The chemical tank 58 is connected to the circulation pipe 29 between the connection point of the supply pipe 55 and the circulation pipe 29 and the water level adjustment tank 37 by the injection pipe 59 having a valve 60. An aqueous solution of hydrazine, which is a pH adjuster, is filled in the chemical tank 58.

[0092] A pH meter 95 is installed in the circulation pipe 29 between the water level adjustment tank 37 and the circulation pump 38.

[0093] A water sampling pipe 82 having a valve 83 is connected to the circulation pipe 29 between the valve 66 and the flow rate control valve 67. A water sampling pipe 84 having a valve 85 is connected to the circulation pipe 29 between the installation position of the oxidation-reduction potentiometer 61 in the circulation pipe 29 and the ultraviolet irradiation device 33. A water sampling pipe 86 having a valve 87 is connected to the circulation pipe 29 between the connection point of the circulation pipe 29 and pipe 80 and the cation exchange resin tower 34. A water sampling pipe 88 having a valve 89 is connected to the circulation pipe 29 between the connection point of the circulation pipe 29 and pipe 75 and the catalyst tower 36.

[0094] There is provided an operation panel (not shown) for operating the chemical decontamination equipment 28. This operation panel is provided with levers or buttons for individually operating all the valves and all the pumps provided in the chemical decontamination equipment 28. In addition, a display device 63 is provided on the operation panel.

[0095] The operation of the BWR plant 1 is stopped after one operation cycle is completed. After this operation shutdown, some of the fuel assemblies loaded in the reactor core 4 are removed as spent fuel assemblies, and new fuel assemblies with a burnup of 0 GWd / t are loaded into the reactor core 4. After such refueling is completed, the BWR plant 1 is restarted for operation in the next operation cycle. Maintenance and inspection of the BWR plant 1 is performed during the period when the BWR plant 1 is shut down for refueling.

[0096] During the period in which the operation of the BWR plant 1 is suspended as described above, the chemical decontamination method of this embodiment is carried out on a stainless steel piping system connected to the RPV 3, which is one of the stainless steel members in the BWR plant 1, such as the recirculation system piping 6. In this chemical method, oxidation decontamination and reduction decontamination are carried out on the recirculation system piping 6 on the inner surface of which an oxide film containing iron oxide, chromium oxide, and radioactive nuclides is formed.

[0097] A chemical decontamination method of this embodiment, which is targeted at the recirculation piping 6 in a BWR plant 1, will be described below based on the procedures shown in Figures 1 and 2. In the chemical decontamination method of this embodiment, a chemical decontamination apparatus 28 is used, and steps S1 to S7 and S21 to S23 shown in Figure 1 are performed. Step S7 includes steps S8 to S10, S16, S17, and S20 shown in Figure 2. Furthermore, step S10 includes steps S11 to S15, S18, and S19 as shown in Figure 2.

[0098] First, a chemical decontamination apparatus is connected to a piping system in which a chemical decontamination method is performed (step S1). During the operation shutdown period of the BWR plant 1 after the operation of the BWR plant 1 is stopped, both ends of the circulation piping 29 of the chemical decontamination apparatus 28, which is a temporary facility, are connected to the recirculation system piping 6 made of stainless steel (constituent member of the nuclear power plant). The recirculation system piping 6 is a constituent member of the BWR plant. The operation of connecting the circulation piping 29 to the recirculation system piping 6 will be specifically described. After the operation of the BWR plant 1 is stopped, for example, the bonnet of the valve 25 installed in the purification system piping 20 connected to the recirculation system piping 6 is opened and the purification system pump 21 side of the bonnet is sealed. One end of the circulation piping 29 of the chemical decontamination apparatus 28, that is, the end of the circulation piping 29 on the side of the on-off valve 71 is connected to the flange of the valve 25. As a result, one end of the circulation piping 29 is connected to the recirculation system piping 6 upstream of the recirculation pump 7. Furthermore, the bonnet of valve 27 provided on branch pipe 26, such as a drain pipe or instrumentation pipe, connected to recirculation system pipe 6 downstream of recirculation pump 7 is opened, and the side of this bonnet opposite to recirculation system pipe 6 is closed. The other end of circulation pipe 29, i.e., the end of circulation pipe 29 on the on-off valve 64 side, is connected to the flange of valve 27. As a result, the other end of circulation pipe 29 is connected to recirculation system pipe 6 downstream of recirculation pump 7.

[0099] In this manner, by connecting both ends of the circulation piping 29 to the recirculation system piping 6, a closed loop including the recirculation system piping 6 and the circulation piping 29 is formed. Each opening of the RPV 3 at both ends of the recirculation system piping 6 is closed with a plug (not shown) so that the chemical decontamination aqueous solution does not flow into the RPV 12.

[0100] Water is filled in the piping system of the object to be chemically decontaminated and in the chemical decontamination equipment, and the temperature of the water is increased after the water is filled (step S2). Water is filled in the recirculation system piping 6 and the circulation piping 29 of the chemical decontamination equipment 28. This water filling is performed, for example, by using a reactor auxiliary cooling water system (not shown). With the on-off valve 64, the valves 65 and 66, the flow rate control valves 67, 68 and 70, and the on-off valve 71 each open and the other valves closed, cooling water is supplied from the reactor auxiliary cooling water system into the circulation piping 29 and the recirculation system piping 6, and these pipings are filled with water. Then, the circulation pumps 30 and 38 are driven, and the water is circulated in a closed loop including the circulation piping 29 and the recirculation system piping 6. The water circulating in this closed loop is heated to 90°C by the heater 31.

[0101] An operator manually operates a lever or button provided on an operation panel that outputs control commands to all valves and all pumps provided in the chemical decontamination apparatus 28. By operating the corresponding lever or button, the above-mentioned control command is output from the operation panel, and the corresponding valve is opened or closed by this control command, and the pump is driven or stopped. For example, when opening the above-mentioned flow rate control valve 67, the operator manually turns a lever provided on the operation panel corresponding to the flow rate control valve 67 in the "open" direction, and a control command to open the flow rate control valve 67 is output from the operation panel. Based on this control command, the flow rate control valve 67 opens. For example, the "drive" and "stop" of the circulation pump 30 are also performed based on a control command output from the operation panel by the operator's button operation. In the specification of the present application, the description of the manual operation of the lever or button by the operator and the output of the control command from the operation panel is omitted, and the name of the device to be operated and the operation of the device to be operated, for example, "open the valve", "close the valve", "drive the pump" and "stop the pump" are simply described.

[0102] Oxidative decontamination is carried out (step S3). In the BWR plant 1 that has been in operation, an oxide film containing iron oxide, chromium oxide and radioactive nuclides is formed on the inner surface of the recirculation system piping 6 that comes into contact with the reactor water in the RPV 3. Chemical decontamination is carried out to dissolve this oxide film. This chemical decontamination includes oxidative decontamination and reduction decontamination.

[0103] First, the oxidative decontamination will be described. By opening the valve 50, the aqueous solution of potassium permanganate (aqueous oxidative decontamination agent) in the oxidative decontamination agent tank 48 is supplied into the tank 54 through the oxidative decontamination agent supply pipe 49. At this time, the valve 53 is closed. The oxidative decontamination agent (potassium permanganate) concentration C of the oxidative decontamination agent aqueous solution (potassium permanganate aqueous solution) flowing in the closed loop including the recirculation system pipe 6 and the circulation pipe 29 is SMn The tank 54 of the chemical decontamination agent supply device 47 is filled with an aqueous solution of an oxidizing decontamination agent so that the concentration of the aqueous solution of the oxidizing decontamination agent in the tank 54 is V (unit: ppm) of 200 to 500 ppm. OMn (unit: L), the oxidizing agent concentration in tank 54 is C OMn (Unit: ppm) is calculated using equation (9).

[0104] C OMn =C SMn ×V S ÷V OMn … (9) With the flow control valves 67, 79, and 81 closed and the flow control valves 76, 77A, and 77 open, the valve 56 is opened to inject the aqueous potassium permanganate solution into the tank 54 through the supply pipe 55 into the water in the circulation pipe 29. The amount of the aqueous potassium permanganate solution supplied to the circulation pipe 29 is controlled by adjusting the opening of the valve 56. By injecting the aqueous potassium permanganate solution from the tank 54, the potassium permanganate concentration of the aqueous potassium permanganate solution flowing through the circulation pipe 29 and the recirculation system pipe 6 becomes within a range of 200 to 500 ppm, for example, 300 ppm.

[0105] An aqueous potassium permanganate solution having a potassium permanganate concentration of 300 ppm at 90° C. is supplied from the circulation pipe 29 to the recirculation pipe 6. In the recirculation pipe 6, the aqueous potassium permanganate solution comes into contact with an oxide film containing iron oxide, chromium oxide and radioactive nuclides formed on the inner surface of the recirculation pipe 6, and performs oxidative decontamination. This oxidative decontamination causes the chromium oxide contained in the oxide film to dissolve into the aqueous potassium permanganate solution. The pH meter 95 measures the pH of the aqueous potassium permanganate solution returned from the recirculation pipe 6 to the circulation pipe 29. Such a pH meter 95 is used to monitor the pH of the aqueous potassium permanganate solution returned from the recirculation pipe 6 to the circulation pipe 29. The aqueous potassium permanganate solution circulates in a closed loop including the circulation pipe 29 and the recirculation pipe 6, and performs oxidative decontamination of the recirculation pipe 6. When the oxidative decontamination time for the recirculation pipe 6 has elapsed a predetermined time, the oxidative decontamination is completed. Specifically, when 2 to 6 hours have passed since the amount of potassium permanganate aqueous solution required for the oxidative decontamination is supplied to the circulation pipe 29, the oxidative decontamination ends.

[0106] The oxidative decontamination agent contained in the oxidative decontamination liquid is decomposed (step S4). When the oxidative decontamination is completed, the valve 53 is opened and the oxalic acid aqueous solution, which is the aqueous solution of the reductive decontamination agent in the reductive decontamination agent tank 51, is supplied to the tank 54 through the reductive decontamination agent supply pipe 52. At this time, the valve 50 is closed. In the process of step S4, the oxalic acid contained in the aqueous oxalic acid solution functions as a decomposition agent of potassium permanganate, which is the oxidative decontamination agent. By opening the valve 56, the aqueous oxalic acid solution in the tank 54 is supplied to the circulation pipe 29 through the supply pipe 55 so that an amount of oxalic acid required to decompose potassium permanganate contained in the aqueous potassium permanganate solution present in the above-mentioned closed loop is supplied to the circulation pipe 29. The amount of the aqueous oxalic acid solution supplied to the circulation pipe 29 is controlled by adjusting the opening degree of the valve 56.

[0107] Potassium permanganate concentration is C OMn (unit: ppm) and the volume of the oxalic acid solution to be filled in the tank 54 is V ROA2(unit: L), the oxalic acid concentration C required to decompose potassium permanganate and produce manganese hydroxide is ROA2 (unit: ppm) is calculated by the following formula (10).

[0108] C ROA2 =(5 / 2)×(90 / 158)×C OMn ×V S ÷V ROA2 …(10) The potassium permanganate contained in the potassium permanganate aqueous solution present in the tank 54 is decomposed by the oxalic acid contained in the oxalic acid aqueous solution supplied from the reduction decontamination agent tank 51 to the tank 54. Furthermore, the potassium permanganate contained in the potassium permanganate aqueous solution present in each of the circulation pipe 29 and the recirculation system pipe 6 is decomposed by the oxalic acid contained in the oxalic acid aqueous solution injected into the circulation pipe 29. The supply of the oxalic acid aqueous solution from the supply pipe 55 to the circulation pipe 29 is performed while the potassium permanganate aqueous solution is flowing in the closed loop so that the oxalic acid aqueous solution is evenly mixed with the potassium permanganate aqueous solution present in the closed loop described above.

[0109] The potassium permanganate contained in the potassium permanganate aqueous solution, which is an oxidative decontamination solution, reacts with the injected oxalic acid (decomposer) as shown in the following formula (11) and is decomposed to form manganese hydroxide (2Mn(OH) 2 )

[0110] 2KMnO 4 +5(COOH) 2 = 2Mn(OH) 2 +2KOH+5CO 2 +2H 2 O…(11) When the potassium permanganate contained in the potassium permanganate aqueous solution is decomposed, the potassium permanganate aqueous solution returned from the recirculation system piping 6 to the circulation piping 29 changes from purple to colorless and transparent. After it is confirmed that the potassium permanganate aqueous solution has become colorless and transparent, the decomposition process of the oxidative decontamination agent is terminated. The fact that the potassium permanganate aqueous solution has become colorless and transparent can be confirmed, for example, by opening the valve 83 and observing the color of the aqueous solution sampled from the circulation piping 29 by the water sampling tube 82. After the decomposition of potassium permanganate is completed, an oxalic acid aqueous solution with a low oxalic acid concentration circulates in the closed loop described above.

[0111] Oxidative decontamination of the inner surface of the recirculation system piping 6 may be performed using an aqueous permanganate solution instead of the aqueous potassium permanganate solution. When performing such oxidative decontamination, the aqueous permanganate solution is injected from the oxidative decontamination agent tank 48 through the tank 54 into the circulation system piping 29. The injected aqueous permanganate solution is supplied from the circulation system piping 29 to the recirculation system piping 6, and oxidative decontamination of the inner surface of the recirculation system piping 6 is performed. After the oxidative decontamination using the aqueous permanganate solution is completed, an aqueous oxalic acid solution in the reduction decontamination agent tank 51 is injected into the circulation system piping 29 through the tank 54 in order to decompose the permanganate contained in the aqueous permanganate solution.

[0112] Permanganate concentration C OMn2 (unit: ppm) and the volume of the oxalic acid solution to be filled in the tank 54 is V ROA2 (unit: L), the oxalic acid concentration C required to decompose the permanganic acid in the permanganic acid working solution to produce manganese hydroxide is ROA2 (unit: ppm) is calculated by the following formula (12).

[0113] C ROA2 =(5 / 2)×(90 / 120)×C OMn2 ×V S ÷V ROA2 …(12) The permanganate contained in the permanganate aqueous solution, which is the oxidative decontamination solution, reacts with the injected oxalic acid (decomposer) as shown in the following formula (13) and is decomposed to form manganese hydroxide (2Mn(OH)2 )

[0114] 2HMnO 4 +5(COOH) 2 = 2Mn(OH) 2 +5CO 2 +4H 2 O …(13) When the permanganic acid contained in the aqueous permanganic acid solution is decomposed, the aqueous permanganic acid solution returned from the recirculation piping 6 to the circulation piping 29 changes color from purple to light brown. After confirming that the aqueous permanganic acid solution has turned light brown, the decomposition process of the oxidative decontamination agent, i.e., permanganic acid, is completed.

[0115] Reduction decontamination and ultraviolet irradiation are carried out (step S5). The flow rate control valves 79 and 70 are closed, and the flow rate control valves 68, 81, 77, and 77A are opened. By opening the valve 53, the oxalic acid aqueous solution, which is the reduction decontamination agent aqueous solution in the reduction decontamination agent tank 51, is supplied to the tank 54 through the reduction decontamination agent supply pipe 52. At this time, the valve 50 is closed. The oxalic acid aqueous solution in the tank 54 is supplied to the circulation pipe 29 through the supply pipe 55 with the supply amount controlled by adjusting the opening degree of the valve 56. The supply of the oxalic acid aqueous solution for reduction decontamination to the circulation pipe 29 is performed after the decomposition of potassium permanganate contained in the potassium permanganate aqueous solution is completed.

[0116] The oxalic acid concentration C of the oxalic acid solution in the tank 54 SOA The tank 54 is filled with the oxalic acid aqueous solution so that the concentration (unit: ppm) is 2000 to 3000 ppm. ROA (unit: L), the oxalic acid concentration C of the oxalic acid solution in the tank 54 is ROA (Unit: mg / L) can be calculated using the following formula (14):

[0117] C ROA =C SOA ×V S ÷V ROA …(14) To adjust the pH of the produced oxalic acid aqueous solution, valve 60 is opened and an aqueous solution of hydrazine, which is a pH adjuster, in chemical tank 58 of pH adjuster injection device 57 is injected into the oxalic acid aqueous solution in circulation pipe 29 through injection pipe 59. The pH of the oxalic acid aqueous solution flowing in circulation pipe 29 is measured by pH meter 95. Based on this measured pH value, the opening of valve 60 is controlled to adjust the amount of hydrazine aqueous solution injected into circulation pipe 29. By adjusting the amount of hydrazine aqueous solution injected, the pH of the oxalic acid aqueous solution is adjusted to 2.5.

[0118] An aqueous oxalic acid solution containing hydrazine, with a pH of 2.5 and at 90°C, is supplied from the circulation pipe 29 to the recirculation pipe 6, and comes into contact with the surface of the oxide film formed on the inner surface of the recirculation pipe 6 and from which chromium oxide has been dissolved by oxidative decontamination. The inner surface of the recirculation pipe 6 is reductively decontaminated by the oxalic acid contained in the aqueous oxalic acid solution, dissolving the oxide film. The iron and radioactive nuclides contained in the oxide film are dissolved into the aqueous oxalic acid solution. The iron ions dissolved from the recirculation pipe 6 into the aqueous oxalic acid solution are converted to Fe 3+ Such reduction decontamination of the recirculation system piping 6 is carried out while circulating the oxalic acid aqueous solution in a closed loop including the circulation piping 29 and the recirculation system piping 6.

[0119] The oxalic acid solution returned from the recirculation pipe 6 to the circulation pipe 29 contains Fe 3+ and radioactive nuclides. The flow rate of the oxalic acid aqueous solution (containing hydrazine) supplied to the ultraviolet irradiation device 33 is adjusted by adjusting the opening of the flow rate control valves 67 and 76, respectively. Since the flow rate control valves 79 and 70 are closed and the flow rate control valves 68, 81, and 77 are open, the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is supplied to the cation exchange resin tower 34, flows through the pipes 80 and 75, and is returned to the circulation pipe 29 downstream of the flow rate control valve 77. The flow rate of the oxalic acid aqueous solution supplied to the ultraviolet irradiation device 33 and the cation exchange resin tower 34 is preferably 30 to 60 times the capacity of the cation exchange resin filled in the cation exchange resin tower 34 per hour.

[0120] While the oxalic acid solution is being supplied to the ultraviolet irradiation device 33, the oxalic acid solution is irradiated with ultraviolet light by the ultraviolet irradiation device 33. Fe contained in the oxalic acid solution 3+ is the Fe 2+ and the oxalic acid contained in the oxalic acid aqueous solution is decomposed (see formula (2) above). In the process of step S5 (reductive decontamination), as in the decomposition process of the reductive decontamination agent (step S7) described later, for example, before irradiation with ultraviolet light, hydrogen peroxide, which is an oxidizing agent, is not supplied from the supply pipe 42 to the oxalic acid aqueous solution in the circulation pipe 29. For this reason, the Fe concentration obtained by the concentration ratio grasping device 62 based on the oxidation-reduction potential measured by the oxidation-reduction potentiometer 61 that measures the oxidation-reduction potential of the oxalic acid aqueous solution irradiated with ultraviolet light is 3+ / Fe 2+ The logarithm of the concentration ratio is less than "1" because the oxalic acid solution irradiated with ultraviolet light does not contain hydrogen peroxide (an oxidizing agent).

[0121] The concentration ratio determination device 62 calculates the concentration ratio of Fe based on the measured oxidation-reduction potential of the oxalic acid solution irradiated with ultraviolet light instead of the logarithm of the concentration ratio. 3+ / Fe 2+ The concentration ratio can also be calculated. When the oxalic acid solution irradiated with ultraviolet light does not contain hydrogen peroxide, the Fe 3+ / Fe 2+ The concentration ratio becomes "10" or less.

[0122] Fe discharged from the ultraviolet irradiation device 33 2+ The oxalic acid aqueous solution containing Fe does not contain hydrogen peroxide and is therefore supplied to the cation exchange resin tower 34. 2+ is adsorbed by the cation exchange resin in the cation exchange resin tower 34 and removed from the oxalic acid aqueous solution. 3+ Since iron (III) oxalate, which is a negative ion, exists in the oxalic acid solution, it is not adsorbed by the cation exchange resin in the cation exchange resin tower 34. 3+ Fe 2+ is reduced to Fe 2+is adsorbed and removed by the cation exchange resin, it is possible to reduce the iron ion concentration in the oxalic acid aqueous solution discharged from the cation exchange resin tower 34. This promotes reduction decontamination of the inner surface of the recirculation system piping 6 by the oxalic acid aqueous solution.

[0123] By irradiating the oxalic acid aqueous solution with ultraviolet light, the oxalic acid contained in the oxalic acid aqueous solution is decomposed, and the oxalic acid concentration of the oxalic acid aqueous solution decreases. In order to compensate for this decrease in the oxalic acid concentration, the oxalic acid aqueous solution is supplied from the tank 54 to the circulation pipe 29. The decrease in the oxalic acid concentration due to ultraviolet light irradiation can be known by opening the valve 85 and sampling the oxalic acid aqueous solution discharged from the ultraviolet light irradiation device 33 through the water sampling pipe 84, and analyzing the sampled oxalic acid aqueous solution. After sampling the oxalic acid aqueous solution, the valve 85 is closed. The reduction decontamination is performed while circulating the oxalic acid aqueous solution in a closed loop including the circulation pipe 29 and the recirculation system pipe 6.

[0124] The end of reduction decontamination is determined (step S6). The reduction decontamination process may be terminated when the surface dose rate of the piping system in which reduction decontamination has been performed, specifically, the recirculation system piping 6, falls to a set dose rate, or when a predetermined time has elapsed since the start of reduction decontamination. Furthermore, the radiation emitted from the cation exchange resin tower 34 that adsorbs metal cations of radioactive nuclides may be measured by a radiation detector, and the end of the reduction decontamination process may be determined based on the dose rate calculated based on the output signal output from the radiation detector.

[0125] For example, when the surface dose rate of the recirculation system piping 6 has fallen to a set dose rate, the determination in step S6 becomes "YES", and the reduction decontamination of the recirculation system piping 6 is terminated. If the surface dose rate of the recirculation system piping 6 has not fallen to the set dose rate, the determination in step S6 becomes "NO", and the process in step S6 is continuously performed until the determination in step S6 becomes "YES". When the determination in step S6 becomes "YES", a decomposition process (step S7) of the reduction decontamination agent is performed.

[0126] The decomposition process of the reducing agent (step S7) includes an ultraviolet irradiation process (step S8), an oxidizing agent supply process (step S9), an oxidizing agent supply amount control process (step S10), a reducing agent aqueous solution supply process to the cation exchange resin tower (step S16), and a reducing agent decomposition process by a catalyst and an oxidizing agent (step S17). The reducing agent decomposition process (step S7) will be described in detail below.

[0127] The aqueous solution for reduction decontamination is irradiated with ultraviolet light (step S8). As in the process of step S5, the Fe 3+ The oxalic acid solution containing Fe and hydrazine is irradiated with ultraviolet light. This ultraviolet light irradiation causes the Fe in the oxalic acid solution to be converted into Fe, as shown in formula (2). 3+ Fe 2+ and some of the oxalic acid in the oxalic acid solution is converted to carbon dioxide (CO 2 ) and water (H 2 O).

[0128] A valve 85 provided on a water sampling pipe 82 connected to the circulation pipe 29 is opened upstream of the connection point between the circulation pipe 29 and the supply pipe 42, and the oxalic acid aqueous solution before being irradiated with ultraviolet light is sampled through the water sampling pipe 82. The sampled oxalic acid aqueous solution is analyzed to determine the metal ion concentration, radioactivity concentration, and oxalic acid concentration of the oxalic acid aqueous solution.

[0129] An oxidizing agent is supplied to the reducing decontamination agent aqueous solution (step S9). The flow rate control valves 68 and 81 are closed, and the flow rate control valves 79, 77A, and 77 are open. An oxidizing agent, for example, hydrogen peroxide, is supplied from the oxidizing agent supply device 39 to the circulation pipe 29 between the flow rate control valve 67 and the ultraviolet irradiation device 33. This supply of hydrogen peroxide will be specifically described. The valve 43 is opened to drive the supply pump 41. At this time, the valve 46 is closed. The hydrogen peroxide in the chemical tank 40 is supplied through the supply pipe 42 to the oxalic acid aqueous solution in the circulation pipe 29 upstream of the ultraviolet irradiation device 33. The hydrogen peroxide in the chemical tank 40 may be supplied through the supply pipe 42 into the ultraviolet irradiation device 33.

[0130] The oxalic acid concentration of the oxalic acid solution present in the circulation pipe 29 upstream of the flow control valve 67 is C SOA (unit: ppm), the equivalent hydrogen peroxide concentration C SHP (Unit: ppm) is expressed by the following formula (15).

[0131] C SHP = C SOA ×(34 / 90) …(15) Hydrogen peroxide is supplied from the oxidizing agent supply device 39, taking into consideration the inflow flow rate of the oxalic acid aqueous solution into the ultraviolet irradiation device 33, so that the hydrogen peroxide concentration of the oxalic acid aqueous solution is, for example, 0.05%. The chemical tank 40 of the oxidizing agent supply device 39 is filled with a 30% undiluted solution of hydrogen peroxide. The flow rate of hydrogen peroxide is adjusted by the supply pump 41 by opening the valve 43, and hydrogen peroxide is supplied from the chemical tank 40 to the circulation pipe 29. The inflow rate of the oxalic acid aqueous solution into the ultraviolet irradiation device 33 is set to F UV (unit: L / h), the supply rate F of hydrogen peroxide to the circulation pipe 29 is HPUV (Unit: L / h) is expressed by the following equation (16).

[0132] F HPUV = 0.05 × C SHP ×10 -6 ×F UV ÷0.3 …(16) The reaction of the above-mentioned formula (2) occurs at least in the ultraviolet irradiation device 33 to which hydrogen peroxide is supplied. 3+ By irradiation with ultraviolet light, Fe 2+ Furthermore, by supplying hydrogen peroxide to the oxalic acid solution, the reaction shown in formula (3) is reduced to OH and OH. - is generated, and Fe generated by UV irradiation 2+ Part of the 3+ Then, as shown in the above formula (3), the oxalic acid and ·OH contained in the oxalic acid aqueous solution react as shown in the following formula (17), and the oxalic acid is decomposed into water and carbon dioxide.

[0133] OH+(COOH)2 → H 2 O+CO 2 …(17) The aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33 is returned to the circulation pipe 29 downstream of the flow control valve 77 through the pipes 78 and 75, and is supplied to the recirculation system pipe 6. Since the flow control valve 68 is closed, the aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33 is not supplied to the cation exchange resin tower 34.

[0134] The valve 85 is opened and the oxalic acid solution irradiated with ultraviolet light is sampled through the water sampling tube 84. The sampled oxalic acid solution is analyzed to determine the Fe content of the oxalic acid solution. 2+ and Fe 3+ Calculate the respective concentrations and the oxalic acid concentration.

[0135] The supply amount of the oxidizing agent is controlled (step S10). As shown in FIG. 2, the supply amount of the oxidizing agent is controlled by steps S11, S21, and S22. 2+ Fe concentration vs. 3+ The ratio of the concentrations of (Fe 3+ / Fe 2+ Step S12: determining the logarithm of the concentration ratio of the oxidizing agent (Fe) or the concentration ratio; Step S13: determining whether the oxidizing agent is contained; Step S14: bypassing the cation exchange resin tower; Step S15: decreasing the amount of the oxidizing agent supplied; 3+ / Fe 2+ The method includes a step S18 of determining whether the logarithm of the concentration ratio is less than a first set value or whether the concentration ratio is less than a second set value, and a step S19 of increasing the supply amount of the oxidizing agent. Each of these steps will be specifically described below.

[0136] The oxidation-reduction potential is measured (step 11). The oxidation-reduction potential of the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is measured by the oxidation-reduction potentiometer 61.

[0137] Fe 2+ Fe concentration vs. 3+ The ratio of the concentrations of (Fe 3+ / Fe 2+The redox potential measured by the redox potentiometer 61 is input to the concentration ratio determining device 62. The concentration ratio determining device 62 determines the Fe concentration in the oxalic acid aqueous solution that corresponds to the input redox potential. 2+ Fe in oxalic acid solution 3+ Concentration ratio of (Fe 3+ / Fe 2+ The logarithm of the concentration ratio (concentration ratio) is calculated based on the characteristics shown in FIG. 6. The data of the characteristics shown in FIG. 6 is stored in a storage device (not shown) of the concentration ratio determination device 62. In addition, the Fe 3+ / Fe 2+ The logarithm of the concentration ratio may be calculated. 0 is determined according to the material of the electrodes of the oxidation-reduction potentiometer 61 and is stored in the storage device of the concentration ratio determination device 62. For example, in the case of formula (1), the standard electrode potential E 0 is Fe 2+ and Fe 3+ is the standard electrode potential of the redox reaction.

[0138] The concentration ratio determination device 62 determines the Fe concentration based on the oxidation-reduction potential of the oxalic acid solution measured by the oxidation-reduction potentiometer 61. 3+ / Fe 2+ The concentration ratio can be calculated by substituting the measured redox potential for E in equation (1). 3+ ] / [Fe 2+ ], i.e., Fe 3+ / Fe 2+ The concentration ratio can be determined.

[0139] Fe determined by concentration ratio determination device 62 3+ / Fe 2+ Either the logarithm of the concentration ratio or the concentration ratio is output to the display device 63 and displayed. 3+ / Fe 2+ The logarithm of the concentration ratio is stored in the storage device of the concentration ratio grasping device 62 .

[0140] It is judged whether the oxidizing agent is contained in the reduction decontamination aqueous solution (step S13). The operator checks the Fe concentration ratio obtained by the concentration ratio determination device 62 and displayed on the display device 63. 3+ / Fe 2+ Based on the logarithm of the concentration ratio, it is determined whether the oxalic acid aqueous solution irradiated with ultraviolet light (the oxalic acid aqueous solution discharged from the ultraviolet light irradiation device 33) contains hydrogen peroxide as an oxidizing agent. When the amount of hydrogen peroxide supplied from the chemical tank 40 of the oxidizing agent supply device 39 to the oxalic acid aqueous solution in step S9 is excessive, the oxalic acid aqueous solution discharged from the ultraviolet light irradiation device 33 contains hydrogen peroxide. As described above, Fe 3+ / Fe 2+ When the logarithm of the concentration ratio is greater than "1", the oxalic acid solution irradiated with ultraviolet light contains hydrogen peroxide (see FIG. 6). 3+ / Fe 2+ When the logarithm of the concentration ratio exceeds "1", for example, when the concentration ratio is "1.3", the determination in step S13 becomes "YES".

[0141] Fe 3+ / Fe 2+ Instead of the logarithm of the concentration ratio, 3+ / Fe 2+ The determination in step S13 can also be performed using the concentration ratio. 3+ / Fe 2+ When the concentration ratio is used, if the concentration ratio is greater than "10", the oxalic acid solution irradiated with ultraviolet light contains hydrogen peroxide. 3+ / Fe 2+ When the concentration ratio exceeds "10", for example, when the concentration ratio is "13", the determination in step S13 becomes "YES".

[0142] In step S13, Fe 3+ / Fe 2+ Judgment of whether the logarithm of the concentration ratio exceeds "1" and Fe 3+ / Fe 2+ This is a step of determining whether the concentration ratio exceeds "10" or not.

[0143] The reduction decontamination aqueous solution is caused to flow so as to bypass the cation exchange resin tower (step S14). When the determination in step S13 becomes "YES", the flow rate control valves 68 and 81 are closed and the flow rate control valves 79, 77A, and 77 are opened. If the flow rate control valves 68 and 81 are already in the "closed" state and the flow rate control valves 79, 77A, and 77 are already in the "open" state in the step S9 when the step S14 is performed, the open and closed states of these flow rate control valves are maintained.

[0144] Since the flow rate control valves 68 and 81 are both "closed," the oxalic acid aqueous solution flowing out from the ultraviolet irradiation device 33 is not supplied to the cation exchange resin tower 34, but flows through the pipes 78 and 75 and is led to the circulation pipe 29 downstream of the flow rate control valve 77. Therefore, the supply of the oxalic acid aqueous solution containing hydrogen peroxide to the cation exchange resin tower 34 is prevented.

[0145] The amount of oxidizing agent supplied to the reduction decontamination aqueous solution is reduced (step S15). Since the determination in step S13 is "YES", Fe 3+ / Fe 2+ The rotation speed of the supply pump 41 is reduced so that the logarithm of the concentration ratio becomes "1", and the amount of hydrogen peroxide supplied from the chemical tank 40 to at least one of the oxalic acid aqueous solution flowing through the circulation pipe 29 between the flow rate control valve 67 and the ultraviolet irradiation device 33 and the oxalic acid aqueous solution in the ultraviolet irradiation device 33 is reduced. The amount of hydrogen peroxide supplied can also be reduced by reducing the opening degree of the valve 43. Each of steps S14, S15, S11, S12, and S13 is repeated until the determination in step S13 is "NO". By repeating the corresponding steps in this way, the Fe concentration obtained by the concentration ratio grasping device 62 to which the output of the oxidation-reduction potentiometer 61 is inputted is reduced. 3+ / Fe 2+The logarithm of the concentration ratio becomes "1", and hydrogen peroxide is no longer contained in the aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33. As a result, the reduction in the rotation speed of the supply pump 41 is stopped, and the reduction in the amount of hydrogen peroxide supplied to the aqueous oxalic acid solution described above is also stopped. Furthermore, the Fe 3+ / Fe 2+ When the logarithm of the concentration ratio is "1", the efficiency of decomposing oxalic acid in the aqueous oxalic acid solution to which hydrogen peroxide is supplied through the supply pipe 42 is highest. Since hydrogen peroxide does not flow out from the ultraviolet irradiation device 33, even when the aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33 is supplied to the cation exchange resin tower 34, hydrogen peroxide does not flow into the cation exchange resin tower 34, and deterioration of the cation exchange resin in the cation exchange resin tower 34 due to hydrogen peroxide does not occur.

[0146] When the determination in step S13 is "YES", 3+ / Fe 2+ The concentration ratio is set to "10". 3+ / Fe 2+ As in the case of the logarithm of the concentration ratio, the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution is reduced. By repeating steps S14, S15, S11, S12, and S13 until the determination in step S13 is "NO", the Fe concentration obtained by the concentration ratio determination device 62 is gradually increased. 3+ / Fe 2+ The concentration ratio becomes "10", and hydrogen peroxide is no longer contained in the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33. As a result, the decrease in the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution is stopped. Furthermore, the calculated Fe 3+ / Fe 2+ When the concentration ratio is "10", the efficiency of decomposing oxalic acid in the aqueous oxalic acid solution to which hydrogen peroxide is supplied is the highest. Since hydrogen peroxide does not flow out from the ultraviolet irradiation device 33, hydrogen peroxide does not flow into the cation exchange resin tower 34 to which the aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33 is supplied, and deterioration of the cation exchange resin in the cation exchange resin tower 34 due to hydrogen peroxide does not occur.

[0147] In the process of step S15, when the determination in step S13 becomes "YES", the Fe in the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is 3+ / Fe 2+ The logarithm of the concentration ratio falls to "1", and the Fe 3+ / Fe 2+ The amount of hydrogen peroxide supplied to the aqueous oxalic acid solution is reduced until either of the following conditions is met: the concentration ratio falls to "10" or the concentration ratio falls to "10".

[0148] When the determination in step S13 becomes "NO", steps S16 and S18 are executed. First, steps S18 and S19 will be described.

[0149] Fe 3+ / Fe 2+ The logarithm of the concentration ratio is less than a first set value, and 3+ / Fe 2+ The first set value is determined whether the concentration ratio is less than the second set value (step S18). 3+ / Fe 2+ This is a setpoint used to determine whether the oxidizer supply rate needs to be increased using the logarithm of the concentration ratio. The second setpoint is Fe 3+ / Fe 2+ This is a set point used to determine whether an increase in the oxidant supply is required using the concentration ratio.

[0150] If the determination in step S13 is "NO", 3+ / Fe 2+ When the logarithm of the concentration ratio is "1", which is the first set value at which no outflow of the oxidizing agent occurs from the ultraviolet irradiation device 33, the determination in step S18 is "NO". 3+ / Fe 2+ When the logarithm of the concentration ratio is "1", which is the first set value at which no outflow of the oxidizing agent occurs from the ultraviolet irradiation device 33, the determination in step S18 is "NO". When the determination in step S18 is "NO", step S16 is performed.

[0151] If the determination in step S18 is "YES", the process in step S19 is carried out.

[0152] The supply amount of the oxidizing agent is increased (step S19). 3+ / Fe 2+ When the logarithm of the concentration ratio is smaller than the first set value "1", for example, "0.9", Fe 3+ / Fe 2+ The opening of valve 43 is increased based on a control command from the operation panel so that the logarithm of the concentration ratio becomes the first set value of "1", and the amount of hydrogen peroxide supplied from the chemical tank 40 to at least one of the oxalic acid aqueous solutions flowing through the circulation piping 29 between the flow control valve 67 and the ultraviolet irradiation device 33, and the oxalic acid aqueous solution in the ultraviolet irradiation device 33 is increased.

[0153] Or, Fe displayed on the display device 63 3+ / Fe 2+ When the concentration ratio is smaller than the second set value "10", for example, "9", Fe 3+ / Fe 2+ Based on the control command from the operation panel, the Fe concentration ratio is adjusted to the second set value of "10". 3+ / Fe 2+ Similarly, when the logarithm of the concentration ratio is smaller than "1", the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution is increased.

[0154] In step S19, Fe 3+ / Fe 2+ When the logarithm of the concentration ratio is smaller than the first set value, the logarithm becomes the first set value, and Fe 3+ / Fe 2+ This is a process of increasing the amount of hydrogen peroxide supplied to the aqueous oxalic acid solution based on a control command from an operation panel so as to achieve either: when the concentration ratio is smaller than a second set value, the concentration ratio becomes the second set value.

[0155] Steps S19, S11, S12, S13, and S18 are repeated until the determination in step S18 is "NO." 3+ / Fe 2+ When the logarithm of the concentration ratio becomes "1", or when the Fe 3+ / Fe 2+ When the concentration ratio becomes "10", the determination in step S18 becomes "NO". Then, the process of step S16 is performed. When the determination in step S18 becomes "NO", the increase in the opening of valve 43 is stopped, and the increase in the amount of hydrogen peroxide supplied to circulation pipe 29 is also stopped.

[0156] When the determination in step S13 becomes "YES", the "reduction in the supply amount of the oxidizing agent" performed in the process of step S15, and when the determination in step S18 becomes "YES", the "increase in the supply amount of the oxidizing agent" performed in the process of step S19 are performed based on the measured oxidation-reduction potential of the reducing decontamination aqueous solution irradiated with ultraviolet light, and ... 2+ Fe 3+ This means that either the logarithm of the concentration ratio or the concentration ratio is calculated, and the amount of oxidant supplied to at least one of the reducing decontamination aqueous solution upstream of the ultraviolet irradiation device 33 and the reducing decontamination aqueous solution in the ultraviolet irradiation device 33 is adjusted based on either the logarithm of the concentration ratio or the concentration ratio.

[0157] The “reduction in the amount of oxidizing agent supplied” performed in step S15 and the “increase in the amount of oxidizing agent supplied” performed in step S19 also mean adjusting the amount of oxidizing agent supplied to at least one of the reducing decontamination aqueous solution upstream of the ultraviolet irradiation device 33 and the reducing decontamination aqueous solution in the ultraviolet irradiation device 33, based on the measured oxidation-reduction potential of the reducing decontamination aqueous solution irradiated with ultraviolet light.

[0158] Fe concentration ratio output from the concentration ratio determination device 62 3+ / Fe 2+When the logarithm of the concentration ratio is "1", the determinations in steps S13 and S18 are "NO", so that Fe 3+ / Fe 2+ The opening of the valve 43 is adjusted so that the logarithm of the concentration ratio is maintained at "1". 3+ / Fe 2+ When the concentration ratio is "10", the determinations in steps S13 and S18 are "NO", so that Fe 3+ / Fe 2+ The opening of the valve 43 is adjusted so that the concentration ratio is maintained at "10".

[0159] If the amount of oxidant supplied in step S19 is increased, the amount of Fe determined in step S12 will be 3+ / Fe 2+ If the logarithm of the concentration ratio exceeds "1", the determination in step S13 becomes "YES", the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 is stopped (step S14), and the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution from the supply pipe 42 is reduced in step S15. When the determination in step S13 becomes "NO" due to the reduction in the amount of hydrogen peroxide supplied, steps S16 and S18 are performed.

[0160] If the amount of oxidant supplied in step S19 is increased, the amount of Fe determined in step S12 will be 3+ / Fe 2+ If the concentration ratio exceeds "10", the determination in step S13 becomes "YES", the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 is stopped (step S14), and the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution from the supply pipe 42 is reduced in step S15. When the determination in step S13 becomes "NO" due to the reduction in the amount of hydrogen peroxide supplied, steps S16 and S18 are performed.

[0161] The reduction decontamination aqueous solution is supplied to the cation exchange resin tower (step S16). When the determination in step S13 becomes "NO", the flow rate control valves 68 and 70 are opened and the flow rate control valve 79 is closed. The oxalic acid aqueous solution irradiated with ultraviolet light by the ultraviolet irradiation device 33 is supplied to the cation exchange resin tower 34. Fe contained in this oxalic acid aqueous solution is 2+ is adsorbed by the cation exchange resin in the cation exchange resin tower 34 and removed from the oxalic acid aqueous solution. Note that even if the determination in step S18 is "NO", the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 is continued.

[0162] The valve 87 is opened and the oxalic acid aqueous solution discharged from the cation exchange resin tower 34 is sampled through the water sampling pipe 86. The sampled oxalic acid aqueous solution is analyzed to determine the amount of Fe in the oxalic acid aqueous solution. 2+ and Fe 3+ The metal ion concentration, radioactivity concentration and oxalic acid concentration are determined.

[0163] The reduction decontamination agent is decomposed by the catalyst and the oxidizing agent (step S17). The oxalic acid aqueous solution discharged from the cation exchange resin tower 34 is supplied to the catalyst tower 36. At this time, the valve 46 is opened and the supply pump 44 is driven. The hydrogen peroxide in the chemical tank 40 is supplied to the circulation pipe 29 between the flow rate control valve 70 and the catalyst tower 36 through the supply pipe 45. By supplying hydrogen peroxide through the supply pipe 45, the hydrogen peroxide is injected into the oxalic acid aqueous solution flowing in the circulation pipe 29. The oxalic acid and hydrazine contained in the oxalic acid aqueous solution supplied to the catalyst tower 36 are decomposed in the catalyst tower 36 by the action of the activated carbon catalyst and the injected hydrogen peroxide. That is, the oxalic acid is decomposed by the reaction of formula (18), and the hydrazine is decomposed by the reaction of formula (19). The oxalic acid is decomposed into carbon dioxide and water, and the hydrazine is decomposed into nitrogen and water. The amount of hydrogen peroxide supplied to the circulation pipe 29 through the supply pipe 45 is desirably 1 to 2 times the reaction equivalent with oxalic acid.

[0164] (COOH) 2 +H 2 O 2 → 2CO2 +2H 2 O ……(18) N 2 H 4 +2H 2 O 2 → N 2 +4H 2 O ……(19) Even when the hydrogen peroxide in the chemical tank 40 is supplied to the circulation pipe 29 via the supply pipe 45 at a position downstream of the cation exchange resin tower 34 and upstream of the catalyst tower 36, the hydrogen peroxide in the chemical tank 40 is supplied to the circulation pipe 29 at a position downstream of the flow control valve 67 and upstream of the ultraviolet irradiation device 33.

[0165] The oxalic acid aqueous solution discharged from the catalyst tower 36 is sampled through the water sampling pipe 88 by periodically opening the valve 89. After the sampling of the oxalic acid aqueous solution is completed, the valve 89 is closed. A test paper is brought into contact with the sampled oxalic acid aqueous solution, and the test paper is used to check whether the oxalic acid aqueous solution discharged from the catalyst tower 36 contains hydrogen peroxide. When the test paper checks that the oxalic acid aqueous solution discharged from the catalyst tower 36 contains hydrogen peroxide, that is, when the outflow of hydrogen peroxide from the catalyst tower 36 is confirmed, the opening of the valve 46 of the oxidizing agent supply device 39 is reduced, and the amount of hydrogen peroxide supplied to the catalyst tower 36 through the supply pipe 45 is reduced. This reduction in the amount of hydrogen peroxide supplied prevents hydrogen peroxide from flowing out from the catalyst tower 36, and prevents the supply of the oxalic acid aqueous solution containing hydrogen peroxide to the cation exchange resin tower 34.

[0166] The valve 89 is opened and the oxalic acid aqueous solution discharged from the catalyst tower 36 is sampled through the water sampling pipe 88. The sampled oxalic acid aqueous solution is analyzed to determine the metal ion concentration, radioactivity concentration and oxalic acid concentration of the oxalic acid aqueous solution.

[0167] It is determined whether to terminate the decomposition of the reduction decontamination agent (step S20). When the decomposition of oxalic acid, which is the reduction decontamination agent, is performed in the process of step S17, the determination in step S20 is performed. As described above, a part of the oxalic acid aqueous solution discharged from the catalyst tower 36 is sampled through the water sampling pipe 88 and analyzed, so that the oxalic acid concentration of the oxalic acid aqueous solution discharged from the catalyst tower 36 can be known. When the oxalic acid concentration of the oxalic acid aqueous solution becomes 10 ppm or less, the determination in the process of step S20 becomes "YES", and the decomposition process of the reduction decontamination agent (step S10) is terminated. Even when the conductivity of the oxalic acid aqueous solution discharged from the catalyst tower 36 becomes 1 mS / m or less, the determination in the process of step S20 becomes "YES", and the decomposition process of the reduction decontamination agent may be terminated. Note that, since hydrazine contained in the oxalic acid aqueous solution is decomposed faster than oxalic acid, when the oxalic acid concentration of the oxalic acid aqueous solution decreases to 10 ppm, the hydrazine concentration of the oxalic acid aqueous solution becomes "0".

[0168] When the determination in step S20 becomes "NO", the following steps are carried out until the determination in step S20 becomes "YES", namely, the ultraviolet irradiation step (step S8), the oxidizing agent supply step (step S9), the oxidizing agent supply amount control step (step S10), the supply step of the reduction decontamination aqueous solution to the cation exchange resin tower (step S16), and the decomposition step of the reduction decontamination agent by a catalyst and an oxidizing agent (step S17).

[0169] Fe 2+ Fe 3+ The case where the first set value of the logarithm of the concentration ratio of Fe is set to a value within a range of 0.8 to 1, for example, "0.9" will be described. In the oxidizer supply amount control process S10, when the determination in step S13 becomes "YES", the Fe 3+ / Fe 2+ The opening of the valve 43 is decreased so that the logarithm of the concentration ratio becomes the first set value, "0.9," and the amount of hydrogen peroxide supplied from the supply pipe 42 to the circulation pipe 29 from the chemical tank 40 is decreased (step S15).3+ / Fe 2+ When the logarithm of the concentration ratio becomes “0.9”, the determination in step S13 becomes “NO”, and hydrogen peroxide stops flowing out of the ultraviolet irradiating device 33.

[0170] However, Fe 3+ / Fe 2+ Even after the logarithm of the concentration ratio becomes "1", Fe 3+ / Fe 2+ The opening of the valve 43 is decreased to decrease the amount of hydrogen peroxide supplied to the circulation pipe 29 until the logarithm of the concentration ratio falls to the first set value, "0.9." 3+ / Fe 2+ When the logarithm of the concentration ratio falls to "0.9", the decrease in the opening of the valve 43 is stopped, and the decrease in the supply amount of hydrogen peroxide is also stopped. 3+ / Fe 2+ The opening control of the valve 43 continues so as to maintain the logarithm of the concentration ratio at "0.9".

[0171] Then, the determination in step S18 is performed. 3+ / Fe 2+ When the logarithm of the concentration ratio falls below the first set value "0.9" and the determination in step S18 becomes "YES", the process in step S19 is performed and the opening degree of the valve 43 is increased. As a result, the hydrogen peroxide concentration of the oxalic acid aqueous solution supplied to the ultraviolet irradiation device 33 also increases, and the oxidation-reduction potential of the oxalic acid aqueous solution measured by the oxidation-reduction potentiometer 61 also increases. 3+ / Fe 2+ The logarithm of the concentration ratio also increases toward the first set value. 3+ / Fe 2+ When the logarithm of the concentration ratio reaches the first set value, "0.9," the increase in the opening of the valve 43 is stopped, and the increase in the supply of hydrogen peroxide to the ultraviolet irradiation device 33 is also stopped. 3+ / Fe 2+ The opening control of the valve 43 continues to maintain the logarithm of the concentration ratio at "0.9." When the determination in step S18 becomes "NO," the process in step S16 continues.

[0172] Fe2+ Fe 3+ The case where the second set value of the concentration ratio of Fe is set to a value within the range of 8 to 10, for example, "9", will be described. When the determination in step S13 becomes "YES", 3+ / Fe 2+ The amount of hydrogen peroxide supplied to the circulation pipe 29 is reduced so that the concentration ratio becomes the second set value of "9." 3+ / Fe 2+ When the concentration ratio falls to “9”, the determination in step S13 becomes “NO”, and hydrogen peroxide stops flowing out of the ultraviolet irradiating device 33.

[0173] However, Fe 3+ / Fe 2+ Even after the concentration ratio reaches "10", Fe 3+ / Fe 2+ The amount of hydrogen peroxide supplied to the circulation pipe 29 is reduced until the concentration ratio falls to the second set value of "9." 3+ / Fe 2+ When the concentration ratio falls to "9", the decrease in the supply of hydrogen peroxide also stops. 3+ / Fe 2+ The opening control of the valve 43 continues to maintain the concentration ratio at "9".

[0174] Then, the determination in step S18 is performed. 3+ / Fe 2+ When the concentration ratio falls below the second set value "9" and the determination in step S18 becomes "YES", the process in step S19 is performed and the opening degree of the valve 43 is increased. As a result, the hydrogen peroxide concentration of the oxalic acid aqueous solution supplied to the ultraviolet irradiation device 33 also increases, and the oxidation-reduction potential of the oxalic acid aqueous solution also increases. 3+ / Fe 2+ The concentration ratio also increases toward the second set point. 3+ / Fe 2+ When the concentration ratio reaches the second set value of "9", the increase in the supply of hydrogen peroxide to the ultraviolet irradiation device 33 is also stopped. 3+ / Fe 2+The opening control of the valve 43 continues to maintain the concentration ratio at "9." When the determination in step S18 becomes "NO," the process in step S16 continues.

[0175] When the determination in step S20 becomes "YES", purification is performed (step S21). The flow rate control valve 79 is opened and the flow rate control valve 68 is closed to stop the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34. The valves 73 and 74 are opened and the valves 65 and 66 are closed, so that the oxalic acid aqueous solution is supplied from the circulation pipe 29 to the cooler 32 and cooled to 60°C or less. At this time, since the flow rate control valve 69 is opened and the flow rate control valve 79 is closed, the oxalic acid aqueous solution at 60°C is supplied to the mixed bed resin tower 35. The cations and anions remaining in the oxalic acid aqueous solution are adsorbed and removed by the cation exchange resin and anion exchange resin in the mixed bed resin tower 35. The oxalic acid contained in the oxalic acid aqueous solution is also removed in the mixed bed resin tower 35. Therefore, the aqueous solution present in the recirculation pipe 6 and the circulation pipe 29 does not substantially contain oxalic acid.

[0176] The end of chemical decontamination is determined (step S22). When the surface dose rate of the piping system where oxidation and reduction decontamination have been performed, specifically, the recirculation system piping 6, drops to the set dose rate, the determination in step S22 becomes "YES", and the chemical decontamination of the recirculation system piping 6 is ended. The drop in the surface dose rate of the recirculation system piping 6 to the set dose rate can be confirmed, for example, by placing a radiation detector (not shown) near the recirculation system piping 6 and measuring the radiation emitted from the recirculation system piping 6 with the radiation detector. Based on the dose rate calculated based on the output signal output from the radiation detector that detected the radiation, it can be confirmed whether the surface dose rate of the recirculation system piping 6 has dropped to the set dose rate.

[0177] If the surface dose rate of the recirculation piping 6 has not decreased to the set dose rate, the determination in step S22 becomes "NO", and steps S3 to S22 are repeated. If the determination in step S22 becomes "YES", the drainage step (step S23) is performed.

[0178] It should be noted that chemical decontamination may be terminated when the number of repetitions of oxidation and reduction decontamination has reached a predetermined number, rather than when the surface dose rate of the recirculation piping 6 is reached.

[0179] In this embodiment, the judgment in step S22 is performed after the purification step (step 21) is completed, so that even if the oxidative decontamination step (step S3) is performed again when the judgment in step S22 is "NO", the potassium permanganate contained in the potassium permanganate aqueous solution injected into the circulation piping 29 in this oxidative decontamination step can be effectively utilized for the oxidative decontamination of the recirculation piping. The reason that the injected potassium permanganate can be effectively utilized in this manner when the oxidative decontamination step is performed again is that at the end of the decontamination step in step 21, the aqueous solution present in the recirculation piping 6 and the circulation piping 29 does not substantially contain oxalic acid, and therefore the injected potassium permanganate is not decomposed by oxalic acid.

[0180] Drainage is carried out (step S23). After the determination in step S22 becomes "YES", the circulation pipe 29 is connected to a waste liquid treatment device (not shown) by a high-pressure hose (not shown) having a pump (not shown). The aqueous solution remaining in the recirculation system pipe 6 and the circulation pipe 29 is discharged from the circulation pipe 29 through the high-pressure hose by driving the pump to the waste liquid treatment device (not shown), and is treated in the waste liquid treatment device.

[0181] After the drainage process in step S22 is completed, the circulation piping 29 of the chemical decontamination equipment 28 is detached from the recirculation system piping 6 to be chemically decontaminated. The recirculation system piping 6 is then restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been chemically decontaminated is started up to start operation in the next operating cycle.

[0182] (1) According to this embodiment, an aqueous solution of reducing decontamination (e.g., an aqueous solution of oxalic acid) containing an oxidizing agent (e.g., hydrogen peroxide) is irradiated with ultraviolet light, and the redox potential of the aqueous solution of reducing decontamination irradiated with ultraviolet light is measured. Based on the measured redox potential, it is possible to quickly confirm that the aqueous solution of reducing decontamination irradiated with ultraviolet light does not contain an oxidizing agent.

[0183] According to the conventional method, the reduction decontamination aqueous solution irradiated with ultraviolet rays is sampled, and the sampled reduction decontamination aqueous solution is analyzed to obtain the concentration of the oxidizing agent contained in the reduction decontamination aqueous solution. Since the reduction decontamination aqueous solution irradiated with ultraviolet rays is confirmed to not contain an oxidizing agent based on the oxidizing agent concentration thus obtained, the time required for this confirmation is long. In this embodiment, the time required for confirming that the reduction decontamination aqueous solution after ultraviolet irradiation does not contain an oxidizing agent can be shortened compared to the conventional method.

[0184] (2) In this embodiment, based on the measured redox potential, Fe in the reduction decontamination solution irradiated with ultraviolet light was 2+ Fe 3+ Since either the logarithm of the concentration ratio or the concentration ratio is obtained, it can be immediately determined that the reduction decontamination aqueous solution irradiated with ultraviolet rays does not contain a peroxidizing agent based on either the logarithm of the concentration ratio or the concentration ratio, that is, if the logarithm of the concentration ratio is either "1" or less or the concentration ratio is either "10" or less. Incidentally, if the logarithm of the concentration ratio is either greater than "1" or greater than "10", it can be immediately determined that the reduction decontamination aqueous solution irradiated with ultraviolet rays contains an oxidizing agent.

[0185] (3) Required, Fe 2+ Fe 3+When the logarithm of the concentration ratio exceeds "1" or exceeds "10", that is, when the ultraviolet-irradiated reduction decontamination aqueous solution (e.g., oxalic acid aqueous solution) contains an oxidizing agent (e.g., hydrogen peroxide), the supply of the reduction decontamination aqueous solution containing the oxidizing agent to the cation exchange resin tower 34 can be stopped. This makes it possible to significantly suppress the deterioration of the cation exchange resin in the cation exchange resin tower 34, and to extend the life of the cation exchange resin.

[0186] (4) In this embodiment, the reduction decontamination agent (e.g., oxalic acid) is decomposed by irradiating the reduction decontamination aqueous solution discharged from the recirculation pipe 6, which is the object of reduction decontamination, with ultraviolet light, and further, by supplying an oxidizing agent to the catalyst tower 36, the reduction decontamination agent can also be decomposed by the action of the catalyst and oxidizing agent in the catalyst tower 36. Therefore, in this embodiment, the decomposition of the reduction decontamination agent is promoted, and the time required for the decomposition is also shortened. Fe 2+ Since the reducing agent is also decomposed by the action of ·OH generated by the reaction of HCl with hydrogen peroxide, the time required for decomposition of the reducing agent is further shortened.

[0187] (5) Fe in the reduction decontamination solution irradiated with ultraviolet light 2+ Fe 3+ When the logarithm of the concentration ratio is "1" or less, or when the concentration ratio is "10" or less, the reduction decontamination aqueous solution does not contain an oxidizing agent. Therefore, the reduction decontamination aqueous solution that has been irradiated with ultraviolet light and does not contain an oxidizing agent can be supplied to the cation exchange resin tower 34. As a result, the Fe contained in the reduction decontamination aqueous solution 2+ The Fe can be removed by being adsorbed on the cation exchange resin in the cation exchange resin tower 34. 2+ The removal of can promote reduction decontamination of the inner surface of the recirculation system piping 6, which is the target of reduction decontamination.

[0188] (6) Required, Fe 2+ Fe 3+When the logarithm of the concentration ratio becomes greater than "1" or when the concentration ratio becomes greater than "10", the flow rate of the oxidizer supplied from the oxidizer supply device 39 to the circulation pipe 29 through the supply pipe 42 can be immediately reduced, and a state in which the reduction decontamination aqueous solution irradiated with ultraviolet rays does not contain any oxidizer can be quickly achieved.

[0189] (7) Required, Fe 2+ Fe 3+ When the logarithm of the concentration ratio becomes smaller than a first set value that is equal to or smaller than "1" or when the concentration ratio becomes smaller than a second set value that is equal to or smaller than "10", the flow rate of the oxidizing agent supplied from the oxidizing agent supply device 39 to the circulation pipe 29 through the supply pipe 42 can be increased, so that the concentration ratio in the reducing decontamination aqueous solution irradiated with ultraviolet rays can be increased to the first set value or the second set value in a short time. This makes it possible to promote decomposition of the reducing decontamination agent contained in the reducing decontamination aqueous solution.

[0190] (8) Fe in the reduction decontamination solution irradiated with ultraviolet light 2+ Fe 3+ When the logarithm of the concentration ratio of Fe is less than the first set value or the concentration ratio is less than the second set value (the determination in step S18 is "YES"), the supply amount of the oxidizing agent is increased (step S19), and then a determination of "whether the oxidizing agent is contained" is made (step S13). 2+ Fe 3+ When the logarithm of the concentration ratio of exceeds "1" or when the concentration ratio of exceeds "10", the supply amount of the oxidizing agent can be immediately reduced (step S15). Therefore, the state in which the oxidizing agent is contained in the reduction decontamination aqueous solution irradiated with ultraviolet rays can be quickly eliminated.

[0191] (9) When the determination in step S22 is "NO", the process in step S3 (oxidative decontamination process) is carried out. In this embodiment, the process in step S22 (determination of the end of chemical decontamination) is carried out after the process in step S21 (purification process) is completed, so that the aqueous solution present in the recirculation system piping 6 and the circulation piping 29 does not substantially contain oxalic acid, and the potassium permanganate contained in the potassium permanganate aqueous solution injected into the circulation piping 29 is hardly decomposed by oxalic acid. The potassium permanganate contained in the potassium permanganate aqueous solution injected to carry out oxidative decontamination again can be effectively used for the oxidative decontamination of the recirculation system piping 6.

[0192] (10) In this embodiment, when the determination in step S13 is "NO", the determination in step S18 and the process in step S16 are performed. When the determination in step S18 is "YES", even when the process in step S19 is being performed, the process in step S16 is performed, and the Fe contained in the oxalic acid aqueous solution is 2+ is removed in the cation exchange resin tower 34. Therefore, Fe in the oxalic acid solution 2+ The concentration of Fe is reduced more quickly, and the reduction decontamination of the recirculation pipe 6 is promoted, and the reduction decontamination can be completed earlier. However, the injection of hydrogen peroxide into the oxalic acid aqueous solution in step S19 causes the concentration of Fe 3+ / Fe 2+ When it is determined in step S13 that the logarithm of the concentration ratio exceeds 1 or that the concentration ratio exceeds 10, the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 can be stopped in the process of step S14. EXAMPLES

[0193] A chemical decontamination method according to a second embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 2, 3, 5 and 8. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a BWR plant.

[0194] In the chemical decontamination method of this embodiment, each of the steps S1 to S23 performed in the embodiment 1 is also performed. In such a chemical decontamination method of this embodiment, a chemical decontamination apparatus 28A shown in FIG. 8 is used. The chemical decontamination apparatus 28A has a configuration in which a control device 90 is added to the chemical decontamination apparatus 28 used in the embodiment 1. The other configuration of the chemical decontamination apparatus 28A is the same as that of the chemical decontamination apparatus 28. In the chemical decontamination method of this embodiment, each of the steps S2 to S23 is performed under automatic control by the control device 90.

[0195] In the chemical decontamination equipment 28A, the control device 90 is connected to the concentration ratio grasping device 62. Furthermore, the control device 90 is also connected to the supply pumps 41 and 44, and the valves 43 and 46, and the flow rate control valves 67, 68, 69, 70, 76, 77, 77A, 79, and 81 in the oxidizer supply device 39. The control device 90 is also connected to each of the pumps and valves other than these pumps and valves in the chemical decontamination equipment 28A.

[0196] In the chemical decontamination method of the present embodiment, the above-mentioned valves and pumps are controlled by the control device 90, and the processes of steps S2 to S23 are carried out in the same manner as in the first embodiment.

[0197] The process of step S1 is carried out in the same manner as in Example 1. In the process of step S2, the on-off valve 64, the valves 65 and 66, the flow rate control valves 67, 68 and 70, and the on-off valve 71 are each opened by the control device 90. The other valves are closed. The circulation pipe 29 and the recirculation system pipe 6 are filled with cooling water. The circulation pumps 30 and 38 are driven by the control device 90, and the cooling water heated to 90° C. by the heater 31 is circulated in a closed loop including the circulation pipe 29 and the recirculation system pipe 6.

[0198] In step S3 (performing oxidative decontamination), valves 50 and 56 are opened by the control device 90, and the potassium permanganate aqueous solution in the oxidative decontamination agent tank 48 is injected into the 90°C water in the circulation pipe 29 via the tank 54, as in the first embodiment. While the permanganate aqueous solution circulates in the closed loop including the circulation pipe 29 and the recirculation pipe 6, oxidative decontamination is performed on the inner surface of the recirculation pipe 6.

[0199] In the process of step S4 (decomposition of the oxidative decontamination agent), the control device 90 opens the valve 53 and closes the valve 50. The aqueous oxalic acid solution in the reductive decontamination agent tank 51 is supplied into the circulation pipe 29 via the tank 54. Potassium permanganate contained in the aqueous potassium permanganate solution in the circulation pipe 29 and the recirculation system pipe 6 is decomposed by oxalic acid contained in the supplied aqueous oxalic acid solution, as in the first embodiment.

[0200] In the process of step S5 (performing reduction decontamination and ultraviolet irradiation), the flow rate control valves 68, 81, and 77 are opened by the control device 90. The flow rate control valves 79 and 70 are closed. The control device 90 opens the valves 53 and 56 of the chemical decontamination agent supply device 47, and opens the valve 60 of the pH adjuster injection device 57. As in the first embodiment, the oxalic acid aqueous solution is injected from the reduction decontamination agent tank 51, and hydrazine is injected by the pH adjuster injection device 57 into the circulation pipe 29. The oxalic acid aqueous solution containing hydrazine at 90° C., the pH of which has been adjusted to 2.5 by the injection of hydrazine, comes into contact with the inner surface of the recirculation system pipe 6 while circulating in the closed loop described above, and reduction decontamination is performed on the inner surface.

[0201] The oxalic acid aqueous solution is irradiated with ultraviolet light in the same manner as in Example 1, and the ultraviolet irradiated oxalic acid aqueous solution is supplied to the cation exchange resin tower 34. Fe contained in the oxalic acid aqueous solution 2+ is removed by the cation exchange resin tower 34. A part of the oxalic acid contained in the oxalic acid aqueous solution is decomposed by the ultraviolet light irradiation.

[0202] When the surface dose rate of the recirculation system piping 6 falls to the set dose rate, or when the reduction decontamination time reaches a predetermined time, the determination in the process of step S6 (determination of the end of reduction decontamination) becomes "YES", and the reduction decontamination of the recirculation system piping 6 is terminated. If the determination in the process of step S6 is "NO", the determination in step S5 is continuously performed until the determination in the process of step S6 becomes "YES".

[0203] Thereafter, a decomposition step (step S7) of the reducing decontamination agent is performed. First, irradiation of ultraviolet light to the oxalic acid aqueous solution is performed (step S8). In the process of step S9 (supply of oxidizing agent), the flow rate control valves 68 and 81 are closed and the flow rate control valves 79, 77A and 77 are opened by the control device 90. Furthermore, the control device 90 opens the valve 43 and drives the supply pump 41. The valve 46 is closed and the supply pump 44 remains stopped. Hydrogen peroxide in the chemical tank 40 is supplied by the supply pipe 42 to the oxalic acid aqueous solution in the circulation pipe 29, for example, upstream of the ultraviolet irradiation device 33. In the ultraviolet irradiation device 33, ultraviolet light is irradiated to the oxalic acid aqueous solution containing hydrogen peroxide. The Fe contained in the oxalic acid aqueous solution is oxidized by the ultraviolet irradiation. 3+ Fe 2+ The oxalic acid is partially decomposed by the OH produced in the reaction of equation (3).

[0204] In the process of step S10 (control of the supply amount of the oxidizing agent), first, the measurement of the oxidation-reduction potential is performed (step S11). The concentration ratio grasping device 62 uses the oxidation-reduction potential measured by the oxidation-reduction potentiometer 61 to obtain the Fe 3+ / Fe 2+ Either the logarithm of the concentration ratio or the concentration ratio is obtained (step S12). 3+ / Fe 2+ Either the logarithm of the concentration ratio or the concentration ratio is input to the control device 90. The control device 90 receives the Fe 3+ / Fe 2+Based on either the logarithm of the concentration ratio or the concentration ratio, it is determined whether or not hydrogen peroxide, which is an oxidizing agent in the oxalic acid aqueous solution irradiated with ultraviolet light, is contained (step S13). 3+ / Fe 2+ When it is determined that the logarithm of the concentration ratio is greater than "1" or the concentration ratio is greater than "10", the control device 90 closes the flow rate control valves 68 and 81 and opens the flow rate control valves 79, 77A and 77. As a result, the supply of the oxalic acid aqueous solution containing hydrogen peroxide irradiated with ultraviolet light to the cation exchange resin tower 34 is prevented (step S14). 3+ / Fe 2+ The amount of hydrogen peroxide supplied to the oxalic acid aqueous solution is reduced so that the logarithm of the concentration ratio becomes "1" or the concentration ratio becomes "10" (step S15). The control device 90 reduces the rotation speed of the supply pump 41 (or reduces the opening of the valve 43). This reduces the amount of hydrogen peroxide supplied.

[0205] When the determination in step S13 becomes "NO", steps S16 and S18 are executed. First, steps S18 and S19 will be described.

[0206] In the control device 90, Fe 3+ / Fe 2+ Concentration ratio is Fe 3+ / Fe 2+ A determination is made as to whether the logarithm of the concentration ratio is less than a first set value or whether the concentration ratio is less than a second set value (step S18). When the control device 90 determines "YES" in step S18, the supply amount of oxidizer is increased (step S19). In step S19, the control device 90 increases the rotation speed of the supply pump 41 (or increases the opening of the valve 43) to increase the supply amount of hydrogen peroxide.

[0207] Fe 2+ Fe 3+It is desirable to set the first set value of the logarithm of the concentration ratio of Fe to be within the range of 0.8 to 1. A case where the first set value is set to "0.9" will be described. When the determination in step S13 by the control device 90 becomes "YES", the Fe 3+ / Fe 2+ The control device 90 reduces the amount of hydrogen peroxide supplied to the circulation pipe 29 so that the logarithm of the concentration ratio becomes the first set value, "0.9" (step S15B). 3+ / Fe 2+ When the logarithm of the concentration ratio becomes “0.9”, the determination in step S13 becomes “NO”, and hydrogen peroxide stops flowing out of the ultraviolet irradiating device 33.

[0208] In step S18, the control device 90 3+ / Fe 2+ When it is determined that the logarithm of the concentration ratio is less than the first set value "0.9", the supply amount of the oxidizing agent is increased (step S19).

[0209] Fe 3+ / Fe 2+ It is desirable to set the second set value of the concentration ratio within the range of 8 to 10. When the second set value is set to "9", the Fe 3+ / Fe 2+ The control device 90 reduces the amount of hydrogen peroxide supplied to the circulation pipe 29 so that the concentration ratio becomes "9" (step S15B). 3+ / Fe 2+ When the concentration ratio becomes "9", the determination in step S13 becomes "NO", and hydrogen peroxide stops flowing out of the ultraviolet irradiation device 33. 3+ / Fe 2+ When it is determined that the concentration ratio is less than "9", the supply amount of the oxidizing agent is increased (step S19).

[0210] Steps S19, S11, S12, S13, and S18 are repeated until the determination in step S18 is "NO." 3+ / Fe 2+ When the logarithm of the concentration ratio becomes the first set value, the control device 90 judges "NO" in step S18. Even when the concentration ratio becomes the first set value, the control device 90 judges "NO" in step S18. When the judgment in step S18 becomes "NO", the process of step S16 is performed. When the judgment in step S18 becomes "NO", the increase in the opening of the valve 43 is stopped, and the increase in the supply amount of hydrogen peroxide to the circulation pipe 29 is also stopped.

[0211] Fe 2+ Fe 3+ When the second set value of the concentration ratio is set to, for example, "9" within the range of 8 to 10, the judgment in step S13, the processes in steps S14 and S15, the judgment in step S18, and the process in step S19 are performed in the same manner as in the first embodiment.

[0212] When the determination in step S13 becomes "NO", the control device 90 opens the flow rate control valves 68 and 70 and closes the flow rate control valve 79. The flow rate control valve 81 is closed. As in the first embodiment, the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is supplied to the cation exchange resin tower 34 (step S16), and the Fe ions contained in the oxalic acid aqueous solution are 2+ is removed in the cation exchange resin tower 34. Note that even if the determination in step S18 is "NO", the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 continues.

[0213] In the catalyst tower 36, the oxalic acid contained in the oxalic acid aqueous solution is decomposed by the action of the catalyst and hydrogen peroxide (step S17). The control device 90 opens the valve 46 and drives the supply pump 44. The hydrogen peroxide in the chemical tank 40 is supplied to the circulation pipe 29 through the supply pipe 45. The oxalic acid aqueous solution containing hydrogen peroxide is introduced into the catalyst tower 36, and the oxalic acid and hydrazine contained in the oxalic acid aqueous solution are decomposed by the action of the activated carbon catalyst and the injected hydrogen peroxide.

[0214] Thereafter, it is determined whether the decomposition of the oxalic acid, which is a reducing decontamination agent, has been completed (step S20), and if this determination is "NO", steps S8, S9, S10, S16, and S17 are performed until the determination in step S20 becomes "YES". When the determination in step S20 becomes "YES", a purification process is performed (step S21), and after the purification process is completed, a determination is made as to whether the chemical decontamination has been completed (step S22), and further, drainage is performed (step S23).

[0215] After the drainage process in step S22 is completed, the circulation piping 29 of the chemical decontamination equipment 28A is detached from the recirculation system piping 6 to be chemically decontaminated. The recirculation system piping 6 is then restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been chemically decontaminated is started up to start operation in the next operating cycle.

[0216] This embodiment can obtain the effects (1) to (10) obtained in the first embodiment.

[0217] In addition, in this embodiment, (11) since the control device 90 is used in this embodiment, each valve and each pump in the chemical decontamination equipment 28A can be automatically controlled, and further, Fe 2+ Fe 3+ By using either the logarithm of the concentration ratio or the concentration ratio, the control of the supply amount of the oxidizing agent to each of the ultraviolet irradiator 33 and the catalyst tower 36 can be automated. EXAMPLES

[0218] A chemical decontamination method according to a third embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 2, 4, 5, and 9. The chemical decontamination method according to this embodiment is applied to the purification system piping of the reactor purification system, which is another component of a BWR plant.

[0219] In the chemical decontamination method of this embodiment, steps S1 to S23 shown in Figs. 1 and 2 are also carried out.

[0220] A chemical decontamination apparatus is connected to a piping system in which a chemical decontamination method is carried out (step S1). During the operation shutdown period of the BWR plant 1 after the operation of the BWR plant 1 is stopped, both ends of a circulation pipe 29 of a chemical decontamination apparatus 28, which is a temporary facility, are connected to a purification system pipe 20 made of carbon steel of a reactor purification system. The connection work of this circulation pipe 29 to the purification system pipe 20 will be specifically described. One end of the purification system pipe 20 is connected to the recirculation system pipe 6 upstream of the recirculation pump 7. The other end of the purification system pipe 20 is connected to the feed water pipe 11 downstream of the high-pressure feed water heater 16. In the purification system pipe 20, a purification system pump 21, a regenerative heat exchanger 22, a non-regenerative heat exchanger 23, and a reactor water purification device 24 are installed in this order from upstream to downstream.

[0221] After the BWR plant 1 is shut down, for example, the bonnet of the valve 93 installed between the purification system piping 20 and the regenerative heat exchanger 22 is opened to close the regenerative heat exchanger 22 side of the bonnet. One end of the circulation piping 29 of the chemical decontamination device 28, i.e., the end of the circulation piping 29 on the side of the on-off valve 64, is connected to the flange of the valve 93. As a result, one end of the circulation piping 29 is connected to the purification system piping 20 upstream of the recirculation pump 7. Furthermore, the bonnet of the valve 25 installed in the purification system piping 20 near the connection point between the recirculation system piping 6 and the purification system piping 20 is opened to close the recirculation system piping 6 side of the bonnet. The other end of the circulation piping 29, i.e., the end of the circulation piping 29 on the side of the on-off valve 71, is connected to the flange of the valve 25. As a result, the other end of the circulation piping 29 is connected to the purification system piping 20.

[0222] In this manner, by connecting both ends of the circulation pipe 29 to the purification system pipe 20, a closed loop including the purification system pipe 20 and the circulation pipe 29 is formed.

[0223] Thereafter, in this embodiment in which the purification system piping 20 is the target of chemical decontamination, the steps of filling with water and increasing temperature (step S2), oxidative decontamination of the purification system piping 20 (step S3), decomposition of the oxidative decontamination agent (step S4), reduction decontamination and ultraviolet irradiation of the purification system piping 20 (step S5), determination of the completion of reduction decontamination (step S6), decomposition of the reduction decontamination agent (step S7), purification (step S21), determination of the completion of chemical decontamination (step S22), and drainage (step S23) are performed in the same manner as in embodiment 1. The determination of the completion of reduction decontamination in step S6 is made based on the dose rate obtained based on the output signal output from the radiation detector 94 arranged near the surface of the purification system piping 20.

[0224] After the drainage process in step S23 is completed, the circulation piping 29 of the chemical decontamination equipment 28 is detached from the purification system piping 20 to be chemically decontaminated. The purification system piping 20 is then restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been chemically decontaminated is started up to start operation in the next operating cycle.

[0225] This embodiment can obtain the effects (1) to (10) obtained in the first embodiment.

[0226] In the third embodiment, a chemical decontamination apparatus 28A may be connected to the purification system piping 20 instead of the chemical decontamination apparatus 28, and the chemical decontamination of the inner surface of the purification system piping 20 may be performed using the chemical decontamination apparatus 28A.

[0227] In each of the above-mentioned embodiments, a chemical decontamination device is connected to a component of a nuclear power plant, for example, a piping system, and chemical decontamination is performed on the inner surface of the piping system. However, a nuclear power plant that is the subject of decommissioning is dismantled. In the dismantling of a nuclear power plant, the component members of the nuclear power plant that are generated by cutting, disassembly, etc. can be chemically decontaminated to remove attached radioactive nuclides and reduce the radiation dose level.

[0228] This section explains chemical decontamination of cut pieces of components of a nuclear power plant, such as cut pieces of piping (recirculation system piping, purification system piping, etc.), generated during the dismantling work of a nuclear power plant to be decommissioned.

[0229] An end portion on the side of the on-off valve 64, which is one end portion of the circulation pipe 29 of the above-mentioned chemical decontamination apparatus 28 used in the first embodiment, is connected to the bottom of the decontamination container, as shown in FIG. 8 of JP 2019-191075 A. An end portion on the side of the on-off valve 71, which is the other end portion of the circulation pipe 29 of the chemical decontamination apparatus 28, is connected to an upper lid that is removably attached to the upper end of the decontamination container. A plurality of cut pieces of the pipe to be chemically decontaminated are stored in the decontamination container by opening the upper lid, and then the upper lid is removably attached to the upper end of the decontamination container. In a closed loop including the decontamination container and the circulation pipe, an oxidizing decontamination aqueous solution (e.g., a potassium permanganate aqueous solution) is circulated to perform oxidizing decontamination on each cut piece of the pipe present in the decontamination container. Furthermore, a reducing decontamination aqueous solution (e.g., an oxalic acid aqueous solution) is circulated in the closed loop to perform reducing decontamination on each cut piece of the pipe present in the decontamination container.

[0230] In the chemical decontamination of each cut piece of piping present in the decontamination container, each process of steps S2 to S23 in the embodiment 1 (see Figs. 1 and 2) is performed. If a decontamination container to which both ends of the circulation piping 29 of the chemical decontamination device 28A are connected is used instead of the chemical decontamination device 28, the chemical decontamination method of the embodiment 2 can be performed on the multiple cut pieces of piping stored in the decontamination container. EXAMPLES

[0231] A chemical decontamination method according to a fourth embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 3, 5, 10 and 11. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a BWR plant.

[0232] The chemical decontamination apparatus 28B (see FIG. 11) used in the chemical decontamination method of this embodiment has a configuration in which the concentration ratio grasping device 62 is removed from the chemical decontamination apparatus 28 (see FIG. 1) used in the first embodiment, and an oxidation-reduction potentiometer 61 is connected to a display device 63. The other configurations of the chemical decontamination apparatus 28B are the same as those of the chemical decontamination apparatus 28.

[0233] In the chemical decontamination method of this embodiment, steps S1 to S6 shown in Fig. 1, step S7A shown in Fig. 10, and steps S21 to S23 shown in Fig. 1 are performed. The decomposition step of the reducing decontamination agent in step S7A includes steps S8, S9, S10A, S16, S17, and S20. Furthermore, the supply flow rate control step of the oxidizing agent in step S10A includes steps S11, S13A, S14, S15A, S18A, and S19A.

[0234] In the chemical decontamination method of this embodiment, each of steps S1 to S6 performed in embodiment 1 is also performed. After the determination in step S6 becomes "YES", the process in step S8 (irradiation of ultraviolet light to the reduction decontamination aqueous solution) and the process in step S9 (supply of oxidizing agent) are performed in the same manner as in embodiment 1.

[0235] Since the on-off valve 64, the valves 65 and 66, and the flow rate control valve 67 are open, the oxalic acid aqueous solution returned from the recirculation system piping 6 to the circulation piping 29 is guided to the ultraviolet irradiation device 33. In the process of step S8, the ultraviolet irradiation device 33 irradiates the oxalic acid aqueous solution with ultraviolet rays. Furthermore, in the process of step S9, hydrogen peroxide is supplied from the oxidizing agent supply device 39 through the supply piping 42 to the oxalic acid aqueous solution in the circulation piping 29 upstream of the ultraviolet irradiation device 33. By irradiating the oxalic acid aqueous solution containing hydrogen peroxide with ultraviolet rays, the Fe contained in the oxalic acid aqueous solution is oxidized. 3+Fe 2+ and part of the oxalic acid contained in the oxalic acid aqueous solution is decomposed into carbon dioxide and water (see formula (2)).

[0236] The aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33 is not supplied to the cation exchange resin tower 34 because the flow control valve 68 is closed, but is returned to the circulation pipe 29 downstream of the flow control valve 77 through pipes 78 and 75, and is supplied to the recirculation system pipe 6.

[0237] Next, the supply amount of the oxidizing agent is controlled (step S10A). In the process of step S10A, each of the processes of steps S11, S13A, S14, S15A, S18A, and S19A is performed.

[0238] The redox potential of the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is measured by the redox potentiometer 61 (step S11). The measured redox potential is displayed on the display device 63. In the process of step S13A (determining whether an oxidizing agent is contained), the operator determines whether the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 contains hydrogen peroxide based on the redox potential displayed on the display device 63. When the displayed redox potential is greater than "400 mV vs SHE", for example, when the redox potential is 450 mV vs SHE, the oxalic acid aqueous solution irradiated with ultraviolet light contains hydrogen peroxide (see FIG. 6), and the determination in step S13A is "YES".

[0239] When the determination in step S13A is "YES", the process of step S14 is carried out as in the first embodiment. That is, the oxalic acid aqueous solution containing hydrogen peroxide bypasses the cation exchange resin tower 34, and is prevented from being supplied to the cation exchange resin tower 34. In order to prevent hydrogen peroxide from being contained in the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33, the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution is reduced (step S15A). Since the determination in step S13A is "YES", the operator operates a lever or button provided on the operation panel, and a control command to reduce the amount of hydrogen peroxide supplied is output from the operation panel. The rotation speed of the supply pump 41 is reduced (or the opening of the valve 43 is reduced) by the control command so that the measured oxidation-reduction potential falls to "400 mV vs. SHE", and the amount of hydrogen peroxide supplied from the chemical tank 40 of the oxidizing agent supply device 39 to the oxalic acid aqueous solution flowing in the circulation pipe 29 at a portion upstream of the ultraviolet irradiation device 33 is reduced.

[0240] Each of steps S14, S15A, S11, and S13A is repeated until the determination of step S13A is "NO". By repeating the corresponding steps in this way, the redox potential measured by the redox potentiometer 61 is eventually reduced to at least "400 mV vs SHE". As a result, hydrogen peroxide is no longer contained in the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33, and the reduction in the rotation speed of the supply pump 41 is stopped. The oxalic acid aqueous solution not containing hydrogen peroxide can be supplied to the cation exchange resin tower 34. Furthermore, when the measured redox potential is "400 mV vs SHE", the decomposition efficiency of oxalic acid in the oxalic acid aqueous solution containing hydrogen peroxide is highest.

[0241] When the determination in step S13A is "NO", the determination in step S18A (determination as to whether the redox potential is less than the third set value) is performed. The third set value is a set value used for determining whether it is necessary to increase the supply amount of the oxidizing agent using the redox potential. The third set value is desirably set to a value within the range of 380 mV vs SHE or more and 400 mV vs SHE or less (range of 380 mV vs SHE to 400 mV vs SHE). Here, the third set value is set to, for example, 390 mV vs SHE.

[0242] When the measured redox potential is less than the third set value "390 mV vs SHE", the determination in step S18A is "YES". When the determination in step S18A is "YES", the process in step S19A (increasing the supply amount of oxidizing agent) is performed. When the redox potential of the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is less than "390 mV vs SHE", for example, 370 mV vs SHE, in the process in step S19A, the rotation speed of the supply pump 41 is increased by the operator so that the redox potential becomes 390 mV vs SHE, and for example, the supply amount of hydrogen peroxide to the oxalic acid aqueous solution guided to the ultraviolet irradiation device 33 is increased. As a result, the redox potential of the oxalic acid aqueous solution increases to 390 mV vs SHE. Since this 390 mV vs. SHE is equal to or lower than 400 mV vs. SHE, the aqueous oxalic acid solution discharged from the ultraviolet irradiator 33 does not contain hydrogen peroxide and can be supplied to the cation exchange resin tower 34.

[0243] Each process of steps S19A, S11, S13A, and S18A is repeated until the judgment of step S18A is "NO". When the judgment of step S13A becomes "NO", the process of step S16 (supply of oxalic acid aqueous solution to the cation exchange resin tower 34) is performed as in the first embodiment. After that, even when the judgment of step S18A becomes "NO", the process of step S16 (supply of oxalic acid aqueous solution to the cation exchange resin tower 34) is continuously performed. Furthermore, as in the first embodiment, the process of step S17 (decomposition of the reduction decontamination agent by the catalyst and the oxidizing agent) and the process of step S20 (judgment of the completion of the decomposition process) are performed. When the judgment of step S20 is "NO", each process of steps S8, S9, S10A, S16, S17, and S20 are performed until the judgment of step S20 becomes "YES".

[0244] When the determination in step S20 becomes "YES", the process in step S21 (purification), the process in step S22 (determination of completion of chemical decontamination), and the process in step S23 (drainage) are performed in the same manner as in the first embodiment. After the drainage process in step S23 is completed, the circulation piping 29 of the chemical decontamination device 28B is detached from the recirculation system piping 6 that is the target of chemical decontamination. Thereafter, the recirculation system piping 6 is restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been subjected to chemical decontamination is started up to start operation in the next operating cycle.

[0245] According to this embodiment, it is possible to obtain the effects (1) and (4) obtained in the embodiment 1. Furthermore, according to this embodiment, it is possible to obtain the effects (12) to (17) shown below.

[0246] (12) In this embodiment, based on the measured redox potential, that is, if the redox potential is 400 mV vs. SHE or less, it can be immediately determined that the ultraviolet-irradiated aqueous solution for reduction decontamination does not contain a peroxidizing agent. Incidentally, if the redox potential exceeds 400 mV vs. SHE, it can be immediately determined that the ultraviolet-irradiated aqueous solution for reduction decontamination contains an oxidizing agent.

[0247] (13) When the measured oxidation-reduction potential exceeds 400 mV vs. SHE, that is, when the ultraviolet-irradiated reduction decontamination aqueous solution (e.g., oxalic acid aqueous solution) contains an oxidizing agent (e.g., hydrogen peroxide), the supply of the reduction decontamination aqueous solution containing the oxidizing agent to the cation exchange resin tower 34 can be stopped. This makes it possible to significantly suppress the deterioration of the cation exchange resin in the cation exchange resin tower 34, and to extend the life of the cation exchange resin.

[0248] (14) When the measured oxidation-reduction potential of the reduction decontamination aqueous solution irradiated with ultraviolet rays is 400 mV vs. SHE or less, this reduction decontamination aqueous solution does not contain an oxidizing agent, so that the reduction decontamination aqueous solution irradiated with ultraviolet rays and not containing an oxidizing agent can be supplied to the cation exchange resin tower 34. As a result, the Fe contained in the reduction decontamination aqueous solution 2+ The Fe can be removed by being adsorbed on the cation exchange resin in the cation exchange resin tower 34. 2+ The removal of can promote reduction decontamination of the inner surface of the recirculation system piping 6, which is the target of reduction decontamination.

[0249] (15) When the measured oxidation-reduction potential becomes greater than 400 mV vs. SHE, the flow rate of the oxidant supplied from the oxidant supply device 39 to the circulation pipe 29 through the supply pipe 42 can be immediately reduced, and a state in which the reduction decontamination aqueous solution irradiated with ultraviolet rays does not contain any oxidant can be quickly achieved.

[0250] (16) When the measured oxidation-reduction potential becomes smaller than the third set value of the oxidation-reduction potential, which is 400 mV vs. SHE or less, the flow rate of the oxidizing agent supplied from the oxidizing agent supply device 39 to the circulation pipe 29 through the supply pipe 42 can be increased, so that the oxidation-reduction potential of the reducing decontamination aqueous solution irradiated with ultraviolet rays can be increased to the third set value in a short time. This can promote decomposition of the reducing decontamination agent contained in the reducing decontamination aqueous solution.

[0251] (17) When the redox potential in the reduction decontamination aqueous solution irradiated with ultraviolet rays is less than the third set value (the determination in step S18A is "YES"), the supply amount of the oxidizing agent is increased (step S19A), and then a determination is made as to whether or not the oxidizing agent is contained (step S13A). Therefore, if the increase in the supply amount of the oxidizing agent causes the redox potential to exceed 400 mV vs. SHE, the supply amount of the oxidizing agent can be immediately decreased (step S15A). Therefore, the state in which the reduction decontamination aqueous solution irradiated with ultraviolet rays contains an oxidizing agent can be quickly eliminated. EXAMPLES

[0252] A chemical decontamination method according to a fifth embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 3, 5, 10, and 12. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a BWR plant.

[0253] In the chemical decontamination method of this embodiment, each process of steps S1 to S6 performed in the first embodiment, step S7A shown in FIG. 10, and each process of steps S21 to S23 shown in FIG. 1 are performed. In the chemical decontamination method of this embodiment, a chemical decontamination apparatus 28C shown in FIG. 12 is used. The chemical decontamination apparatus 28C has a configuration in which a control device 90 is added to the chemical decontamination apparatus 28B (see FIG. 11) used in the fourth embodiment. The other configurations of the chemical decontamination apparatus 28C are the same as those of the chemical decontamination apparatus 28B. An oxidation-reduction potentiometer 61 is connected to the control device 90. The control device 90 is also connected to each of the supply pumps 41 and 44, the valves 43 and 46, and the flow rate control valves 67, 68, 69, 70, 76, 77, 77A, 79, and 81 in the oxidizing agent supply device 39. The control device 90 is also connected to each of the pumps and valves other than these pumps and valves in the chemical decontamination apparatus 28C.

[0254] In the chemical decontamination method of this embodiment, steps S2 to S6, S7A, and S21 to S23 are performed under automatic control by the control device 90.

[0255] In the chemical decontamination method of this embodiment, steps S1 to S6 shown in Fig. 1, step S7A shown in Fig. 10, and steps S21 to S23 shown in Fig. 1 are performed in the same manner as in the third embodiment, except that the above-mentioned valves and pumps are controlled by the control device 90. The decomposition step of the reducing decontamination agent in step S7A includes steps S8, S9, S10A, S16, S17, and S20. Furthermore, the supply flow rate control step of the oxidizing agent in step S10A includes steps S11, S13A, S14, S15A, S18A, and S19A.

[0256] As in the second embodiment, the control device 90 controls the opening and closing of the corresponding valves and the driving of the circulation pumps 30 and 38, and performs each process of steps S1 to S6. When the determination in step S6 becomes "YES", the reduction decontamination process for the inner surface of the recirculation system piping 6 is completed.

[0257] Thereafter, the decomposition step (step S7A) of the reduction decontamination agent is performed. First, the irradiation of ultraviolet light to the oxalic acid aqueous solution (step S8) and the step S9 (supply of oxidizing agent) are performed in the same manner as in the second embodiment. In the step S9, the flow rate control valves 68 and 81 are closed and the flow rate control valves 79, 77A and 77 are opened by the control device 90. Furthermore, the control device 90 opens the valve 43 and drives the supply pump 41. The valve 46 is closed and the supply pump 44 remains stopped. The hydrogen peroxide in the chemical tank 40 is supplied to the oxalic acid aqueous solution in the circulation pipe 29, for example, upstream of the ultraviolet irradiation device 33, through the supply pipe 42. The hydrogen peroxide may be directly supplied to the ultraviolet irradiation device 33. In the ultraviolet irradiation device 33, the oxalic acid aqueous solution containing hydrogen peroxide is irradiated with ultraviolet light. The Fe in the oxalic acid aqueous solution is irradiated with ultraviolet light. 3+ Fe 2+ The oxalic acid is further decomposed by the OH produced in the reaction of equation (3).

[0258] In the process of step S10A (control of the supply amount of the oxidizing agent), first, the measurement of the oxidation-reduction potential (step S11) is performed. The oxidation-reduction potential measured by the oxidation-reduction potentiometer 61 is input to the control device 90. The control device 90 judges whether the oxalic acid aqueous solution irradiated with ultraviolet light contains hydrogen peroxide as an oxidizing agent based on the input oxidation-reduction potential (step S13A). When it is judged that the oxalic acid aqueous solution irradiated with ultraviolet light contains hydrogen peroxide (the oxidation-reduction potential is greater than "400 mV vs. SHE"), the control device 90 closes the flow rate control valves 68 and 81 and opens the flow rate control valves 79, 77A, and 77. Therefore, the supply of the oxalic acid aqueous solution irradiated with ultraviolet light and containing hydrogen peroxide to the cation exchange resin tower 34 is prevented (step S14). Then, the supply amount of hydrogen peroxide to the oxalic acid aqueous solution is reduced so that the oxidation-reduction potential becomes "400 mV vs. SHE" (step S15A). The control device 90 reduces the rotation speed of the supply pump 41 (or decreases the opening of the valve 43), thereby decreasing the amount of hydrogen peroxide supplied.

[0259] When the determination in step S13A is "NO", steps S16 and S18A are executed. First, steps S18A and S19A will be described.

[0260] The control device 90 determines whether the redox potential is less than a third set value of the redox potential (step S18A). The third set value is desirably set to a value within the range of 380 mV vs SHE to 400 mV vs SHE (range of 380 mV vs SHE to 400 mV vs SHE), similar to the fourth embodiment. When the control device 90 determines "YES" in step S18A, the supply amount of the oxidizing agent is increased (step S19A). In step S19A, the control device 90 increases the rotation speed of the supply pump 41 (or increases the opening of the valve 43) to increase the supply amount of hydrogen peroxide.

[0261] Steps S19A, S11, S13A, and S18A are repeated until the determination in step S18A is "NO." When the oxidation-reduction potential input from the oxidation-reduction potentiometer 61 eventually reaches the third set value of the oxidation-reduction potential, the control device 90 determines "NO" in step S18A. Then, the process in step S16 is performed. When the determination in step S18A is "NO," the increase in the opening of the valve 43 is stopped, and the increase in the amount of hydrogen peroxide supplied to the circulation pipe 29 is also stopped.

[0262] When the determination in step S13A becomes "NO", the control device 90 opens the flow rate control valves 68 and 70 and closes the flow rate control valve 79. The flow rate control valve 81 is closed. As in the second embodiment, the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is supplied to the cation exchange resin tower 34 (step S16), and the Fe ions contained in the oxalic acid aqueous solution are 2+ is removed in the cation exchange resin tower 34. Note that even if the determination in step S18A is "NO", the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 continues.

[0263] In the catalyst tower 36, the oxalic acid contained in the oxalic acid aqueous solution is decomposed by the action of the catalyst and hydrogen peroxide (step S17). The control device 90 opens the valve 46 and drives the supply pump 44. The hydrogen peroxide in the chemical tank 40 is supplied to the circulation pipe 29 through the supply pipe 45. The oxalic acid aqueous solution containing hydrogen peroxide is introduced into the catalyst tower 36, and the oxalic acid and hydrazine contained in the oxalic acid aqueous solution are decomposed by the action of the activated carbon catalyst and the injected hydrogen peroxide.

[0264] Thereafter, it is determined whether the decomposition of the oxalic acid, which is a reducing decontamination agent, has been completed (step S20), and if this determination is "NO", steps S8, S9, S10A, S16, and S17 are performed until the determination in step S20 becomes "YES". When the determination in step S20 becomes "YES", a purification step (step S21) is performed, a step of determining the completion of chemical decontamination (step S22) is performed, and further, a drainage step (step S23) is performed.

[0265] After the drainage process in step S23 is completed, the circulation piping 29 of the chemical decontamination device 28C is detached from the recirculation system piping 6 to be chemically decontaminated. The recirculation system piping 6 is then restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been chemically decontaminated is started up to start operation in the next operating cycle.

[0266] This embodiment can obtain the effects (1) and (4) obtained in embodiment 1, and the effects (12) to (17) obtained in embodiment 4. In addition, (18) this embodiment uses the control device 90, so that the valves and pumps in the chemical decontamination device 28C can be automatically controlled, and further, the amount of oxidant supplied to each of the ultraviolet irradiation device 33 and the catalyst tower 36 can be automatically controlled using the measured oxidation-reduction potential. EXAMPLES

[0267] A chemical decontamination method according to a sixth embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 3, 5, 13 and 14. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a BWR plant.

[0268] The chemical decontamination apparatus 28D (see FIG. 14) used in the chemical decontamination method of this embodiment has a configuration in which the concentration ratio determining device 62 in the chemical decontamination apparatus 28 (see FIG. 1) used in Example 1 is replaced with a concentration ratio determining device 62A, the oxidation-reduction potentiometer 61 is replaced with a corrosion potentiometer 96, and the corrosion potentiometer 96 is connected to the concentration ratio determining device 62A. The other configurations of the chemical decontamination apparatus 28D are the same as those of the chemical decontamination apparatus 28.

[0269] In the chemical decontamination method of this embodiment, steps S1 to S6 shown in Fig. 1, step S7B shown in Fig. 13, and steps S21 to S23 shown in Fig. 1 are performed. The decomposition step of the reducing decontamination agent in step S7B includes steps S8, S9, S10B, S16, S17, and S20. Furthermore, the supply flow rate control step of the oxidizing agent in step S10B includes steps S11A, S12A, S13B, S14, S15B, S18B, and S19B.

[0270] In the chemical decontamination method of this embodiment, each of steps S1 to S6 performed in embodiment 1 is also performed. After the determination in step S6 becomes "YES", step S7B is performed. In step S7B, first, step S8 (irradiating the aqueous solution for reduction decontamination with ultraviolet rays) and step S9 (supply of oxidizing agent) are performed in the same manner as in embodiment 1.

[0271] Since the on-off valve 64, the valves 65 and 66, and the flow rate control valve 67 are open, the oxalic acid aqueous solution returned from the recirculation system piping 6 to the circulation piping 29 is guided to the ultraviolet irradiation device 33. In the process of step S8, the ultraviolet irradiation device 33 irradiates the oxalic acid aqueous solution with ultraviolet light. Furthermore, in the process of step S9, hydrogen peroxide is supplied from the oxidizing agent supply device 39 through the supply piping 42 to the oxalic acid aqueous solution in the circulation piping 29 upstream of the ultraviolet irradiation device 33. The hydrogen peroxide may be supplied directly into the ultraviolet irradiation device 33 through the supply piping 42. The Fe ions contained in the oxalic acid aqueous solution are irradiated with ultraviolet light by the oxalic acid aqueous solution containing hydrogen peroxide. 3+ Fe 2+ and part of the oxalic acid contained in the oxalic acid aqueous solution is decomposed into carbon dioxide and water (see formula (2)).

[0272] The aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33 is not supplied to the cation exchange resin tower 34 because the flow control valve 68 is closed, but is returned to the circulation pipe 29 downstream of the flow control valve 77 through pipes 78 and 75, and is supplied to the recirculation system pipe 6.

[0273] Next, the supply amount of the oxidizing agent is controlled (step S10B). In the process of step S10B, each of the processes of steps S11A, S12A, S13B, S14, S15B, S18B, and S19B is performed.

[0274] The corrosion potential of the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is measured by the corrosion potentiometer 96 (step S11A). The concentration ratio grasping device 62A uses the corrosion potential measured by the corrosion potentiometer 96 to grasp the Fe concentration in the oxalic acid aqueous solution. 2+ Fe in oxalic acid solution 3+ Concentration ratio of (Fe 3+ / Fe 2+ Either the logarithm of Fe (concentration ratio) or the concentration ratio is calculated (step S12A). 3+ / Fe 2+ The logarithm of the Fe concentration ratio is calculated based on the characteristics shown in FIG. 3+ / Fe 2+ The logarithm of the concentration ratio is displayed on the display device 63. Moreover, the data of the characteristics shown in Fig. 7 is stored in a storage device (not shown) of the concentration ratio grasping device 62A.

[0275] The reduction decontamination solution contains Fe ions, and the corrosion potential of the reduction decontamination solution (E e ) is the divalent Fe ion concentration [Fe 2+ ] versus trivalent Fe ion concentration [Fe 3+ This can be expressed by the Nernst equation, which includes the ratio of

[0276] E e =E 0 +(RT / nF) log([Fe 3+ ] / [Fe 2+ ]) …(20) Here, E 0 is the standard electrode potential, R is the gas constant, T is the temperature of the reduction decontamination aqueous solution, n is the valence, and F is the Faraday constant.

[0277] In the concentration ratio grasping device 62A, the input corrosion potential is calculated by E eBy substituting into 3+ / Fe 2+ The logarithm of the concentration ratio can be determined.

[0278] The concentration ratio determination device 62 determines the Fe concentration based on the corrosion potential of the oxalic acid aqueous solution measured by the corrosion potential meter 96. 3+ / Fe 2+ The concentration ratio may be calculated by Eq. (20). e By substituting the measured corrosion potential into [Fe 3+ ] / [Fe 2+ ], i.e., Fe 3+ / Fe 2+ The concentration ratio can be determined.

[0279] Fe determined by concentration ratio determination device 62 3+ / Fe 2+ Either the logarithm of the concentration ratio or the concentration ratio is output to a display device 63 and displayed.

[0280] In step S13B (determining whether an oxidizing agent is included), the operator reads the Fe 3+ / Fe 2+ Based on either the logarithm of the concentration ratio or the concentration ratio, it is determined whether or not the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 contains hydrogen peroxide (step S13B). 3+ / Fe 2+ When the logarithm of the concentration ratio is greater than "1", the oxalic acid solution irradiated with ultraviolet light contains hydrogen peroxide (see FIG. 7), and the determination in step S13B is "YES". 3+ / Fe 2+ When the concentration ratio is greater than "10", the oxalic acid solution irradiated with ultraviolet light also contains hydrogen peroxide, and the determination in step S13B is "YES".

[0281] When the determination in step S13B is "YES", the process of step S14 is carried out as in the first embodiment. That is, as described above, the aqueous oxalic acid solution containing hydrogen peroxide bypasses the cation exchange resin tower 34, and is prevented from being supplied to the cation exchange resin tower 34. In order to prevent hydrogen peroxide from being contained in the aqueous oxalic acid solution discharged from the ultraviolet irradiator 33, the amount of hydrogen peroxide supplied to the aqueous oxalic acid solution is reduced (step S15B). Since the determination in step S13B is "YES", the operator operates a lever or button provided on the operation panel, and a control command to reduce the amount of hydrogen peroxide supplied is output from the operation panel. The calculated Fe 3+ / Fe 2+ The control command reduces the rotation speed of the supply pump 41 (or reduces the opening of the valve 43) so that the logarithm of the concentration ratio falls to "1", thereby reducing the amount of hydrogen peroxide supplied from the chemical tank 40 of the oxidizer supply device 39 to the oxalic acid aqueous solution flowing in the circulation piping 29, for example, upstream of the ultraviolet irradiation device 33.

[0282] In step S15B, Fe 3+ / Fe 2+ Instead of the logarithm of the concentration ratio, 3+ / Fe 2+ For example, the amount of hydrogen peroxide supplied to the aqueous oxalic acid solution flowing in the circulation pipe 29 upstream of the ultraviolet irradiation device 33 may be reduced so that the concentration ratio falls to "10".

[0283] Steps S14, S15B, S11A, S12A, and S13B are repeated until the determination in step S13B is "NO." By repeating the corresponding steps in this manner, the Fe concentration ratio obtained by the concentration ratio determination device 62A will be 3+ / Fe 2+ The logarithm of the concentration ratio becomes "1" or the concentration ratio becomes "10". As a result, hydrogen peroxide is no longer contained in the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33, and the reduction in the rotation speed of the supply pump 41 is stopped. The oxalic acid aqueous solution not containing hydrogen peroxide can be supplied to the cation exchange resin tower 34 by opening the flow rate control valve 68. Furthermore, Fe3+ / Fe 2+ When the logarithm of the concentration ratio is "1" or the concentration ratio is "10", the decomposition efficiency of oxalic acid in the oxalic acid aqueous solution containing hydrogen peroxide is the highest.

[0284] When the determination in step S13B is "NO", the process in step S16 is carried out, and further, the determination in step S18B (Fe 3+ / Fe 2+ The determination in step S18B is performed as to whether the logarithm of the concentration ratio is less than a first set value or whether the concentration ratio is less than a second set value. The determination in step S18B will be described. 3+ / Fe 2+ When the logarithm of the concentration ratio is a first set value equal to or less than "1" at which no outflow of the oxidizing agent occurs from the ultraviolet irradiation device 33, for example, "0.9", the determination in step S18B becomes "NO". When the determination in step S18B is "NO", step S16 is performed.

[0285] If the required Fe 3+ / Fe 2+ When the logarithm of the concentration ratio is less than a first set value equal to or less than "1", for example, less than "0.9", the determination in step S18B becomes "YES", and the process in step S19B is performed. 3+ / Fe 2+ When the logarithm of the concentration ratio is "0.8", in the process of step S19B, the Fe 3+ / Fe 2+ The rotation speed of the supply pump 41 is increased by the operator operating the operation panel so that the logarithm of the concentration ratio becomes "0.9", and for example, the amount of hydrogen peroxide supplied to the oxalic acid solution introduced to the ultraviolet irradiation device 33 is increased. 3+ / Fe 2+ The logarithm of the concentration ratio increases to "0.9". 3+ / Fe 2+Since the first set value of the logarithm of the concentration ratio is "0.9", the aqueous oxalic acid solution discharged from the ultraviolet irradiator 33 does not contain hydrogen peroxide and can be supplied to the cation exchange resin tower .

[0286] Fe 3+ / Fe 2+ The second set value of the concentration ratio is preferably set within a range of 8 to 10. For example, the second set value is "9", and the obtained Fe 3+ / Fe 2+ When the concentration ratio is less than the second set value, for example, "9", the determination in step S18B becomes "YES" and the process in step S19B is performed. 3+ / Fe 2+ When the concentration ratio is "8", in the process of step S19B, the Fe 3+ / Fe 2+ The amount of hydrogen peroxide supplied to the oxalic acid solution is increased so that the concentration ratio becomes "9". 3+ / Fe 2+ The concentration ratio increases to “9”, and the aqueous oxalic acid solution discharged from the ultraviolet irradiator 33 can be supplied to the cation exchange resin tower 34.

[0287] Each of steps S19B, S11A, S12A, S13B, and S18B is repeated until the determination of step S18B is "NO". When the determination of step S13B is "NO" and the determination of step S18B is "NO", the process of step S16 (supply of the oxalic acid aqueous solution to the cation exchange resin tower 34) is continuously performed. Furthermore, similar to the first embodiment, the process of step S17 (decomposition of the reduction decontamination agent by the catalyst and the oxidizing agent) and the process of step S20 (determination of the completion of the decomposition process) are performed. When the determination of step S20 is "NO", each of steps S8, S9, S10B, S16, S17, and S20 is performed until the determination of step S20 is "YES".

[0288] When the determination in step S20 becomes "YES", the process in step S21 (purification), step S22 (determination of completion of chemical decontamination), and the process in step S23 (drainage) are performed in the same manner as in the first embodiment. After the drainage process in step S23 is completed, the circulation piping 29 of the chemical decontamination device 28D is detached from the recirculation system piping 6 that is the target of chemical decontamination. Thereafter, the recirculation system piping 6 is restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been subjected to chemical decontamination is started up to start operation in the next operating cycle.

[0289] According to this embodiment, it is possible to obtain the respective effects (3) to (8) obtained in the embodiment 1. Furthermore, according to this embodiment, it is possible to obtain the respective effects (19) and (20) shown below.

[0290] (19) According to this embodiment, an aqueous solution for reducing decontamination (e.g., an aqueous solution of oxalic acid) containing an oxidizing agent (e.g., hydrogen peroxide) is irradiated with ultraviolet light, and the oxidation-reduction potential of the aqueous solution for reducing decontamination irradiated with ultraviolet light is measured. Based on the measured corrosion potential, it is possible to quickly confirm that the aqueous solution for reducing decontamination irradiated with ultraviolet light does not contain an oxidizing agent.

[0291] According to the conventional method, the reduction decontamination aqueous solution irradiated with ultraviolet rays is sampled, and the sampled reduction decontamination aqueous solution is analyzed to obtain the concentration of the oxidizing agent contained in the reduction decontamination aqueous solution. Since the reduction decontamination aqueous solution irradiated with ultraviolet rays is confirmed to not contain an oxidizing agent based on the oxidizing agent concentration thus obtained, the time required for this confirmation is long. In this embodiment, the time required for confirming that the reduction decontamination aqueous solution after ultraviolet irradiation does not contain an oxidizing agent can be shortened compared to the conventional method.

[0292] (20) In this example, based on the measured corrosion potential, Fe in the reduction decontamination solution irradiated with ultraviolet light was 2+ Fe 3+Since either the logarithm of the concentration ratio or the concentration ratio is obtained, it can be immediately determined that the reduction decontamination aqueous solution irradiated with ultraviolet rays does not contain a peroxidizing agent based on either the logarithm of the concentration ratio or the concentration ratio, that is, if the logarithm of the concentration ratio is "1" or less or the concentration ratio is "10" or less. Incidentally, if the logarithm of the concentration ratio exceeds "1" or the concentration ratio exceeds "10", it can be immediately determined that the reduction decontamination aqueous solution irradiated with ultraviolet rays contains an oxidizing agent. EXAMPLES

[0293] A chemical decontamination method according to a seventh embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 3, 5, 13 and 15. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a BWR plant.

[0294] In the chemical decontamination method of this embodiment, each process of steps S1 to S6 shown in FIG. 1, step S7B shown in FIG. 13, and each process of steps S21 to S23 shown in FIG. 1 are performed. In the chemical decontamination method of this embodiment, a chemical decontamination apparatus 28E shown in FIG. 15 is used. The chemical decontamination apparatus 28E has a configuration in which a control device 90 is added to the chemical decontamination apparatus 28D (see FIG. 14) used in the sixth embodiment, and the concentration ratio grasping device 62A is connected to the control device 90. The other configuration of the chemical decontamination apparatus 28E is the same as that of the chemical decontamination apparatus 28D. The control device 90 is also connected to each of the supply pumps 41 and 44, the valves 43 and 46, and the flow rate control valves 67, 68, 69, 70, 76, 77, 77A, 79, and 81 in the oxidizing agent supply device 39. The control device 90 is also connected to each of the pumps and valves other than these pumps and valves in the chemical decontamination apparatus 28E.

[0295] In the chemical decontamination method of this embodiment, each of steps S2 to S6, S7B, and S21 to S23 is performed under automatic control by the control device 90. In the chemical decontamination method of this embodiment, each of the valves and pumps described above is controlled by the control device 90.

[0296] The decomposition process of the reducing decontamination agent in step S7B includes steps S8, S9, S10B, S16, S17, and S20. Furthermore, the supply flow rate control process of the oxidizing agent in step S10B includes steps S11A, S12A, S13B, S14, S15B, S18B, and S19B.

[0297] As in the second embodiment, the control device 90 controls the opening and closing of the corresponding valves and the driving of the circulation pumps 30 and 38, and performs each process of steps S1 to S6. When the determination in step S6 becomes "YES", the oxidative decontamination and reductive decontamination of the inner surface of the recirculation system piping 6 is completed.

[0298] Thereafter, the decomposition step (step S7B) of the reduction decontamination agent is performed. First, the irradiation of ultraviolet light to the oxalic acid aqueous solution (step S8) and the step S9 (supply of oxidizing agent) are performed in the same manner as in the second embodiment. In the step S9, the flow rate control valves 68 and 81 are closed and the flow rate control valves 79, 77A and 77 are opened by the control device 90. Furthermore, the control device 90 opens the valve 43 and drives the supply pump 41. The valve 46 is closed and the supply pump 44 remains stopped. The hydrogen peroxide in the chemical tank 40 is supplied to the oxalic acid aqueous solution in the circulation pipe 29, for example, upstream of the ultraviolet irradiation device 33, through the supply pipe 42. The hydrogen peroxide may be directly supplied to the ultraviolet irradiation device 33. In the ultraviolet irradiation device 33, the oxalic acid aqueous solution containing hydrogen peroxide is irradiated with ultraviolet light. The Fe in the oxalic acid aqueous solution is irradiated with ultraviolet light. 3+ Fe 2+ The oxalic acid is partially decomposed by the OH produced in the reaction of equation (3).

[0299] In the process of step S10B (control of the supply amount of the oxidizing agent), first, a corrosion potential is measured (step S11A). The corrosion potential measured by the corrosion potentiometer 96 is input to the concentration ratio determining device 62A. The concentration ratio determining device 62A determines the Fe 3+ / Fe 2+Obtain either the logarithm of the concentration ratio or the concentration ratio. 3+ / Fe 2+ Either the logarithm of the concentration ratio or the concentration ratio is input to the control device 90. The control device 90 calculates the Fe 3+ / Fe 2+ Based on either the logarithm of the concentration ratio or the concentration ratio, it is determined whether or not the oxalic acid aqueous solution irradiated with ultraviolet light contains hydrogen peroxide, which is an oxidizing agent (step S13B). 3+ / Fe 2+ When it is determined that the logarithm of the concentration ratio is greater than "1" or the concentration ratio is greater than "10"), the control device 90 closes the flow rate control valves 68 and 81 and opens the flow rate control valves 79, 77A, and 77. As a result, the supply of the oxalic acid aqueous solution containing hydrogen peroxide irradiated with ultraviolet light to the cation exchange resin tower 34 is prevented (step S14). 3+ / Fe 2+ The amount of hydrogen peroxide supplied to the oxalic acid aqueous solution is reduced so that the logarithm of the concentration ratio becomes "1" or the concentration ratio becomes "10" (step S15B). The control device 90 reduces the rotation speed of the supply pump 41 (or decreases the opening of the valve 43). This reduces the amount of hydrogen peroxide supplied. Steps S14, S15B, S11A, S12A, and S13B are repeated until the determination in step S13B is "NO".

[0300] When the determination in step S13B is "NO," steps S16 and S18B are executed. First, steps S18B and S19B are described.

[0301] In the control device 90, Fe 3+ / Fe 2+ The logarithm of the concentration ratio is less than a first set value, and 3+ / Fe 2+In step S18B, it is determined whether the logarithm of the concentration ratio is less than the first set value "1" or the concentration ratio is less than the second set value "10" (step S18B). If the logarithm of the concentration ratio is less than the first set value "1" or the concentration ratio is less than the second set value "10", the controller 90 determines "YES" in step S18B and increases the supply amount of oxidizer (step S19B). Then, in step S19B, the controller 90 increases the rotation speed of the supply pump 41 (or increases the opening of the valve 43) to increase the supply amount of hydrogen peroxide.

[0302] Steps S19B, S11A, S12A, S13B, and S18B are repeated until the determination in step S18B is "NO." 3+ / Fe 2+ When the logarithm of the concentration ratio becomes "1" or when the concentration ratio becomes "10," control device 90 determines "NO" in step S18B. Then, the process of step S16 is performed. When the determination in step S18B becomes "NO," the increase in the opening of valve 43 is stopped, and the increase in the amount of hydrogen peroxide supplied to circulation pipe 29 is also stopped.

[0303] When the determination in step S13B becomes "NO", the control device 90 opens the flow rate control valves 68 and 70 and closes the flow rate control valve 79. The flow rate control valve 81 is closed. As in the second embodiment, the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is supplied to the cation exchange resin tower 34 (step S16), and the Fe ions contained in the oxalic acid aqueous solution are 2+ is removed in the cation exchange resin tower 34. Note that even if the determination in step S18B is "NO", the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 continues.

[0304] In the catalyst tower 36, the oxalic acid contained in the oxalic acid aqueous solution is decomposed by the action of the catalyst and hydrogen peroxide (step S17). The control device 90 opens the valve 46 and drives the supply pump 44. The hydrogen peroxide in the chemical tank 40 is supplied to the circulation pipe 29 through the supply pipe 45. The oxalic acid aqueous solution containing hydrogen peroxide is introduced into the catalyst tower 36, and the oxalic acid and hydrazine contained in the oxalic acid aqueous solution are decomposed by the action of the activated carbon catalyst and the injected hydrogen peroxide.

[0305] Thereafter, it is determined whether the decomposition of the oxalic acid, which is a reducing decontamination agent, has been completed (step S20), and if this determination is "NO", steps S8, S9, S10B, S16, and S17 are performed until the determination in step S20 becomes "YES". When the determination in step S20 becomes "YES", a purification step is performed (step S21), a step of determining the completion of chemical decontamination (step S22), and further, drainage is performed (step S23).

[0306] After the drainage process in step S23 is completed, the circulation piping 29 of the chemical decontamination equipment 28E is detached from the recirculation system piping 6 to be chemically decontaminated. The recirculation system piping 6 is then restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been chemically decontaminated is started up to start operation in the next operating cycle.

[0307] This embodiment can obtain the effects (3) to (8) of the embodiment 1 and the effects (19) and (20) of the embodiment 6. In addition, (21) in this embodiment, since the control device 90 is used, each valve and each pump in the chemical decontamination equipment 28E can be automatically controlled, and further, the Fe 3+ / Fe 2+ Based on the concentration ratio, the amount of oxidant supplied to each of the ultraviolet irradiator 33 and the catalyst tower 36 can be automatically controlled. EXAMPLES

[0308] A chemical decontamination method according to a fourth embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 3, 5, 16 and 17. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a BWR plant.

[0309] The chemical decontamination apparatus 28F (see FIG. 17) used in the chemical decontamination method of this embodiment has a configuration in which the concentration ratio grasping device 62 is removed from the chemical decontamination apparatus 28 (see FIG. 1) used in the first embodiment, and a corrosion potentiometer 96 is connected to a display device 63. The other configurations of the chemical decontamination apparatus 28F are the same as those of the chemical decontamination apparatus 28.

[0310] In the chemical decontamination method of this embodiment, steps S1 to S6 shown in Fig. 1, step S7C shown in Fig. 16, and steps S21 to S23 shown in Fig. 1 are performed. The decomposition step of the reducing decontamination agent in step S7C includes steps S8, S9, S10C, S16, S17, and S20. Furthermore, the supply flow rate control step of the oxidizing agent in step S10C includes steps S11A, S13C, S14, S15C, S18C, and S19C.

[0311] In the chemical decontamination method of this embodiment, each of steps S1 to S6 performed in embodiment 1 is also performed. After the determination in step S6 becomes "YES", the process of step S8 (irradiating the aqueous solution for reduction decontamination with ultraviolet light) and the process of step S9 (supply of oxidizing agent) included in step S7C are performed in the same manner as in embodiment 1.

[0312] Since the on-off valve 64, the valves 65 and 66, and the flow rate control valve 67 are open, the oxalic acid aqueous solution returned from the recirculation system piping 6 to the circulation piping 29 is guided to the ultraviolet irradiation device 33. In the process of step S8, the ultraviolet irradiation device 33 irradiates the oxalic acid aqueous solution with ultraviolet rays. Furthermore, in the process of step S9, hydrogen peroxide is supplied from the oxidizing agent supply device 39 through the supply piping 42 to the oxalic acid aqueous solution in the circulation piping 29 upstream of the ultraviolet irradiation device 33. By irradiating the oxalic acid aqueous solution containing hydrogen peroxide with ultraviolet rays, the Fe contained in the oxalic acid aqueous solution is oxidized. 3+ Fe 2+and some of the oxalic acid in the oxalic acid solution is decomposed into carbon dioxide and water (see formula (2)). Some of the oxalic acid is also decomposed by the ·OH produced in the reaction of formula (3).

[0313] The aqueous oxalic acid solution discharged from the ultraviolet irradiation device 33 is not supplied to the cation exchange resin tower 34 because the flow control valve 68 is closed, but is returned to the circulation pipe 29 downstream of the flow control valve 77 through pipes 78 and 75, and is supplied to the recirculation system pipe 6.

[0314] Next, the supply amount of the oxidizing agent is controlled (step S10C). In the process of step S10C, each of the processes of steps S11A, S13C, S14, S15C, S18C, and S19C is performed.

[0315] The corrosion potential of the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is measured by the corrosion potentiometer 96 (step S11A). The measured corrosion potential is displayed on the display device 63. In the process of step S13C (determining whether an oxidizing agent is contained), the operator determines whether the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 contains hydrogen peroxide based on the corrosion potential displayed on the display device 63. When the displayed corrosion potential is greater than "400 mV vs SHE", for example, when the corrosion potential is 460 mV vs SHE, the oxalic acid aqueous solution irradiated with ultraviolet light contains hydrogen peroxide (see FIG. 7), and the determination in step S13C is "YES".

[0316] When the determination in step S13C is "YES", the process of step S14 is carried out as in the first embodiment. That is, the oxalic acid aqueous solution containing hydrogen peroxide bypasses the cation exchange resin tower 34, and is prevented from being supplied to the cation exchange resin tower 34. In order to prevent hydrogen peroxide from being contained in the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33, the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution is reduced (step S15C). Since the determination in step S13C is "YES", the operator operates a lever or button provided on the operation panel, and a control command to reduce the amount of hydrogen peroxide supplied is output from the operation panel. The rotation speed of the supply pump 41 is reduced (or the opening of the valve 43 is reduced) by the control command so that the measured corrosion potential falls to "400 mV vs SHE", and the amount of hydrogen peroxide supplied from the chemical tank 40 of the oxidizing agent supply device 39 to the oxalic acid aqueous solution flowing in the circulation pipe 29 at a portion upstream of the ultraviolet irradiation device 33 is reduced.

[0317] Each of steps S14, S15C, S11A, and S13C is repeated until the determination of step S13C is "NO". By repeating such corresponding steps, the corrosion potential measured by the corrosion potentiometer 96 is eventually reduced to at least "400 mV vs SHE". As a result, hydrogen peroxide is no longer contained in the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33, and the reduction in the rotation speed of the supply pump 41 is stopped. The oxalic acid aqueous solution not containing hydrogen peroxide can be supplied to the cation exchange resin tower 34. Furthermore, when the measured corrosion potential is "400 mV vs SHE", the decomposition efficiency of oxalic acid in the oxalic acid aqueous solution containing hydrogen peroxide is highest.

[0318] When the determination in step S13C is "NO", the process in step S16 and the determination in step S18C (determination as to whether the corrosion potential is less than a fourth set value) are performed. The fourth set value is a set value used to determine whether it is necessary to increase the supply amount of the oxidizing agent using the corrosion potential. The fourth set value is desirably set to a value within a range of 380 mV vs SHE to 400 mV vs SHE (a range of 380 mV vs SHE to 400 mV vs SHE). Here, the fourth set value is, for example, 380 mV vs SHE. When the measured corrosion potential is less than "380 mV vs SHE", the determination in step S18C is "YES". When the determination in step S18C is "YES", the process in step S19C (increasing the supply amount of the oxidizing agent) is performed. When the corrosion potential of the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is, for example, 360 mV vs SHE, in the process of step S19C, the rotation speed of the supply pump 41 is increased by the operator so that the corrosion potential becomes 380 mV vs SHE, and for example, the amount of hydrogen peroxide supplied to the oxalic acid aqueous solution guided to the ultraviolet irradiation device 33 is increased. As a result, the corrosion potential of the oxalic acid aqueous solution increases to 380 mV vs SHE. Since this 380 mV vs SHE is equal to or lower than 400 mV vs SHE, the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 does not contain hydrogen peroxide and can be supplied to the cation exchange resin tower 34.

[0319] Each process of steps S19C, S11A, S13C, and S18C is repeated until the determination of step S18C is "NO". When the determination of step S13C is "NO", the process of step S16 (supply of oxalic acid aqueous solution to the cation exchange resin tower 34) is performed as in the first embodiment. After that, even when the determination of step S18C is "NO", the process of step S16 (supply of oxalic acid aqueous solution to the cation exchange resin tower 34) is continuously performed. Furthermore, as in the first embodiment, the process of step S17 (decomposition of the reduction decontamination agent by the catalyst and the oxidizing agent) and the process of step S20 (determination of the completion of the decomposition process) are performed. When the determination of step S20 is "NO", each process of steps S8, S9, S10C, S16, S17, and S20 are performed until the determination of step S20 is "YES".

[0320] When the determination in step S20 becomes "YES", the process in step S21 (purification), the process in step S22 (determination of completion of chemical decontamination), and the process in step S23 (drainage) are performed in the same manner as in the first embodiment. After the drainage process in step S23 is completed, the circulation piping 29 of the chemical decontamination apparatus 28F is detached from the recirculation system piping 6 that is the target of chemical decontamination. Thereafter, the recirculation system piping 6 is restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been subjected to chemical decontamination is started up to start operation in the next operating cycle.

[0321] According to this embodiment, it is possible to obtain the effects (1) and (4) obtained in the embodiment 1. Furthermore, according to this embodiment, it is possible to obtain the effects (22) to (27) shown below.

[0322] (22) In this embodiment, based on the measured corrosion potential, that is, if the corrosion potential is 400 mV vs. SHE or less, it can be immediately determined that the reduction decontamination aqueous solution irradiated with ultraviolet rays does not contain a peroxidizing agent. Incidentally, if the corrosion potential exceeds 400 mV vs. SHE, it can be immediately determined that the reduction decontamination aqueous solution irradiated with ultraviolet rays contains an oxidizing agent.

[0323] (23) When the measured corrosion potential exceeds 400 mV vs. SHE, that is, when the ultraviolet-irradiated reduction decontamination aqueous solution (e.g., oxalic acid aqueous solution) contains an oxidizing agent (e.g., hydrogen peroxide), the supply of the reduction decontamination aqueous solution containing the oxidizing agent to the cation exchange resin tower 34 can be stopped. This makes it possible to significantly suppress deterioration of the cation exchange resin in the cation exchange resin tower 34, and to extend the life of the cation exchange resin.

[0324] (24) When the measured corrosion potential of the reduction decontamination aqueous solution irradiated with ultraviolet rays is 400 mV vs. SHE or less, this reduction decontamination aqueous solution does not contain an oxidizing agent, so the reduction decontamination aqueous solution irradiated with ultraviolet rays and not containing an oxidizing agent can be supplied to the cation exchange resin tower 34. As a result, the Fe contained in the reduction decontamination aqueous solution 2+ The Fe can be removed by being adsorbed on the cation exchange resin in the cation exchange resin tower 34. 2+ The removal of can promote reduction decontamination of the inner surface of the recirculation system piping 6, which is the target of reduction decontamination.

[0325] (25) When the measured corrosion potential becomes greater than 400 mV vs. SHE, the flow rate of the oxidant supplied from the oxidant supply device 39 to the circulation pipe 29 through the supply pipe 42 can be immediately reduced, and a state in which the reduction decontamination aqueous solution irradiated with ultraviolet rays does not contain any oxidant can be quickly achieved.

[0326] (26) When the measured corrosion potential becomes smaller than the fourth set value, which is equal to or less than 400 mV vs. SHE, the flow rate of the oxidizing agent supplied from the oxidizing agent supply device 39 to the circulation pipe 29 through the supply pipe 42 can be increased, so that the corrosion potential of the reducing decontamination aqueous solution irradiated with ultraviolet rays can be increased to the fourth set value in a short time. This can promote the decomposition of the reducing decontamination agent contained in the reducing decontamination aqueous solution.

[0327] (27) When the corrosion potential in the reducing decontamination aqueous solution irradiated with ultraviolet rays is less than the fourth set value (the determination in step S18C is "YES"), the supply amount of the oxidizing agent is increased (step S19C), and then a determination is made as to whether "the oxidizing agent is contained" (step S13C). Therefore, if the corrosion potential exceeds 400 mV vs. SHE due to the increase in the supply amount of the oxidizing agent, the supply amount of the oxidizing agent can be immediately decreased (step S15C). Therefore, the state in which the reducing decontamination aqueous solution irradiated with ultraviolet rays contains the oxidizing agent can be quickly eliminated. EXAMPLES

[0328] A chemical decontamination method according to a ninth embodiment, which is another preferred embodiment of the present invention, will be described with reference to Figures 1, 3, 5, 16 and 18. The chemical decontamination method according to this embodiment is applied to the recirculation system piping of a BWR plant.

[0329] In the chemical decontamination method of this embodiment, each process of steps S1 to S6 shown in FIG. 1, S7C shown in FIG. 16, and S21 to S23 shown in FIG. 1 are performed. In the chemical decontamination method of this embodiment, a chemical decontamination apparatus 28G shown in FIG. 18 is used. The chemical decontamination apparatus 28G has a configuration in which a control device 90 is added to the chemical decontamination apparatus 28F (see FIG. 17) used in the eighth embodiment. The other configurations of the chemical decontamination apparatus 28G are the same as those of the chemical decontamination apparatus 28F. A corrosion potentiometer 96 is connected to the control device 90. The control device 90 is also connected to the supply pumps 41 and 44, the valves 43 and 46, and the flow rate control valves 67, 68, 69, 70, 76, 77, 77A, 79, and 81 in the oxidizer supply device 39. The control device 90 is also connected to each of the pumps and valves other than these pumps and valves in the chemical decontamination apparatus 28G.

[0330] In the chemical decontamination method of this embodiment, steps S2 to S6, S7C, and S21 to S23 are performed under automatic control by the control device 90.

[0331] In the chemical decontamination method of this embodiment, steps S1 to S6 shown in Fig. 1, step S7C shown in Fig. 16, and steps S21 to S23 shown in Fig. 1 are performed in the same manner as in the eighth embodiment, except that the above-mentioned valves and pumps are controlled by the control device 90. The decomposition step of the reducing decontamination agent in step S7C includes steps S8, S9, S10C, S16, S17, and S20. Furthermore, the supply flow rate control step of the oxidizing agent in step S10C includes steps S11A, S13C, S14, S15C, S18C, and S19C.

[0332] As in the second embodiment, the control device 90 controls the opening and closing of the corresponding valves and the driving of the circulation pumps 30 and 38, and performs each process of steps S1 to S6. When the determination in step S6 becomes "YES", the oxidative decontamination and reductive decontamination of the inner surface of the recirculation system piping 6 is completed.

[0333] Thereafter, the decomposition step (step S7C) of the reduction decontamination agent is performed. First, the irradiation of ultraviolet light to the oxalic acid aqueous solution (step S8) and the step S9 (supply of oxidizing agent) are performed in the same manner as in the second embodiment. In the step S9, the flow rate control valves 68 and 81 are closed and the flow rate control valves 79, 77A and 77 are opened by the control device 90. Furthermore, the valve 43 is opened and the supply pump 41 is driven by the control device 90. The valve 46 is closed and the supply pump 44 remains stopped. The hydrogen peroxide in the chemical tank 40 is supplied to the oxalic acid aqueous solution in the circulation pipe 29, for example, upstream of the ultraviolet irradiation device 33, by the supply pipe 42. The hydrogen peroxide may be directly supplied to the ultraviolet irradiation device 33. In the ultraviolet irradiation device 33, the oxalic acid aqueous solution containing hydrogen peroxide is irradiated with ultraviolet light. The Fe in the oxalic acid aqueous solution is irradiated with ultraviolet light. 3+ Fe 2+ and part of the oxalic acid is decomposed in reaction (2). Part of the oxalic acid is also decomposed by the OH produced in reaction (3).

[0334] In the process of step S10C (control of the supply amount of the oxidizing agent), first, the corrosion potential is measured (step S11A). The corrosion potential measured by the corrosion potentiometer 96 is input to the control device 90. The control device 90 judges whether the oxalic acid aqueous solution irradiated with ultraviolet light contains hydrogen peroxide, which is an oxidizing agent, based on the input corrosion potential (step S13C). When it is judged that the oxalic acid aqueous solution irradiated with ultraviolet light contains hydrogen peroxide (corrosion potential is greater than "400 mV vs. SHE"), the control device 90 closes the flow rate control valves 68 and 81 and opens the flow rate control valves 79, 77A, and 77. Therefore, the supply of the oxalic acid aqueous solution irradiated with ultraviolet light and containing hydrogen peroxide to the cation exchange resin tower 34 is prevented (step S14). Then, the supply amount of hydrogen peroxide to the oxalic acid aqueous solution is reduced so that the corrosion potential becomes "400 mV vs. SHE" (step S15C). The control device 90 reduces the rotation speed of the supply pump 41 (or decreases the opening of the valve 43), thereby decreasing the amount of hydrogen peroxide supplied.

[0335] When the determination in step S13C is "NO", steps S16 and S18C are executed. First, steps S18C and S19C will be described.

[0336] The control device 90 determines whether the corrosion potential is less than a fourth set value of the corrosion potential (step S18C). The fourth set value is desirably set to a value within the range of 380 mV vs SHE to 400 mV vs SHE (range of 380 mV vs SHE to 400 mV vs SHE), similar to the fourth embodiment. When the control device 90 determines "YES" in step S18C, the supply amount of the oxidizing agent is increased (step S19C). In step S19C, the control device 90 increases the rotation speed of the supply pump 41 (or increases the opening of the valve 43) to increase the supply amount of hydrogen peroxide.

[0337] Steps S19C, S11A, S13C, and S18C are repeated until the determination in step S18C is "NO." When the corrosion potential input from the corrosion potentiometer 96 eventually reaches the fourth set value of the corrosion potential, the control device 90 determines "NO" in step S18C. Then, the process in step S16 is performed. When the determination in step S18C is "NO," the increase in the opening of the valve 43 is stopped, and the increase in the amount of hydrogen peroxide supplied to the circulation pipe 29 is also stopped.

[0338] When the determination in step S13C becomes "NO", the control device 90 opens the flow rate control valves 68 and 70 and closes the flow rate control valve 79. The flow rate control valve 81 is closed. As in the second embodiment, the oxalic acid aqueous solution discharged from the ultraviolet irradiation device 33 is supplied to the cation exchange resin tower 34 (step S16), and the Fe ions contained in the oxalic acid aqueous solution are 2+ is removed in the cation exchange resin tower 34. Note that even if the determination in step S18C is "NO", the supply of the oxalic acid aqueous solution to the cation exchange resin tower 34 continues.

[0339] In the catalyst tower 36, the oxalic acid contained in the oxalic acid aqueous solution is decomposed by the action of the catalyst and hydrogen peroxide (step S17). The control device 90 opens the valve 46 and drives the supply pump 44. The hydrogen peroxide in the chemical tank 40 is supplied to the circulation pipe 29 through the supply pipe 45. The oxalic acid aqueous solution containing hydrogen peroxide is introduced into the catalyst tower 36, and the oxalic acid and hydrazine contained in the oxalic acid aqueous solution are decomposed by the action of the activated carbon catalyst and the injected hydrogen peroxide.

[0340] Thereafter, it is determined whether the decomposition of the oxalic acid, which is a reducing decontamination agent, has been completed (step S20), and if this determination is "NO", steps S8, S9, S10C, S16, and S17 are performed until the determination in step S20 becomes "YES". When the determination in step S20 becomes "YES", a purification step is performed (step S21), a step of determining the completion of chemical decontamination (step S22), and further, drainage is performed (step S23).

[0341] After the drainage process in step S23 is completed, the circulation piping 29 of the chemical decontamination device 28G is detached from the recirculation system piping 6 to be chemically decontaminated. The recirculation system piping 6 is then restored. After the fuel exchange and maintenance and inspection of the BWR plant 1 are completed, the BWR plant 1 that has been chemically decontaminated is started up to start operation in the next operating cycle.

[0342] This embodiment can obtain the effects (1) and (4) obtained in embodiment 1, and the effects (22) to (27) obtained in embodiment 8. In addition, (28) this embodiment uses the control device 90, so that each valve and each pump in the chemical decontamination device 28G can be automatically controlled, and further, the amount of oxidant supplied to each of the ultraviolet irradiation device 33 and the catalyst tower 36 can be automatically controlled using the measured corrosion potential. EXAMPLES

[0343] A chemical decontamination method according to embodiment 10, which is another preferred embodiment of the present invention, will be described with reference to Fig. 19. The chemical decontamination method of this embodiment is applied to the recirculation system piping of a boiling water reactor plant (BWR plant).

[0344] In the chemical decontamination method of this embodiment, steps S1 to S7 and steps S21 to S23 shown in Fig. 1 are performed, which are performed in the chemical decontamination method of Example 1. However, the order of the steps performed in the chemical decontamination method of this embodiment is different from the order of the steps performed in the chemical decontamination method of Example 1. In the chemical decontamination method of Example 1, steps S21, S22, and S23 are performed in this order after step S7, whereas in the chemical decontamination method of this embodiment, steps S22, S21, and S23 are performed in this order after step S7, as shown in Fig. 19.

[0345] In the chemical decontamination method of this embodiment, in step S7, each of steps S8 to S20 shown in FIG. 2 is performed, and a chemical decontamination apparatus 28 having the structure shown in FIGS. 4 and 5 is connected to the recirculation system piping 6 as shown in FIG. 3.

[0346] In the chemical decontamination method of this embodiment, as shown in FIG. 19, the above-mentioned steps S1 to S7 are carried out in order, and further, steps S22, S21 and S23 are carried out in the same order.

[0347] According to this embodiment, it is possible to obtain the effects (1) to (10) obtained in the first embodiment. Furthermore, in this embodiment, the judgment in step S22 is performed after the judgment in step S20 in the reduction decontamination agent decomposition step (step S7) is completed, so even if the judgment in step S22 becomes "NO" and the oxidation decontamination step (step S3) is performed again, the potassium permanganate contained in the potassium permanganate aqueous solution injected into the circulation pipe 29 in this oxidation decontamination step can be effectively used for the oxidation decontamination of the recirculation pipe. However, in this embodiment, at the end of the judgment in step S20 in the reduction decontamination agent decomposition step (step S7), the oxalic acid concentration of the oxalic acid aqueous solution may be up to 10 ppm, so the potassium permanganate injected into the circulation pipe 29 in the oxidation decontamination step performed again is first decomposed by oxalic acid until the oxalic acid concentration of the oxalic acid aqueous solution present in the recirculation pipe 6 and the circulation pipe 29 becomes 0. Therefore, the degree of effective utilization of the injected potassium permanganate in the re-execution of the oxidative decontamination process is lower than in Example 1. However, since the oxalic acid concentration of the oxalic acid aqueous solution at the end of the determination in step S20 is extremely low, the potassium permanganate injected in the re-execution of the oxidative decontamination process can be effectively utilized.

[0348] In this embodiment, instead of the decomposition process (step S7) of the reducing decontamination agent, any of the decomposition process (step S7A) of the reducing decontamination agent shown in FIG. 10, the decomposition process (step S7B) of the reducing decontamination agent shown in FIG. 13, and the decomposition process (step S7C) of the reducing decontamination agent shown in FIG. 16 may be performed. Furthermore, when performing the decomposition process (step S7) of the reducing decontamination agent, the chemical decontamination apparatus 28A shown in FIG. 8 may be used instead of the chemical decontamination apparatus 28 shown in FIG. 4. When performing the decomposition process (step S7A) of the reducing decontamination agent shown in FIG. 10, any of the chemical decontamination apparatus 28B shown in FIG. 11 and the chemical decontamination apparatus 28C shown in FIG. 12 may be used. When performing the decomposition process (step S7B) of the reducing decontamination agent shown in FIG. 13, any of the chemical decontamination apparatus 28D shown in FIG. 14 and the chemical decontamination apparatus 28E shown in FIG. 15 may be used. When the decomposition step (step S7C) of the reducing decontamination agent shown in FIG. 16 is performed, either the chemical decontamination apparatus 28F shown in FIG. 17 or the chemical decontamination apparatus 28G shown in FIG. 18 may be used.

[0349] Each of the chemical decontamination methods of the first to tenth embodiments can also be applied to the chemical decontamination of components of nuclear power plants other than BWR plants, such as pressurized water nuclear power plants. In addition, by storing a plurality of cut pieces of components of a nuclear power plant other than BWR plants, such as pressurized water nuclear power plants, in the above-mentioned decontamination container to which both ends of the circulation piping of any one of the chemical decontamination devices 28 to 28G are connected, each of the chemical decontamination methods of the first to tenth embodiments can also be applied to these cut pieces. [Explanation of symbols]

[0350] 1...BWR plant, 2...reactor, 3...reactor pressure vessel, 6...recirculation system piping, 9...turbine, 11...supply water piping, 28,28A...chemical decontamination equipment, 29...circulation piping, 30,38...circulation pump, 33...ultraviolet irradiation equipment, 34...cation exchange resin tower, 35...mixed bed resin tower, 36...catalyst tower, 37...water level adjustment tank, 39...oxidizer supply equipment, 47...chemical decontamination agent supply equipment, 48...oxidation decontamination agent tank, 51...reduction decontamination agent tank, 57...pH adjuster injection equipment, 61...oxidation-reduction potentiometer, 62...concentration ratio grasping device, 90...control device, 96...corrosion potentiometer.

Claims

1. bringing an aqueous solution of a reduction decontamination agent into contact with a surface of a component of a nuclear power plant that is in contact with reactor water to perform reduction decontamination of the component; In the step of decomposing the reducing decontamination agent contained in the aqueous solution, irradiating the aqueous solution with ultraviolet light, measuring either an oxidation-reduction potential or a corrosion potential of the aqueous solution irradiated with the ultraviolet light, and adjusting an amount of an oxidizing agent supplied to at least one of the aqueous solution upstream of an ultraviolet irradiation device that irradiates the ultraviolet light and the aqueous solution in the ultraviolet irradiation device based on the measured oxidation-reduction potential or the measured corrosion potential; the adjustment of the amount of the oxidant supplied to the aqueous solution is performed by determining, based on the measured oxidation-reduction potential or the measured corrosion potential, either a logarithm of a concentration ratio of Fe 3+ to Fe 2+ in the aqueous solution irradiated with the ultraviolet light or the concentration ratio, and adjusting, based on the determined logarithm of the concentration ratio or the concentration ratio, an amount of the oxidant supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device and the aqueous solution in the ultraviolet irradiation device; The chemical decontamination method includes determining whether the aqueous solution irradiated with ultraviolet light contains the oxidizing agent based on either the logarithm of the concentration ratio or the concentration ratio.

2. The chemical decontamination method according to claim 1 , further comprising displaying either the logarithm of the determined concentration ratio or the determined concentration ratio.

3. The chemical decontamination method according to claim 1 , wherein the aqueous solution irradiated with the ultraviolet light is determined to contain the oxidizing agent when the logarithm of the concentration ratio is greater than 1 or when the concentration ratio is greater than 10.

4. 2. The chemical decontamination method according to claim 1, wherein when the aqueous solution irradiated with the ultraviolet light contains the oxidant, the supply of the aqueous solution to a cation exchange resin tower is stopped, and an amount of the oxidant supplied to the aqueous solution is reduced.

5. 2. The chemical decontamination method according to claim 1, wherein an amount of the oxidant supplied to the aqueous solution is increased so that, when it is determined that the aqueous solution does not contain the oxidant and that the logarithm of the concentration ratio is smaller than a first set value that is equal to or less than 1, the logarithm of the concentration ratio becomes the first set value, or, when it is determined that the concentration ratio is smaller than a second set value that is equal to or less than 10, the concentration ratio becomes the second set value.

6. 2. The chemical decontamination method according to claim 1, wherein the reducing decontamination agent contained in the aqueous solution is decomposed by irradiating the aqueous solution with ultraviolet light, and the reducing decontamination agent is decomposed in a catalytic tower in which a catalyst is present by the catalyst and the oxidizing agent supplied to the catalytic tower.

7. 7. The chemical decontamination method according to claim 6, wherein, when it is determined that the aqueous solution does not contain the oxidizing agent, the aqueous solution is supplied to a cation exchange resin tower, metal cations contained in the aqueous solution are removed in the cation exchange resin tower, the oxidizing agent is supplied to the aqueous solution from which the metal cations have been removed, and the aqueous solution containing the oxidizing agent and the reduction decontamination agent is supplied to the catalyst tower.

8. A second pipe separate from a first pipe that is a component of a nuclear power plant and is connected to a reactor pressure vessel and is a target for chemical decontamination is connected to the first pipe to form a closed loop including the first pipe and the second pipe; Supplying an aqueous solution containing a reduction decontamination agent from the second pipe to the first pipe to perform reduction decontamination on an inner surface of the first pipe; In the step of decomposing the reducing decontamination agent contained in the aqueous solution, irradiating ultraviolet light onto the aqueous solution which is connected to the second pipe and is returned from the first pipe to the second pipe, and measuring either an oxidation-reduction potential or a corrosion potential of the aqueous solution which has been irradiated with the ultraviolet light; adjusting an amount of an oxidizing agent supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device and the aqueous solution in the ultraviolet irradiation device based on the measured oxidation-reduction potential or the measured corrosion potential; the adjustment of the amount of the oxidant supplied to the aqueous solution is performed by determining, based on the measured oxidation-reduction potential or the measured corrosion potential, either a logarithm of a concentration ratio of Fe 3+ to Fe 2+ in the aqueous solution irradiated with the ultraviolet light or the concentration ratio, and adjusting, based on the determined logarithm of the concentration ratio or the concentration ratio, an amount of the oxidant supplied to at least one of the aqueous solution upstream of the ultraviolet irradiation device and the aqueous solution in the ultraviolet irradiation device; The chemical decontamination method includes determining whether the aqueous solution irradiated with ultraviolet light contains the oxidizing agent based on either the logarithm of the concentration ratio or the concentration ratio.

9. 9. The chemical decontamination method according to claim 8, wherein the reducing decontamination agent contained in the aqueous solution is decomposed by irradiating the aqueous solution with ultraviolet light, and the reducing decontamination agent is decomposed in a catalyst tower in which a catalyst is present by the catalyst and the oxidizing agent supplied to the catalyst tower.

10. A circulation pipe for supplying an aqueous solution containing a reduction decontamination agent to a piping system to be chemically decontaminated, which is a component of a nuclear power plant; an oxidation-reduction potential measuring device for measuring an oxidation-reduction potential of the aqueous solution in the circulation pipe, the oxidation-reduction potential being irradiated with ultraviolet light, or a corrosion potential measuring device for measuring a corrosion potential of the aqueous solution in the circulation pipe, the corrosion potential being irradiated with ultraviolet light; a control device that controls an amount of an oxidant supplied to the aqueous solution based on either the oxidation-reduction potential measured by the oxidation-reduction potential measuring device or the corrosion potential measured by the corrosion potential measuring device; and a concentration ratio determining device that determines either a logarithm of a concentration ratio of Fe 3+ to Fe 2+ in the aqueous solution or the concentration ratio based on the measured oxidation-reduction potential or the measured corrosion potential; The control device includes: adjusting the amount of the oxidant supplied to the aqueous solution based on either the logarithm of the concentration ratio obtained by the concentration ratio determining device or the concentration ratio, by controlling the amount of the oxidant supplied to the aqueous solution; The chemical decontamination apparatus determines whether the aqueous solution irradiated with ultraviolet light contains the oxidant based on either the logarithm of the concentration ratio or the concentration ratio.

11. The chemical decontamination apparatus according to claim 10 , further comprising an ultraviolet irradiator that irradiates the aqueous solution with ultraviolet light.

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