Chemical decontamination method and chemical decontamination equipment

Abstract

To provide a chemical decontamination method and chemical decontamination equipment that enable decontamination to be performed at a low cost and in a short time, by suppressing deterioration of ion exchange resin.SOLUTION: A chemical decontamination method has the steps of: chemically decontaminating a decontamination object; and decomposing a decontamination agent. The step of decomposing the decontamination agent is the step of performing decomposition processing of decomposing the decontamination agent, and ion exchange processing of collecting an ion. In the step of decomposing the decontamination agent, when iron ion concentration of decontamination liquid is equal to or greater than a reference value, the step is configured to perform the decontamination processing after performing the ion exchange processing, and when the iron ion concentration of the decontamination liquid is less than the reference value, the step thereof is configured to perform the ion exchange processing after performing the decomposition processing. Chemical decontamination equipment is configured in such a way that, when the iron ion concentration of the decontamination liquid is equal to or greater than the reference value, a flow channel of the decontamination liquid is switched to a flow channel in the following order via an ion exchange resin tower and a decontamination equipment, and when the iron ion concentration of the decontamination liquid is less than the reference value, the flow channel is switched to a flow channel in following order via the decontamination equipment and ion exchange resin tower.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a chemical decontamination method and a chemical decontamination apparatus for chemically decontaminating the inside of a reactor or piping of a nuclear power plant. [Background technology]

[0002] A boiling water nuclear power plant (hereinafter referred to as "BWR plant") has a nuclear reactor with a core built inside a reactor pressure vessel (hereinafter referred to as "RPV"). Inside the RPV, reactor water (cooling water) is supplied to the core by a recirculation pump (or internal pump). The reactor water is heated by heat generated by nuclear fission of nuclear fuel material in the fuel assemblies loaded in the core, and some of it becomes steam. The steam is led from the RPV to a turbine, which rotates the turbine. The steam discharged from the turbine becomes water in a condenser. The condensed water is supplied to the reactor as feedwater through the feedwater system piping. In order to suppress the generation of radioactive corrosion products in the RPV, the feedwater is mainly freed of metallic impurities by a filtration and demineralization device installed in the feedwater piping.

[0003] Corrosion products that are the source of radioactive corrosion products are generated on the surfaces of BWR plant components such as RPVs and recirculation system piping that come into contact with reactor water. For this reason, stainless steel, nickel-based alloys, and other materials with excellent corrosion resistance are used as materials for the main primary system components. In addition, RPVs made of low-alloy steel are overlaid with stainless steel on the inside to prevent the low-alloy steel from coming into direct contact with reactor water. In addition, the filtration and demineralization equipment of the reactor purification system purifies a portion of the reactor water and actively removes the metal impurities contained in the reactor water in small amounts.

[0004] However, even if the above-mentioned corrosion countermeasures are taken, the presence of very small amounts of metal impurities in the reactor water is unavoidable, and some of the metal impurities adhere to the surfaces of the fuel rods in the fuel assemblies as metal oxides. The metal elements contained in the metal oxides undergo nuclear reactions with neutrons emitted from the nuclear fuel material in the fuel rods, and become radioactive nuclides such as cobalt-60, cobalt-58, chromium-51, and manganese-54. Most of these radioactive nuclides remain attached to the surfaces of the fuel rods in the form of oxides, but some of the radioactive nuclides dissolve as ions in the reactor water or are re-released into the reactor water as insoluble solids called crud, depending on the solubility of the oxides. If any radioactive material remains unremoved in this way, it circulates through the recirculation system piping together with the reactor water and accumulates on the surfaces of the components that come into contact with the reactor water. As a result, high doses of radiation are emitted from the surfaces of the components, causing radiation exposure to workers during routine inspection work. The radiation exposure dose of workers is controlled so as not to exceed the prescribed value for each individual. In recent years, this standard has been lowered, creating a need to keep each person's radiation exposure as low as economically possible.

[0005] Therefore, chemical decontamination is being carried out to remove oxide films containing radioactive nuclides such as cobalt 60 and cobalt 58 that form on the surfaces of structural members of nuclear power plants that have been in operation, such as piping, through chemical reactions using chemical agents. Generally, chemical decontamination involves reduction decontamination, in which a reducing agent is used as a decontamination agent to remove the metal oxide film, and oxidation decontamination, in which an oxidizing agent is used as a decontamination agent to oxidize and dissolve the chromium in the metal oxide as hexavalent chromium.

[0006] As a technology related to chemical decontamination, for example, Patent Document 1 (JP Patent Publication No. 2000-105295) discloses a chemical decontamination method including a process of performing reduction decontamination using a reduction decontamination agent containing at least two or more components (oxalic acid, hydrazine), and a process of decomposing the reduction decontamination agent using a decomposition device that decomposes at least two or more chemical substances in the reduction decontamination agent after the process. The decomposition device that decomposes the reduction decontamination agent is located upstream (at the inlet side of the purification device) of the cation resin tower and the mixed bed resin tower, and in the reduction decontamination agent decomposition mode, the component captured in the cation resin tower (hydrazine) is decomposed together with the other component (oxalic acid). The decomposition of the reduction decontamination agent suppresses the load on the cation resin tower and the mixed bed resin tower.

[0007] In chemical decontamination, the decontamination liquid used to clean the inside of the reactor and piping is passed through an ion exchange resin to capture the ions dissolved in the decontamination liquid. However, if the decontamination liquid is passed through an ion exchange resin when hydrogen peroxide is present in the decontamination liquid, it can cause deterioration of the ion exchange resin. When decontamination is performed during decommissioning, radiation emitted from the reactor equipment in the RPV tends to cause radiolysis of water, which easily generates hydrogen peroxide. If the hydrogen peroxide concentration in the decontamination liquid is high, oxidation deterioration of the ion exchange resin becomes a problem.

[0008] When ion exchange resins deteriorate, it becomes difficult to capture ions of radionuclides. In addition, resin components such as polystyrene sulfonic acid become more likely to elute. The cationic resin components are adsorbed to the anion exchange resin used to purify the decontamination waste liquid, and the ion exchange capacity of the anion exchange resin decreases. If the hydrogen peroxide concentration in the decontamination liquid is high, it will lead to an increase in the amount of ion exchange resin used, which will increase the decontamination cost. As various issues increase, it is desirable to reduce the hydrogen peroxide contained in the decontamination waste liquid as much as possible.

[0009] In Patent Document 2 (JP Patent Publication No. 2021-148478), a process is performed to decompose hydrogen peroxide contained in the reduction decontamination liquid after the reduction decontamination process. The process to decompose hydrogen peroxide is performed in a hydrogen peroxide decomposition device separately from the process to decompose the reduction decontamination liquid. The hydrogen peroxide decomposition device is installed upstream of the cation exchange resin tower, between the outlet at the top of the RPV and the cation exchange resin tower. Hydrogen peroxide generated in the high-dose RPV is decomposed by a hydrogen peroxide decomposition device filled with a catalyst. The decontamination liquid in which hydrogen peroxide has been decomposed in the hydrogen peroxide decomposition device is introduced into the cation exchange resin tower. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2000-105295 A [Patent Document 2] Patent Publication No. 2021-148478 Summary of the Invention [Problem to be solved by the invention]

[0011] In chemical decontamination, it is desirable to reduce the hydrogen peroxide concentration in the decontamination liquid before the liquid is passed through the ion exchange resin. There are two possible sources of hydrogen peroxide. The first is (1) hydrogen peroxide generated by radiolysis of water in a high-dose reactor. The second is (2) hydrogen peroxide injected into the reduction decontamination liquid for the purpose of decomposing the reducing agent. The hydrogen peroxide injected into the reduction decontamination liquid may flow into the ion exchange resin tower via the RPV. It is necessary to reduce this hydrogen peroxide before it flows into the ion exchange resin tower.

[0012] In Patent Document 1, hydrogen peroxide is injected upstream of the cation exchange resin tower into the decontamination liquid into which hydrazine has been injected. Therefore, in the technology of Patent Document 1, not only hydrogen peroxide generated by radiolysis of water but also hydrogen peroxide injected into the decontamination liquid may flow into the cation exchange resin tower.

[0013] In Patent Document 2, hydrogen peroxide contained in the reduction decontamination liquid is decomposed upstream of the cation exchange resin tower. Therefore, the technology of Patent Document 2 can prevent high-concentration hydrogen peroxide from flowing into the cation exchange resin tower. However, the technology of Patent Document 2 requires a new hydrogen peroxide decomposition device to be prepared in addition to the reduction decontamination liquid decomposition device that processes the used reduction decontamination liquid. If a new hydrogen peroxide decomposition device is installed, it may take time to circulate the decontamination liquid, which may increase the time required for chemical decontamination. In addition, there is a problem that the equipment costs and operating costs of the decontamination system increase.

[0014] It has been confirmed that hydrogen peroxide is decomposed not only by the impregnated catalyst etc. described in Patent Document 2, but also by reaction with iron ions. When the concentration of iron ions is 15 ppm or more, it has been obtained that almost all hydrogen peroxide contained in the reduction decontamination solution is decomposed. Since iron ions are eluted from corrosion products during chemical decontamination, the concentration is higher in the early stages of chemical decontamination. Until the concentration of iron ions decreases due to the continuation of chemical decontamination, there is a possibility that the catalyst of a newly installed hydrogen peroxide decomposition device will be poisoned by iron ions, and operation may be restricted. In conventional technology, the effective operation period of the catalyst tower that decomposes hydrogen peroxide is limited, so improvements are desired from the viewpoints of the time required for chemical decontamination, equipment costs, operating costs, etc.

[0015] In addition, in conventional chemical decontamination equipment, when checking the concentration of hydrogen peroxide flowing into the ion exchange resin tower, it is necessary to sample the decontamination liquid flowing out from the catalyst tower that decomposes hydrogen peroxide. A colorimetric analysis method using test strips is generally used to measure the concentration of hydrogen peroxide. Therefore, manual analysis is required after sampling. Conventional chemical decontamination equipment requires workers to perform the analysis, which takes time, increases the risk of exposure, and increases operating costs. As for peripheral technologies for chemical decontamination, there is an advantage to forms that require less manual labor.

[0016] In view of the above circumstances, an object of the present invention is to provide a chemical decontamination method and chemical decontamination apparatus that can suppress deterioration of ion exchange resin and perform decontamination at low cost and in a short time. [Means for solving the problem]

[0017] In order to solve the above problems, the chemical decontamination method according to the present invention is a method for chemically decontaminating a component of a nuclear power plant using a decontamination agent as a decontamination target, the method comprising the steps of: connecting a chemical decontamination device to the decontamination target; supplying a decontamination liquid containing a decontamination agent from the chemical decontamination device to the decontamination target to chemically decontaminate the decontamination target; and supplying hydrogen peroxide to the decontamination liquid to decompose the decontamination agent. The decontamination step is a step of passing the decontamination liquid through a decomposition device to decompose the decontamination agent, and passing the decontamination liquid through an ion exchange resin tower to collect ions. In the decomposition step, when the iron ion concentration of the decontamination liquid is equal to or higher than a preset reference value, the ion exchange step is performed before the decomposition step, and when the iron ion concentration of the decontamination liquid is less than a preset reference value, the decomposition step is performed before the ion exchange step.

[0018] Further, the chemical decontamination apparatus according to the present invention is a chemical decontamination apparatus that chemically decontaminates a component of a nuclear power plant using a decontamination agent as a decontamination target, and is provided with a circulation pipe that circulates a decontamination liquid containing a decontamination agent, a decomposition device that performs a decomposition process that decomposes the decontamination agent, and an ion exchange resin tower that performs an ion exchange process that captures ions, and a bypass system that bypasses a part of the circulation pipe, and when the iron ion concentration of the decontamination liquid is equal to or greater than a preset reference value, the flow path of the decontamination liquid is switched from the circulation pipe to a flow path that passes through the ion exchange resin tower and the decomposition device in this order and returns to the circulation pipe, and when the iron ion concentration of the decontamination liquid is less than the preset reference value, the flow path is switched from the circulation pipe to a flow path that passes through the decomposition device and the ion exchange resin tower in this order and returns to the circulation pipe. Effect of the Invention

[0019] According to the present invention, it is possible to provide a chemical decontamination method and a chemical decontamination apparatus that are capable of suppressing deterioration of ion exchange resin and performing decontamination at low cost and in a short time.

[0020] Problems, configurations and effects other than those described above will become apparent from the following description of the embodiments. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram showing a schematic diagram of a nuclear power plant equipped with a chemical decontamination apparatus. [Diagram 2] FIG. 2 is a diagram showing the relationship between a decomposition process and an ion exchange process in the chemical decontamination method according to the embodiment of the present invention. [Diagram 3] FIG. 2 is a diagram showing details of the chemical decontamination apparatus according to the first embodiment. [Figure 4] 2 is a flowchart showing a chemical decontamination method according to the first embodiment. [Diagram 5] 4 is a flowchart showing details of a reduction decontamination agent decomposition step of the chemical decontamination method according to the first embodiment. [Figure 6] 4 is a flowchart showing details of a reduction decontamination agent decomposition step of the chemical decontamination method according to the first embodiment. [Figure 7] FIG. 1 is a graph showing the dependence of hydrogen peroxide concentration on iron ion concentration. [Figure 8] 4 is a flowchart showing details of a reduction decontamination step of the chemical decontamination method according to the first embodiment. [Figure 9] FIG. 11 is a diagram showing details of a chemical decontamination apparatus according to a second embodiment. [Figure 10] 13 is a flowchart showing details of a reduction decontamination agent decomposition step of the chemical decontamination method according to the second embodiment. [Figure 11] 13 is a flowchart showing details of a reduction decontamination agent decomposition step of the chemical decontamination method according to the second embodiment. [Figure 12]FIG. 1 is a graph showing the relationship between the oxidation-reduction potential of an oxidation-reduction system involving iron ions and the detection state of hydrogen peroxide. [Figure 13] FIG. 11 is a diagram showing details of a chemical decontamination apparatus according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present inventors have conducted intensive studies to solve the above problems. In the course of their studies, the present inventors have focused on the change in hydrogen peroxide concentration during the decomposition of the reduction decontamination agent. Even at the end of reduction decontamination, the reduction decontamination solution contains 2000 ppm oxalic acid and about 500 ppm hydrazine. In addition, iron ions, nickel ions, chromium ions, etc., which are eluted from the oxide film, which is a corrosion product, are present. The concentrations of these chemical species gradually decrease with the decomposition of the reduction decontamination agent and the removal of the oxide film during chemical decontamination, but a test was conducted to examine the change in hydrogen peroxide concentration over time using the concentrations of these chemical species as parameters.

[0023] In the test, aqueous solutions were prepared with oxalic acid (H2C2O4) concentrations ranging from 0 ppm to 2000 ppm and hydrazine (N2H4) concentrations ranging from 2 ppm to 56 ppm. Iron (Fe) ions were added to the prepared aqueous solutions in the range of 0 ppm to 60 ppm, heated to 90°C, and hydrogen peroxide (H2O2) was added to approximately 10 ppm to prepare simulated decontamination solutions of various concentrations. The simulated decontamination solutions were sampled over time from the start of hydrogen peroxide addition to measure the hydrogen peroxide concentration.

[0024] Figure 7 shows the dependence of hydrogen peroxide concentration on iron ion concentration. Figure 7 shows the results of analyzing the simulated decontamination liquid sampled 60 minutes after the start of hydrogen peroxide addition in the above test. Figure 7 shows the analysis results obtained when the oxalic acid concentration was in the range of 0 ppm to 2000 ppm and the hydrazine concentration was in the range of 2 ppm to 56 ppm. The iron ion concentration was quantified as the iron concentration without distinguishing between divalent iron ions and trivalent iron ions.

[0025] As shown in Figure 7, when the iron ion concentration was approximately 15 ppm or more, the hydrogen peroxide concentration was approximately zero. When the iron ion concentration was high, even if radiolysis of water progressed in the RPV due to a high dose of gamma rays, most of the hydrogen peroxide generated in the RPV was decomposed by the iron ions. When the iron ion concentration was high, the decontamination liquid discharged from the RPV did not substantially contain hydrogen peroxide, and deterioration of the ion exchange resin used in the decontamination system was suppressed.

[0026] On the other hand, when the iron ion concentration was less than about 15 ppm, hydrogen peroxide remained in the simulated decontamination liquid. It was found that when the iron ion concentration was low, there was a high possibility that hydrogen peroxide generated in the RPV would flow into the decontamination system. When decontamination liquid containing a high concentration of residual hydrogen peroxide was passed through an ion exchange resin, oxidative decomposition of the ion exchange resin occurred, reducing the ion exchange capacity and causing resin components to elute. When resin components bound to cation exchange groups were eluted, they were adsorbed by the anion exchange resin used downstream, and there was a high possibility that this would cause problems in purifying the decontamination liquid.

[0027] In response to such problems, in the chemical decontamination method according to the present embodiment, when decomposing the decontamination agent contained in the decontamination liquid used for chemical decontamination of the object to be decontaminated, the order of decomposition processing in which the decontamination agent contained in the decontamination liquid is decomposed by hydrogen peroxide and ion exchange processing in which the ions contained in the decontamination liquid are captured by ion exchange are switched between each other depending on the iron ion concentration of the decontamination liquid. Also, in the chemical decontamination apparatus according to the present embodiment, the order of passing the decontamination liquid between the reduction decontamination agent decomposition apparatus that performs the decomposition processing and the ion exchange resin tower that performs the ion exchange processing is switched between each other.

[0028] A preferred embodiment of a chemical decontamination method capable of suppressing deterioration of an ion exchange resin, which reflects the above-mentioned study results, will be described below. In addition, in each of the following drawings, the same reference numerals are used for common configurations, and duplicated explanations will be omitted.

[0029] First Embodiment First, a chemical decontamination apparatus according to a first embodiment of the present invention will be described with reference to Figures 1, 2, and 3. The chemical decontamination apparatus according to this embodiment is applied to a boiling water nuclear power plant (BWR plant).

[0030] A schematic diagram of a nuclear power plant equipped with a chemical decontamination device is shown in Figure 1. Figure 1 shows a BWR plant in the decommissioning stage with a chemical decontamination device connected. As shown in FIG. 1, a BWR plant 1 includes a reactor 2, a turbine 9, a condenser 10, a recirculation system including recirculation piping 6, a reactor purification system (not shown), a feedwater system including feedwater piping 11, and the like.

[0031] The reactor 2 is a steam generating device that generates steam by heating cooling water with heat from the nuclear fission reaction of nuclear fuel. The reactor 2 has a reactor pressure vessel (RPV) 3 that houses a core 4, a jet pump 5, and the like. The core 4 is where fuel assemblies are loaded, and is surrounded by a core shroud within the RPV 3. An annular downcomer is formed between the outer surface of the core shroud and the inner surface of the RPV 3. A plurality of jet pumps 5 are installed within the downcomer. The fuel assembly is composed of a plurality of fuel rods, etc. A fuel rod is a plurality of fuel pellets made of processed nuclear fuel material housed in a fuel cladding tube.

[0032] The recirculation system has a recirculation system piping 6 made of stainless steel and a recirculation pump 7 installed in the recirculation system piping 6. The feedwater system is composed of a condensate pump 12, a condensate purification device 13 such as a condensate demineralizer, a low-pressure feedwater heater 14, a feedwater pump 15, and a high-pressure feedwater heater 16. These devices are installed in this order in feedwater piping 11 connecting the condenser 10 and the RPV 3. The RPV 3 is installed in a reactor containment vessel 18. The reactor containment vessel 18 is installed in a reactor building (not shown).

[0033] Cooling water (hereinafter referred to as reactor water) in the RPV 3 is pressurized by a recirculation pump 7 and is sprayed into the jet pump 5 through the 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 supplied to the reactor core 4 together with the cooling water in the recirculation system. The reactor water supplied to the reactor core 4 is heated by the heat of the nuclear fission reaction of the nuclear fuel material, and a part of it becomes steam. The generated steam is guided from the RPV 3 through the main steam piping 8 to the turbine 9, causing it to rotate. A generator connected to the turbine 9 rotates, generating electricity.

[0034] The steam discharged from the turbine 9 is condensed into water in a condenser 10. The condensed water is supplied as feed water through a feed water piping 11 into the RPV 3. The feed water 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. Extracted steam extracted from the turbine 9 is sent to a high-pressure feed water heater 16 through an extraction piping 17, and then sent to a low-pressure feed water heater 14.

[0035] The feedwater pressurized by the feedwater pump 15 is heated by heat exchange with extracted steam in the low-pressure feedwater heater 14 and the high-pressure feedwater heater 16, and then led into the RPV 3. Drain water recovery piping 73 for recovering drain water condensed by heat exchange is connected to the high-pressure feedwater heater 16 and the low-pressure feedwater heater 14. The drain water recovery piping 73 is connected between the high-pressure feedwater heater 16 and the low-pressure feedwater heater 14, and between the low-pressure feedwater heater 14 and the condenser 10.

[0036] 1, in the chemical decontamination method according to this embodiment, a temporary circulation pipe 19 is connected to the lower and upper parts of the RPV 3, and a chemical decontamination apparatus 20 is connected via the circulation pipe 19. As the circulation pipe 19, a pipe, a hose, or the like can be used.

[0037] Candidates for connecting the lower part of the RPV3 include the CRD housing 78, the ICM housing 79, and the RPV bottom drain line 80. Although each of these connection points may be used alone or in combination, it is preferable to use a combination of a plurality of them from the viewpoint of increasing the circulation flow rate. Candidates for connecting the upper part of the RPV3 include a steam outlet nozzle 81, a feedwater system nozzle 82, and a core spray nozzle (not shown). When the inside of the RPV3 is completely filled with the decontamination liquid, the steam outlet nozzle 81 is preferable because it can reduce the amount of gas stagnating in the upper part of the RPV3. In FIG. 1, the connection points of the lower part of the RPV3 are the CRD housing 78, the ICM housing 79, and the RPV bottom drain line 80, and the connection point of the upper part of the RPV3 is the steam outlet nozzle 81.

[0038] Fig. 2 is a diagram showing the relationship between the decomposition process and the ion exchange process in the chemical decontamination method according to the embodiment of the present invention. In Fig. 2, the solid arrows indicate the flow of the decontamination liquid when the iron ion concentration exceeds the reference value of X ppm. The dashed arrows indicate the flow of the decontamination liquid when the iron ion concentration is below the reference value of X ppm. As shown in FIG. 2, in the chemical decontamination method according to this embodiment, the order of execution of the decomposition process and the ion exchange process is switched between each other depending on the iron ion concentration of the decontamination liquid.

[0039] The chemical decontamination method according to the present embodiment is a method for chemically decontaminating a component of a nuclear power plant using a decontamination agent as a decontamination target. The chemical decontamination method according to the present embodiment includes a step of connecting a chemical decontamination device to the decontamination target, a step of supplying a decontamination liquid containing a decontamination agent from the chemical decontamination device to the decontamination target to chemically decontaminate the decontamination target, and a step of supplying hydrogen peroxide to the decontamination liquid to decompose the decontamination agent.

[0040] The process of decomposing the decontamination agent involves a decomposition process in which the decontamination agent is decomposed by passing the decontamination liquid through a decomposition device, and an ion exchange process in which the decontamination liquid is passed through an ion exchange resin tower to capture ions, which are successively carried out in a purification device provided in series in the decontamination system. However, the ion exchange process can be temporarily stopped depending on the properties of the decontamination liquid.

[0041] In addition, in the chemical decontamination apparatus according to this embodiment, in order to switch between the execution order of the decomposition process and the ion exchange process, in the decontamination system that cyclically supplies the decontamination liquid to decontaminate the object to be decontaminated, the order of the decontamination liquid flow is switched between the reduction decontamination agent decomposition apparatus and the ion exchange resin tower according to the iron ion concentration of the decontamination liquid. However, the flow of water to the ion exchange resin tower can be temporarily stopped according to the properties of the decontamination liquid.

[0042] The decomposition process is a process in which the decontamination agent contained in the decontamination liquid is decomposed by hydrogen peroxide. The decomposition process is performed in a reduction decontamination agent decomposition device. A catalyst tower filled with a specific catalyst can be used as the reduction decontamination agent decomposition device. In the decomposition process, oxalic acid, which is a reducing agent contained in the reduction decontamination liquid, and hydrazine, which is a pH adjuster, are decomposed. This decomposition reduces secondary waste generated during reduction decontamination and suppresses deterioration of the ion exchange resin. As the decomposition process, a catalytic reaction process in which the decontamination agent contained in the decontamination liquid is decomposed by a catalytic reaction is preferably performed.

[0043] The ion exchange process is a process for capturing ions contained in the decontamination liquid by ion exchange. The ion exchange process is carried out in an ion exchange resin tower. A cation exchange resin tower filled with a cation exchange resin is used as the ion exchange resin tower. In the ion exchange process, metal ions and the like eluted from an oxide film, which is a corrosion product, are captured. By capturing the ions, metal ions and the like of radionuclides dissolved in the decontamination liquid during chemical decontamination are recovered.

[0044] The order of treatment and the order of passing the decontamination liquid are determined by comparing the measurement result of the iron ion concentration of the decontamination liquid with a preset reference value X. The reference value X can be set as a limit concentration of iron ions such that hydrogen peroxide generated by radiolysis of water does not substantially remain in the decontamination liquid due to reaction with iron ions. The reference value X is an iron ion concentration or iron concentration not distinguishing between valences, and is preferably a value of 10 ppm or more and 20 ppm or less, more preferably a value of 15 ppm or more and 20 ppm or less.

[0045] As shown in FIG. 2, when the iron ion concentration of the decontamination liquid is equal to or higher than the reference value X as a result of comparing the measurement result of the iron ion concentration with the reference value, the decomposition process is performed after the ion exchange process. That is, the decontamination liquid is passed through the ion exchange resin tower and then the reduction decontamination agent decomposition device. When the iron ion concentration is high, hydrogen peroxide is sufficiently decomposed by reaction with the iron ions. Therefore, the ion exchange process is performed first to collect metal ions and the like eluted in the decontamination liquid. In addition, the decomposition process is performed later to decompose the reduction decontamination agent used in the reduction decontamination.

[0046] According to this order, when the reducing agent used in the reducing decontamination is decomposed, it is possible to continue ion capture and decomposition of the reducing agent while avoiding deterioration of the ion exchange resin caused by hydrogen peroxide. By carrying out the ion exchange process first, it is possible to reduce the risk that the hydrogen peroxide injected into the decontamination liquid will come into contact with the ion exchange resin.

[0047] On the other hand, when the iron ion concentration of the decontamination liquid is less than the reference value X as a result of comparing the measurement result of the iron ion concentration with the reference value, the decomposition process is performed and then the ion exchange process is performed. That is, the decontamination liquid is passed through the reduction decontamination agent decomposition device and then the ion exchange resin tower in that order. When the iron ion concentration is low, hydrogen peroxide is not sufficiently decomposed by reaction with the iron ions. Therefore, the decomposition process is performed first to decompose the reduction decontamination agent and hydrogen peroxide used in the reduction decontamination. In addition, the ion exchange process is performed later to collect metal ions and the like dissolved in the decontamination liquid.

[0048] According to this order, when the reducing agent used in the reducing decontamination is decomposed, the concentration of ions contained in the decontamination liquid can be reduced while continuing to decompose the reducing agent and hydrogen peroxide. By carrying out the decomposition process first, both the hydrogen peroxide generated in RPV3 and the hydrogen peroxide injected into the decontamination liquid can be decomposed before coming into contact with the ion exchange resin.

[0049] In the configuration where the order of processing and the order of passing the decontamination liquid are switched, the reduction decontamination agent decomposition device and the ion exchange resin tower used when the iron ion concentration of the decontamination liquid is equal to or higher than the reference value, and the reduction decontamination agent decomposition device and the ion exchange resin tower used when the iron ion concentration of the decontamination liquid is less than the reference value are the same device. In order to prevent the deterioration of the ion exchange resin due to hydrogen peroxide, the existing reduction decontamination agent decomposition device can be used, and there is no need to add a new decomposition device. Therefore, the equipment cost and operation cost of the decontamination system can be reduced.

[0050] Fig. 3 is a diagram showing details of the chemical decontamination apparatus according to the first embodiment. Fig. 3 shows a partial configuration for bypassing the circulation piping 19 of the chemical decontamination apparatus 20. The chemical decontamination apparatus 20 according to the first embodiment includes the circulation piping 19 for circulating the decontamination agent, a circulation pump 21, a cooler 22, a mixed bed resin tower 23, an ion exchange resin tower 24, a filter 25, a heater 26, a reduction decontamination agent decomposition device 27, a multipurpose tank 28, a hydrogen peroxide injection device 29, an oxidative decontamination agent injection device 30, a pH adjuster supply device 31, and the like.

[0051] The circulation pipe 19 is a pipe that connects the chemical decontamination equipment 20 to the object to be decontaminated and circulates a decontamination liquid containing a decontamination agent. The circulation pipe 19 is provided with an on-off valve 32, a circulation pump 21, a valve 33, a valve 40, a heater 26, a valve 69, a valve 72, and an on-off valve 50 in this order from upstream to downstream.

[0052] A bypass line is connected to the circulation pipe 19, bypassing the valves 33 and 40 and passing through the cooler 22 and the mixed bed resin tower 23. This bypass line is formed by an upstream pipe 60 and a downstream pipe 61. One end of the upstream pipe 60 is connected to the circulation pipe 19 downstream of the circulation pump 21 and upstream of the valve 33, and the other end is connected to the cooler 22. One end of the downstream pipe 61 is connected to the cooler 22, and the other end is connected to the circulation pipe 19 downstream of the valve 40. The upstream pipe 60 is provided with a valve 34. The downstream pipe 61 is provided with a valve 36, a mixed bed resin tower 23, and a valve 37 in this order from upstream to downstream. The mixed bed resin tower 23 is filled with an anion exchange resin and a cation exchange resin.

[0053] A reduction decontamination agent decomposition device 27 and an ion exchange resin tower 24 are installed in the circulation piping 19, and a bypass system that bypasses a part of the circulation piping is connected to the circulation piping. The reduction decontamination agent decomposition device 27 and the ion exchange resin tower 24 are installed in the bypass system. In the bypass system, the reduction decontamination agent decomposition device 27 and the ion exchange resin tower 24 are connected in parallel to each other.

[0054] The bypass system is equipped with a first bypass line in which a reductive decontamination agent decomposition device 27 is installed and which bypasses part of the circulation piping 19, a second bypass line in which an ion exchange resin tower 24 is installed and which bypasses part of the circulation piping, and relay lines which interconnect the upstream and downstream sides of the reductive decontamination agent decomposition device 27 and the upstream and downstream sides of the ion exchange resin tower 24.

[0055] The upstream side of the bypass system is formed by a branched pipe 84. The pipe 84 is connected to the circulation pipe 19 downstream of the valve 33 so as to bypass the valve 40. One end of the pipe 84 is connected to the circulation pipe 19 downstream of the valve 33 and upstream of the valve 40, one of the other branched ends is connected to the ion exchange resin tower 24, and the other branched end is connected to the reduction decontamination agent decomposition device 27.

[0056] The first bypass line is formed by an upstream pipe 84 and a downstream pipe 87. One end of the downstream pipe 87 is connected to the reductive decontamination agent decomposition device 27, and the other end is connected to the downstream side of the valve 37 of the pipe 61 connected to the mixed bed resin tower 23. The upstream pipe 84 is provided with a valve 35, a branch point, and a valve 44 in this order from upstream to downstream. The downstream pipe 87 is provided with a valve 91. The valve 44 constitutes an upstream first valve that opens and closes the flow path from the circulation pipe 19 to the first bypass line. The valve 91 constitutes a downstream first valve that opens and closes the flow path from the first bypass line to the circulation pipe 19. The reductive decontamination agent decomposition device 27 is filled with a catalyst that decomposes the reductive decontamination agent, for example, an activated carbon catalyst in which ruthenium is impregnated on the surface of activated carbon.

[0057] The second bypass line is formed by an upstream pipe 84 and a downstream pipe 62. One end of the downstream pipe 62 is connected to the ion exchange resin tower 24, and the other end is connected downstream of the valve 37 of the pipe 61 connected to the mixed bed resin tower 23 and downstream of the junction of the pipe 87 connected to the reduction decontamination agent decomposition device 27. The upstream pipe 84 is provided with a valve 35, a branch point, and a valve 38 in this order from upstream to downstream. The downstream pipe 62 is provided with a valve 90. The valve 38 constitutes an upstream second valve that opens and closes the flow path from the circulation pipe 19 to the second bypass line. The valve 90 constitutes a downstream second valve that opens and closes the flow path from the second bypass line to the circulation pipe 19. The ion exchange resin tower 24 is filled with a cation exchange resin that ion-exchanges cations such as metal ions.

[0058] The relay line is composed of a first relay line connecting the downstream side of the first bypass line from the reductive decontamination agent decomposition device 27 to the upstream side of the ion exchange resin tower 24 of the second bypass line, and a second relay line connecting the downstream side of the ion exchange resin tower 24 of the second bypass line to the upstream side of the reductive decontamination agent decomposition device 27 of the first bypass line.

[0059] The first relay line forms a flow path capable of supplying the decontamination liquid treated in the reduction decontamination agent decomposition device 27 to the ion exchange resin tower 24. The second relay line forms a flow path capable of supplying the decontamination liquid treated in the ion exchange resin tower 24 to the reduction decontamination agent decomposition device 27. These relay lines are formed by an upstream pipe 89, an intermediate pipe 86, and a downstream pipe 85.

[0060] The upstream pipe 89 is connected between the downstream side of the reductive decontamination agent decomposition device 27 and the upstream side of the valve 91 of the pipe 87 connected to the reductive decontamination agent decomposition device 27, and the downstream side of the ion exchange resin tower 24 and the upstream side of the valve 90 of the pipe 62 connected to the ion exchange resin tower 24. The downstream pipe 85 is connected between the downstream side of the valve 44 of the pipe 84 connected to the reductive decontamination agent decomposition device 27 and the upstream side of the reductive decontamination agent decomposition device 27, and the downstream side of the valve 38 of the pipe 84 connected to the ion exchange resin tower 24 and the upstream side of the ion exchange resin tower 24. One end of the intermediate pipe 86 is connected to the upstream pipe 89, and the other end is connected to the downstream pipe 85.

[0061] In the upstream pipe 89, a valve 88 is provided between a branch point from the pipe 87 connected to the reduction decontamination agent decomposition device 27 and a branch point to the intermediate pipe 86, for controlling the flow of the decontamination liquid treated in the reduction decontamination agent decomposition device 27. In addition, a valve 39 is provided between a branch point from the pipe 62 connected to the ion exchange resin tower 24 and a branch point to the intermediate pipe 86, for controlling the flow of the decontamination liquid treated in the ion exchange resin tower 24. The valve 88 constitutes a first flow path valve that opens and closes the first relay line. The valve 39 constitutes a second flow path valve that opens and closes the second relay line.

[0062] In the downstream pipe 85, a valve 83 for controlling the flow of the decontamination liquid treated in the ion exchange resin tower 24 is provided between the branch point from the intermediate pipe 86 and the junction point with the pipe 84 on the upstream side of the reduction decontamination agent decomposition device 27. In addition, a valve 43 for controlling the flow of the decontamination liquid treated in the reduction decontamination agent decomposition device 27 is provided between the branch point from the intermediate pipe 86 and the junction point with the pipe 84 on the upstream side of the ion exchange resin tower 24. The valve 83 constitutes a second flow path valve for opening and closing the second relay line. The valve 43 constitutes a first flow path valve for opening and closing the first relay line.

[0063] The circulation pipe 19 is provided with a heater 26 downstream of the valve 40 and downstream of the junction with the pipe 61 connected to the mixed bed resin tower 23. A line that bypasses the valve 69 and passes through the filter 25 is connected to the downstream side of the heater 26. This line is formed by a pipe 63 branched off from the circulation pipe 19. One end of this pipe 63 is connected to the circulation pipe 19 upstream of the valve 69, and the other end is connected to the circulation pipe 19 downstream of the valve 69. The pipe 63 is provided with a valve 42, a filter 25, and a valve 41 in this order from upstream.

[0064] A line that bypasses the valve 72 and passes through the multipurpose tank 28 is connected to the circulation pipe 19 downstream of the valve 69 and downstream of the junction with the pipe 63 connected to the filter 25. This line is formed by an upstream pipe 48 and a downstream pipe 49 branched from the circulation pipe 19. One end of the upstream pipe 48 is connected to the circulation pipe 19 downstream of the junction with the pipe 63 connected to the filter 25 and upstream of the valve 72, and the other end is connected to the multipurpose tank 28. One end of the downstream pipe 49 is connected to the multipurpose tank 28, and the other end is connected to the circulation pipe 19 downstream of the valve 72. The upstream pipe 48 is provided with a valve 71. The downstream pipe 49 is provided with a supply pump 74 and a valve 73 in this order from upstream. The multipurpose tank 28 is provided with a hopper for feeding oxalic acid used as a reduction decontamination agent. A heater 45 is installed inside the multipurpose tank 28.

[0065] A pipe 67 capable of supplying an oxidizing decontamination agent to the decontamination system is connected downstream of the junction of the piping 49 connected to the multipurpose tank 28 of the circulation piping 19. One end of this pipe 67 is connected downstream of the valve 72 of the circulation piping 19, and the other end is connected to the oxidizing decontamination agent injection device 30. The oxidizing decontamination agent injection device 30 is composed of a chemical tank 54 that stores a chemical used as an oxidizing decontamination agent, and a supply pump 55 that supplies the chemical. An aqueous solution of permanganic acid, which is an oxidizing agent, is prepared in the chemical tank 54. A valve 56 that controls the supply of the chemical is provided in the piping 67.

[0066] A pipe 68 capable of supplying a pH adjuster to the decontamination system is connected to the circulation pipe 19 downstream of the junction with the pipe 67 connected to the oxidizing decontamination agent injection device 30. One end of this pipe 68 is connected downstream of the junction with the pipe 67 connected to the oxidizing decontamination agent injection device 30, and the other end is connected to the pH adjuster supply device 31. The pH adjuster supply device 31 is composed of a chemical tank 57 that stores a chemical solution used as a pH adjuster, and a supply pump 58 that supplies the chemical solution. An aqueous solution of hydrazine, which is a weakly basic reducing agent, is prepared in the chemical tank 57. A valve 59 that controls the supply of the chemical solution is provided in the pipe 68.

[0067] A pipe 64 that bypasses the circulation pump 21 and passes through a pH meter 70 is connected to the circulation pipe 19. One end of this pipe 64 is connected downstream of the on-off valve 32 of the circulation pipe 19, and the other end is connected upstream of the branch point to the pipe 60 that passes through the cooler 22. A valve 46, a pH meter 70, and a valve 47 are provided in this pipe 64 in this order from upstream. The pH meter 70 measures the pH of the decontamination liquid circulating through the circulation pipe 19. During reduction decontamination, the pH of the reduction decontamination liquid is adjusted to about 2.5.

[0068] A pipe 66 capable of supplying hydrogen peroxide, which is an oxidizing agent, to the decontamination liquid is connected to the upstream side of the reduction decontamination agent decomposition device 27. One end of this pipe 66 is connected to the downstream side of the valve 44 of the pipe 84 and the upstream side of the reduction decontamination agent decomposition device 27, and the other end is connected to the hydrogen peroxide injection device 29. The hydrogen peroxide injection device 29 is composed of a chemical tank 51 that stores a chemical solution containing hydrogen peroxide, and a supply pump 52 that supplies the chemical solution. The pipe 66 is provided with a valve 53 that controls the supply of the chemical solution. The hydrogen peroxide is supplied to the reduction decontamination agent decomposition device 27. The hydrogen peroxide is used to decompose oxalic acid and hydrazine.

[0069] Fig. 4 is a flow chart showing the chemical decontamination method according to the first embodiment. Fig. 5 and Fig. 6 are flow charts showing details of the reduction decontamination agent decomposition step (S6) of the chemical decontamination method according to the first embodiment. In the chemical decontamination method according to this embodiment, the chemical decontamination apparatus 20 shown in Fig. 3 is used, and each of steps S1 to S9 shown in Fig. 4 is carried out.

[0070] During chemical decontamination, first, the chemical decontamination equipment 20 is connected to the object to be decontaminated (step S1). For example, when decontaminating the RPV 3, for the lower part of the RPV 3, the lower side of the CRD housing 78 and the ICM housing 79 is cut, a connection jig is attached, and the circulation piping 19 is connected. For the upper part of the RPV 3, one of the multiple steam outlet nozzles 81 is cut, a connection jig is attached, and the circulation piping 19 is connected. For the steam outlet nozzle 81 that is not cut, the core spray nozzle, etc., a closing jig is attached and closed so that the decontamination liquid does not flow to the outside. By connecting the chemical decontamination equipment 20 to the piping leading to the inside of the RPV 3 via the circulation piping 19, a closed loop that can circulate the decontamination liquid is formed.

[0071] In this embodiment, the chemical decontamination apparatus 20 is connected to the CRD housing 78, the ICM housing 79, and the steam outlet nozzle 81, but may be connected to piping leading to the inside of the RPV 3 other than the above. In addition, in this embodiment, the RPV 3 is the object of decontamination, but the chemical decontamination apparatus 20 may be connected to any of the piping of the residual heat removal system, the reactor core isolation cooling system and the core spray system, the feedwater system, and the reactor cooling water recirculation system to be the object of decontamination.

[0072] Each of steps S2 to S8 described below is performed on the RPV 3 by the chemical decontamination apparatus 20.

[0073] After connecting the chemical decontamination equipment 20 to the object to be decontaminated, water to be used for chemical decontamination is filled and heated (step S2). The water is filled into the chemical decontamination equipment 20 and the RPV 3, for example, by using a reactor component cooling water system (RCW). The on-off valve 32, valves 33, 40, 69, 72, and on-off valve 50 are opened, the other valves are closed, and the circulation pump 21 is operated. Then, the heater 26 is operated to heat the water in the closed loop to 90°C. The water heated to 90°C is circulated and supplied to the circulation piping 19 and the RPV 3.

[0074] Next, oxidative decontamination is performed by passing an oxidative decontamination liquid through the object to be decontaminated (step S3). After the water circulating in the closed loop reaches 90°C, the valve 56 of the pipe 67 connected to the oxidative decontamination agent injection device 30 is opened, and the supply pump 55 is operated to inject the aqueous solution of permanganic acid in the chemical tank 54 into the water in the closed loop. Injection of the aqueous solution of permanganic acid produces an oxidative decontamination liquid, which is an aqueous solution of permanganic acid, in the closed loop. The concentration of permanganic acid in the oxidative decontamination liquid is adjusted to, for example, 200 ppm.

[0075] The injection of the aqueous solution of permanganic acid is terminated by stopping the supply pump 55 and closing the valve 56 after the water circulating in the closed loop reaches a predetermined concentration of permanganic acid. The oxidizing decontamination liquid circulates in the closed loop, thereby carrying out oxidizing decontamination of the object to be decontaminated. The oxidizing decontamination ends when a predetermined time has elapsed since the start of circulation of the oxidizing decontamination liquid.

[0076] The oxidizing decontamination liquid circulating in the closed loop is sampled periodically to measure the concentration of each component. The oxidizing decontamination liquid is sampled at appropriate time intervals, for example, every hour. Components to be measured include permanganic acid, oxalic acid used in the post-process, hydrazine, metals such as iron, nickel, and chromium, and radioactive nuclides such as cobalt 60 and cobalt 58. The concentration measurement results are used for status management of chemical decontamination.

[0077] When monitoring the state of the decontamination liquid, the sampling location is preferably the outlet side of the RPV 3 in the closed loop and upstream of a purification device such as the ion exchange resin tower 24 and the reduction decontamination agent decomposition device 27. When monitoring the state of the purification device, the sampling location is preferably the inlet and outlet sides of the purification device. The state of the purification device can be evaluated based on the difference between the measurement results on the inlet side and the measurement results on the outlet side.

[0078] Next, the oxidative decontamination agent contained in the oxidative decontamination liquid is decomposed (step S4). In order to decompose permanganic acid, which is an oxidative decontamination agent contained in the used oxidative decontamination liquid, a predetermined amount of oxalic acid is introduced into the multipurpose tank 28. By introducing oxalic acid, an aqueous solution of oxalic acid is generated in the multipurpose tank 28. The valve 73 of the pipe 49 connected to the multipurpose tank 28 is opened, and the supply pump 74 is operated to inject the aqueous solution of oxalic acid in the multipurpose tank 28 into the oxidative decontamination liquid in the closed loop.

[0079] The injection of the aqueous solution of oxalic acid is completed by stopping the supply pump 74 and closing the valve 73 after injecting a predetermined amount of the aqueous solution of oxalic acid. The aqueous solution of oxalic acid circulates in the closed loop together with the oxidizing decontamination liquid, thereby decomposing the oxidizing decontamination agent contained in the oxidizing decontamination liquid. The decomposition of the oxidizing decontamination agent is completed when it is confirmed that the oxidizing decontamination liquid has changed from purple to colorless and transparent.

[0080] Next, reduction decontamination is performed by passing the reduction decontamination liquid through the object to be decontaminated (step S5). In preparation for reduction decontamination, valves 35, 38, and 90 are opened, valve 40 is closed, and the decontamination liquid in which the oxidative decontamination agent has been decomposed is passed through the ion exchange resin tower 24. Also, similar to step S4, an aqueous solution of oxalic acid is generated in the multipurpose tank 28. The aqueous solution of oxalic acid is injected into the closed loop. The injection of the aqueous solution of oxalic acid is completed by stopping the supply pump 74 and closing the valve 73 after a predetermined amount of the aqueous solution of oxalic acid has been injected.

[0081] After preparation for reduction decontamination is completed, an aqueous solution of hydrazine is injected into the reduction decontamination liquid to adjust the pH of the reduction decontamination liquid. The valve 59 of the pipe 68 connected to the pH adjuster supply device 31 is opened, and the supply pump 58 is operated to inject the aqueous solution of hydrazine in the chemical tank 57 into the reduction decontamination liquid in the closed loop. The aqueous solution of oxalic acid, which is the reduction decontamination liquid, is adjusted to pH 2.5 by injecting the aqueous solution of hydrazine, which acts as a pH adjuster.

[0082] The amount of hydrazine injected can be adjusted by controlling the discharge rate of the supply pump 58 and the opening of the valve 59 based on the measurement result of the pH of the reduction decontamination liquid by the pH meter 70. The reduction decontamination of the object to be decontaminated is performed by circulating the reduction decontamination liquid at pH 2.5 and 90°C in the closed loop. The oxalic acid contained in the reduction decontamination liquid dissolves the oxide film containing radioactive nuclides formed on the inner surface of the RPV3 and the inner surface of the piping.

[0083] The reduction decontamination ends when the dose rate of the area to be decontaminated in the RPV3 falls to a preset dose rate, or when a predetermined time has elapsed since the start of circulation of the reduction decontamination liquid. Whether the dose rate of the area to be decontaminated has fallen to the preset dose rate can be confirmed based on the measurement results of the radiation dose of the area to be decontaminated. The radiation dose of the area to be decontaminated can be measured by installing a radiation detector in the vicinity.

[0084] Next, the reducing agent contained in the reducing decontamination liquid is decomposed (step S6). In this process, oxalic acid, which is a reducing agent contained in the reducing decontamination liquid, and hydrazine, which is a pH adjuster, are decomposed using hydrogen peroxide. The order of processing and the order of passing the decontamination liquid are switched in this process. Details of this process are shown in Figures 5 and 6.

[0085] Next, it is determined whether or not the termination condition of the chemical decontamination is satisfied (step S7). The termination condition of the chemical decontamination can be determined, for example, whether or not the dose rate at the decontamination target area falls below a preset target value, or whether or not the number of repetitions of steps S3 to S6 exceeds a preset value.

[0086] The target dose rate can be set to, for example, a surface dose rate that results in a decontamination factor (DF) of 10 for the area to be decontaminated. The set value for the number of repetitions can be set to, for example, 2, 3, etc. Even if the target dose rate is not achieved, if the set value for the number of repetitions is achieved, it can be determined that the termination condition is met on the condition that the surface dose rate of the designated area to be decontaminated has been reduced to the background level.

[0087] If the end condition of the chemical decontamination is not met (step S7; NO), the process returns to step S3 and repeats steps S3 to S7. On the other hand, if the end condition of the chemical decontamination is met (step S7; YES), the process proceeds to step S8.

[0088] Next, the decontamination waste liquid remaining in the closed loop is cooled and drained (step S8). With the valves 34 and 40 open and the valves 35, 90, and 91 closed, the circulation pump 21 is operated to pass the decontamination waste liquid remaining in the circulation pipe 19 through the cooler 22. The decontamination waste liquid is cooled to 60°C or less by the cooler 22. Thereafter, the valves 36 and 37 are opened, the valve 40 is adjusted open to reduce the opening degree partially, and the decontamination waste liquid is passed through the mixed bed resin tower 23. When the water quality in the closed loop meets the drainage standard and the oxalic acid concentration is below the standard value, the water passing through the mixed bed resin tower 23 is stopped. Then, the water in the closed loop is drained to the outside.

[0089] Next, the chemical decontamination apparatus 20 is removed (step S9) from the object to be decontaminated, such as the RPV 3. By removing the chemical decontamination apparatus 20, the chemical decontamination of the object to be decontaminated, such as the RPV 3, is completed.

[0090] 5 and 6 are flow charts showing the details of the reduction decontamination agent decomposition step of the chemical decontamination method according to the first embodiment. 5 and 6, the order of processing and the order of passing the decontamination liquid can be switched in the reducing agent decomposition step (step S6) in which the reducing agent contained in the reducing agent decomposition step is decomposed. The reducing agent decomposition step (step S6) is performed using an ion exchange resin tower 24, a reducing agent decomposition device 27, a hydrogen peroxide injection device 29, etc.

[0091] First, the reducing decontamination liquid circulating in the closed loop is passed through the ion exchange resin tower 24 (step S601). The ion exchange resin tower 24 maintains an in-service state during decomposition of the reducing decontamination agent. Next, the reducing decontamination liquid circulating in the closed loop is passed through the reducing decontamination agent decomposition device 27 (step S602).

[0092] At the start of the flow of the reducing decontamination liquid, the valves 35, 38, 39, 83, and 91 are opened, and the valves 40, 44, 43, and 88 are closed. Also, the valve 90 is adjusted open to reduce the opening degree. By such adjustment, a part of the reducing decontamination liquid is sent from the ion exchange resin tower 24 to the reducing decontamination agent decomposition device 27. The reducing decontamination liquid passes through the ion exchange resin tower 24 from the circulation pipe 19, then passes through the valves 39 and 83, and returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27.

[0093] While the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27 from the ion exchange resin tower 24, hydrogen peroxide is injected into the reducing decontamination agent decomposition device 27 (step S603). The valve 53 is opened and the supply pump 52 is operated to supply the aqueous solution of hydrogen peroxide in the chemical tank 51 to the reducing decontamination agent decomposition device 27. The oxalic acid and hydrazine contained in the reducing decontamination liquid are decomposed in the reducing decontamination agent decomposition device 27 by the catalyst and hydrogen peroxide.

[0094] The decomposition reactions of oxalic acid and hydrazine are represented by the following equations (1) and (2). (COOH)2+H2O2→2CO2+2H2O…Formula (1) N2H4+2H2O2→N2+4H2O…Equation (2)

[0095] The decomposition of oxalic acid and hydrazine is performed while circulating the reducing agent in a closed loop. The amount of hydrogen peroxide injected into the reducing agent decomposition device 27 is adjusted by controlling the supply pump 52 so that hydrogen peroxide does not flow downstream of the reducing agent decomposition device 27. The hydrogen peroxide injected into the reducing agent is completely consumed in the reducing agent decomposition device 27.

[0096] Next, while the reducing decontamination liquid is passed from the ion exchange resin tower 24 to the reducing decontamination agent decomposition device 27, the hydrogen peroxide concentration of the reducing decontamination waste liquid discharged from the reducing decontamination agent decomposition device 27 is measured to determine whether the hydrogen peroxide concentration is less than 1 ppm (step S604). In the first embodiment, the measurement and determination of the hydrogen peroxide concentration are performed manually. The hydrogen peroxide concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0097] If the hydrogen peroxide concentration is less than 1 ppm (step S604; YES), continue passing water from the ion exchange resin tower 24 to the reduction decontamination agent decomposition device 27, and proceed to step S609. On the other hand, if the hydrogen peroxide concentration is 1 ppm or more (step S604; NO), switch to a flow path that bypasses the ion exchange resin tower 24 (step S605).

[0098] When switching to a flow path that bypasses the ion exchange resin tower 24, the valves 35, 44, and 91 are opened, and the valves 40, 38, 90, 83, and 88 are closed. The reducing decontamination liquid returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27 without passing through the ion exchange resin tower 24 from the circulation pipe 19. This control prevents the deterioration of the ion exchange resin due to the hydrogen peroxide generated in the RPV 3 or the hydrogen peroxide injected into the decontamination liquid.

[0099] Next, while the reduction decontamination liquid bypasses the ion exchange resin tower 24, the injection amount of hydrogen peroxide to the reduction decontamination agent decomposition device 27 is reduced (step S606). The injection amount of hydrogen peroxide can be adjusted by lowering the rotation speed of the supply pump 52. By reducing the injection amount of hydrogen peroxide, deterioration of the ion exchange resin caused by hydrogen peroxide is suppressed, so that the bypassed ion exchange resin tower 24 can be restored.

[0100] Next, while decreasing the amount of hydrogen peroxide injected into the reductive decontamination agent decomposition apparatus 27, the hydrogen peroxide concentration of the reductive decontamination waste liquid discharged from the reductive decontamination agent decomposition apparatus 27 is measured to determine whether the hydrogen peroxide concentration is less than 1 ppm (step S607). The hydrogen peroxide concentration can be measured on the outlet side of the reductive decontamination agent decomposition apparatus 27, etc.

[0101] If the hydrogen peroxide concentration is 1 ppm or more (step S607; NO), the process returns to step S606, and the operation of reducing the amount of hydrogen peroxide injected into the reduction decontamination agent decomposition device 27 is continued. On the other hand, if the hydrogen peroxide concentration is less than 1 ppm (step S607; YES), the process switches to a flow path passing through the ion exchange resin tower 24 (step S608).

[0102] When switching to the flow path via the ion exchange resin tower 24, the valves 35, 38, 39, 83, and 91 are opened, and the valves 40, 44, 43, and 88 are closed. Also, the valve 90 is adjusted open to reduce the opening degree of the valve 90. The reduction decontamination liquid passes through the ion exchange resin tower 24 from the circulation pipe 19, then passes through the valves 39 and 83, and returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27. By such control, the ion exchange resin tower 24, which had been bypassed, is restored, and the collection of metal ions of radioactive nuclides and the like is resumed.

[0103] If the hydrogen peroxide concentration is less than 1 ppm (step S604; YES) or if the flow path is switched to one passing through the ion exchange resin tower 24 (step S608), the iron ion concentration of the reduction decontamination liquid discharged from the RPV 3 is measured to determine whether the iron ion concentration exceeds 15 ppm (step S609). As the iron ion concentration, the most recent measurement result measured within about one hour among the measurement results measured by periodic sampling can be used.

[0104] When the iron ion concentration exceeds 15 ppm (step S609; YES), hydrogen peroxide is sufficiently consumed by reaction with iron ions, so water is continued to be passed from the ion exchange resin tower 24 to the reductive decontamination agent decomposition device 27 without switching the water passing order to the ion exchange resin tower 24 and the reductive decontamination agent decomposition device 27, and the process proceeds to step S610. The reductive decontamination liquid returns to the circulation pipe 19 via the ion exchange resin tower 24 and the reductive decontamination agent decomposition device 27 in this order.

[0105] Next, while the reducing decontamination liquid is passed through the ion exchange resin tower 24 and the reducing decontamination agent decomposition device 27 in this order, the oxalic acid concentration of the reducing decontamination liquid is measured to determine whether the oxalic acid concentration is less than 10 ppm (step S610). The oxalic acid concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0106] When the iron ion concentration exceeds 15 ppm and the oxalic acid concentration is 10 ppm or more (step S610; NO), the reducing agent is not sufficiently decomposed, so the process returns to step S601 and the decomposition of the reducing agent is continued. On the other hand, when the oxalic acid concentration is less than 10 ppm (step S610; YES), the reducing agent is sufficiently decomposed to a trace amount, so the decomposition of the reducing agent is terminated (step S611).

[0107] On the other hand, when the iron ion concentration is 15 ppm or less (step S609; NO), hydrogen peroxide is not sufficiently consumed by reaction with iron ions, so the order of water flow to the ion exchange resin tower 24 and the reduction decontamination agent decomposition device 27 is switched (step S612). The flow path of the reduction decontamination liquid is switched from the circulation pipe 19 to a flow path returning to the circulation pipe 19 via the reduction decontamination agent decomposition device 27 and the ion exchange resin tower 24 in this order.

[0108] When switching the water flow order from the ion exchange resin tower 24 to the reduction decontamination agent decomposition device 27, the valves 44, 88, 43, and 90 are opened, and the valves 38, 83, 39, and 91 are closed. The reduction decontamination liquid returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27 and the ion exchange resin tower 24 in this order from the circulation pipe 19. This control makes it possible to continue the decomposition process and the ion exchange process while avoiding deterioration of the ion exchange resin due to hydrogen peroxide. By carrying out the decomposition process first, both the hydrogen peroxide generated in the RPV 3 and the hydrogen peroxide injected into the decontamination liquid can be decomposed before coming into contact with the ion exchange resin.

[0109] Next, while the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27 and the ion exchange resin tower 24 in this order, the oxalic acid concentration of the reducing decontamination liquid is measured to determine whether the oxalic acid concentration is less than 10 ppm (step S613). The oxalic acid concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0110] If the oxalic acid concentration is 10 ppm or more (step S613; NO), the reducing agent is not sufficiently decomposed, so proceed to step S614 and continue decomposing the reducing agent. On the other hand, if the oxalic acid concentration is less than 10 ppm (step S613; YES), the reducing agent is sufficiently decomposed to a trace amount, so terminate the decomposition of the reducing agent (step S611).

[0111] Next, if the oxalic acid concentration is 10 ppm or more (step S613; NO), the reducing decontamination liquid circulating in the closed loop is passed through the reducing decontamination agent decomposition device 27 (step S614). Next, while the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27, hydrogen peroxide is injected into the reducing decontamination agent decomposition device 27 (step S615).

[0112] Next, while the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27 and then through the ion exchange resin tower 24, the hydrogen peroxide concentration of the reducing decontamination waste liquid discharged from the reducing decontamination agent decomposition device 27 is measured to determine whether the hydrogen peroxide concentration is less than 1 ppm (step S616). In the first embodiment, the measurement and determination of the hydrogen peroxide concentration are performed manually. The hydrogen peroxide concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0113] If the hydrogen peroxide concentration is less than 1 ppm (step S616; YES), continue passing water from the reduction decontamination agent decomposition device 27 to the ion exchange resin tower 24, and proceed to step S617. On the other hand, if the hydrogen peroxide concentration is 1 ppm or more (step S616; NO), switch to a flow path that bypasses the ion exchange resin tower 24 (step S618).

[0114] When switching to a flow path that bypasses the ion exchange resin tower 24, the valves 35, 44, and 91 are opened, and the valves 40, 38, 90, 83, and 88 are closed. The reducing decontamination liquid returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27 from the circulation pipe 19 without passing through the ion exchange resin tower 24. By such control, even after the iron ion concentration has decreased, deterioration of the ion exchange resin due to hydrogen peroxide generated in the RPV 3 or hydrogen peroxide injected into the decontamination liquid is avoided.

[0115] Next, while the reduction decontamination liquid bypasses the ion exchange resin tower 24, the injection amount of hydrogen peroxide to the reduction decontamination agent decomposition device 27 is reduced (step S619). The injection amount of hydrogen peroxide is adjusted by lowering the rotation speed of the supply pump 52. By reducing the injection amount of hydrogen peroxide, deterioration of the ion exchange resin caused by hydrogen peroxide is suppressed, so that the bypassed ion exchange resin tower 24 can be restored.

[0116] Next, while decreasing the amount of hydrogen peroxide injected into the reductive decontamination agent decomposition apparatus 27, the hydrogen peroxide concentration of the reductive decontamination waste liquid discharged from the reductive decontamination agent decomposition apparatus 27 is measured to determine whether the hydrogen peroxide concentration is less than 1 ppm (step S620). The hydrogen peroxide concentration can be measured on the outlet side of the reductive decontamination agent decomposition apparatus 27, etc.

[0117] If the hydrogen peroxide concentration is 1 ppm or more (step S620; NO), the process returns to step S619, and the operation of reducing the amount of hydrogen peroxide injected into the reduction decontamination agent decomposition device 27 continues. On the other hand, if the hydrogen peroxide concentration is less than 1 ppm (step S620; YES), the process switches to a flow path passing through the ion exchange resin tower 24 (step S617).

[0118] When switching to the flow path via the ion exchange resin tower 24, the valves 35, 44, 88, 43, and 90 are opened, and the valves 40, 38, 83, and 39 are closed. In addition, the valve 91 is adjusted open to reduce the opening degree of the valve 91. The reduction decontamination liquid passes through the reduction decontamination agent decomposition device 27 from the circulation pipe 19, then passes through the valves 88 and 43, and returns to the circulation pipe 19 via the ion exchange resin tower 24. By such control, the ion exchange resin tower 24, which had been bypassed, is restored, and the collection of metal ions of radioactive nuclides and the like is resumed.

[0119] Next, while the reducing decontamination liquid is passed through the ion exchange resin tower 24 from the reducing decontamination agent decomposition device 27, the oxalic acid concentration of the reducing decontamination liquid is measured to determine whether the oxalic acid concentration is less than 10 ppm (step S621). The oxalic acid concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0120] If the oxalic acid concentration is 10 ppm or more (step S621; NO), the reducing agent is not sufficiently decomposed, so the process returns to step S614 and the decomposition of the reducing agent is continued. On the other hand, if the oxalic acid concentration is less than 10 ppm (step S621; YES), the reducing agent is sufficiently decomposed to a trace amount, so the decomposition of the reducing agent is terminated (step S622).

[0121] 5 and 6, the treatment of the reduction decontamination liquid is switched depending on whether the hydrogen peroxide concentration is less than 1 ppm or whether the oxalic acid concentration is less than 10 ppm, but the treatment of the reduction decontamination liquid can be switched depending on appropriate reference values. The reference values ​​of the hydrogen peroxide concentration and the oxalic acid concentration can be set arbitrarily depending on the effect on the ion exchange resin, the goal of chemical decontamination, the detection limit of each component, etc.

[0122] 5 and 6, the order of processing and the order of passing the decontamination liquid are changed depending on whether the iron ion concentration exceeds 15 ppm, but the order of processing and the order of passing the decontamination liquid can be changed depending on an appropriate reference value of the iron ion concentration as long as hydrogen peroxide is sufficiently consumed. The reference value is preferably in the range of 10 ppm to 20 ppm in terms of iron ion concentration.

[0123] The reference value of 15 ppm is based on the results shown in FIG. 7. In the measurement test shown in FIG. 7, when the iron ion concentration is 15 ppm or more, the hydrogen peroxide concentration after 60 minutes is below the detection limit. The time required for the decontamination liquid to circulate through the closed loop is about 20 to 60 minutes depending on the output of the circulation pump. When the iron ion concentration is less than 15 ppm, it is considered that the hydrogen peroxide generated in the RPV 3 is not completely decomposed while circulating through the closed loop and reaches the ion exchange resin tower 24. Therefore, it is preferable to switch the order of processing and the order of passing the decontamination liquid when the concentration falls below the reference value after circulating through the closed loop.

[0124] According to the above chemical decontamination method and chemical decontamination apparatus, the order of decomposition and ion exchange, and the order of water flow through the reduction decontamination agent decomposition apparatus and the ion exchange resin tower can be switched according to the concentration of iron ions reacting with hydrogen peroxide in the reduction decontamination agent decomposition process, so that the decomposition of the reduction decontamination agent and hydrogen peroxide can be continued while suppressing deterioration of the ion exchange resin. Since the deterioration of the ion exchange resin can be suppressed by the appropriate decomposition of hydrogen peroxide, additional work such as removal of total organic carbon (TOC) and additional use of consumables such as ion exchange resin can be reduced. In addition, since the decomposition of hydrogen peroxide can be performed in a short time, the time for chemical decontamination can be shortened. In addition, since the order of decomposition and ion exchange can be changed, hydrogen peroxide generated in the RPV3 and hydrogen peroxide injected into the reduction decontamination solution can be decomposed early at an appropriate stage. Therefore, the installation of a new hydrogen peroxide decomposition apparatus can be omitted, the equipment cost and operating cost of the decontamination system can be reduced, and the extension of the time required for chemical decontamination can be avoided. Therefore, it is possible to provide a chemical decontamination method and chemical decontamination apparatus that can suppress deterioration of the ion exchange resin and perform decontamination at low cost in a short time.

[0125] FIG. 8 is a flowchart showing the details of the reduction decontamination step of the chemical decontamination method according to the first embodiment. As shown in Fig. 8, in a chemical decontamination method in which the order of processing and the order of passing the decontamination liquid are switched, in the reduction decontamination step (step S5) in which the reduction decontamination liquid is passed through the object to be decontaminated, it is also possible to switch whether or not to perform ion exchange processing. The reduction decontamination step (step S5) is performed using an ion exchange resin tower 24, an oxidative decontamination agent injection device 30, a pH adjuster supply device 31, etc.

[0126] Switching between the implementation and non-implementation of ion exchange treatment in the reduction decontamination step (step S5) can be applied, for example, when the iron ion concentration of the reduction decontamination liquid is not high and hydrogen peroxide generated in the RPV 3 is not completely decomposed. By using a predetermined bypass system, it is possible to switch between the implementation and non-implementation of ion exchange treatment and to switch the order of treatment and the order of passing the decontamination liquid at the same time.

[0127] First, the decomposition liquid of the oxidative decontamination liquid circulating in the closed loop is passed through the ion exchange resin tower 24 (step S501). In preparation for reduction decontamination, the valves 35, 38, and 90 are opened, and the valve 40 is closed to pass the decomposition liquid of the oxidative decontamination liquid in which the oxidative decontamination agent has been decomposed through the ion exchange resin tower 24. Manganese ions derived from permanganic acid and the like are captured by the ion exchange resin tower 24.

[0128] Next, oxalic acid is added to the decontamination liquid circulating in the closed loop (step S502). As in step S4, an aqueous solution of oxalic acid is generated in the multipurpose tank 28. The aqueous solution of oxalic acid is injected into the decontamination liquid, which is a decomposition liquid of the oxidizing decontamination liquid in the closed loop. The injection of the aqueous solution of oxalic acid is completed by stopping the supply pump 74 and closing the valve 73 after injecting a predetermined amount of the aqueous solution of oxalic acid.

[0129] Next, hydrazine is added to the reducing decontamination liquid circulating in the closed loop (step S503). The valve 59 of the pipe 68 connected to the pH adjuster supply device 31 is opened, and the supply pump 58 is operated to inject the aqueous solution of hydrazine in the chemical tank 57 into the reducing decontamination liquid in the closed loop. The aqueous solution of oxalic acid, which is the reducing decontamination liquid, is adjusted to pH 2.5 by injecting the aqueous solution of hydrazine, which acts as a pH adjuster.

[0130] The amount of hydrazine injected can be adjusted by controlling the discharge rate of the supply pump 58 and the opening of the valve 59 based on the measurement results of the pH of the reduction decontamination liquid by the pH meter 70. The reduction decontamination of the object to be decontaminated is performed by circulating the reduction decontamination liquid at pH 2.5 and 90°C in a closed loop. The oxalic acid contained in the reduction decontamination liquid dissolves the oxide film containing radioactive nuclides formed on the inner surface of the RPV3 and the inner surface of the piping. The elution of metals from the components is suppressed by adjusting the pH.

[0131] Next, the iron ion concentration of the reduction decontamination waste liquid discharged from the RPV 3 is measured to determine whether the iron ion concentration exceeds 10 ppm (step S504). As the iron ion concentration, the most recent measurement result measured within about one hour among the measurement results measured by periodic sampling can be used.

[0132] During reduction decontamination, the metal ion concentration of the reduction decontamination liquid increases as the oxide film dissolves. The metal ions contained in the reduction decontamination liquid are captured by the cation exchange resin filled in the ion exchange resin tower 24. The reduction decontamination liquid discharged from the ion exchange resin tower 24 is resupplied to the RPV 3 and the like to perform reduction decontamination on the object to be decontaminated. As the dissolution of the oxide film on the surface of the object to be decontaminated progresses, the iron ion concentration of the reduction decontamination liquid gradually decreases due to ion exchange.

[0133] In the RPV3 undergoing reduction decontamination during the decommissioning process, radiolysis of water occurs under high radiation doses, and hydrogen peroxide is likely to be generated. In the early stages of reduction decontamination, iron ions derived from the oxide film are supplied to the reduction decontamination solution circulating in the closed loop. Therefore, most of the hydrogen peroxide is decomposed by reaction with the iron ions. On the other hand, as reduction decontamination progresses, the concentration of iron ions decreases compared to the early stages of reduction decontamination. This is because iron ions are less likely to elute from the oxide film and are captured by ion exchange.

[0134] Therefore, as reduction decontamination progresses, hydrogen peroxide becomes difficult to decompose. As shown in Fig. 7, when the iron ion concentration falls below a certain concentration, hydrogen peroxide remains in the closed loop. When hydrogen peroxide reaches the ion exchange resin tower 24, the ion exchange resin deteriorates, which is a problem. Therefore, during reduction decontamination, the ion exchange resin tower 24 is set as an out-service according to the iron ion concentration of the reduction decontamination liquid, so that the reduction decontamination liquid is not passed through the ion exchange resin.

[0135] The reduction decontamination liquid is sampled periodically at the outlet side of the RPV3, and the concentration of the reduction decontamination agent, the concentration of metal ions such as iron, nickel, and chromium, and the concentration of radioactive nuclides such as cobalt 60 and cobalt 58 are measured. Based on the most recent measurement results obtained within about one hour from such periodic sampling, the flow path of the decontamination liquid can be switched.

[0136] If the iron concentration exceeds 10 ppm (step S504; YES), hydrogen peroxide is sufficiently consumed by reaction with iron ions, so the ion exchange resin tower 24 is maintained in an in-service state, reduction decontamination is continued, and the process proceeds to step S509. On the other hand, if the iron concentration is 10 ppm or less (step S504; NO), the process proceeds to step S505, where a flow path is switched to one that bypasses the ion exchange resin tower 24, and the ion exchange resin tower 24 is put in an out-of-service state (step S505).

[0137] When switching to a flow path that bypasses the ion exchange resin tower 24, the valves 35, 44, and 91 are opened, and the valves 40, 38, 90, 83, and 88 are closed. The reducing decontamination liquid returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27 without passing through the ion exchange resin tower 24 from the circulation pipe 19. This control prevents the deterioration of the ion exchange resin due to the hydrogen peroxide generated in the RPV 3 or the hydrogen peroxide injected into the decontamination liquid.

[0138] Next, while bypassing the ion exchange resin tower 24, the iron ion concentration of the reduction decontamination waste liquid discharged from the RPV 3 is measured to determine whether the iron ion concentration exceeds 10 ppm (step S506). As the iron ion concentration, the most recent measurement result measured within about one hour among the measurement results measured by periodic sampling can be used.

[0139] If the iron concentration exceeds 10 ppm (step S506; YES), hydrogen peroxide is sufficiently consumed by reaction with iron ions, and so flow of the reduction decontamination solution into the ion exchange resin tower 24 is resumed (step S507). Valves 35, 38, and 90 are opened, and valve 40 is closed to place the ion exchange resin tower 24 in an in-service state. On the other hand, if the iron concentration is 10 ppm or less (step S504; NO), it is determined whether the reduction decontamination termination condition is met (step S508).

[0140] The conditions for ending the reduction decontamination can be determined by whether the dose rate of the area to be decontaminated falls below a target value or whether a predetermined time has elapsed since the start of the reduction decontamination. For example, the target value of the dose rate can be set to a surface dose rate that results in a decontamination factor (DF) of 10 for the area to be decontaminated.

[0141] If the reduction decontamination end condition is not satisfied (step S508; NO), the process returns to step S505. On the other hand, if the reduction decontamination end condition is satisfied (step S508; YES), the reduction decontamination is terminated and the process proceeds to decomposition of the reduction decontamination agent.

[0142] When the iron concentration exceeds 10 ppm (step S504; YES) or when the flow of the reduction decontamination liquid to the ion exchange resin tower 24 is resumed (step S507), it is determined whether the reduction decontamination end condition is met (step S509). As the reduction decontamination end condition, the same condition as in step S508 can be determined.

[0143] If the reduction decontamination end condition is not met (step S509; NO), the process returns to step S504. On the other hand, if the reduction decontamination end condition is met (step S509; YES), the reduction decontamination is ended and the process proceeds to decomposition of the reduction decontamination agent.

[0144] In Fig. 8, the flow path of the reducing decontamination liquid is switched depending on whether the iron ion concentration exceeds 10 ppm, but the flow path of the reducing decontamination liquid can be switched depending on an appropriate reference value of the iron ion concentration as long as the iron ion concentration is such that hydrogen peroxide is sufficiently consumed. The reference value is preferably in the range of 10 ppm to 20 ppm in terms of iron ion concentration.

[0145] According to the above chemical decontamination method and chemical decontamination apparatus, the flow path of the reduction decontamination liquid is switched according to the concentration of iron ions that react with hydrogen peroxide in the reduction decontamination process, so that the reduction decontamination can be continued while suppressing the deterioration of the ion exchange resin. Since the deterioration of the ion exchange resin can be suppressed by the appropriate decomposition of hydrogen peroxide, the additional use of consumables such as ion exchange resin can be reduced even when the object to be decontaminated emits a high dose that causes radiolysis of water. Furthermore, in the reduction decontamination agent decomposition process (step S6) performed after the reduction decontamination process (step S5), it is possible to switch the order of processing and the order of passing the decontamination liquid, and common equipment and piping can be used. Since the deterioration of the ion exchange resin can be suppressed by the appropriate decomposition of hydrogen peroxide, it is possible to reduce additional work such as removing total organic carbon (TOC) and additional use of consumables such as ion exchange resin. The installation of a new hydrogen peroxide decomposition device can be omitted, reducing the equipment cost and operating cost of the decontamination system and avoiding the extension of the time required for chemical decontamination. Therefore, it is possible to provide a chemical decontamination method and chemical decontamination apparatus that can suppress deterioration of the ion exchange resin and perform decontamination at low cost in a short time.

[0146] <Second embodiment> Next, a chemical decontamination apparatus according to a second embodiment of the present invention will be described with reference to Fig. 9. The chemical decontamination apparatus according to this embodiment is applied to a boiling water nuclear power plant (BWR plant).

[0147] Fig. 9 is a diagram showing details of the chemical decontamination apparatus according to the second embodiment. Fig. 9 shows a partial configuration for bypassing the circulation piping 19 of the chemical decontamination apparatus 20. The chemical decontamination apparatus 20 according to the second embodiment includes the circulation piping 19 for circulating the decontamination agent, the circulation pump 21, the cooler 22, the mixed bed resin tower 23, the ion exchange resin tower 24, the filter 25, the heater 26, the reduction decontamination agent decomposition device 27, the multipurpose tank 28, the hydrogen peroxide injection device 29, the oxidation decontamination agent injection device 30, the pH adjuster supply device 31, the ultraviolet irradiation device 92, the oxidation reduction potentiometer 93, and the like.

[0148] The chemical decontamination apparatus 20 according to the second embodiment differs from the chemical decontamination apparatus 20 according to the first embodiment in that an ultraviolet ray irradiation device 92 and an oxidation-reduction potentiometer 93 are installed downstream of the reduction decontamination agent decomposition device 27. In addition, a pipe 95 in which a valve 94 is installed connects the downstream side of the supply pump 52 and the upstream side of the valve 53 of the pipe 66 connected to the hydrogen peroxide injection device 29 to the downstream side of the reduction decontamination agent decomposition device 27 and the upstream side of the ultraviolet ray irradiation device 92 of the pipe 87 connected to the reduction decontamination agent decomposition device 27.

[0149] The ultraviolet irradiation device 92 is a device that irradiates the reducing decontamination liquid with ultraviolet light to decompose the reducing decontamination agent contained in the reducing decontamination liquid. In the ultraviolet irradiation device 92, an ultraviolet irradiation process is performed in which the reducing decontamination agent is decomposed by irradiation with ultraviolet light. The ultraviolet irradiation process decomposes the oxalic acid remaining after the catalytic reaction process. As the ultraviolet irradiation device 92, a device in which an ultraviolet source such as a mercury lamp is installed in a water-passing treatment tower can be used. A pipe 95 in which a valve 94 is installed is used to inject hydrogen peroxide from the hydrogen peroxide injection device 29 to the ultraviolet irradiation device 92. The injection of hydrogen peroxide can promote the decomposition of oxalic acid. The amount of hydrogen peroxide injected into the ultraviolet irradiation device 92 can be adjusted by adjusting the opening of the valve 94 installed in this pipe 95.

[0150] 10 and 11 are flow charts showing details of the reductive decontamination agent decomposition step of the chemical decontamination method according to the second embodiment. In the chemical decontamination method according to this embodiment, the chemical decontamination apparatus 20 shown in Fig. 9 is used, and each of steps S1 to S9 shown in Fig. 4 is performed. The reductive decontamination agent decomposition step (step S6) is performed according to the procedure shown in Figs. 10 and 11.

[0151] The chemical decontamination method according to the second embodiment differs from the chemical decontamination method according to the first embodiment in that the decomposition of oxalic acid, which is a reductive decontamination agent, is performed by irradiation with ultraviolet light, and the order of processing and the order of passing the decontamination liquid are switched based on the oxidation-reduction potential of the reductive decontamination liquid. The chemical decontamination method according to the second embodiment can be applied when the iron ion concentration of the reductive decontamination liquid is reduced to a level where hydrogen peroxide generated in the RPV 3 remains.

[0152] First, the reducing decontamination liquid circulating in the closed loop is passed through the ion exchange resin tower 24 (step SA601). The ion exchange resin tower 24 maintains an in-service state during decomposition of the reducing decontamination agent. Next, the reducing decontamination liquid circulating in the closed loop is passed through the reducing decontamination agent decomposition device 27 (step SA602).

[0153] At the start of the flow of the reducing decontamination liquid, the valves 35, 38, 39, 83, and 91 are opened, and the valves 40, 44, 43, and 88 are closed. Also, the valve 90 is adjusted open to reduce the opening degree. By such adjustment, a part of the reducing decontamination liquid is sent from the ion exchange resin tower 24 to the reducing decontamination agent decomposition device 27. The reducing decontamination liquid passes through the ion exchange resin tower 24 from the circulation pipe 19, then passes through the valves 39 and 83, and returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27.

[0154] While the reducing decontamination liquid is being passed through the reducing decontamination agent decomposition device 27 from the ion exchange resin tower 24, hydrogen peroxide is injected into the reducing decontamination agent decomposition device 27 (step SA603). The valve 53 is opened and the supply pump 52 is operated to supply the aqueous solution of hydrogen peroxide in the chemical tank 51 to the reducing decontamination agent decomposition device 27. The oxalic acid and hydrazine contained in the reducing decontamination liquid are decomposed in the reducing decontamination agent decomposition device 27 by the catalyst and hydrogen peroxide.

[0155] Next, the reducing decontamination liquid circulating in the closed loop is passed through the ultraviolet irradiation device 92 (step SA604). In the ultraviolet irradiation device 92, the reducing decontamination liquid is irradiated with ultraviolet light. Oxalic acid contained in the reducing decontamination liquid is decomposed by irradiation with ultraviolet light in the ultraviolet irradiation device 92. Hydrogen peroxide may be injected into the ultraviolet irradiation device 92 from the hydrogen peroxide injection device 29 within a range in which the oxidation-reduction potential of the reducing decontamination liquid does not exceed 400 mV vs SHE.

[0156] The decomposition reaction of oxalic acid by ultraviolet light is expressed by the following equation (3). 2[Fe(III)(C2O4)3] 3- +hν →2[Fe(II)(C2O4)2] 2- +C2O4 2- +2CO2…Formula (3)

[0157] Next, the oxidation-reduction potential of the reducing decontamination liquid passed through the ultraviolet irradiation device 92 is measured to determine whether or not the oxidation-reduction potential of the reducing decontamination liquid is less than 400 mV vs. SHE (step SA605). The oxidation-reduction potential of the reducing decontamination liquid is measured by an oxidation-reduction potentiometer 93. The oxidation-reduction potentiometer 93 measures the oxidation-reduction potential of divalent iron ions (Fe 2+ ) and trivalent iron ions (Fe 3+ ) is involved in the measurement of the redox potential of the redox system.

[0158] FIG. 12 is a diagram showing the relationship between the oxidation-reduction potential of an oxidation-reduction system involving iron ions and the detection state of hydrogen peroxide. 2+ ) and trivalent iron ions (Fe 3+The results of investigating whether hydrogen peroxide was detected in response to changes in the redox potential of the redox system involving ferric oxalate were shown below. Iron (II) oxalate dihydrate was added to an aqueous solution with an oxalic acid concentration of 2000 ppm and a temperature of 90°C so that the iron concentration was 30 to 100 ppm, and then hydrogen peroxide was added. The redox potential and iron ion concentration of the aqueous solution were then measured, and the presence or absence of hydrogen peroxide was examined.

[0159] As shown in FIG. 12, when the redox potential was 400 mV vs. SHE or less, hydrogen peroxide was below the detection limit. On the other hand, when the redox potential exceeded 400 mV vs. SHE, hydrogen peroxide was detected. This result indicates that hydrogen peroxide is likely to remain when the redox potential is high and there are few divalent iron ions. 3+ ) and divalent iron ions (Fe 2+ ) and the concentration ratio ([Fe 3+ ] / [Fe 2+ When the redox potential of a decontamination solution containing oxalic acid and iron ions exceeds approximately 10, the amount of divalent iron ions that consume hydrogen peroxide decreases, resulting in significant hydrogen peroxide remaining. It can be said that the presence or absence of hydrogen peroxide can be detected by measuring the redox potential of a decontamination solution containing oxalic acid and iron ions.

[0160] If the result of the determination is that the oxidation-reduction potential of the reduction decontamination solution is less than 400 mV vs. SHE (step SA605; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, so water is continued to be passed through the ion exchange resin tower 24. In this case, hydrogen peroxide is decomposed by the reduction decontamination agent decomposition device 27 and the ultraviolet irradiator 92, so there is no need to reduce the amount of hydrogen peroxide injected.

[0161] On the other hand, if the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA605; NO), the divalent iron ion (Fe 2+ ), the flow path is switched to bypass the ion exchange resin tower 24 (step SA606). 2+) has been consumed by the Fenton reaction, and hydrogen peroxide is likely to remain.

[0162] The Fenton reaction is a reaction in which iron ions catalyze the decomposition of hydrogen peroxide, producing highly oxidizing hydroxyl radicals. The Fenton reaction is represented by the following formula (4). Fe 2+ +H2O2→Fe 3+ +OH - +·OH…Formula (4)

[0163] When switching to a flow path that bypasses the ion exchange resin tower 24, the valves 35, 44, and 91 are opened, and the valves 40, 38, 90, 83, and 88 are closed. The reducing decontamination liquid returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27 and the ultraviolet irradiator 92 without passing through the ion exchange resin tower 24 from the circulation pipe 19. This control prevents the deterioration of the ion exchange resin due to the hydrogen peroxide generated in the RPV 3 or the hydrogen peroxide injected into the decontamination liquid.

[0164] Next, while the reduction decontamination liquid bypasses the ion exchange resin tower 24, the injection amount of hydrogen peroxide to the reduction decontamination agent decomposition device 27 is reduced (step SA607). The injection amount of hydrogen peroxide can be adjusted by lowering the rotation speed of the supply pump 52. By reducing the injection amount of hydrogen peroxide, deterioration of the ion exchange resin caused by hydrogen peroxide is suppressed, so that the bypassed ion exchange resin tower 24 can be restored.

[0165] Next, while reducing the amount of hydrogen peroxide injected into the reductive decontamination agent decomposition device 27, the redox potential of the reductive decontamination agent is measured to determine whether or not the redox potential of the reductive decontamination agent is less than 400 mV vs. SHE (step SA608). The redox potential of the reductive decontamination agent is measured by the redox potentiometer 93.

[0166] If the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA608; NO), the divalent iron ion (Fe 2+), the process returns to step SA607, and the operation of decreasing the amount of hydrogen peroxide injected into the reduction decontamination agent decomposition device 27 is continued. On the other hand, if the oxidation-reduction potential of the reduction decontamination agent is less than 400 mV vs. SHE (step SA608; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, so the flow path is switched to one passing through the ion-exchange resin tower 24 (step SA609).

[0167] When switching to the flow path via the ion exchange resin tower 24, the valves 35, 38, 39, 83, and 91 are opened, and the valves 40, 44, 43, and 88 are closed. Also, the valve 90 is adjusted open to reduce the opening degree of the valve 90. The reduction decontamination liquid passes through the ion exchange resin tower 24 from the circulation pipe 19, then passes through the valves 39 and 83, and returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27 and the ultraviolet ray irradiation device 92. By such control, the ion exchange resin tower 24 that was bypassed is restored, and the collection of metal ions of radioactive nuclides and the like is resumed.

[0168] When the oxidation-reduction potential of the reduction decontamination liquid is less than 400 mV vs. SHE (step SA605; YES) or when the flow path is switched to one passing through the ion exchange resin tower 24 (step SA609), the iron ion concentration of the reduction decontamination liquid discharged from the RPV 3 is measured to determine whether the iron ion concentration exceeds 15 ppm (step SA610). As the iron ion concentration, the most recent measurement result measured within about one hour among the measurement results measured by periodic sampling can be used.

[0169] If the iron ion concentration exceeds 15 ppm (step SA610; YES), hydrogen peroxide is sufficiently consumed by reaction with iron ions, so water is continued to be passed from the ion exchange resin tower 24 to the reductive decontamination agent decomposition device 27 without switching the water passing order to the ion exchange resin tower 24 and the reductive decontamination agent decomposition device 27, and the process proceeds to step SA611. The reductive decontamination liquid returns to the circulation pipe 19 via the ion exchange resin tower 24, the reductive decontamination agent decomposition device 27, and the ultraviolet ray irradiation device 92 in this order.

[0170] On the other hand, if the iron ion concentration is 15 ppm or less (step SA610; NO), hydrogen peroxide is not sufficiently consumed by reaction with iron ions, so the order of water flow to the ion exchange resin tower 24 and the reduction decontamination agent decomposition device 27 is switched (step SA613). The flow path of the reduction decontamination liquid is switched to a flow path that returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27, the ultraviolet irradiator 92, and the ion exchange resin tower 24 in this order.

[0171] When switching the water flow order from the ion exchange resin tower 24 to the reduction decontamination agent decomposition device 27, the valves 44, 88, 43, and 90 are opened, and the valves 38, 83, 39, and 91 are closed. The reduction decontamination liquid returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27, the ultraviolet ray irradiation device 92, and the ion exchange resin tower 24 in this order from the circulation pipe 19. This control makes it possible to continue the decomposition process and the ion exchange process while avoiding the deterioration of the ion exchange resin due to hydrogen peroxide. By carrying out the decomposition process first, both the hydrogen peroxide generated in the RPV 3 and the hydrogen peroxide injected into the decontamination liquid can be decomposed before coming into contact with the ion exchange resin.

[0172] Thereafter, similarly to the first embodiment, the oxalic acid concentration of the reducing decontamination liquid is measured and determined (step SA611, step SA614), the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27 (step SA615), and hydrogen peroxide is injected into the reducing decontamination agent decomposition device 27 (step SA616). Thereafter, the reducing decontamination liquid is passed through the ultraviolet irradiator 92 (step SA617).

[0173] Next, while the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27, the ultraviolet irradiator 92, and the ion exchange resin tower 24 in this order, the oxidation-reduction potential of the reducing decontamination liquid in which the reducing decontamination agent has been decomposed is measured, and it is determined whether the oxidation-reduction potential of the reducing decontamination liquid is less than 400 mV vs. SHE (step SA618). The oxidation-reduction potential of the reducing decontamination liquid is measured by an oxidation-reduction potentiometer 93. The oxidation-reduction potentiometer 93 measures the oxidation-reduction potential of divalent iron ions (Fe 2+ ) and trivalent iron ions (Fe 3+) is involved in the measurement of the redox potential of the redox system.

[0174] If the oxidation-reduction potential of the reduction decontamination solution is less than 400 mV vs. SHE (step SA618; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, so water is continued to be passed through the ion exchange resin tower 24. On the other hand, if the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA618; NO), the divalent iron ion (Fe 2+ ) is small, the flow path is switched to bypass the ion-exchange resin tower 24 (step SA620).

[0175] When switching to a flow path that bypasses the ion exchange resin tower 24, the valves 35, 44, and 91 are opened, and the valves 40, 38, 90, 83, and 88 are closed. The reducing decontamination liquid returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27 and the ultraviolet irradiator 92 from the circulation pipe 19 without passing through the ion exchange resin tower 24. By such control, even after the iron ion concentration has decreased, deterioration of the ion exchange resin due to hydrogen peroxide generated in the RPV 3 and hydrogen peroxide injected into the decontamination liquid is avoided.

[0176] Next, while the reduction decontamination liquid bypasses the ion exchange resin tower 24, the injection amount of hydrogen peroxide to the reduction decontamination agent decomposition device 27 is reduced (step SA621). The injection amount of hydrogen peroxide can be adjusted by lowering the rotation speed of the supply pump 52. By reducing the injection amount of hydrogen peroxide, deterioration of the ion exchange resin caused by hydrogen peroxide is suppressed, so that the bypassed ion exchange resin tower 24 can be restored.

[0177] Next, while the amount of hydrogen peroxide injected into the reductive decontamination agent decomposition device 27 is being reduced, the redox potential of the reductive decontamination agent decomposed is measured to determine whether the redox potential of the reductive decontamination agent is less than 400 mV vs. SHE (step SA622). The redox potential of the reductive decontamination agent is measured by the redox potentiometer 93. The redox potentiometer 93 measures the redox potential of divalent iron ions (Fe 2+) and trivalent iron ions (Fe 3+ ) is involved in the measurement of the redox potential of the redox system.

[0178] If the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA622; NO), the divalent iron ion (Fe 2+ ), the process returns to step SA621, and the operation of decreasing the injection amount of hydrogen peroxide into the reduction decontamination agent decomposition device 27 is continued. On the other hand, if the oxidation-reduction potential of the reduction decontamination agent is less than 400 mV vs. SHE (step SA622; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, so the flow path is switched to one passing through the ion-exchange resin tower 24 (step SA619).

[0179] When switching to the flow path via the ion exchange resin tower 24, the valves 35, 44, 88, 43, and 90 are opened, and the valves 40, 38, 83, and 39 are closed. Also, the valve 91 is adjusted open to reduce the opening degree of the valve 91. The reduction decontamination liquid passes through the reduction decontamination agent decomposition device 27 and the ultraviolet ray irradiation device 92 from the circulation pipe 19, and then passes through the valves 88 and 43 and the ion exchange resin tower 24 to return to the circulation pipe 19. By such control, the ion exchange resin tower 24, which had been bypassed, is restored, and the collection of metal ions of radioactive nuclides and the like is resumed.

[0180] Next, while the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27, the ultraviolet irradiator 92, and the ion exchange resin tower 24 in this order, the oxalic acid concentration of the reducing decontamination liquid is measured to determine whether the oxalic acid concentration is less than 10 ppm (step SA623). The oxalic acid concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0181] If the oxalic acid concentration is 10 ppm or more (step SA623; NO), the reducing agent has not been sufficiently decomposed, so the process returns to step SA615 and continues decomposing the reducing agent. On the other hand, if the oxalic acid concentration is less than 10 ppm (step SA623; YES), the reducing agent has been sufficiently decomposed to a trace amount, so the decomposition of the reducing agent is terminated (step SA624).

[0182] In addition, in Fig. 10 and Fig. 11, the treatment of the reduction decontamination liquid is switched depending on whether the redox potential is 400 mV vs. SHE or more and whether the oxalic acid concentration is less than 10 ppm, but the treatment of the reduction decontamination liquid can be switched depending on an appropriate reference value. The reference values ​​of the redox potential and the oxalic acid concentration can be set arbitrarily depending on the effect on the ion exchange resin, the goal of chemical decontamination, the detection limit of each component, etc. The reference value of the redox potential is preferably in the range of 350 mV vs. SHE or more and 450 mV vs. SHE or less.

[0183] 10 and 11, the order of processing and the order of passing the decontamination liquid are changed depending on whether the iron ion concentration exceeds 15 ppm, but the order of processing and the order of passing the decontamination liquid can be changed depending on an appropriate reference value of the iron ion concentration as long as hydrogen peroxide is sufficiently consumed. The reference value is preferably in the range of 10 ppm to 20 ppm in terms of iron ion concentration.

[0184] According to the above chemical decontamination method and chemical decontamination apparatus, the order of decomposition and ion exchange, and the order of water flow through the reduction decontamination agent decomposition apparatus and the ion exchange resin tower can be switched according to the concentration of iron ions reacting with hydrogen peroxide in the reduction decontamination agent decomposition process, so that the decomposition of the reduction decontamination agent and hydrogen peroxide can be continued while suppressing deterioration of the ion exchange resin. Since the deterioration of the ion exchange resin can be suppressed by the appropriate decomposition of hydrogen peroxide, additional work such as removal of total organic carbon (TOC) and additional use of consumables such as ion exchange resin can be reduced. In addition, since the decomposition of hydrogen peroxide can be performed in a short time, the time for chemical decontamination can be shortened. In addition, since the order of decomposition and ion exchange can be changed, hydrogen peroxide generated in the RPV3 and hydrogen peroxide injected into the reduction decontamination solution can be decomposed early at an appropriate stage. Therefore, the installation of a new hydrogen peroxide decomposition apparatus can be omitted, the equipment cost and operating cost of the decontamination system can be reduced, and the extension of the time required for chemical decontamination can be avoided. Therefore, it is possible to provide a chemical decontamination method and chemical decontamination apparatus that can suppress deterioration of the ion exchange resin and perform decontamination at low cost in a short time.

[0185] In addition, an ultraviolet irradiation device is installed downstream of the reduction decontamination agent decomposition catalyst device, so that the oxalic acid contained in the decontamination solution can be decomposed quickly. Also, an oxidation-reduction potentiometer is installed, so that the divalent iron ion (Fe 2+ ) and trivalent iron ions (Fe 3+ It is possible to measure the redox potential of the redox system involving ferrous iron ions, and the amount of iron ions that are effective in decomposing hydrogen peroxide can be changed. When the redox potential is low, the iron ions that are effective in decomposing hydrogen peroxide are dominant, and the Fenton reaction can be caused by irradiation with ultraviolet light. Therefore, the decomposition action of hydroxyl radicals can shorten the time for chemical decontamination.

[0186] For example, oxalic acid can be decomposed by the decomposition action of hydroxyl radicals. The reaction between hydroxyl radicals and oxalic acid is shown in the following formula (5). C2O42- + OH → CO2+ CO2 - +OH - …Equation (5)

[0187] In addition, because the order of processing and the order of passing the decontamination liquid are switched based on the redox potential of the reduction decontamination liquid, the measurement results required to determine the switching can be obtained online as a measurement signal from an redox potentiometer. Since there is no need to sample the decontamination liquid or perform manual analysis, there is no need to deploy workers to operate the valves or perform manual analysis. This reduces the risk of exposure and operating costs during chemical decontamination.

[0188] <Third embodiment> Next, a chemical decontamination apparatus according to a third embodiment of the present invention will be described with reference to Fig. 13. The chemical decontamination apparatus according to this embodiment is applied to a boiling water nuclear power plant (BWR plant).

[0189] Fig. 13 is a diagram showing details of the chemical decontamination apparatus according to the third embodiment. Fig. 13 shows a part of the configuration for bypassing the circulation piping 19 of the chemical decontamination apparatus 20. In Fig. 13, the dashed lines indicate signal lines. The chemical decontamination apparatus 20 according to the third embodiment includes the circulation piping 19 for circulating the decontamination agent, the circulation pump 21, the cooler 22, the mixed bed resin tower 23, the ion exchange resin tower 24, the filter 25, the heater 26, the reduction decontamination agent decomposition device 27, the multipurpose tank 28, the hydrogen peroxide injection device 29, the oxidation decontamination agent injection device 30, the pH adjuster supply device 31, the ultraviolet irradiation device 92, the oxidation reduction potentiometer 93, the arithmetic and control device 96, and the like.

[0190] The chemical decontamination apparatus 20 according to the third embodiment differs from the chemical decontamination apparatus 20 according to the second embodiment in that each valve for switching the flow path is controlled by an arithmetic and control device 96. In addition, devices provided in piping that bypasses the circulation piping 19 are connected to the arithmetic and control device 96 via a signal line 95.

[0191] In the second embodiment, it is assumed that the judgment based on the redox potential, the iron ion concentration, and the oxalic acid concentration and the operation of each valve are performed manually. In contrast, in the third embodiment, the judgment based on the redox potential, the iron ion concentration, and the oxalic acid concentration and the operation of each valve are performed automatically.

[0192] The signal line 95 transmits signals indicating the state of the devices and the measurement results to the arithmetic and control device 96, and transmits signals for controlling the devices from the arithmetic and control device 96. The signal line 95 connects the arithmetic and control device 96 to the valves 35, 38, 43, 44, 83, 53, 94, 39, 88, 90, and 91 for switching the flow paths, the supply pump 52, the ultraviolet radiation decomposition device 92, and the oxidation-reduction potentiometer 93.

[0193] The arithmetic and control device 96 executes control based on an analysis result input 97. The arithmetic and control device 96 is composed of an arithmetic device such as a CPU (Central Processing Unit), a main memory device such as a RAM (Random Access Memory) and a ROM (Read Only Memory), a secondary memory device such as a hard disk, an input device such as a keyboard and a mouse, and an output device such as a display. The analysis result input 97 is performed by an operator based on an analysis by sampling. Examples of data for which the analysis result input 97 is performed include the iron ion concentration and oxalic acid concentration of the reduction decontamination solution.

[0194] In the chemical decontamination apparatus 20 according to the third embodiment, the steps up to the reduction decontamination step (step S5) are performed in the same manner as in the second embodiment. At the start of the reduction decontamination agent decomposition step (step S6), an instruction to start the step is given to the arithmetic and control device 96. The reduction decontamination agent decomposition step (step S6) is performed according to the procedure shown in Figs. 10 and 11.

[0195] The arithmetic and control device 96 controls the flow of water to the ion exchange resin tower 24 (step SA601), the flow of water to the reduction decontamination agent decomposition device 27 (step SA602), the injection of hydrogen peroxide to the reduction decontamination agent decomposition device 27 (step SA603), and the flow of water to the ultraviolet ray irradiation device 92 (step SA604). The arithmetic and control device 96 performs control to open valves 35, 38, 39, 83, 91, and 53, close valve 40, and operate supply pump 52.

[0196] Next, the measurement result of the oxidation-reduction potential of the reducing decontamination solution by the oxidation-reduction potentiometer 93 is received, and it is determined whether the oxidation-reduction potential of the reducing decontamination solution is less than 400 mV vs. SHE (step SA605). 2+ ) and trivalent iron ions (Fe 3+ ) is involved in the measurement of the redox potential of the redox system.

[0197] If the oxidation-reduction potential of the reduction decontamination solution is less than 400 mV vs. SHE (step SA605; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, so water is continued to be passed through the ion exchange resin tower 24. On the other hand, if the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA605; NO), the divalent iron ion (Fe 2+ ) is small, the flow path is switched to bypass the ion exchange resin tower 24 (step SA606).

[0198] When switching to a flow path that bypasses the ion exchange resin tower 24, the arithmetic and control device 96 controls the valves 35, 44, and 91 to be open and the valves 40, 38, 90, 83, and 88 to be closed. The reducing decontamination liquid returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27 and the ultraviolet irradiator 92 without passing through the ion exchange resin tower 24 from the circulation pipe 19. This control prevents the deterioration of the ion exchange resin due to the hydrogen peroxide generated in the RPV 3 or the hydrogen peroxide injected into the decontamination liquid.

[0199] Next, the amount of hydrogen peroxide injected into the reduction decontamination agent decomposition device 27 is reduced (step SA607). The arithmetic and control device 96 controls the valve 53 of the pipe 66 connected to the hydrogen peroxide injection device 29 to be opened less, or controls the discharge rate of the supply pump 52 to be reduced.

[0200] Next, while the injection amount of hydrogen peroxide to the reduction decontamination agent decomposition device 27 is being reduced, the measurement result of the oxidation-reduction potential of the reduction decontamination agent by the oxidation-reduction potentiometer 93 is received, and it is determined whether the oxidation-reduction potential of the reduction decontamination agent is less than 400 mV vs. SHE (step SA608). 2+ ) and trivalent iron ions (Fe 3+ ) is involved in the measurement of the redox potential of the redox system.

[0201] If the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA608; NO), the divalent iron ion (Fe 2+ ), the process returns to step SA607, and the operation of decreasing the amount of hydrogen peroxide injected into the reduction decontamination agent decomposition device 27 is continued. On the other hand, if the oxidation-reduction potential of the reduction decontamination agent is less than 400 mV vs. SHE (step SA608; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, the flow path is switched to the flow path via the ion exchange resin tower 24 (step SA609). The arithmetic and control device 96 controls the valves 35, 38, 39, 83, and 91 to be open and the valves 40, 44, 43, and 88 to be closed.

[0202] When switching to the flow path via the ion exchange resin tower 24, the arithmetic and control device 96 controls the valves 35, 38, 39, 83, and 91 to be open and the valves 40, 44, 43, and 88 to be closed. In addition, the valve 90 is adjusted open to reduce the opening degree of the valve 90. The reduction decontamination liquid passes through the ion exchange resin tower 24 from the circulation pipe 19, then passes through the valves 39 and 83, and returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27 and the ultraviolet ray irradiation device 92. By such control, the ion exchange resin tower 24 that was bypassed is restored, and the collection of metal ions of radioactive nuclides and the like is resumed.

[0203] When the oxidation-reduction potential of the reduction decontamination liquid is less than 400 mV vs. SHE (step SA605; YES) or when the flow path is switched to one passing through the ion exchange resin tower 24 (step SA609), the iron ion concentration of the reduction decontamination liquid discharged from the RPV 3 is measured to determine whether the iron ion concentration exceeds 15 ppm (step SA610). As the iron ion concentration, the most recent measurement result measured within about one hour among the measurement results measured by periodic sampling can be used.

[0204] The iron ion concentration is manually analyzed by sampling at the outlet side of the RPV 3. After the manual analysis, an operator inputs 97 the analysis result into the arithmetic and control device 96. The arithmetic and control device 96 refers to a preset reference value for the iron ion concentration, and compares the analysis result of the iron ion concentration based on the analysis result input 97 with the preset reference value for the iron ion concentration to determine whether the iron ion concentration exceeds 15 ppm.

[0205] If the iron ion concentration exceeds 15 ppm (step SA610; YES), the hydrogen peroxide is sufficiently consumed by reaction with the iron ions, so the water continues to flow from the ion exchange resin tower 24 to the reduction decontamination agent decomposition device 27 without switching the order of water flow to the ion exchange resin tower 24 and the reduction decontamination agent decomposition device 27, and proceed to step SA611.

[0206] On the other hand, if the iron ion concentration is 15 ppm or less (step SA610; NO), hydrogen peroxide is not sufficiently consumed by reaction with iron ions, so the order of water flow to the ion exchange resin tower 24 and the reduction decontamination agent decomposition device 27 is switched (step SA613). The flow path of the reduction decontamination liquid is switched to a flow path that returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27, the ultraviolet irradiator 92, and the ion exchange resin tower 24 in this order.

[0207] When switching the water flow order from the ion exchange resin tower 24 to the reduction decontamination agent decomposition device 27, the arithmetic and control device 96 controls the valves 44, 88, 43, and 90 to be open and the valves 38, 83, 39, and 91 to be closed. The reduction decontamination liquid returns to the circulation pipe 19 via the reduction decontamination agent decomposition device 27, the ultraviolet ray irradiation device 92, and the ion exchange resin tower 24 in this order from the circulation pipe 19. This control makes it possible to continue the decomposition process and the ion exchange process while avoiding the deterioration of the ion exchange resin due to hydrogen peroxide. By carrying out the decomposition process first, both the hydrogen peroxide generated in the RPV 3 and the hydrogen peroxide injected into the decontamination liquid can be decomposed before coming into contact with the ion exchange resin.

[0208] Next, while the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27, the ultraviolet ray irradiation device 92, and the ion exchange resin tower 24 in this order, the oxalic acid concentration of the reducing decontamination liquid is measured to determine whether the oxalic acid concentration is less than 10 ppm (step SA614). The oxalic acid concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0209] The oxalic acid concentration is manually analyzed by sampling at the outlet side of the reduction decontamination agent decomposition device 27 or the like. After the manual analysis, an operator inputs 97 the analysis result into the arithmetic and control device 96. The arithmetic and control device 96 refers to a preset reference value for the oxalic acid concentration, and compares the analysis result of the oxalic acid concentration based on the analysis result input 97 with the preset reference value for the oxalic acid concentration to determine whether the oxalic acid concentration is less than 10 ppm.

[0210] If the oxalic acid concentration is 10 ppm or more (step SA614; NO), the reducing agent has not been sufficiently decomposed, so proceed to step SA615 and continue decomposing the reducing agent. On the other hand, if the oxalic acid concentration is less than 10 ppm (step SA614; YES), the reducing agent has been sufficiently decomposed to a trace amount, so terminate the decomposition of the reducing agent (step SA612).

[0211] Next, the arithmetic and control device 96 controls the flow of water to the reductive decontamination agent decomposition device 27 (step SA615), the injection of hydrogen peroxide to the reductive decontamination agent decomposition device 27 (step SA616), and the flow of water to the ultraviolet irradiator 92 (step SA617). The arithmetic and control device 96 controls the valves 44, 88, 43, and 90 to be open and the valves 38, 83, 39, and 91 to be closed, and controls the supply pump 52 to operate.

[0212] Next, while the reducing agent is passed through the reducing agent decomposition device 27, the ultraviolet ray irradiation device 92, and the ion exchange resin tower 24 in this order, the measurement result of the redox potential of the reducing agent by the redox potentiometer 93 is received, and it is judged whether the redox potential of the reducing agent is less than 400 mV vs. SHE (step SA618). 2+ ) and trivalent iron ions (Fe 3+ ) is involved in the measurement of the redox potential of the redox system.

[0213] If the oxidation-reduction potential of the reduction decontamination solution is less than 400 mV vs. SHE (step SA618; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, so water is continued to be passed through the ion exchange resin tower 24. On the other hand, if the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA618; NO), the divalent iron ion (Fe 2+ ) is small, the flow path is switched to bypass the ion-exchange resin tower 24 (step SA620).

[0214] When switching to a flow path that bypasses the ion exchange resin tower 24, the arithmetic and control device 96 controls the valves 35, 44, and 91 to be open and the valves 40, 38, 90, 83, and 88 to be closed. The reducing decontamination liquid returns to the circulation pipe 19 via the reducing decontamination agent decomposition device 27 and the ultraviolet irradiator 92 from the circulation pipe 19 without passing through the ion exchange resin tower 24. By such control, even after the iron ion concentration has decreased, deterioration of the ion exchange resin due to hydrogen peroxide generated in the RPV 3 or hydrogen peroxide injected into the decontamination liquid is avoided.

[0215] Next, while the reducing decontamination liquid bypasses the ion exchange resin tower 24, the injection amount of hydrogen peroxide to the reducing decontamination agent decomposition device 27 is reduced (step SA621). The arithmetic and control device 96 controls to reduce the opening of the valve 53 of the pipe 66 connected to the hydrogen peroxide injection device 29, or controls to reduce the discharge amount of the supply pump 52.

[0216] Next, while the injection amount of hydrogen peroxide to the reduction decontamination agent decomposition device 27 is being reduced, the measurement result of the oxidation-reduction potential of the reduction decontamination agent by the oxidation-reduction potentiometer 93 is received, and it is determined whether or not the oxidation-reduction potential of the reduction decontamination agent is less than 400 mV vs. SHE (step SA622). 2+ ) and trivalent iron ions (Fe 3+ ) is involved in the measurement of the redox potential of the redox system.

[0217] If the oxidation-reduction potential of the reduction decontamination solution is 400 mV vs. SHE or more (step SA622; NO), the divalent iron ion (Fe 2+ ), the process returns to step SA621, and the operation of decreasing the injection amount of hydrogen peroxide into the reduction decontamination agent decomposition device 27 is continued. On the other hand, if the oxidation-reduction potential of the reduction decontamination agent is less than 400 mV vs. SHE (step SA622; YES), the divalent iron ion (Fe 2+ ) is dominant and hydrogen peroxide is unlikely to remain, so the flow path is switched to one passing through the ion-exchange resin tower 24 (step SA619).

[0218] When switching to the flow path via the ion exchange resin tower 24, the arithmetic and control device 96 controls the valves 35, 44, 88, 43, 90 to be open and the valves 40, 38, 83, 39 to be closed. In addition, the valve 91 is adjusted open to reduce the opening degree of the valve 91. The reduction decontamination liquid passes through the reduction decontamination agent decomposition device 27 and the ultraviolet ray irradiation device 92 from the circulation pipe 19, and then passes through the valves 88 and 43 and returns to the circulation pipe 19 via the ion exchange resin tower 24. By such control, the ion exchange resin tower 24 that was bypassed is restored, and the collection of metal ions of radioactive nuclides and the like is resumed.

[0219] Next, while the reducing decontamination liquid is passed through the reducing decontamination agent decomposition device 27, the ultraviolet irradiator 92, and the ion exchange resin tower 24 in this order, the oxalic acid concentration of the reducing decontamination liquid is measured to determine whether the oxalic acid concentration is less than 10 ppm (step SA623). The oxalic acid concentration can be measured at the outlet side of the reducing decontamination agent decomposition device 27, etc.

[0220] The oxalic acid concentration is manually analyzed by sampling at the outlet side of the reduction decontamination agent decomposition device 27 or the like. After the manual analysis, an operator inputs 97 the analysis result into the arithmetic and control device 96. The arithmetic and control device 96 refers to a preset reference value for the oxalic acid concentration, and compares the analysis result of the oxalic acid concentration based on the analysis result input 97 with the preset reference value for the oxalic acid concentration to determine whether the oxalic acid concentration is less than 10 ppm.

[0221] If the oxalic acid concentration is 10 ppm or more (step SA623; NO), the reducing agent has not been sufficiently decomposed, so the process returns to step SA615 and continues decomposing the reducing agent. On the other hand, if the oxalic acid concentration is less than 10 ppm (step SA623; YES), the reducing agent has been sufficiently decomposed to a trace amount, so the decomposition of the reducing agent is terminated (step SA624).

[0222] According to the above chemical decontamination method and chemical decontamination apparatus, the order of decomposition and ion exchange, and the order of water flow through the reduction decontamination agent decomposition apparatus and the ion exchange resin tower can be switched according to the concentration of iron ions reacting with hydrogen peroxide in the reduction decontamination agent decomposition process, so that the decomposition of the reduction decontamination agent and hydrogen peroxide can be continued while suppressing deterioration of the ion exchange resin. Since the deterioration of the ion exchange resin can be suppressed by the appropriate decomposition of hydrogen peroxide, additional work such as removal of total organic carbon (TOC) and additional use of consumables such as ion exchange resin can be reduced. In addition, since the decomposition of hydrogen peroxide can be performed in a short time, the time for chemical decontamination can be shortened. In addition, since the order of decomposition and ion exchange can be changed, hydrogen peroxide generated in the RPV3 and hydrogen peroxide injected into the reduction decontamination solution can be decomposed early at an appropriate stage. Therefore, the installation of a new hydrogen peroxide decomposition apparatus can be omitted, the equipment cost and operating cost of the decontamination system can be reduced, and the extension of the time required for chemical decontamination can be avoided. Therefore, it is possible to provide a chemical decontamination method and chemical decontamination apparatus that can suppress deterioration of the ion exchange resin and perform decontamination at low cost in a short time.

[0223] In addition, because judgments based on iron ion concentration and redox potential and operation of each valve are performed automatically, there is no need to deploy workers to operate the valves, reducing human error and the time required for valve operation.

[0224] In addition, since the analysis results are input 97 to the arithmetic and control device 96, the measurement results of the redox potential, iron ion concentration, and oxalic acid concentration, which are periodically input, can be stored as time-series data. Based on the stored time-series data, it is possible to perform regression analysis of the time-varying redox potential, iron ion concentration, and oxalic acid concentration. By obtaining a regression equation showing the time-varying concentration, the future time until the target value is reached can be estimated.

[0225] Based on such an estimation result, pumps and valves can be controlled, so that the time required for chemical decontamination can be shortened. For example, if the iron ion concentration changes by an average of 10 ppm / h while it decreases from 50 ppm to 20 ppm, it can be estimated that the iron ion concentration will decrease to 15 ppm 30 minutes after that point. In such a case, the control by the arithmetic and control device 96 can be scheduled so that the flow path will be switched to one that bypasses the ion exchange resin tower 24 30 minutes later.

[0226] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the present invention. For example, the present invention is not necessarily limited to having all the configurations of the above embodiment. It is possible to replace a part of the configuration of an embodiment with another configuration, add a part of the configuration of an embodiment to another form, or omit a part of the configuration of an embodiment. [Explanation of symbols]

[0227] 1...Boiling water nuclear power plant, 2...Reactor, 3...Reactor pressure vessel, 4...Reactor core, 5...Jet pump, 6...Recirculation system piping, 7...Recirculation pump, 8...Main steam piping, 9...Turbine, 10...Condenser, 11...Feedwater piping, 12...Condensate pump, 13...Condensate purification system, 14...Low pressure feedwater heater, 15...Feedwater pump, 16...High pressure feedwater heater, 17...Bleed piping, 19...Circulation piping, 20...Chemical decontamination equipment, 21...Circulation pump, 22...Cooler , 23...mixed bed resin tower, 24...ion exchange resin tower, 26...heater, 27...reductive decontamination agent decomposition catalyst device, 28...multipurpose tank, 29...hydrogen peroxide injection device, 30...oxidative decontamination agent injection device, 31...pH adjuster supply device, 73...drain water recovery piping, 78...CRD housing, 79...ICM housing, 80...RPV bottom drain line, 81...steam outlet nozzle, 82...water supply system nozzle, 92...ultraviolet irradiation device, 93...oxidation-reduction potentiometer

Claims

1. A method for chemically decontaminating components of a nuclear power plant using a decontamination agent, comprising: connecting a chemical decontamination device to the object to be decontaminated; A step of chemically decontaminating the object by supplying a decontamination liquid containing a decontamination agent from the chemical decontamination apparatus to the object to be decontaminated; supplying hydrogen peroxide to the decontamination liquid to decompose the decontamination agent; The step of decomposing the decontamination agent includes a decomposition process in which the decontamination liquid is passed through a decomposition device to decompose the decontamination agent, and an ion exchange process in which the decontamination liquid is passed through an ion exchange resin tower to capture ions, A chemical decontamination method in which, in a step of decomposing the decontamination agent, when an iron ion concentration of the decontamination liquid is equal to or higher than a preset reference value, the ion exchange treatment is performed before the decomposition treatment, and when an iron ion concentration of the decontamination liquid is less than a preset reference value, the decomposition treatment is performed before the ion exchange treatment.

2. 2. The chemical decontamination method according to claim 1, A chemical decontamination method in which the reference value is a limit concentration of iron ions at which hydrogen peroxide produced by radiolysis of water no longer remains in the decontamination solution due to a reaction between the iron ions and the hydrogen peroxide.

3. 2. The chemical decontamination method according to claim 1, The chemical decontamination method, wherein the reference value is 15 ppm or more and 20 ppm or less in terms of iron ion concentration.

4. 2. The chemical decontamination method according to claim 1, The decomposition treatment is a chemical decontamination method in which the decontamination agent is decomposed by a catalytic reaction.

5. 2. The chemical decontamination method according to claim 1, The decomposition treatment is a chemical decontamination method in which a catalytic reaction treatment is performed to decompose the decontamination agent by a catalytic reaction, and an ultraviolet irradiation treatment is performed to decompose the decontamination agent after the catalytic reaction treatment by irradiation with ultraviolet rays.

6. The chemical decontamination method according to claim 5, A chemical decontamination method comprising: measuring an oxidation-reduction potential of the decontamination solution after the ultraviolet irradiation treatment; and switching whether or not to perform the ion exchange treatment depending on the measured oxidation-reduction potential.

7. 2. The chemical decontamination method according to claim 1, A chemical decontamination method, wherein the decomposition device and the ion exchange resin tower used when the iron ion concentration of the decontamination liquid is equal to or higher than a reference value, and the decomposition device and the ion exchange resin tower used when the iron ion concentration of the decontamination liquid is less than the reference value are identical devices.

8. In a chemical decontamination device for chemically decontaminating components of a nuclear power plant using a decontamination agent, A circulation pipe that connects a chemical decontamination apparatus to the object to be decontaminated and circulates a decontamination liquid containing a decontamination agent; A decomposition device that performs a decomposition process for decomposing the decontamination agent and an ion exchange resin tower that performs an ion exchange process for capturing ions are installed, and a bypass system that bypasses a part of the circulation piping is provided; a flow path of the decontamination liquid that is switched from the circulation piping to a flow path that passes through the ion exchange resin tower and the decomposition device in this order and returns to the circulation piping when the iron ion concentration of the decontamination liquid is equal to or higher than a predetermined reference value, and that is switched from the circulation piping to a flow path that passes through the decomposition device and the ion exchange resin tower in this order and returns to the circulation piping when the iron ion concentration of the decontamination liquid is less than a predetermined reference value.

9. 9. The chemical decontamination apparatus according to claim 8, A chemical decontamination apparatus, wherein the reference value is a limit concentration of iron ions at which hydrogen peroxide produced by radiolysis of water no longer remains in the decontamination solution due to a reaction between the iron ions and the hydrogen peroxide.

10. 9. The chemical decontamination apparatus according to claim 8, The reference value is an iron ion concentration of 15 ppm or more and 20 ppm or less in a chemical decontamination apparatus.

11. 9. The chemical decontamination apparatus according to claim 8, The decomposition device is a chemical decontamination device that is a catalyst tower filled with a catalyst for a reaction that decomposes the decontamination agent.

12. 9. The chemical decontamination apparatus according to claim 8, The decomposition device is a chemical decontamination device that is a combination of a catalytic tower filled with a catalyst for a reaction that decomposes the decontamination agent, and an ultraviolet irradiation device that is installed downstream of the catalytic tower and decomposes the decontamination agent by irradiating it with ultraviolet rays.

13. 13. The chemical decontamination apparatus of claim 12, An oxidation-reduction potentiometer is installed downstream of the ultraviolet irradiation device, A chemical decontamination apparatus in which the presence or absence of water flow through the ion exchange resin tower is switched depending on the measurement result by the oxidation-reduction potentiometer.

14. 9. The chemical decontamination apparatus according to claim 8, The bypass system includes: a first bypass line in which the decomposition apparatus is installed and which bypasses a part of the circulation piping; a second bypass line in which the ion exchange resin tower is installed and which is arranged in parallel with the first bypass line and bypasses a part of the circulation piping; a first relay line connecting a downstream side of the first bypass line from the decomposition device and an upstream side of the second bypass line from the ion exchange resin tower; a second relay line connecting the second bypass line downstream of the ion exchange resin tower and the first bypass line upstream of the decomposition device; an upstream first valve that opens and closes a flow path from the circulation pipe to the first bypass line; a second upstream valve that opens and closes a flow path from the circulation pipe to the second bypass line; a downstream first valve that opens and closes a flow path from the first bypass line to the circulation pipe; a downstream second valve that opens and closes a flow path from the second bypass line to the circulation pipe; a first flow passage valve that opens and closes the first relay line; a second flow passage valve that opens and closes the second relay line, A chemical decontamination apparatus in which a flow path of the decontamination liquid is switched by the first upstream valve, the second upstream valve, the first downstream valve, the second downstream valve, a first flow path valve, and the second flow path valve.

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