Chemical decontamination method and chemical decontamination equipment

Abstract

When chemically decontaminating carbon steel members in a nuclear power plant, it is possible to both inhibit corrosion of the carbon steel members and maintain the decontamination performance of a decontamination solution. [Solution] A method for chemically decontaminating carbon steel components in a nuclear power plant using a decontamination liquid and a cation exchange resin, comprising a dissolution process in which metal oxides adhering to the carbon steel components are dissolved with the decontamination liquid, the decontamination liquid circulating between the carbon steel components and the cation exchange resin, and the dissolution process adjusting the amount of decontamination liquid that has passed through the carbon steel components and passed through the cation exchange resin so that the pH of the decontamination liquid at the point where the decontamination liquid is supplied to the carbon steel components is within the standard pH range of 2.3 to 3.3.

Description

[Technical Field]

[0001] The present disclosure relates to chemical decontamination methods and apparatus. [Background technology]

[0002] In boiling water nuclear power plants (hereinafter referred to as "BWR plants"), metallic impurities contained in the reactor water adhere to the surface of the fuel rods. These metallic impurities then undergo nuclear reactions on the surface of the fuel rods due to neutrons emitted from the fuel rods, turning into radioactive nuclides such as cobalt-54, cobalt-58, chromium-51, and manganese-54. These radioactive nuclides are re-released into the reactor water in the form of ions or in the form of an insoluble solid called cladding, and accumulate in oxide coatings formed on the inner walls of structural materials such as piping. As a result, radiation is emitted from the structural materials.

[0003] In particular, in BWR plants, the radiation dose from carbon steel components such as reactor water purification systems is high, causing workers to be exposed to radiation during periodic inspections (hereinafter referred to as "regular inspections").

[0004] Therefore, chemical decontamination is carried out to reduce the radiation exposure of workers during periodic inspection work. Chemical decontamination is the removal of oxide coatings containing radioactive nuclides from the inner wall surfaces of structural materials through chemical reactions using chemical agents.

[0005] For example, Patent Document 1 discloses a decontamination solution containing formic acid, ascorbic acid and / or erythorbic acid (hereinafter referred to as ascorbic acid, etc.), and a corrosion inhibitor, which provides a high decontamination effect without forming a film such as iron oxalate on the surface of carbon steel. Patent Document 1 also discloses a chemical decontamination method including a dissolution step in which the decontamination solution is used to dissolve radioactive insoluble matter containing metal oxides attached to an object to be decontaminated, including carbon steel, and a metal ion removal step in which the metal ion-containing decontamination solution produced by the dissolution step is contacted with a cation exchange resin to remove the metal ions. Patent Document 1 also discloses that the dissolution step includes a reduction dissolution step using the decontamination solution, and that in the reduction dissolution step, metal oxides are dissolved using a decontamination solution containing 1,000 to 10,000 mg / L of formic acid, 400 to 4,000 mg / L of ascorbic acid or the like, and 100 to 500 mg / L of a corrosion inhibitor. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-151210 Summary of the Invention [Problem to be solved by the invention]

[0007] In chemical decontamination, in order to remove radionuclides accumulated in the structural materials of a plant, the iron oxide film containing radionuclides is dissolved with a decontamination solution. Acid is usually used to dissolve iron oxide. The dissolution reaction is expressed by the following equation (1):

[0008] Fe3O4+8H + →Fe 2+ +2Fe 3+ +4H2O …(1) The dissolution reaction of the above formula (1) proceeds more easily as the pH (hydrogen ion exponent) decreases. However, acids can corrode not only iron oxides but also carbon steel components themselves. The corrosion reaction of carbon steel caused by decontamination solutions is expressed by the following formula (2):

[0009] Fe+2H + →Fe 2+ +H2…(2) The corrosion reaction of the above formula (2) also progresses more easily as the pH decreases. Corrosion should be suppressed to maintain the strength and other properties of structural materials. To suppress corrosion, it is desirable to use a low-concentration decontamination solution and perform decontamination under high pH conditions.

[0010] As can be seen from the above formula (1), the dissolution of iron oxides also consumes hydrogen ions, increasing the pH and slowing the dissolution rate. In chemical decontamination, radionuclides contained in iron oxides are removed by dissolving them together with the iron oxides, so a decrease in dissolution rate means a decrease in decontamination performance. Therefore, using a low-concentration decontamination solution to suppress corrosion leads to a decrease in decontamination performance.

[0011] If decontamination performance declines, radionuclides cannot be sufficiently removed, and worker exposure cannot be sufficiently reduced. It may also result in a longer decontamination period. From the perspective of improving equipment utilization, it is desirable to shorten the periodic inspections that are carried out by shutting down an operating plant. Therefore, it is also desirable to shorten the period for decontamination of in-service carbon steel that is carried out during the periodic inspections. Here, in-service carbon steel refers to carbon steel components such as piping that are currently in use at nuclear power plants.

[0012] In order to maintain the decontamination performance of the decontamination liquid in the chemical decontamination method described in Patent Document 1, it is conceivable to use a highly concentrated decontamination liquid to suppress the increase in pH that accompanies the dissolution of oxides. However, in this case, there is a concern that the reaction of the above formula (2) may increase corrosion of carbon steel members.

[0013] In fact, in the study by the inventors described below, it was confirmed that corrosion increases when a high-concentration decontamination solution is used. On the other hand, when corrosion inhibition is desired, it is possible to use a low-concentration decontamination solution, but in this case, there is a concern that the dissolution of iron oxides attached to the decontamination target will not progress, resulting in a decrease in decontamination performance. Therefore, it is difficult to simultaneously inhibit corrosion of carbon steel members and maintain decontamination performance by simply changing the concentration of the decontamination solution used.

[0014] In view of the above circumstances, an object of the present disclosure is to achieve both corrosion suppression of carbon steel members and maintenance of the decontamination performance of a decontamination solution when chemically decontaminating carbon steel members in a nuclear power plant. [Means for solving the problem]

[0015] The chemical decontamination method disclosed herein is a method for chemically decontaminating carbon steel components of a nuclear power plant using a decontamination liquid and a cation exchange resin, and includes a dissolution step in which metal oxides adhering to the carbon steel components are dissolved with the decontamination liquid. The decontamination liquid circulates between the carbon steel components and the cation exchange resin, and the dissolution step adjusts the amount of the decontamination liquid that has passed through the carbon steel components and passed through the cation exchange resin so that the pH of the decontamination liquid at the point where the decontamination liquid is supplied to the carbon steel components is within the standard pH range of 2.3 to 3.3. [Effects of the Invention]

[0016] According to the present disclosure, when chemically decontaminating carbon steel members in a nuclear power plant, it is possible to both inhibit corrosion of the carbon steel members and maintain the decontamination performance of the decontamination liquid.

[0017] Problems, configurations, and effects other than those described above will be described in the following description of the embodiments. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a graph showing the relationship between the concentration of the decontamination agent and the amount of corrosion of carbon steel. [Figure 2] 1 is a graph showing the decontamination agent concentration dependency of the change in pH and iron concentration over time in a dissolution test. [Figure 3] 1 is a graph showing the results of thermodynamic calculation of the pH dependence of the solubility of iron oxide Fe3O4. [Figure 4] 1 is a graph showing the dissolution of iron oxide into a decontamination solution in a dissolution step, and the effect of passing the solution through a cation exchange resin on the pH and iron concentration of a metal ion-containing decontamination solution. [Figure 5] 1 is a configuration diagram showing an example of a chemical decontamination apparatus according to a first embodiment. [Figure 6] FIG. 1 is a flow chart showing a chemical decontamination method according to a first embodiment. [Figure 7] FIG. 3 is a flow chart showing a dissolving step in the chemical decontamination method according to the first embodiment. [Figure 8] FIG. 3 is a flow chart showing an iron concentration adjustment step in the chemical decontamination method according to the first embodiment. [Figure 9] FIG. 3 is a flow chart showing a decontamination agent decomposition step in the chemical decontamination method according to the first embodiment. [Figure 10] FIG. 2 is a flow chart showing a purification step in the chemical decontamination method according to the first embodiment. [Figure 11] FIG. 10 is a flow chart showing a dissolving step in the chemical decontamination method according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present disclosure relates to a method for chemically decontaminating carbon steel members inside reactors, piping, and the like of nuclear power plants.

[0020] In the following description, carbon steel is used as an example, but the carbon steel according to the present disclosure is not limited to this and includes a type of steel that is an alloy of iron and carbon, and contains elements other than carbon in amounts below which it is not classified as an alloy steel. The carbon content of such carbon steel is in the range of 0.02 to 2.14 mass%, as is the usual definition.

[0021] <Study related to the embodiment> In order to solve the above problems, an experimental study was conducted.

[0022] First, to investigate the relationship between the concentration of the decontamination agent contained in the decontamination solution and the amount of corrosion of carbon steel, a test was conducted to examine the effect of changing the concentration of the decontamination agent on the amount of corrosion of carbon steel.

[0023] The test used a decontamination solution containing formic acid, ascorbic acid, and an anticorrosive. The standard decontamination solution concentrations were 1750 ppm formic acid, 750 ppm ascorbic acid, and 100 ppm anticorrosive, with the formic acid concentration varied from 1750 ppm to 3000 ppm. The concentrations of the other components were varied to maintain the same concentration ratio with the formic acid. The anticorrosion solution used was Ivit 30AR manufactured by Asahi Chemical Industry Co., Ltd. These decontamination solutions were passed through a container containing a carbon steel (SS400) test piece at a liquid temperature of 90°C under nitrogen purging conditions, and the amount of corrosion was measured.

[0024] Figure 1 is a graph showing the relationship between the concentration of the decontamination agent and the amount of corrosion of carbon steel. The horizontal axis represents the formic acid concentration, and the vertical axis represents the amount of corrosion.

[0025] As shown in this figure, the amount of corrosion increases when the formic acid concentration exceeds 2000 ppm. This is presumably because the corrosion reaction of the decontamination agent, expressed by the above formula (2), is accelerated as the decontamination agent concentration increases and the pH of the decontamination solution decreases. Therefore, it is clear that using a decontamination solution with a low formic acid concentration of 2000 ppm or less is effective in suppressing the amount of corrosion in the decontamination process.

[0026] Next, to investigate the relationship between the pH of the decontamination solution and the decontamination performance, a dissolution test was conducted to examine the changes in pH and iron concentration during the dissolution of iron oxides.

[0027] In the dissolution test, a decontamination solution containing formic acid, ascorbic acid, and a corrosion inhibitor was contacted with a test specimen coated with an iron oxide film. The iron oxide film was formed by immersing the test specimen in high-temperature water under reactor water conditions (280°C, 100 ppb dissolved oxygen concentration, and 200 ppb hydrogen peroxide concentration) for 500 hours. These reactor water conditions simulated the conditions experienced by the inner wall surfaces of actual BWR plant piping. When the test specimen came into contact with the decontamination solution, the iron oxide film dissolved, increasing the iron concentration in the decontamination solution. The pH and iron concentration of the decontamination solution were recorded over time during the test.

[0028] Figure 2 is a graph showing the decontamination agent concentration dependence of the changes in pH and iron concentration over time in the dissolution test. The horizontal axis represents the dissolution time. Here, the dissolution time refers to the elapsed time from when the decontamination solution was brought into contact with the test piece in the dissolution test.

[0029] The decontamination agent concentrations were low (formic acid 1750 ppm, ascorbic acid 750 ppm, anticorrosive agent 100 ppm), medium (formic acid 3500 ppm, ascorbic acid 1500 ppm, anticorrosive agent 200 ppm), and high (formic acid 7000 ppm, ascorbic acid 3000 ppm, anticorrosive agent 400 ppm), and the results shown were those passed through the test specimen.

[0030] As shown in this figure, when the decontamination agent concentration is low, the pH rises significantly and then stagnates after rising to around pH 3.3. Furthermore, the iron concentration decreases as the decontamination agent concentration decreases. Since the iron oxide dissolution reaction is expressed by the above formula (1), an increase in pH indicates the progress of iron oxide dissolution, and a stagnation in the pH increase indicates the stagnation of iron oxide dissolution. Therefore, under low-concentration conditions (formic acid 1750 ppm, ascorbic acid 750 ppm, corrosion inhibitor 100 ppm), it can be seen that iron oxide dissolution progresses and then stagnates once the pH rises to 3.3.

[0031] FIG. 3 is a graph showing the pH dependence of the solubility of iron oxide Fe3O4 determined by thermodynamic calculation.

[0032] This figure shows that the higher the pH, the lower the iron solubility (iron concentration in the solution).

[0033] Therefore, when decontamination is performed using a low-concentration decontamination solution, the pH rises as decontamination progresses, and the dissolution of iron oxides stagnates. In chemical decontamination, the radionuclides contained in the iron oxides are dissolved and removed along with the iron oxides, so the stagnation of iron oxide dissolution means a decline in decontamination performance.

[0034] Therefore, next, we investigated a method to prevent the stagnation of dissolution of iron oxides in low-concentration decontamination solutions.

[0035] The dissolution reaction of iron oxides is expressed by the above formula (1), so stagnation of dissolution can be prevented by lowering the pH of the decontamination solution, which has risen due to the dissolution of iron oxides. However, if the pH drops significantly, the corrosion reaction of the above formula (2) will be accelerated. For this reason, we investigated a method to maintain the pH of the decontamination solution within an appropriate range.

[0036] As a means of lowering the pH of the decontamination liquid, we considered contacting the decontamination liquid (hereinafter referred to as "metal ion-containing decontamination liquid") produced by dissolving iron oxides (metal oxides) in the dissolution process described below with an ion exchange resin.

[0037] The metal ion-containing decontamination solution contains iron ions. When the metal ion-containing decontamination solution is brought into contact with a proton-type cation exchange resin (hereinafter referred to as "cation exchange resin"), the pH decreases due to an exchange reaction between iron ions and protons, as shown in the following formula (3).

[0038] 2(Resin-H)+Fe 2+ → (2Resin-Fe) + 2H + …(3) In the formula, Resin represents the functional group of the ion exchange resin.

[0039] It is believed that this reaction can lower the pH of the metal ion-containing decontamination solution, and that the pH can be maintained within an appropriate range by adjusting the amount of metal ion-containing decontamination solution that is brought into contact with the cation exchange resin.

[0040] Therefore, to verify that the pH of metal ion-containing decontamination solutions can be lowered using cation exchange resins, a cation exchange resin flow test was conducted. In this test, iron oxide was dissolved in the decontamination solution, and the solution was passed through a cation exchange resin tower to examine the changes in pH and iron concentration.

[0041] 4 is a graph showing the effect of passing the metal ion-containing decontamination solution through a cation exchange resin on the dissolution of iron oxides in the decontamination solution during the dissolution process, as well as on the pH and iron concentration of the metal ion-containing decontamination solution. The horizontal axis represents the elapsed time.

[0042] In the cation exchange resin liquid flow test shown in this figure, the dissolution process was started by circulating the decontamination liquid through a pipe (made of carbon steel) on which an iron oxide film had been formed, and 30 minutes after the start, the process was switched to passing the liquid through a cation exchange resin tower by opening and closing a valve.

[0043] This figure shows that the iron concentration and pH increase due to the dissolution of iron oxides up to 30 minutes after the start of the dissolution process, but the iron ions are removed by passing the solution through the cation exchange resin tower, causing the pH to decrease.

[0044] Based on the above findings, when chemically decontaminating carbon steel components at nuclear power plants, it was determined that an effective method for achieving both corrosion inhibition of carbon steel components and maintaining the decontamination performance of the decontamination solution would be to use a low-concentration decontamination solution that is less corrosive to carbon steel components, and to monitor the pH of the decontamination solution during the dissolution process and adjust the amount of decontamination solution passed through the cation exchange resin tower to maintain an appropriate pH range.

[0045] In the above discussion, iron ions are used as a representative example, but the present disclosure is not limited to this, and other heavy metal ions and the like are also targets for removal by cation exchange resins.

[0046] An embodiment of chemical decontamination that reflects the above considerations will be described below as a first embodiment.

[0047] First Embodiment FIG. 5 is a configuration diagram showing an example of a chemical decontamination apparatus according to the first embodiment.

[0048] In this figure, the chemical decontamination equipment includes a surge tank T1, an ultraviolet irradiation tank T2 (UV irradiation tank) having an ultraviolet irradiation section (UV irradiation section), a heat exchanger T3, a cation exchange resin tower T4, a mixed bed resin tower T5 (a mixture of cation exchange resin and anion exchange resin), and tanks T6 and T7 for added chemicals.

[0049] The portion to be decontaminated A1 is, for example, a pipe made of carbon steel. A dosimeter D1 is installed on the outer surface of the portion to be decontaminated A1.

[0050] The surge tank T1 is connected to the inlet of the area A1 to be decontaminated via piping. A bypass piping having an ultraviolet irradiation tank T2 is connected downstream of the outlet of the area A1 to be decontaminated. A heat exchanger T3, a cation exchange resin tower T4, and a mixed bed resin tower T5 are connected in series in this order downstream of the ultraviolet irradiation tank T2. The heat exchanger T3, the cation exchange resin tower T4, and the mixed bed resin tower T5 are each installed in a bypass piping connected in parallel to the main pipe. This configuration is similar to that of the ultraviolet irradiation tank T2.

[0051] Between the surge tank T1 and the inlet of the section to be decontaminated A1, an additive chemical tank T6 is connected so that the chemical can be injected. Furthermore, between the outlet of the section to be decontaminated A1 and the ultraviolet irradiation tank T2, an additive chemical tank T7 is connected so that the chemical can be injected. Between the outlet of the section to be decontaminated A1 and the injection part of the additive chemical tank T7, a sampling part A2 for collecting the decontamination liquid is provided. A heater A3 for heating the liquid stored inside the surge tank T1 is installed. A decontamination agent (including formic acid, ascorbic acid, and anticorrosive agent) can be injected into the surge tank T1 from a hopper A4 using an ejector A5.

[0052] A pH meter M1, an oxidation-reduction potentiometer M2 (ORP meter), and a conductivity meter M3 are installed in series in the bypass piping connected between the surge tank T1 and the inlet of the area to be decontaminated A1. In addition, a valve V2 is installed upstream of the pH meter M1, the oxidation-reduction potentiometer M2, and the conductivity meter M3, and a valve V3 is installed downstream of them.

[0053] A circulation pump P1 and a valve V1 are installed between the surge tank T1 and the inlet of the portion to be decontaminated A1.

[0054] A chemical injection pump P2 is connected to the added chemical tank T6, and a chemical injection pump P3 is connected to the added chemical tank T7.

[0055] Valve V4 is installed in the main pipe arranged in parallel to ultraviolet irradiation tank T2, valve V5 is installed upstream of ultraviolet irradiation tank T2, and valve V6 is installed downstream.

[0056] Valve V7 is installed in the main pipe arranged in parallel to heat exchanger T3, valve V8 is installed upstream of heat exchanger T3, and valve V9 is installed downstream.

[0057] A valve V10 is installed in the main pipe arranged in parallel to the cation exchange resin tower T4, a valve V11 is installed upstream of the cation exchange resin tower T4, and a valve V12 is installed downstream.

[0058] Valve V13 is installed in the main pipe arranged in parallel to mixed-bed resin tower T5, valve V14 is installed upstream of mixed-bed resin tower T5, and valve V15 is installed downstream.

[0059] A valve 16 is installed in the pipe connecting the main pipe and the ejector A5.

[0060] A valve 17 is installed in the piping connecting chemical injection pump P2 to the main pipe.

[0061] A valve 18 is installed in the pipe connecting the chemical injection pump P3 and the bypass pipe of the ultraviolet irradiation tank T2.

[0062] A valve 19 is installed upstream of the sampling section A2.

[0063] In addition, in this figure, a chemical decontamination apparatus is described that includes piping and a valve installed in the piping, but the chemical decontamination apparatus according to the present disclosure is not limited to this, and may include a liquid flow path instead of the piping, and a dam plate that opens and closes the flow path instead of the valve.

[0064] FIG. 6 is a flow chart showing the chemical decontamination method according to the first embodiment.

[0065] In this figure, the chemical decontamination method is carried out in the order of a dissolving step S601, an iron concentration adjusting step S602, a decontamination agent decomposing step S603, and a purification step S604.

[0066] Each step of the chemical decontamination method will be described below with reference to the drawings.

[0067] FIG. 7 is a flow chart showing details of the dissolving step S601 in FIG.

[0068] As shown in Figure 7, first, the chemical decontamination equipment (see Figure 5) is connected to the area to be decontaminated A1 to form a circulation flow path (S1). Next, water is filled and circulation begins (S2). Water is poured into the system connecting the chemical decontamination equipment and the area to be decontaminated, filling the area to be decontaminated A1 with water. The circulation pump P1 is started, and valves V1, V4, V7, V10, and V13 are opened to begin circulating system water that fills the entire system connecting the chemical decontamination equipment and the area to be decontaminated A1. Valves V2 and V3 are also opened, and the water is passed through the bypass flow path equipped with a pH meter M1, an oxidation-reduction potentiometer M2, and a conductivity meter M3.

[0069] The portion where the decontamination liquid is supplied to the carbon steel component, which is the portion to be decontaminated A1, includes a flow path in the circulation flow path that is upstream of and near the portion, and also includes, for example, a pH meter M1, an oxidation-reduction potentiometer M2, and a conductivity meter M3 shown in Figure 5.

[0070] Next, heater A3 is started and the temperature is raised to a target of 85 to 95°C (S3). Next, a decontamination agent containing formic acid, ascorbic acid, and an anticorrosive is added from hopper A4 (S4). Based on the results of the aforementioned study that corrosion can be suppressed at low concentrations, the amounts of decontamination agent added are adjusted to achieve a formic acid concentration of 1500 to 2000 ppm, an ascorbic acid concentration of 600 to 850 ppm, and an anticorrosive concentration of 90 to 110 ppm in order to suppress corrosion.

[0071] During the dissolution process, the surface dose rate is monitored by a dosimeter D1 installed on the outer surface of the area to be decontaminated A1, and it is determined whether it is below a predetermined target dose (S5). If the dose rate is below the target dose in step S5, the dissolution process is terminated and the iron concentration adjustment process is carried out. The dissolution process is continued until the target dose is achieved.

[0072] In order to maintain the decontamination performance of the decontamination liquid during the dissolution process, the pH is monitored using a pH meter M1, and it is determined whether the pH of the decontamination liquid is outside a preset standard pH range (S6). If the pH is outside the standard pH range in step S6, the amount of liquid passed through the cation exchange resin tower T4 is adjusted (S7). In step S7, the amount of anticorrosive agent added is also adjusted based on the amount of liquid passed through the cation exchange resin tower T4. Then, the process returns to step S6.

[0073] On the other hand, if the pH is within the reference pH range in step S6, the process returns to step S5.

[0074] In this manner, the pH of the decontamination solution is adjusted to fall within the standard pH range.

[0075] In step S7, if the pH exceeds the upper limit of the standard pH range, the flow rate to the cation exchange resin tower is increased to decrease the pH through the reaction of formula (3) above, whereas if the pH is below the lower limit of the standard pH range, the flow rate to the cation exchange resin tower is decreased to increase the pH through the reaction of formula (2) above.

[0076] The reference pH range is preferably set to pH 2.3 to 3.3, because, as shown in Figure 2, when the concentration of the decontamination solution is low, dissolution of iron oxides progresses in the pH range of 2.3 to 3.3.

[0077] The flow rate to the cation exchange resin tower may be adjusted by any method that allows the pH to be within the above-mentioned reference pH range. For example, the following methods (A) and (B) are possible.

[0078] (A) Method by operator operating valve An operator monitors pH meter M1, and if the pH exceeds the upper limit of the standard pH range, they manually operate valves V10, V11, and V12 to increase the flow rate to cation exchange resin tower T4 until the pH falls within the standard range.If the pH falls below the lower limit of the standard pH range, they manually operate valves V10, V11, and V12 to reduce the flow rate to cation exchange resin tower T4 until the pH falls within the standard pH range.

[0079] (B) Method using an arithmetic and control device An arithmetic and control unit is added to the configuration of the chemical decontamination equipment in Figure 5, and the arithmetic and control unit is electrically connected to the pH meter M1 and the valves V10, V11, and V12. Based on the signal from the pH meter M1, the arithmetic and control device calculates the flow rate to the cation exchange resin tower T4, sends signals to the valves V10, V11, and V12, and controls the cation exchange resin tower T4, thereby automatically controlling the flow rate.

[0080] The cation exchange resin also removes the anticorrosive agent. Because a decrease in the concentration of the anticorrosive agent leads to increased corrosion during the dissolution process, in this embodiment, the amount of anticorrosive agent removed by the cation exchange resin is added during the dissolution process (S7). The anticorrosive agent is placed in the additive chemical tank T6, valve V17 is opened, and the chemical injection pump P2 is started to add the anticorrosive agent. Adding an excessive amount of anticorrosive agent will extend the subsequent decontamination agent decomposition process, so an amount equivalent to the amount removed by the cation exchange resin is added. Assuming that the entire amount of anticorrosive agent is removed after passing through the cation exchange resin, the addition rate a (L / min) of the anticorrosive agent is calculated using the following equation (4), where u (L / min) is the flow rate into the cation exchange resin tower and C (ppm) is the concentration of the anticorrosive agent in the decontamination solution before removal.

[0081] a = C × u …(4) The iron concentration adjustment step is a step of adjusting the iron concentration in the decontamination solution for the decontamination agent decomposition step.

[0082] FIG. 8 is a flow diagram showing details of the iron concentration adjusting step S602 in FIG.

[0083] In Figure 8, first, valve V19 is opened to sample the decontamination solution from sampling unit A2 (S9). Then, the iron concentration is measured and the measured value is determined to be within a preset standard range of iron concentration (S10). If the iron concentration is within the standard range in step S10, the process proceeds directly to the decontamination solution decomposition step (S14). The standard range of iron concentration is preferably 10 to 100 ppm.

[0084] If the iron concentration is outside the reference range in step S10, the process proceeds to step S11.

[0085] In step S11, it is determined whether the iron concentration is above the upper limit or below the lower limit of the standard range (S11). If the iron concentration is above the upper limit of the standard range, the solution is passed through a cation exchange resin tower to reduce the iron concentration to within the standard range (S12). When passing the solution through a cation exchange resin tower, an anticorrosive agent is added in the amount that would be removed based on the above formula (4), as in the dissolution step. On the other hand, if the iron concentration is below the lower limit of the standard range, an Fe compound is added from a hopper to increase the iron concentration (S13). As the Fe compound, oxides or hydroxides such as Fe3O4 and FeOOH are preferable.

[0086] After steps S12 and S13, the process returns to step S10.

[0087] The decontamination agent decomposition process is a process to decompose the decontamination agent contained in the decontamination solution used in the dissolution process and reduce its concentration. The decontamination agent is decomposed using the Fenton reaction shown in the following formula (5) and the UV photochemical reaction shown in the following formula (6). The hydroxyl radical (·OH) generated by the reactions shown in the following formulas (5) and (6) oxidizes and decomposes the decontamination agent.

[0088] Fe 2+ +H2O2→Fe 3+ +·OH+OH - …(5) Fe 3+ +H2O+hν→Fe 2+ +H + +·OH …(6) FIG. 9 is a flow chart showing details of the decontamination agent decomposition step S603 in FIG.

[0089] 9, first, valves V5 and V6 are opened to allow the decontamination liquid to flow through the system including ultraviolet irradiation tank T2 (S15). Next, the UV irradiation unit of ultraviolet irradiation tank T2 is activated to start UV irradiation, hydrogen peroxide is placed in added chemical tank T7, valve V18 is opened, and chemical injection pump P3 is activated to start adding hydrogen peroxide (S16).

[0090] In the decontamination agent decomposition process, hydrogen peroxide is added to decompose the decontamination agent. However, if excess hydrogen peroxide is added and is not consumed in the decontamination of the decontamination agent and leaks into the area A1 to be decontaminated, the carbon steel members may be corroded by the reaction represented by the following formula (7).

[0091] Fe+H2O2+2H + →Fe 2+ +2H2O …(7) Therefore, the amount of hydrogen peroxide added is adjusted to suppress corrosion. The leakage of hydrogen peroxide can be detected when the ORP exceeds the reference value. Therefore, the amount of hydrogen peroxide added is adjusted based on the ORP. To do this, first, the ORP of the decontamination solution is monitored using the oxidation-reduction potentiometer M2, and it is determined whether it is below the reference value (S17). Here, the reference value of ORP is preferably set to 400 mV.

[0092] If the ORP is below a preset reference value in step S17, the decontamination agent decomposition process continues. Thereafter, valve V19 is periodically opened to sample the decontamination agent from sampling unit A2 (S18). The decontamination agent concentration of the sampled decontamination agent is measured, and it is determined whether the measured value is below a preset target concentration (S20). If the measured value is below the target concentration, the addition of hydrogen peroxide, UV irradiation, and addition of anticorrosive agent are stopped, and the decontamination agent decomposition process is terminated (S21).

[0093] On the other hand, if the measured value of the decontamination agent concentration exceeds the target concentration, the process returns to step S17, and the decomposition of the decontamination agent is continued until the measured value of the decontamination agent concentration becomes equal to or less than the target concentration.

[0094] Here, the target concentrations are, for example, a formic acid concentration of 10 ppm or less and an ascorbic acid concentration of 10 ppm or less.

[0095] If the ORP exceeds the reference value in step S17, the amount of hydrogen peroxide added is adjusted so that the ORP is equal to or less than the reference value (S19). After the adjustment, the process returns to step S17.

[0096] When measuring the hydrogen peroxide concentration, the decontamination liquid may be sampled from the sampling unit A2. However, this increases the workload of the workers because they have to sample the decontamination liquid and measure the hydrogen peroxide concentration. In this regard, using ORP is simpler and requires less work.

[0097] The purification process is a process for removing impurities that could not be decomposed in the decontamination agent decomposition process, and purifying the decontamination solution to a quality that allows it to be discharged.

[0098] FIG. 10 is a flow diagram showing details of the purification step S604 in FIG.

[0099] 10, first, the decontamination liquid after decomposition of the decontamination agent (hereinafter referred to as "decomposed decontamination liquid") is cooled to a temperature that allows it to be passed through the mixed-bed resin tower T5 (S22). At this time, valves V8 and V9 are opened to pass the decontamination liquid through the heat exchanger T3, thereby cooling the decomposed decontamination liquid.

[0100] Next, the liquid temperature is measured to determine whether it is below a reference temperature (S23). If the liquid temperature is below the reference temperature, purification using mixed bed resin is initiated. The reference temperature is set to 60°C or below, which is the upper limit of the usable temperature of anion exchange resin. Purification using mixed bed resin is performed by opening valves V14 and V15 and passing the liquid through mixed bed resin tower T5 (S24). On the other hand, if the liquid temperature exceeds the reference temperature in step S23, cooling of the decontamination liquid after decomposition continues.

[0101] Next, UV irradiation is performed while the decomposed decontamination solution is passing through the mixed bed resin to ionize the non-ionic components of the anticorrosive agent, which are among the impurities contained in the decontamination solution after decomposition, so that they can be removed by the mixed bed resin. Irradiation is started after confirming that the purification has progressed and the pH has exceeded the reference pH. This is because, when the pH is low at the start of purification, the anticorrosive agent components are maintained, thereby suppressing corrosion of carbon steel components.

[0102] While the solution is passing through the mixed-bed resin tower, the pH is monitored with a pH meter M1 to determine whether the pH is below the reference pH (S25). If the pH is below the reference pH, the solution continues to pass through the mixed-bed resin tower T5. On the other hand, if the pH exceeds the reference pH, the solution is passed through the ultraviolet irradiation tank T2, the UV irradiation unit is activated, and UV irradiation begins (S26). The reference pH is, for example, pH 4.

[0103] The decontamination liquid undergoing purification is periodically sampled (S27). The sampled decontamination liquid is measured to determine whether it is below the wastewater standard, i.e., whether it can be discharged (S28). If it is below the wastewater standard, UV irradiation and passing the liquid through the mixed-bed resin tower are stopped, and the purification process is terminated (S29). The wastewater standard varies depending on the plant, but is set, for example, at a pH of 5.3 to 8.6, a conductivity of 0.2 mS / m or less, a formic acid concentration of 1 ppm or less, and an ascorbic acid concentration of 1 ppm or less. The liquid purified to below the wastewater standard is discharged from the system (S30), and the object to be decontaminated is disconnected from the chemical decontamination equipment, completing the chemical decontamination (S31).

[0104] According to this embodiment, by adjusting the flow rate to the cation exchange resin tower based on the pH during the dissolution process, it is possible to maintain decontamination performance even with a low concentration decontamination solution, thereby achieving both corrosion inhibition and decontamination performance. In addition, the method of this embodiment is effective in shortening the decontamination process.

[0105] In this embodiment, the pH of the decontamination solution is maintained within a standard range by passing the decontamination solution through a cation exchange resin during the dissolution process. However, the pH can also be maintained within the standard range by adding a pH adjuster. However, if a pH adjuster is added, the pH adjuster must be decomposed in a decontamination solution decomposition process following the dissolution process, which lengthens the process accordingly. In this embodiment, the dissolution process is shortened by maintaining decontamination performance, and the use of a cation exchange resin instead of a pH adjuster prevents the subsequent decontamination solution decomposition process from being extended, thereby shortening the decontamination process.

[0106] In this embodiment, the addition of an excessive amount of anticorrosive agent leads to an extension of the subsequent decontamination agent decomposition process, so the amount of anticorrosive agent added during the cation exchange resin flow is determined by calculating the amount equivalent to the amount removed by the cation exchange resin. However, this method requires changing the addition rate of the anticorrosive agent depending on changes in the flow rate to the cation exchange resin tower, making the operation complicated. To simplify the operation, it is possible to maintain both decontamination performance and corrosion suppression even if the addition rate of the anticorrosive agent is always constant. Therefore, an embodiment in such a case is described below as the second embodiment.

[0107] Second Embodiment The second embodiment can be implemented using the configuration of FIG. 5, similarly to the first embodiment, and the steps other than the dissolving step are the same as those of the first embodiment.

[0108] In the following, only the differences between the second embodiment and the first embodiment regarding the dissolving step will be described.

[0109] FIG. 11 is a flow diagram showing the dissolving step in the chemical decontamination method according to the second embodiment.

[0110] In order to maintain the decontamination performance of the decontamination liquid during the dissolution process, the pH is monitored using a pH meter M1, and it is determined whether the pH of the decontamination liquid is outside a preset standard pH range (S6). If the pH is outside the standard pH range in step S6, the amount of liquid passing through the cation exchange resin tower T4 is adjusted (S7a). In addition, in step S7a, the amount of anticorrosive agent added is also adjusted. At this time, the addition rate of the anticorrosive agent is always constant.

[0111] The rate of addition of the anticorrosive agent is determined by calculation on the assumption that the entire amount of the decontamination solution is passed through the cation exchange resin and that the entire amount of the anticorrosive agent is removed by the cation exchange resin so that the concentration of the anticorrosive agent does not fall below the initial concentration.

[0112] The addition rate a (L / min) of the anticorrosive agent is calculated using the following formula (8), where U (L / min) is the circulation flow rate of the entire system, and C (ppm) is the concentration of the anticorrosive agent in the decontamination solution before it is removed.

[0113] a = C × U …(8) Unlike the first embodiment, in which the addition rate of the anticorrosive agent is calculated using the above formula (4), this embodiment does not depend on the amount of solution passed through the cation exchange resin. Since the circulation flow rate U of the entire system is usually constant, in this embodiment, the anticorrosive agent is added at a constant rate. As a result, in this embodiment, the flow rate adjustment operation is less required and the work is simpler than in the method of the first embodiment.

[0114] This embodiment also provides the same effects as the first embodiment.

[0115] In this embodiment, the operation is simpler than in the first embodiment because there is no need to change the addition rate of the anticorrosive agent in response to adjustment of the amount of cation exchange resin, but in terms of shortening the decontamination process, the first embodiment is more effective. Therefore, when deciding whether to adopt the first or second embodiment, it is desirable to select the one that has the greater advantage when implementing the chemical decontamination method according to the present disclosure.

[0116] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the embodiments, and various modifications are possible within the scope of the present disclosure. For example, the present disclosure is not necessarily limited to those including all of the configurations of the above-described embodiments. Part of the configuration of an embodiment may be replaced with another configuration, part of the configuration of an embodiment may be added to another form, or part of the configuration of an embodiment may be omitted. [Explanation of symbols]

[0117] A1: part to be decontaminated, A2: sampling unit, A3: heater, A4: hopper, A5: ejector, D1: dosimeter, M1: pH meter, M2: oxidation-reduction potentiometer, M3: conductivity meter, P1: circulation pump, P2, P3: chemical injection pump, T1: surge tank, T2: ultraviolet irradiation tank, T3: heat exchanger, T4: cation exchange resin tower, T5: cation exchange resin tower, V1, V2, V3, V4, V5, V6, V7, V8, V9, V10, V11, V12, V13, V14, V15, V16, V17, V18, V19: valves.

Claims

1. A method for chemical decontamination of carbon steel components in a nuclear power plant using a decontamination solution and a cation exchange resin, comprising: a dissolving step of dissolving metal oxides adhered to the carbon steel member with the decontamination solution, the decontamination liquid circulates between the carbon steel member and the cation exchange resin; The dissolving step is a chemical decontamination method in which the amount of the decontamination liquid that has passed through the carbon steel member and passed through the cation exchange resin is adjusted so that the pH of the decontamination liquid at a portion where the decontamination liquid is supplied to the carbon steel member is within a standard pH range of 2.3 to 3.

3.

2. 2. The chemical decontamination method according to claim 1, wherein an anticorrosive agent is added to the decontamination solution downstream of the cation exchange resin while the solution is passed through the cation exchange resin, and the solution is returned to the carbon steel member.

3. The chemical decontamination method of claim 1 , wherein the decontamination solution comprises formic acid, ascorbic acid, and a corrosion inhibitor.

4. 4. The chemical decontamination method according to claim 3, further comprising, after the dissolving step, a decontamination agent decomposition step of adding hydrogen peroxide to the decontamination solution and irradiating the solution with ultraviolet light.

5. 5. The chemical decontamination method according to claim 4, wherein the decontamination agent decomposition step includes the steps of measuring an oxidation-reduction potential of the decontamination solution at the site where the decontamination solution is supplied to the carbon steel member, and changing the amount of hydrogen peroxide added based on the measured oxidation-reduction potential.

6. 5. The chemical decontamination method according to claim 4, further comprising a purification step of passing the decontamination agent through a mixed-bed resin tower and removing residual impurities by irradiating the decontamination agent with ultraviolet light after the decontamination agent decomposition step.

7. 7. The chemical decontamination method according to claim 6, wherein the purifying step measures a pH of the decontamination solution, and if the pH is higher than a reference pH, the ultraviolet light is irradiated.

8. An apparatus for chemical decontamination of carbon steel components in a nuclear power plant using a decontamination solution and a cation exchange resin, dissolving metal oxides adhering to the carbon steel member with the decontamination liquid; a cation exchange resin tower having the cation exchange resin; a configuration in which the decontamination liquid circulates between the carbon steel member and the cation exchange resin tower; a pH meter for measuring pH, located upstream of the carbon steel member and downstream of the cation exchange resin tower.

9. a bypass flow path for the decontamination liquid to bypass the cation exchange resin tower; The chemical decontamination apparatus according to claim 8 , further comprising: a configuration for changing a flow rate of the bypass flow path.

10. 9. The chemical decontamination apparatus according to claim 8, further comprising a configuration for adding an anticorrosive to the decontamination solution upstream of the carbon steel member and downstream of the cation exchange resin tower.

11. The chemical decontamination apparatus according to claim 10 , further comprising a configuration for adjusting a rate of addition of the anticorrosive agent based on an amount of liquid passing through the cation exchange resin tower.

12. 9. The chemical decontamination apparatus according to claim 8, further comprising a configuration for adding hydrogen peroxide to the decontamination solution downstream of the carbon steel member and upstream of the cation exchange resin tower, and irradiating the solution with ultraviolet light.

13. The chemical decontamination apparatus according to claim 12 , further comprising a configuration for measuring an oxidation-reduction potential of the decontamination liquid at a location where the decontamination liquid is supplied to the carbon steel member.

14. 9. The chemical decontamination apparatus according to claim 8, further comprising a configuration for adjusting the amount of the decontamination liquid that has passed through the carbon steel members and passed through the cation exchange resin tower so that the pH of the decontamination liquid at a portion where the decontamination liquid is supplied to the carbon steel members is within a reference pH range of 2.3 to 3.3.

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