Chemical decontamination method for carbon steel components of nuclear power plants and chemical decontamination equipment

Abstract

To provide a chemical decontamination method and equipment for carbon steel and stainless steel components of nuclear power plants capable of reducing process time compared to conventional methods with high decontamination efficiency on both carbon steel and stainless steel in chemical decontamination of systems containing carbon steel and stainless steel in chemical decontamination of systems containing carbon steel and stainless steel.SOLUTION: The chemical decontamination method for carbon steel components of nuclear power plants includes: a decontamination process of purification system piping 67, RHR piping 82 of decontaminating a purification system piping 67 and an RHR piping 82 of the decontamination target area by supplying a formic acid, an ascorbic acid, and a corrosion inhibitor; after the decontamination process of the purification system piping 67 and the RHR piping 82, a formic acid decomposition step in which hydrogen peroxide is supplied and formic acid is decomposed using the oxidation-reduction potential as an indicator; and after the formic acid decomposition step, a residual organic matter decomposition step that decomposes residual organic matter by supplying compounds containing permanganate ions.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a chemical decontamination method and chemical decontamination apparatus suitable for chemical decontamination of carbon steel members used in nuclear power plants, particularly boiling water nuclear power plants. [Background technology]

[0002] As an example of a chemical decontamination method that uses a small amount of cation exchange resin and can perform decontamination efficiently, Patent Document 1 describes a chemical decontamination method that includes a dissolution step in which radioactive insoluble matter containing metal oxides attached to an object to be decontaminated, including carbon steel, is dissolved with a decontamination liquid, and a metal ion removal step in which the metal ion-containing decontamination liquid produced by the dissolution step is brought into contact with a cation exchange resin to remove the metal ions, and that the dissolution step includes a reduction dissolution step using a decontamination liquid containing formic acid, ascorbic acid and / or erythorbic acid, and a corrosion inhibitor.

[0003] As an example of a technology that can suppress corrosion thinning of structural components in a nuclear power plant and efficiently remove radioactive nuclides, Patent Document 2 describes that first, oxidative decontamination is performed, and an aqueous potassium permanganate solution is supplied from a circulation line to the reactor pressure vessel, which is a stainless steel structural component, and the purification system piping and drain piping, which are carbon steel structural components, and these structural components are oxidatively decontaminated by the action of potassium permanganate, and then the structural components are reductively decontaminated using an aqueous oxalic acid solution, which contains hydrazine. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-151210 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-74887 Summary of the Invention [Problem to be solved by the invention]

[0005] For example, a boiling water reactor plant (hereinafter referred to as a BWR plant) has a reactor with a core housed in a reactor pressure vessel (hereinafter referred to as an RPV).

[0006] The reactor water supplied to the reactor core by the recirculation pump or internal pump is heated by the heat generated by the nuclear fission of the nuclear fuel material in the fuel assemblies loaded in the reactor core, and part of it becomes steam. This steam is led from the RPV to the turbine, where it rotates. The steam discharged from the turbine is condensed in the condenser and becomes water. This water is supplied to the reactor as feedwater.

[0007] In order to prevent the generation of radioactive corrosion products in the RPV, feedwater is filtered and demineralized in a demineralization unit installed downstream of the condenser to remove mainly metallic impurities. Reactor water is the cooling water present in the RPV.

[0008] In addition, since the corrosion products that are the source of radioactive corrosion products are generated on the surfaces of BWR plant components such as the RPV and recirculation system piping that come into contact with reactor water, stainless steels such as stainless steels and nickel-based alloys, which are less susceptible to corrosion, are used for the main primary system components.

[0009] In addition, the low-alloy steel RPV has a stainless steel cladding on the inside to prevent the low-alloy steel from coming into direct contact with the reactor water. Furthermore, a portion of the reactor water is purified by a filtration and demineralization device in the reactor cleanup system, actively removing the small amounts of metal impurities present in the reactor water.

[0010] However, even with the above-mentioned corrosion prevention measures, the presence of trace amounts of metal impurities in the reactor water is unavoidable, and some of these metal impurities adhere to the surfaces of the fuel rods contained in the fuel assemblies as metal oxides. The impurities (e.g., metal elements) that adhere to the surfaces of the fuel rods undergo nuclear reactions when irradiated with neutrons released by nuclear fission of the nuclear fuel material in the fuel rods, and become radioactive nuclides such as cobalt-60, cobalt-58, chromium-51, and manganese-54.

[0011] These radionuclides remain attached to the fuel rod surfaces in the form of oxides. However, some radionuclides are eluted as ions into the reactor water depending on the solubility of the incorporated oxides, or are re-released into the reactor water as insoluble solids called crud. Radioactive materials contained in the reactor water are removed by the reactor cleanup system connected to the RPV.

[0012] Radioactive materials that are not removed by the reactor cleanup system accumulate on the surfaces of components of the nuclear power plant (such as piping) that come into contact with the reactor water while circulating together with the reactor water through recirculation systems, etc. As a result, radiation is emitted from the surfaces of the components, causing radiation exposure to workers during periodic inspection work.

[0013] The radiation exposure dose of workers is managed so that it does not exceed a specified value for each individual. In recent years, this specified value has been lowered, making it necessary to keep each individual's radiation exposure dose as low as possible.

[0014] Therefore, when it is expected that the radiation exposure dose during routine inspection work will be high, chemical decontamination may be carried out to dissolve and remove radioactive nuclides attached to the piping. Examples of such techniques include those described in Patent Documents 1 and 2.

[0015] In the above-mentioned Patent Document 1, reduction decontamination of carbon steel members is performed using an organic corrosion inhibitor in addition to formic acid and ascorbic acid, and after adjusting the iron ion concentration by passing water through a cationic resin, hydrogen peroxide is added to decompose the formic acid, and then ascorbic acid is decomposed by ultraviolet irradiation and hydrogen peroxide, and corrosion inhibitors are added in both decomposition steps to suppress corrosion of the base material.

[0016] In Patent Document 2, after oxidative decontamination using an aqueous potassium permanganate solution, reduction decontamination is performed using an aqueous oxalic acid solution containing hydrazine. Therefore, when oxalic acid comes into contact with carbon steel, iron (II) oxalate dihydrate is formed, which may cover an oxide film containing radioactive materials.

[0017] The chemical decontamination method described in Patent Document 1 does not use oxalic acid, as used in Patent Document 2, and therefore does not encounter the problem of forming an iron (II) oxalate dihydrate film when dissolving an oxide film that has absorbed radioactive materials. Furthermore, when hydrogen peroxide is used to decompose formic acid or ascorbic acid, consideration is given to inhibiting corrosion of the base material by adding an organic rust inhibitor.

[0018] On the other hand, because it does not contain oxidizing chemical substances, it is thought that the decontamination effect on stainless steel, which requires oxidative decontamination, is low. For this reason, if the decontamination targets include stainless steel in addition to carbon steel, one possible method would be to decontaminate the carbon steel using the method of Patent Document 1, and then decontaminate the stainless steel using the decontamination method of Patent Document 2.

[0019] Specifically, after decontamination of carbon steel is completed, formic acid and ascorbic acid are decomposed before potassium permanganate is added. This is because if formic acid and ascorbic acid remain, the added permanganate ions will decompose and will not be able to exhibit an oxidizing effect. Therefore, it is necessary to decompose formic acid and ascorbic acid using the method described in Patent Document 1, which uses hydrogen peroxide and ultraviolet light. However, because ultraviolet light decomposition is a photochemical reaction, the efficiency of the photoreaction is generally lower than that of chemical reactions, and therefore the decomposition takes a long time.

[0020] Therefore, an object of the present invention is to provide a chemical decontamination method and chemical decontamination apparatus for carbon steel components of a nuclear power plant, which have high decontamination efficiency and can shorten the process time compared to conventional methods, in the chemical decontamination of systems containing carbon steel. [Means for solving the problem]

[0021] The present invention includes multiple means for solving the above-mentioned problems, and one example thereof is a chemical decontamination method for a part to be decontaminated at a nuclear power plant, which includes a carbon steel member decontamination step of supplying formic acid, ascorbic acid, and a corrosion inhibitor to decontaminate the carbon steel member of the part to be decontaminated, a formic acid decomposition step of decomposing formic acid using a decomposition liquid, decomposition agent, or decomposition method capable of decomposing the formic acid, after the carbon steel member decontamination step, and a residual organic matter decomposition step of supplying a compound containing permanganate ions to decompose residual organic matter, after the formic acid decomposition step. [Effects of the Invention]

[0022] According to the present invention, the decontamination efficiency is high and the process time can be shortened compared to the conventional method. Problems, configurations and effects other than those described above will become clear from the following description of the examples. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a graph showing the relationship between the change in concentration of ascorbic acid formate and ORP (Oxidation Reduction Potential) during decomposition of hydrogen peroxide. [Figure 2] 1 is a diagram showing the configuration of a primary cooling water system of a BWR plant, which is one of the preferred suitable subjects of the present invention. [Figure 3] FIG. 2 is a detailed configuration diagram of a chemical decontamination apparatus used in the chemical decontamination method for carbon steel members and stainless steel members of a nuclear power plant according to the first embodiment. [Figure 4] 1 is a flowchart showing the steps performed in the chemical decontamination method for carbon steel members and stainless steel members of a nuclear power plant according to the first embodiment. [Figure 5] FIG. 10 is a detailed configuration diagram of a chemical decontamination apparatus used in the chemical decontamination method for carbon steel members and stainless steel members of a nuclear power plant according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0024] The following describes an embodiment of the chemical decontamination method and chemical decontamination apparatus for carbon steel and stainless steel components of a nuclear power plant according to the present invention, with reference to the drawings. In the drawings used in this specification, identical or similar reference numerals are used to designate identical or corresponding components, and repeated explanations of these components may be omitted.

[0025] First, the process by which the present invention was completed will be described with reference to Figure 1. Figure 1 shows the time course of formic acid and total organic carbon concentrations and redox potential in water quality during the decomposition process of a carbon steel decontamination agent using ascorbic acid formate.

[0026] The present inventors have investigated a method for promoting the decomposition of formic acid and ascorbic acid by hydrogen peroxide in the chemical decontamination of carbon steel using an aqueous solution (reducing decontamination solution) containing formic acid, ascorbic acid, and an organic corrosion inhibitor.

[0027] The specific conditions were an aqueous solution containing 4% thiourea, 1-5% quaternary ammonium salt, and 1-5% organic sulfur compounds as corrosion inhibitors. The decontamination agent consisted of 1750 ppm formic acid, 750 ppm ascorbic acid, and 200 ppm corrosion inhibitor, with magnetite added to achieve an iron concentration of 100 ppm. The solution was dissolved at 90°C, and then carbon steel specimens were immersed in the solution. Furthermore, hydrogen peroxide equivalent to the total decomposition of formic acid and ascorbic acid was added over a 3-hour period, and the addition of hydrogen peroxide was terminated when the redox potential exceeded 500 mV. The changes over time in formic acid, total organic carbon concentration, and redox potential were examined.

[0028] Formic acid is decomposed by hydroxyl radicals formed in the Fenton reaction (1) between iron(II) ions and hydrogen peroxide according to reaction equation (2). Fe 2+ +H2O2 → Fe 3+ +OH - +OH * ……(1) HCOOH+2OH * → CO2+2H2O ……(2)

[0029] As shown in the following reaction formula (3), ascorbic acid reduces iron (III) ions to iron (II) ions, and then becomes dehydroascorbic acid. 2Fe 3+ +C6H8O6 → 2Fe 2+ +C6H6O6+2H + ...(3)

[0030] Ascorbic acid and dehydroascorbic acid undergo oxidation reactions with hydroxyl radicals to produce various intermediate products, but are ultimately decomposed into carbon dioxide and water, as shown in reaction equations (4) and (5) below. C6H8O6+20OH * → 6CO2+14H2O ……(4) C6H6O6+18OH * → 6CO2+12H2O ……(5)

[0031] Similarly, organic matter in corrosion inhibitors is ultimately decomposed into carbon dioxide and water by hydroxyl radicals.

[0032] In the test, the addition of hydrogen peroxide gradually promoted these decomposition reactions, leading to a decrease in the concentrations of formic acid, ascorbic acid, and the organic matter in the corrosion inhibitor. This process is shown in Figure 1. Figure 1 also shows the analysis of total organic carbon (TOC) as an indicator of the decomposition of ascorbic acid, its decomposition products, and the corrosion inhibitor.

[0033] As can be seen from Figure 1, the redox potential increases as the decomposition of formic acid and TOC progresses. More specifically, the redox potential increases gradually up to 200 mV (120 minutes after the start), then increases rapidly to exceed 500 mV (180 minutes after the start), at which point the decomposition of formic acid is nearly complete and about 55 ppm of TOC remains.

[0034] Regarding the treatment of this residual organic matter, since there is a possibility that components that cannot be treated with ion exchange resins may be generated, it is possible to combine a decomposition process that combines ultraviolet irradiation and hydrogen peroxide with purification using ion exchange resins. However, this would require incorporating an ultraviolet irradiation device into the decontamination equipment, and furthermore, since it is a photochemical reaction, the efficiency of the decomposition process would depend on the output of the ultraviolet irradiation device, and it was thought that the efficiency would not be as high as that of a chemical reaction, and decomposition might take time.

[0035] Therefore, we investigated decomposition treatment using chemicals that use oxidizing agents. For example, when 1750 ppm (0.038 mol / L) of formic acid and 750 ppm (0.0043 mol / L) of ascorbic acid are oxidized with permanganate ions, decomposition is expected to occur according to the following reaction formulas (6) and (7). 2MnO4 - + 5HCOOH + 6H + = 2Mn 2+ + 5CO2+ 8H2O ……(6) 4MnO4 - + C6H8O6+ 12H + = 4Mn 2+ + 6CO2+ 10H2O ……(7)

[0036] In this case, the permanganate ion concentration required for decomposition is 0.032 mol / L, which is equivalent to approximately 5100 ppm for potassium permanganate. When potassium permanganate is used in chemical decontamination, the concentration is approximately 300 ppm, so this is more than 10 times higher, and it is clear that decomposing formic acid and ascorbic acid with permanganate ions is not realistic.

[0037] On the other hand, the results of this test showed that when the Fenton reaction is used to decompose formic acid and ascorbic acid, most of the formic acid is decomposed when the redox potential exceeds 200 mV, and when it exceeds 500 mV, the formic acid is almost completely decomposed, with approximately 55 ppm remaining as a TOC component.

[0038] This TOC component is 8.3 x 10 -4If mol / L is considered as ascorbic acid, its concentration is 1.4 × 10 -4 mol / L, the required permanganate ion concentration is 5.6 x 10 -4 Assuming potassium permanganate in mol / L, this corresponds to 88 ppm. Because this is a lower value than the concentration in chemical decontamination using potassium permanganate, it was thought that permanganate ions could be used to decompose residual organic matter.

[0039] Based on the above results, after decontaminating carbon steel with a decontamination solution containing formic acid, ascorbic acid, and a corrosion inhibitor, the formic acid and ascorbic acid can be decomposed before moving on to decontamination of stainless steel. Decomposition can be carried out first using the Fenton reaction with the addition of hydrogen peroxide, using the redox potential as an indicator. When the redox potential reaches 500 mV, most of the formic acid is decomposed, leaving approximately 55 ppm of TOC components. Using permanganate ions for decomposition will enable rapid decomposition, and the process can be shortened by carrying out the subsequent oxidation decontamination process of the stainless steel oxide film in parallel.

[0040] Oxidative decontamination is performed under time management, with an oxidative decontamination solution containing permanganate ions circulating within the decontamination system for a specified period of time, for example, about four hours, after which oxalic acid is added to decompose the permanganate ions and perform reductive decontamination of the stainless steel oxide film, thereby carrying out reductive decontamination.

[0041] Furthermore, when the dose rate of the stainless steel piping falls to the target value or after a predetermined time has passed, the reduction decontamination is terminated and the oxalic acid decomposition process is carried out, followed by purification. If the dose rate of the stainless steel falls to the target value, the decontamination is terminated, but if the target is not reached, oxidation decontamination and reduction decontamination are carried out again.

[0042] Next, a first embodiment and a second embodiment of the chemical decontamination method and chemical decontamination apparatus for carbon steel members of a nuclear power plant according to the present invention will be described.

[0043] Example 1 A first embodiment of the chemical decontamination method and chemical decontamination apparatus for carbon steel members of a nuclear power plant according to the present invention will be described with reference to Figures 2 to 4. In this embodiment, the chemical decontamination method for carbon steel members and stainless steel members of a nuclear power plant is applied to, for example, a boiling water reactor (BWR) plant.

[0044] First, the schematic configuration of a BWR plant to which the chemical decontamination method for carbon steel members and stainless steel members of a nuclear plant of this embodiment is applied will be described with reference to Figure 2. Figure 2 is a diagram showing the configuration of a primary coolant water system of a BWR plant, which is one of the preferred targets of the present invention.

[0045] The BWR plant shown in FIG. 2 includes a nuclear reactor 49, a turbine 56, a condenser 57, a recirculation system, a reactor cleaning system, a feedwater system, and the like.

[0046] The reactor 49 installed in the containment vessel 11 has a reactor pressure vessel 50 (hereinafter referred to as RPV 50) that houses a reactor core 51, and a jet pump 52 is installed in the RPV 50. A plurality of fuel assemblies (not shown) are loaded into the reactor core 51. The nuclear fuel assembly includes a plurality of fuel rods filled with a plurality of fuel pellets made from nuclear fuel material.

[0047] The recirculation system includes a recirculation pump 53 and a recirculation pipe 54 made of stainless steel, and the recirculation pump 53 is installed on the recirculation pipe 54 .

[0048] The feedwater system is configured by installing a condensate pump 59, a condensate purification device 60, a low-pressure feedwater heater 61, a feedwater pump 63, and a high-pressure feedwater heater 62 in this order on a feedwater piping 58 that connects the condenser 57 and the RPV 50. A hydrogen injection device 66 is connected to the feedwater piping 58 between the condenser 57 and the condensate pump 59. A bypass piping 65 that bypasses the condensate purification device 60 is connected to the feedwater piping 58.

[0049] The reactor water purification system is configured by installing a purification system pump 68, a regenerative heat exchanger 69, a non-regenerative heat exchanger 70, and a reactor water purification device 71 in purification system piping 67 that connects the recirculation system piping 54 and the feedwater piping 58. The purification system piping 67 is connected to the recirculation system piping 54 upstream of the recirculation system pump 53.

[0050] The cooling water in the RPV 50 is pressurized by a recirculation system pump 53, passes through a recirculation system piping 54, and is ejected from a nozzle (not shown) of a jet pump 52 into a bell mouth (not shown) of the jet pump 52. The reactor water present around this nozzle is also sucked into the bell mouth by the action of the jet flow ejected from the nozzle.

[0051] The reactor water discharged from the jet pumps 52 is supplied to the reactor core 51 and heated by the heat generated by the nuclear fission of the nuclear fuel material in the fuel rods. Part of the heated reactor water becomes steam. This steam is led from the RPV 50 through the main steam pipe 55 to the turbine 56, causing it to rotate. A generator (not shown) connected to the turbine 56 is rotated, generating electricity. The steam discharged from the turbine 56 is condensed in the condenser 57 and becomes water.

[0052] This water is supplied as feedwater through feedwater piping 58 into the RPV 50. The feedwater flowing through feedwater piping 58 is pressurized by a condensate pump 59, impurities are removed in a condensate purification device 60, the pressure is further increased by a feedwater pump 63, and the water is heated by a low-pressure feedwater heater 61 and a high-pressure feedwater heater 62. Extracted steam extracted from the main steam pipe 55 and the turbine 56 through an extraction pipe 74 is supplied to the low-pressure feedwater heater 61 and the high-pressure feedwater heater 62, respectively, and serves as a heating source for the feedwater.

[0053] The system connecting the recirculation system and the RPV 50 includes a residual heat removal system (RHR system) that removes residual heat from the core when the reactor is shut down. The RHR system includes carbon steel RHR piping 82, a heat exchanger (not shown), and a recirculation system pump 83.

[0054] 2, one end of the RHR piping 82 is connected to the recirculation system piping 54 upstream of the recirculation system pump 53. The other end of the RHR piping 82 is connected to the RPV 50. In this way, the RHR piping 82 is connected to a core spray spurger (not shown) having a plurality of core spray nozzles (not shown), which is provided at the upper end of the core shroud above the core within the core shroud installed in the RPV 50. The core spray nozzles and the core spray spurger are part of the components that make up the high-pressure spray system.

[0055] 2, the RHR system is only shown on the right side of the RPV 50, but it is connected to the recirculation system piping 54 upstream of the recirculation system pump 53 on the left side, and the other end of the RHR piping 82 is connected to a system (not shown) that is connected to the RPV 50. These two RHR systems have the same configuration.

[0056] Next, the detailed configuration of the chemical decontamination apparatus 1 used in the chemical decontamination method for carbon steel members and stainless steel members of a nuclear plant according to this embodiment will be described with reference to Fig. 3. Fig. 3 is a detailed configuration diagram of the chemical decontamination apparatus used in the chemical decontamination method for carbon steel members and stainless steel members of a nuclear plant according to embodiment 1.

[0057] As shown in Figure 3, the chemical decontamination equipment 1 is an equipment for chemical decontamination of a system in which the decontamination target parts, including carbon steel components such as purification system piping 67 and RHR piping 82 of a nuclear power plant and stainless steel components such as recirculation system piping 54, are installed in part, and is equipped with circulation piping 2A, 2B, a hydrogen peroxide injection device 7, a decontamination agent supply unit 6, a surge tank 17, a corrosion inhibitor addition device 12, circulation pumps 20, 26, a filter 21, a catalyst tower 25, a cation exchange resin tower 23, and a mixed bed resin tower 24, etc.

[0058] An on-off valve 27, a circulation pump 20, and valves 28, 29, 30, and 31 are provided in this order on the circulation pipe 2A from the upstream purification system pipe 67, and via a surge tank 17, a circulation pump 26, a valve 32, and an on-off valve 33 are provided in this order on the circulation pipe 2B from the upstream side, and are connected to the downstream recirculation system pipe 54.

[0059] A valve 35 and a filter 21 are installed in a pipe 34 that bypasses the valve 28 and is connected to the circulation pipe 2A.

[0060] A pipe 36 bypassing the valve 29 is connected to the circulation pipe 2A, and a cooler 22 and a valve 37 are installed on the pipe 36. The other end of the pipe 36 is connected to a pipe 39 upstream of the mixed-bed resin tower 24.

[0061] A cation exchange resin tower 23 and a valve 40 are installed in a pipe 38 whose both ends are connected to the circulation pipe 2A and which bypasses the valve 30. A mixed bed resin tower 24 and a valve 41 are installed in a pipe 39 whose both ends are connected to the pipe 38 and which bypasses the cation exchange resin tower 23 and the valve 40. The cation exchange resin tower 23 has a resin layer filled with a cation exchange resin therein. The mixed bed resin tower 24 has a resin layer filled with a cation exchange resin and an anion exchange resin therein.

[0062] A pipe 42 in which a valve 43 and a catalyst tower 25 are installed bypasses the valve 31 and is connected to the circulation pipe 2A.

[0063] Hydrogen peroxide injection device 7 is a device for supplying hydrogen peroxide, which is a decontamination agent decomposition solution and oxidizing agent that decomposes formic acid and ascorbic acid, which are decontamination agents for carbon steel, and oxalic acid, which is a decontamination agent for stainless steel, and has chemical liquid tank 8, injection pump 9, and injection piping 44. Hydrogen peroxide is supplied from hydrogen peroxide injection device 7 to catalyst tower 25 and surge tank 17.

[0064] The chemical tank 8 is connected to the pipe 42 upstream of the catalyst tower 25 by an injection pipe 44 having an injection pump 9 and a valve 45. The chemical tank 8 is filled with an oxidizing agent (for example, hydrogen peroxide or ozone water).

[0065] The catalyst tower 25 is primarily used to decompose oxalic acid. When hydrazine is used during the reduction decontamination of stainless steel (recirculation system piping 54) with oxalic acid, it is also used to decompose hydrazine. The reduction decontamination agent used in chemical decontamination is an organic acid that can be decomposed into water and carbon dioxide, taking into consideration the reduction of waste volumes. Among these, formic acid, a monocarboxylic acid with few C—H bonds, is used when decontaminating carbon steel, as it forms complexes with iron and is less likely to form precipitates. However, other monocarboxylic acids can also be used.

[0066] The surge tank 17 is installed downstream of the circulation pipe 2A and upstream of the circulation pipe 2B between the valve 31 and the circulation pump 26, and a heater 19 is installed inside the surge tank 17.

[0067] A pipe 75 is connected to the upper end of the surge tank 17, and this pipe 75 is connected to the circulation pipe 2B between the circulation pump 26 and the valve 32. A valve 3 and an ejector 4 are installed on the pipe 75.

[0068] The decontamination agent supply unit 6 is a part that supplies decontamination agents including formic acid and ascorbic acid, which are decontamination agents for carbon steel, oxalic acid, which is a decontamination agent for stainless steel, and permanganic acid, which is used to decompose organic matter remaining in the decontamination agent decomposition solution, and has a hopper 5, an ejector 4, etc.

[0069] Hopper 5 is connected to ejector 4, and when decontaminating carbon steel (cleaning system piping 67 and RHR piping 82), formic acid, ascorbic acid, and a corrosion inhibitor are added, and by adding water appropriately and opening valve 3, the reagents in hopper 5 are sucked by the water flow from ejector 4 and supplied to surge tank 17. In addition, permanganate is supplied to hopper 5 when oxidative decontamination of stainless steel (recirculation system piping 54) is performed, and oxalic acid and hydrazine are supplied when reductive decontamination is performed.

[0070] The oxidation-reduction potentiometer 18, which measures the oxidation-reduction potential in the carbon steel decontamination agent decomposition solution, is installed downstream of the surge tank 17 on the circulation pipe 2B that connects the surge tank 17 to the purification system pipe 67, between the valve 32 and the on-off valve 33. The purpose of measuring the oxidation-reduction potential is to measure the decomposition state of formic acid and ascorbic acid, which are carbon steel decontamination agents, during decomposition. Hydrogen peroxide added to the decontamination solution just before the surge tank 17 decomposes formic acid, ascorbic acid, and the like in the surge tank according to reaction formulas (1) to (6). When the decontamination solution in the surge tank 17 is drawn out by the pump 26, if there is unreacted hydrogen peroxide at this time, the Fe 2+ is completely consumed and Fe 2+ Fe 3+ When the redox potential is elevated, hydrogen peroxide remains in the decontamination solution drawn from the surge tank 17, and when this decontamination solution reaches the carbon steel piping, corrosion of the oxygen steel piping progresses. Therefore, it is desirable to install an oxidation-reduction potentiometer 18 between the outlet of the surge tank 17 and the object to be decontaminated, which can detect the presence of residual hydrogen peroxide.

[0071] In the chemical decontamination apparatus 1 of the present invention, the decomposition state of formic acid in the decontamination agent decomposition solution is determined based on the oxidation-reduction potential of the decontamination agent decomposition solution measured by this oxidation-reduction potentiometer 18, and the timing to stop supplying hydrogen peroxide by the hydrogen peroxide injector 7 is determined. For example, if the oxidation-reduction potential measured by the oxidation-reduction potentiometer 18 exceeds a reference value, the injection of hydrogen peroxide can be stopped. This will be described in detail later.

[0072] The corrosion inhibitor adding device 12 is composed of a chemical tank 13 that holds a chemical solution containing a corrosion inhibitor, an injection pump 14 that delivers the chemical solution containing the corrosion inhibitor, injection piping 16, and a valve 15, and injects the corrosion inhibitor into the reduction decontamination waste liquid circulating from the circulation piping 2B via the injection piping 16. In addition to the corrosion inhibitor, liquid chemicals such as a permanganate solution or a hydrazine solution, and a pH adjuster can also be injected from the corrosion inhibitor adding device 12.

[0073] Both ends of the bypass piping 47 equipped with an on-off valve 46 are connected to the circulation piping 2B located between the oxidation-reduction potentiometer 18 and the on-off valve 33, and to the circulation piping 2A located between the on-off valve 27 and the circulation pump 20, respectively, and are arranged in parallel to the piping to be decontaminated, namely the purification system piping 67 and the recirculation system piping 54. In this bypass piping 47, the on-off valve 46 is normally kept closed, and after the decontamination agent decomposition liquid is injected, the on-off valve 46 can be opened to circulate the decontamination agent decomposition liquid through the bypass piping 47 in a closed loop including the surge tank 17.

[0074] The cooling water in the RPV 50 is irradiated with radiation generated by the nuclear fission of the nuclear fuel material contained in the fuel assemblies loaded in the reactor core 51, causing radiolysis and the generation of oxidizing species such as hydrogen peroxide and oxygen. These oxidizing species increase the corrosion potential of the components of the nuclear plant that come into contact with the cooling water. For this reason, some BWR plants inject hydrogen into the feedwater from a hydrogen injector 66 as a measure to mitigate the corrosive environment against stress corrosion cracking. This hydrogen reacts with the oxidizing species, such as hydrogen peroxide and oxygen, contained in the cooling water using radiation, thereby reducing the concentration of oxidizing species in the cooling water and lowering the corrosion potential of the components of the nuclear plant.

[0075] In BWR plants, operation with hydrogen injection into the feedwater is called hydrogen water chemistry (HWC) operation, while operation without hydrogen injection is called normal water chemistry (NWC) operation. While continuous operation of a BWR plant is desirable, which reduces the corrosion potential through hydrogen injection, there are cases where hydrogen injection is interrupted. When hydrogen injection is interrupted, the BWR plant operates in NWC operation, which results in a high corrosion potential for nuclear power plant components. Oxide films formed on stainless steel piping in plants operated under HWC conditions tend to develop inner oxide films rich in chromium. For this reason, the use of permanganate, a highly oxidizing agent, is preferred for decontamination. Conversely, the formation of inner oxide films rich in chromium is less likely on stainless steel piping in plants operated under NWC conditions, so the use of inexpensive potassium permanganate as an oxidizing agent is also possible.

[0076] A portion of the reactor water flowing through the recirculation system piping 54 flows into the purification system piping 67 by driving the purification system pump 68, is cooled in the regenerative heat exchanger 69 and the non-regenerative heat exchanger 70, is purified in the reactor water purification device 71, is heated by the regenerative heat exchanger 69, is supplied to the feedwater flowing through the feedwater piping 58, and is returned to the RPV 50.

[0077] A BWR plant is shut down after completing one operation cycle. After this shutdown, a periodic inspection is conducted on the BWR plant. After this periodic inspection is completed, the BWR plant is started up again. During this periodic inspection, some of the fuel assemblies in the reactor core 51 are replaced with new fuel assemblies. That is, some of the fuel assemblies in the reactor core 51 are removed from the RPV 50 as spent fuel assemblies, and new fuel assemblies with a burnup of 0 GWd / t are loaded into the reactor core 51. After the BWR plant is shut down and before a periodic inspection is conducted, chemical decontamination may be performed on the piping, etc. of the BWR plant.

[0078] A specific description will now be given of the chemical decontamination method for carbon steel members of a nuclear power plant according to the first embodiment, which is carried out according to the procedure shown in Fig. 4. Fig. 4 is a flowchart showing the procedure carried out in the chemical decontamination method for carbon steel members of a nuclear power plant according to the first embodiment.

[0079] For example, in a periodic inspection in which inspection and maintenance work is planned for the recirculation system pump, valve 72, etc., provided in the recirculation system piping 54, which is a piping made of stainless steel, if there is a purification system piping 67 around these valves and pumps, chemical decontamination may be performed on the recirculation system piping 54 and the purification system piping 67 in order to reduce radiation exposure of inspection or maintenance workers, including the recirculation system piping 54 and the purification system piping 67. In this case, steps S1 to S11 shown in FIG. 4 are performed on the recirculation system piping 54 and the purification system piping 67.

[0080] In BWR plants that have been in operation, oxide films containing radioactive nuclides are formed on the inner surfaces of the recirculation system piping 54 and purification system piping 67 through which cooling water flows in the RPV 50, and these oxide films are removed by chemical decontamination.

[0081] In order to grasp the status of this decontamination, a radiation detector 76 is placed outside the purification system piping 67, which is the object to be chemically decontaminated, to detect radiation emitted from the purification system piping 67. Similarly, a radiation detector 86 is placed outside the recirculation system piping 54 to detect radiation emitted from the recirculation system piping 54.

[0082] The chemical decontamination method of this embodiment is performed on carbon steel and stainless steel members of a BWR plant, and therefore is a process for removing oxide films from the inner surface of the purification system piping 67, which is a carbon steel piping, and the recirculation system piping 54, which is a stainless steel piping.

[0083] For chemical decontamination of the purification system piping 67 and the recirculation system piping 54, a chemical decontamination apparatus 1 shown in FIG. 3 is used.

[0084] First, the chemical decontamination apparatus 1 is connected to a piping system of a nuclear power plant whose operation has been stopped, which is an object to be chemically decontaminated (step S1).

[0085] The upstream end of circulation piping 2A and the downstream end of circulation piping 2B of chemical decontamination equipment 1, which is temporary equipment, are connected to carbon steel purification system piping 67 and stainless steel recirculation system piping 54, which are objects to be chemically decontaminated. The work of connecting these circulation piping 2A, 2B to purification system piping 67 and recirculation system piping 54 will now be described in detail.

[0086] After the BWR plant is shut down, for example, the hood of a valve 85 of instrumentation system piping 84, which is connected to the upper part of the vertical piping of recirculation system piping 54 downstream of recirculation system pump 53, is opened to seal the side opposite to recirculation system piping 54. An end of circulation piping 2B of chemical decontamination equipment 1 is connected to the flange of valve 85. As a result, the end of circulation piping 2B is connected to recirculation system piping 54 downstream of recirculation system pump 53 through instrumentation system piping 84.

[0087] On the other hand, the hood of valve 73 installed in purification system piping 67 downstream of purification system pump 68 is opened to close the regenerative heat exchanger 69 side. An end of circulation piping 2A of the chemical decontamination equipment 1 is connected to the flange of valve 73. As a result, one end of circulation piping 2B is connected to purification system piping 67 downstream of purification system pump 68, and a closed loop including instrumentation system piping 84, purification system piping 67, and circulation piping A and 2B is formed.

[0088] After the chemical decontamination equipment 1 is connected to the purification system piping 67 and the recirculation system piping 54, and before the circulation pumps 20, 26 are driven in step S2, water is filled into the circulation piping 2A, 2B, the surge tank 17, and the purification system piping 67 and the recirculation system piping 54 between the valves 85 and 73.

[0089] 3 are first closed, then surge tank 17 is filled with water, and on-off valve 46 and valves 32, 28, 29, 30, and 31 are opened to circulate the water through the circulation system of chemical decontamination equipment 1. Next, on-off valves 27 and 33 are opened, and on-off valve 46 is closed to fill recirculation system piping 54 and purification system piping 67 with water (step S2). At this time, water supply to surge tank 17 continues.

[0090] Next, the temperature of the circulating water is adjusted (step S2). The circulating water circulating in the circulation pipes 2A, 2B and the purification system pipe 67 is heated by the heater 19 installed in the surge tank 17, and the temperature of the circulating water is adjusted to about 90°C.

[0091] After the temperature is raised, reduction decontamination is performed on the inner surface of the purification system piping 67 using a carbon steel decontamination liquid containing formic acid and ascorbic acid (step S3). This step S3 is a carbon steel decontamination step.

[0092] The reduction decontamination in the chemical decontamination method of this embodiment will be described in detail below. Specifically, formic acid, ascorbic acid, and a corrosion inhibitor are added to produce a carbon steel decontamination solution. The corrosion inhibitor may be, for example, IBIT® "30AR" manufactured by Asahi Chemical Industry Co., Ltd.

[0093] In this step S3, preferably, formic acid, ascorbic acid, and a corrosion inhibitor are added to the hopper 5 of the decontamination agent supply unit 6, and water is added as needed. Then, the valve 3 is opened, and the reagent in the hopper 5 is sucked in by the water flow of the ejector 4 and supplied to the surge tank 17.

[0094] The supplied reagent is supplied into the system of the chemical decontamination equipment 1 by driving the circulation pumps 26, 20, dissolving the oxide film formed on the inner surface of the purification system piping 67 and also dissolving radioactive nuclides such as Co-60 that are trapped in the oxide film. At this time, the decontamination liquid also circulates inside the recirculation system piping 54, so that part of the oxide film formed on the stainless steel also dissolves, but because the stainless steel also has an oxide film containing nickel and chromium formed on it, only a small part of the oxide film on the stainless steel dissolves.

[0095] Here, the higher the concentration of formic acid or ascorbic acid, the more effective it is in dissolving the oxide film. However, since this increases the load on the decomposition in the subsequent carbon steel decontamination agent decomposition process, it is not practical to decontaminate at such a high concentration.

[0096] For example, formic acid should be 3500 ppm and ascorbic acid should be about half that, 1750 ppm, and the respective concentration ranges should be adjusted to between 1500 ppm and 7000 ppm for formic acid and between 750 ppm and 3500 ppm for ascorbic acid.

[0097] As the oxide film dissolves, the iron concentration and Co-60 concentration in the reduction decontamination solution increase, so the opening of valves 40 and 30 is adjusted to pass the chemical decontamination solution through cation exchange resin tower 23 to remove the cation components. At this time, the corrosion inhibitor is also removed, so an amount equivalent to the amount removed is supplied from hopper 5. The carbon steel decontamination solution from which the metal cations have been removed in cation exchange resin tower 23 is mixed with the carbon steel decontamination solution that has passed through valve 30 and is introduced into surge tank 17.

[0098] The carbon steel decontamination aqueous solution is circulated in the closed loop formed by the purification system piping 67, the recirculation system piping 54, and the circulation piping 2A, 2B, while reduction decontamination is performed on the inner surface of the purification system piping 67.

[0099] While the carbon steel decontamination is being carried out, a part of the aqueous solution for reduction decontamination is introduced into the cation exchange resin tower 23, where metal cations contained in the aqueous solution for reduction decontamination are removed.

[0100] At this time, the radiation detector 76 arranged outside the purification system piping 67, which is the object to be chemically decontaminated, detects the radiation emitted from the purification system piping 67, which is the object to be chemically decontaminated, and outputs the dose rate.

[0101] If the dose rate determined by the removal of radionuclides is the dose rate required for post-decontamination work, for example, the dose rate required for post-decontamination exposure dose management is 0.1 mSv / h, carbon steel decontamination will end when the dose rate falls below this rate, or when the downward trend in the dose rate stops, for example, when the hourly dose rate decrease becomes less than 1% of the initial dose rate. Alternatively, carbon steel decontamination will end when a predetermined time has passed since the start of reduction decontamination.

[0102] After the carbon steel decontamination is completed, hydrogen peroxide is supplied and the formic acid used as the carbon steel decontamination agent is decomposed using the oxidation-reduction potential as an indicator (step S4). Because this step is a preparation for the next stainless steel decontamination, the carbon steel decontamination agent must be decomposed in advance so that the permanganate injected in the stainless steel decontamination is not decomposed by the carbon steel decontamination agent.

[0103] Note that step S4 is not limited to supplying hydrogen peroxide and using the oxidation-reduction potential as an indicator, and may be performed by decomposing formic acid using a decomposition liquid, decomposition agent, or decomposition method capable of decomposing formic acid. Examples of decomposition liquids and decomposition agents other than hydrogen peroxide include ozone bubbling. Methods other than adding chemicals include ultraviolet irradiation and gamma ray irradiation.

[0104] When decontaminating stainless steel, oxalic acid decontamination may be performed first, but if the system includes carbon steel, iron (II) oxalate dihydrate will form on the surface of the carbon steel, which will decompose the permanganate in the subsequent oxidation decontamination process, so it is better to start with oxidation decontamination.

[0105] The formic acid decomposition process in step S4 decomposes the formic acid and ascorbic acid used as decontamination agents for carbon steel by injecting hydrogen peroxide from hydrogen peroxide injection device 7 just before surge tank 17. Valve 88 is opened to drive injection pump 9, and hydrogen peroxide in chemical tank 8 is injected through pipe 87 into circulation pipe 2A between valve 31 and surge tank 17.

[0106] The added hydrogen peroxide undergoes the Fenton reaction shown in reaction formula (1) above with the iron (II) ions in the carbon steel decontamination agent to generate hydroxyl radicals. These hydroxyl radicals decompose formic acid in reaction formula (2) above and ascorbic acid in reaction formula (4). Furthermore, the iron (III) ions generated in reaction formula (1) are reduced to iron (II) ions by ascorbic acid in reaction formula (3) above. These reactions take place in surge tank 17.

[0107] Furthermore, the oxidation-reduction potentiometer 18 measures the oxidation-reduction potential of the carbon steel decontamination solution being decomposed and drawn out from the surge tank 17 by the circulation pump 26 .

[0108] If hydrogen peroxide is consumed in the surge tank 17 at the beginning of decomposition, the redox potential will be about 100 mV as shown in Figure 1, but if excessive hydrogen peroxide is injected, the redox potential will increase. This indicates that the reduction of iron (III) ions by ascorbic acid according to reaction formula (3) cannot keep up, and iron (III) ions are in excess of iron (II) ions in the carbon steel decontamination solution.

[0109] In this state, there is a possibility that hydrogen peroxide may remain in the carbon steel decontamination solution during decomposition, and if this reaches the carbon steel to be decontaminated, it may accelerate corrosion, so the amount of hydrogen peroxide injected is reduced by, for example, lowering the flow rate of the injection pump 9. On the other hand, if the oxidation-reduction potential remains low, there will not be enough hydrogen peroxide for decomposition, so in that case the amount of hydrogen peroxide injected is temporarily increased, and when the oxidation-reduction potential begins to increase, the increase in the amount of hydrogen peroxide injected is stopped and hydrogen peroxide injection is continued at a constant rate.

[0110] As the decomposition of formic acid and ascorbic acid progresses, the redox potential increases as shown in Figure 1 even if hydrogen peroxide is injected at a constant rate, so in this case the hydrogen peroxide injection rate should be further reduced.

[0111] In this way, even if the injection of hydrogen peroxide is stopped, it is determined that the decomposition of formic acid is almost complete as shown in FIG. 1 when the oxidation-reduction potential exceeds a predetermined potential, for example, 200 mV, more preferably 500 mV (step S5).

[0112] Here, the oxidation-reduction potential (E) of carbon steel decontamination wastewater is determined by the iron (II) ion concentration [Fe 2+ ], iron(III) ion concentration [Fe 3+ ], ascorbic acid concentration [C6H8O6] and its oxidized form, dehydroascorbic acid concentration [C6H6O6], which can be expressed by the Nernst equation shown below in equation (8). E = E0 + (RT / nF) log([Fe 3+ ][C6H8O6] / {[Fe 2+ ][C6H6O6]}) ……(8)

[0113] Here, in formula (8), E0 is the standard electrode potential, R is the gas constant, T is the temperature of the aqueous solution for reduction decontamination, n is the valence, and F is the Faraday constant.

[0114] As the injection rate of hydrogen peroxide increases, the consumption of iron (II) ions in reaction (1) increases, and the redox potential tends to increase. Conversely, as the injection rate of hydrogen peroxide decreases, the consumption of iron (II) ions decreases, and the redox potential tends to decrease.

[0115] The amount of hydrogen peroxide injected is less than the equivalent required to decompose formic acid and ascorbic acid, for example, 0.5 equivalent, so that the hydrogen peroxide concentration is about 3000 ppm when the formic acid concentration is 1750 ppm and the ascorbic acid concentration is 750 ppm.

[0116] The added hydrogen peroxide reacts with the iron (II) ions contained in the reduced decontamination waste liquid to form hydroxyl radicals and iron (III) ions according to reaction formula (1). The hydroxyl radicals decompose formic acid according to reaction formula (2) and ascorbic acid according to reaction formula (4).

[0117] Meanwhile, iron (III) ions are reduced to iron (II) ions by ascorbic acid contained in the reduced decontamination waste liquid according to reaction formula (3). If hydrogen peroxide remains, it reacts with the iron (II) ions formed in reaction formula (3), further promoting the decomposition of formic acid.

[0118] Through these reactions (1), (2), (3), and (4), the decomposition of formic acid and ascorbic acid proceeds in surge tank 17. The hydroxyl radicals used to decompose formic acid and ascorbic acid have low reaction selectivity and also decompose corrosion inhibitors, so adding a corrosion inhibitor to compensate for this is effective in inhibiting corrosion of the carbon steel piping downstream.

[0119] However, the amount of hydrogen peroxide added is 0.5 times the equivalent amount required to decompose the formic acid and ascorbic acid in the reduction decontamination waste liquid flowing through the circulation pipes 2A and 2B, and the impact of residual hydrogen peroxide on carbon steel corrosion is not significant, so the addition of a corrosion inhibitor is not necessarily required. Valve 15 of the corrosion inhibitor addition device 12 is opened, and injection pump 14 is driven to inject the corrosion inhibitor in the chemical tank 13 into the carbon steel decontamination waste liquid circulating through injection pipe 16 connected to circulation pipe 2B.

[0120] The carbon steel decontamination waste liquid accumulates in surge tank 17, and while it is there, the decomposition reactions of formic acid and ascorbic acid progress. The amount of hydrogen peroxide added is 0.5 times the equivalent amount required to decompose the formic acid and ascorbic acid in the carbon steel decontamination waste liquid flowing through circulation pipes 2A and 2B, so while it is retained in surge tank 17, the hydrogen peroxide disappears first.

[0121] As a result, ascorbic acid remains in the carbon steel decontamination waste liquid flowing out of surge tank 17, and iron ions are reduced to iron (II) ions according to reaction formula (3). As a result, the value of redox potentiometer 18 indicates a low potential where iron (II) ions predominate. As the decomposition of formic acid and ascorbic acid progresses, there is no more ascorbic acid to reduce the iron (III) ions in reaction formula (3), so the iron (II) ions decrease, iron (III) ions become dominant, and the redox potential increases.

[0122] Therefore, as described above, in this formic acid decomposition step, it is desirable to stop the supply of hydrogen peroxide when the oxidation-reduction potential, which is an indicator, exceeds 200 mV, more preferably 500 mV.

[0123] The determination of whether the redox potential, which is this indicator, has exceeded 200 mV, more preferably 500 mV, may be made automatically by a control device or by an operator observing the measurement results of the redox potentiometer 18, and is not particularly limited.

[0124] Furthermore, if hydrogen peroxide remains in the decomposing reduction decontamination liquid in the surge tank 17 even after the addition of hydrogen peroxide has been stopped, one method is to open the on-off valve 46, close the on-off valve 33 and the on-off valve 27, bypass the object to be decontaminated, and wait for the hydrogen peroxide to decompose by circulating it in the bypass system for a while.

[0125] Furthermore, if the remaining hydrogen peroxide does not decompose easily, an activated carbon filter can be introduced into the filter 21 and water can be passed through it to decompose the remaining hydrogen peroxide.

[0126] Even after hydrogen peroxide injection has ceased, if the oxidation-reduction potential remains above 500 mV, it is necessary to decompose the organic matter among the remaining formic acid, ascorbic acid, and ascorbic acid decomposition products. At this point, the concentration of the remaining organic matter, as shown in Figure 1, is about 55 ppm total organic carbon, which is about 1 / 10 of the initial total organic carbon concentration of about 750 ppm.

[0127] As a method for decomposing the residual organic matter, a compound containing permanganate ions, which have a fast decomposition reaction, is supplied (step S6, residual organic matter decomposition step).

[0128] Specifically, 55 ppm of total organic carbon is considered to be derived from ascorbic acid, and the ascorbic acid concentration is 7.6 x 10 -4 The amount required for decomposition of 10 mol / L was calculated based on the above reaction equation (7), which is 3.1 × 10 -3A permanganate solution is prepared in an amount that will give a permanganate concentration of mol / L (367 ppm), and is poured into a hopper 5 . The valve 3 is opened, and the solution is sucked into an ejector 4 and supplied to a surge tank 17 .

[0129] Further, a permanganate solution in an amount of 200 ppm as permanganate for oxidative decontamination of stainless steel is charged into the hopper 5 and supplied to the surge tank 17 in the same manner.

[0130] The permanganate supplied in the surge tank 17 decomposes the total organic carbon of approximately 55 ppm contained in the carbon steel decontamination agent waste liquid during decomposition into water and carbon dioxide, and the remaining permanganate is sent from the surge tank 17 to the system to be decontaminated by the circulation pump 26. This simultaneously performs the decomposition treatment of the carbon steel decontamination waste liquid and the oxidative decontamination of stainless steel, shortening the decomposition process of the carbon steel decontamination waste liquid.

[0131] When permanganate reaches the stainless steel piping to be decontaminated, it oxidizes the chromium (III) oxide in the oxide film formed on the stainless steel, and dissolves it as chromate (VI) ions.

[0132] The concentration of permanganate in this residual organic matter decomposition process is adjusted to, for example, 200 ppm within the system, but the amount consumed in the decomposition of organic matter in the carbon steel decontamination agent waste liquid cannot be accurately estimated due to the chemical form of the decomposition residue of ascorbic acid and residual organic matter derived from corrosion inhibitors, so the permanganate concentration after the organic matter in the carbon steel decontamination agent waste liquid has been decomposed may deviate from the target value.

[0133] Therefore, it is desirable to analyze the permanganate concentration in the oxidizing decontamination solution after permanganate has been supplied, and supply permanganate so that the concentration of permanganate ions is 100 ppm or more, more preferably 200 ppm or more.

[0134] The completion of oxidative decontamination of stainless steel piping using permanganate is time-managed, for example, four hours after the permanganate ion concentration has been adjusted to 200 ppm. Oxidative decontamination is completed four hours after the permanganate ion concentration has reached 200 ppm.

[0135] Next, permanganate ions are decomposed by oxalic acid and the stainless steel piping is subjected to reduction decontamination (step S7, reduction decontamination process). The oxalic acid is placed in a hopper 5 and dissolved in a small amount of water. Then, valve 3 is opened to allow water to pass through ejector 4, which sucks the oxalic acid from the hopper 5 and supplies it to surge tank 17. The supplied oxalic acid initially reacts with permanganate in the oxidizing decontamination solution and decomposes according to the reaction shown in formula (9) below. 2MnO4 - + 5(COOH)2+6H + → 2Mn 2+ + 8H2O + 10CO2...(9)

[0136] When the concentration of permanganate remaining in the oxidative decontamination wastewater is 200 ppm, the oxalic acid required for decomposition of permanganate is 1.7 × 10 -3 mol / L to oxalic acid concentration 4.2 × 10 -3 mol / L (375 ppm) is the equivalent. Furthermore, oxalic acid is charged into the hopper 5 to achieve the oxalic acid concentration required for reduction decontamination (for example, 2000 ppm) and is supplied to the surge tank 17 in the same manner.

[0137] The oxalic acid supplied to the surge tank 17 reacts with the permanganic acid in the oxidative decontamination waste liquid according to reaction formula (9), and an amount of oxalic acid equivalent to the amount of permanganic acid is decomposed.

[0138] The remaining oxalic acid is circulated through the decontamination system by circulation pumps 26 and 20 to perform reduction decontamination on the oxide film formed on the stainless steel, which dissolves the oxide film formed on the stainless steel. Note that in order to suppress corrosion of the object to be decontaminated, hydrazine is sometimes added to the reduction decontamination solution containing oxalic acid to raise the pH from 1.8 (2000 ppm oxalic acid) to 2.5.

[0139] When reduction decontamination of stainless steel is carried out using an oxalic acid reduction decontamination agent, the oxide film formed on the stainless steel dissolves in the reduction decontamination solution, and the dose rate measured by the radiation detector 86 in the stainless steel recirculation system piping 54 decreases.

[0140] At the same time, the concentration of radioactive nuclides, mainly Co-60, in the reduction decontamination solution increases. Therefore, in order to remove radioactive nuclides, mainly Co-60, the reduction decontamination solution is passed through the cation exchange resin tower. This is done by opening valve 40 and adjusting valve 30 to open, and introducing some or all of the circulating reduction decontamination solution into cation exchange resin tower 23 through pipe 38. The reduction decontamination solution from which Co-60, some of the iron ions, and manganese ions derived from the permanganate used in the oxidative decontamination have been removed is discharged from the outlet of cation exchange resin tower 23, and this reduction decontamination solution with these reduced concentrations reaches the area to be decontaminated again, dissolves the oxide film, and continues the reduction decontamination.

[0141] As time passes during the reduction decontamination treatment, the rate of decrease in the dose rate measured by the radiation detector 86 decreases. When the rate of decrease in the dose rate has decreased to, for example, 1 / 100 of the initial rate, or when the reduction decontamination time determined based on the time planned for the entire decontamination process has been reached, the reduction decontamination treatment of the stainless steel is terminated.

[0142] Next, a decomposition process of oxalic acid is carried out (step S8). Valve 43 is opened and valve 31 is adjusted open to pass a portion of the reduction decontamination solution through catalyst tower 25. Catalyst tower 25 is filled with a ruthenium-supported activated carbon catalyst. By opening valve 45 and driving injection pump 9, hydrogen peroxide is added to the reduction decontamination solution flowing into catalyst tower 25. The amount of hydrogen peroxide injected is at least one equivalent based on the equivalent of the decomposition reaction of oxalic acid shown in the following formula (10). (COOH)2+ H2O2= 2CO2+ 2H2O ……(10)

[0143] In the case of 2000 ppm of oxalic acid, the equivalent amount is 760 ppm of hydrogen peroxide, so the injection rate of the injection pump 9 is adjusted so that the concentration at the injection point of hydrogen peroxide is equal to or higher than this concentration.

[0144] If hydrazine is added to the reduction decontamination solution to suppress corrosion, hydrogen peroxide is injected taking into account the decomposition equivalent of hydrazine. The decomposition reaction of hydrazine and hydrogen peroxide is shown in the following equation (11). N2H4+ 2H2O2= N2+ 4H2O ……(11)

[0145] The hydrazine concentration required to bring the pH of the reduction decontamination solution to 2.5 also depends on the concentration of dissolved iron ions, but if we assume it is approximately 500 ppm, the decomposition equivalent of hydrogen peroxide is calculated to be approximately 1000 ppm, so it is desirable to inject hydrogen peroxide so that the total concentration, including the amount of oxalic acid decomposition, is 1760 ppm.

[0146] As the decomposition of oxalic acid progresses, there is a possibility that dissolved iron ions may precipitate, so in this case filter 21 is operated. By opening valve 35 and adjusting valve 28 to open, a portion of the reducing decontamination agent decomposition solution is passed through filter 21 to remove the precipitate. The decomposition of oxalic acid is confirmed by analyzing the reducing decontamination agent decomposition solution, which is sampled periodically, and the decomposition is stopped when the oxalic acid concentration falls below 10 ppm.

[0147] After confirming the decomposition of oxalic acid, the completion of decontamination is determined (step S9). If the dose rate in the recirculation system piping measured by the radiation detector 86 reaches the target, the decontamination is completed, and the process proceeds to the purification and drainage process in step S10. On the other hand, if the target has not been reached, the process returns to the oxidation decontamination process using permanganate in step S6.

[0148] Thereafter, the previously explained permanganate decomposition and reduction decontamination step S7 and oxalic acid decomposition purification step S8 are performed, and the completion of decontamination is again determined based on the dose rate in the recirculation system piping measured by the radiation detector 86 and the dose rate of the decontamination target. In this way, it is desirable to repeat the oxidation decontamination process (step S6) and reduction decontamination process (step S7) multiple times.

[0149] However, from the second time onwards, a decision will also be made based on the number of times the stainless steel decontamination process is repeated, which is set from the decontamination process period allocated in the periodic inspection process.For example, if the stainless steel decontamination process is repeated twice, the decontamination may be considered complete and the process may move on to the next purification and drainage process even if the target dose rate has not been reached.

[0150] After determining that decontamination is complete, the reduction decontamination agent waste liquid is purified and drained (step S10). First, injection pump 9 of hydrogen peroxide injection device 7 is stopped and valve 45 is closed. Valve 31 is opened and valve 43 is closed to stop the flow of the reduction decontamination agent decomposition liquid into catalyst tower 25. Next, water is passed through mixed bed resin tower 24 to remove ionic components remaining in the reduction decontamination agent decomposition liquid.

[0151] The anion exchange resin contained in the mixed bed resin has a heat resistance of about 60°C, so the reduced decontamination waste liquid is cooled. Valve 37 is opened, and valve 29 is adjusted open. A portion of the reduced decontamination waste liquid is passed through pipe 36 to cooler 22, lowering the temperature of the passed reduced decontamination waste liquid to below 60°C. After confirming that the temperature has dropped below 60°C, valve 41 is opened, and the reduced decontamination waste liquid coming out of cooler 22 is passed through pipe 39 to mixed bed resin tower 24. This removes the ionic components contained in the reduced decontamination waste liquid, gradually reducing its conductivity, ultimately reaching 2 μS / cm. After analyzing sampled water to confirm that the conductivity, pH, and metal and other impurity concentrations of the reduced decontamination waste liquid meet the discharge standards, the purified decontamination waste liquid is discharged.

[0152] After the drainage is completed, the chemical decontamination equipment 1 temporarily attached to the reactor water purification system piping and the recirculation system piping is removed from the system, and the decontamination is completed.

[0153] Next, the effects of this embodiment will be described.

[0154] The chemical decontamination method for the area to be decontaminated, including the purification system piping 67 and the RHR piping 82 at the nuclear power plant in the above-described first embodiment of the present invention, includes a purification system piping 67 and the RHR piping 82 decontamination step in which formic acid, ascorbic acid, and a corrosion inhibitor are supplied to decontaminate the purification system piping 67 and the RHR piping 82 of the area to be decontaminated, a formic acid decomposition step in which, after the purification system piping 67 and the RHR piping 82 decontamination step, formic acid is decomposed using a decomposition liquid, decomposition agent, or decomposition method capable of decomposing formic acid, and a residual organic matter decomposition step in which, after the formic acid decomposition step, a compound containing permanganate ions is supplied to decompose the residual organic matter.

[0155] Furthermore, the chemical decontamination apparatus 1 for the area to be decontaminated, including the purification system piping 67 and RHR piping 82 at the nuclear power plant in Example 1 of the present invention, is equipped with a decontamination agent supply unit 6 that supplies a decontamination agent containing formic acid and ascorbic acid, a hydrogen peroxide injection device 7 that supplies a decontamination agent decomposition solution containing hydrogen peroxide that decomposes formic acid and ascorbic acid, and the decontamination agent supply unit 6 that supplies permanganate ions to decompose organic matter remaining in the decontamination agent decomposition solution, and determines whether or not to supply hydrogen peroxide from the hydrogen peroxide injection device 7 based on the oxidation-reduction potential of the decontamination agent decomposition solution measured by an oxidation-reduction potentiometer 18.

[0156] This avoids the formation of a precipitated film that would prevent the dissolution of the oxide film on the carbon steel, while allowing for a smaller amount of permanganate ion to be used from the start of decomposition using formic acid and ascorbic acid than when permanganate ion is used, and also shortens the decomposition time compared to conventional decomposition using ultraviolet light. In addition, the oxidative decontamination of stainless steel components, which begins after the decomposition of organic decomposition products, can be carried out simultaneously with the decomposition of the organic decomposition products. These two effects enable the decontamination process to be significantly shortened compared to conventional methods.

[0157] Furthermore, in the formic acid decomposition step, hydrogen peroxide is supplied and formic acid is decomposed using the oxidation-reduction potential as an indicator, so that formic acid can be decomposed more efficiently, and the overall decontamination step can be more reliably shortened in time.

[0158] Furthermore, since the areas to be decontaminated include the recirculation system piping 54 in addition to the purification system piping 67 and the RHR piping 82, it is possible to decontaminate stainless steel as well as carbon steel.

[0159] Furthermore, by further including a reduction decontamination step in which oxalic acid is supplied after the residual organic matter decomposition step to decompose permanganate ions and perform reduction decontamination, the carbon steel decontamination agent can be used without forming iron(II) oxalate dihydrate on the carbon steel surface, which is generated when oxalic acid, a decontamination agent for stainless steel components, is brought into contact with carbon steel, thereby further improving the decontamination efficiency of carbon steel. Furthermore, when decontaminating stainless steel after decontaminating carbon steel with ascorbic formate, it is possible to start with oxidative decontamination that also decomposes residual organic matter, so that the oxidative decontamination agent can be used without forming iron(II) oxalate dihydrate on the carbon steel surface, and since decomposition of permanganate by iron(II) oxalate dihydrate does not occur, consumption of permanganate used can be reduced. Furthermore, if reductive decontamination is started, the added oxalic acid will form iron (II) oxalate dihydrate on the surface of the carbon steel, and in the subsequent oxidative decontamination process using permanganate, some of the added permanganate will be consumed by the decomposition of the oxalic acid portion of the iron (II) oxalate dihydrate formed on the surface of the carbon steel. Therefore, performing oxidative decontamination before the formation of iron (II) oxalate dihydrate reduces the consumption of permanganate, which does not affect the decontamination of stainless steel, and is therefore advantageous in terms of the efficiency of permanganate utilization.

[0160] Furthermore, by repeating the oxidation decontamination step and the reduction decontamination step multiple times, the decontamination of stainless steel can be achieved more reliably.

[0161] Furthermore, in the formic acid decomposition step, by stopping the supply of hydrogen peroxide when the oxidation-reduction potential, which is an indicator, exceeds 200 mV, particularly 500 mV, it becomes possible to stop the formic acid decomposition at the optimal timing.

[0162] Furthermore, in the residual organic matter decomposition process, by supplying permanganate ions so that the concentration of permanganate ions after supply is 100 ppm or more, particularly 200 ppm or more, it is possible to reliably decompose residual organic matter and carry out oxidative decontamination in parallel.

[0163] Furthermore, by further providing a decontamination agent supply section 6 that supplies a decontamination agent containing oxalic acid, a catalyst tower 25 for decomposing oxalic acid, and a hydrogen peroxide injection device 7 that supplies hydrogen peroxide to the catalyst tower 25, reduction decontamination of stainless steel can also be performed.

[0164] In addition, a surge tank 17 is further provided, and an oxidation-reduction potentiometer 18 is installed downstream of the surge tank 17. When the oxidation-reduction potential measured by the oxidation-reduction potentiometer 18 exceeds a reference value, the injection of hydrogen peroxide is stopped. This allows the oxidation-reduction potential to be monitored for leakage of hydrogen peroxide from the surge tank 17, which is the decomposition reaction site, and when leakage is detected, the on-off valve 46 is opened and the on-off valve 33 is closed, allowing the carbon steel decontamination agent waste liquid containing hydrogen peroxide to flow into the bypass piping 47. This prevents the carbon steel decontamination agent waste liquid containing hydrogen peroxide from reaching the purification system piping 67, which is the target of decontamination, and contributes to corrosion suppression.

[0165] Furthermore, the system is further provided with a bypass pipe 47 arranged in parallel to the decontamination targets of the purification system pipe 67, the RHR pipe 82, and the recirculation system pipe 54. After the decontamination agent decomposition liquid is injected, the decontamination agent decomposition liquid is circulated in a closed loop including the surge tank 17 by the bypass pipe 47, thereby preventing the decontamination agent decomposition liquid from flowing outside the system where no decontamination targets are located. This prevents the introduction of more decontamination agent decomposition liquid than necessary and further improves the decomposition efficiency.

[0166] In addition, the system is further provided with a hydrogen peroxide injection device 7 arranged in the circulation piping 2A where the flow runs from the target to be decontaminated in the purification system piping 67, RHR piping 82, and recirculation system piping 54 to the surge tank 17, and an oxidation-reduction potentiometer 18 is installed in the circulation piping 2B where the flow runs from the surge tank 17 to the target to be decontaminated in the purification system piping 67, RHR piping 82, and recirculation system piping 54. This makes it possible to set up an injection point for hydrogen peroxide upstream of the surge tank 17, and even if there is an excess of hydrogen peroxide at the injection point, it is used to decompose the formic acid and ascorbic acid in the surge tank 17. This further advances the decomposition of formic acid and ascorbic acid in the carbon steel decontamination solution as a whole, thereby enabling the time to complete the decontamination process to be shortened.

[0167] <Example 2> A chemical decontamination method and chemical decontamination apparatus for carbon steel and stainless steel members of a nuclear plant according to a second embodiment of the present invention will be described with reference to Fig. 5. Fig. 5 is a detailed configuration diagram of the chemical decontamination apparatus used in the chemical decontamination method for carbon steel members and stainless steel members of a nuclear plant according to the second embodiment. The chemical decontamination method for carbon steel members of a nuclear plant according to this embodiment is also an example applied to a boiling water reactor (BWR) plant.

[0168] 5, the chemical decontamination apparatus 1A used in the chemical decontamination method of this embodiment is configured in such a way that, in addition to the above-described chemical decontamination apparatus 1, a permanganate injection device 89 is added via piping 93 having a valve 92 at a position upstream of the piping to be decontaminated and downstream of the surge tank 17 where the permanganate injection device 89 joins the circulation piping 2B. The other configurations of the chemical decontamination apparatus 1A are the same as those of the chemical decontamination apparatus 1.

[0169] In Example 1, permanganate was injected from hopper 5 into surge tank 17 by ejector 4. The injection amount of permanganate was determined so that the circulating liquid volume would be 367 ppm for decomposition of residual organic carbon (based on the results shown in Figure 1) and 200 ppm for oxidative decontamination, for a total of 567 ppm. Adding this amount over one circulation time achieved by circulation pumps 20 and 26 minimizes concentration variations due to circulation position, so we chose to add this amount over one circulation time. In this case, the surge tank 17 contains a large volume of liquid, so there is also a large amount of residual organic carbon to be decomposed. Initially, the 200 ppm permanganate added for oxidative decontamination is consumed in the oxidation treatment of residual organic carbon, and permanganate for oxidative decontamination treatment with a concentration lower than 200 ppm is delivered to the decontamination target by circulation pump 26. This state continues until the initially added permanganate circulates around the decontamination target and returns to surge tank 17.

[0170] On the other hand, in Example 2, the amount of permanganate consumed by total organic carbon at the injection point is only 367 ppm, which is the total organic carbon concentration, so the remaining permanganate remains at about 200 ppm and can be used for oxidative decontamination. Therefore, oxidative decontamination of the decontamination target becomes effective immediately after the start of permanganate injection, and the oxidative decontamination time can be shorter than in Example 1.

[0171] The other configurations and operations are substantially the same as those of the chemical decontamination method and chemical decontamination apparatus for carbon steel and stainless steel members of a nuclear power plant according to the first embodiment, and therefore details are omitted here.

[0172] The chemical decontamination method and chemical decontamination apparatus for carbon steel and stainless steel members of a nuclear power plant according to the second embodiment of the present invention also provides substantially the same effects as those of the chemical decontamination method and chemical decontamination apparatus for carbon steel and stainless steel members of a nuclear power plant according to the first embodiment described above.

[0173] Furthermore, by installing a permanganate injection device 89 on the circulation piping 2B between the surge tank 17 and the purification system piping 67, the RHR piping 82, and the recirculation system piping 54, which are the targets of decontamination, the time required for oxidation decontamination of stainless steel can be further shortened.

[0174] <Other> It should be noted that the present invention is not limited to the above-described embodiment, and includes various modifications. The above-described embodiment has been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to an embodiment having all of the described configurations.

[0175] It is also possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment, or to add, delete, or replace part of the configuration of each embodiment with the configuration of another embodiment.

[0176] For example, the chemical decontamination apparatus and chemical decontamination method according to the first or second embodiment can also be applied to a pressurized water nuclear power plant. [Explanation of symbols]

[0177] 1,1A...Chemical decontamination equipment 2A…Circulation piping (first piping) 2B…Circulation piping (second piping) 3,15,28,29,30,31,32,35,37,40,41,43,45,72,73,85,88,92...Valves 4...Ejector 5...Hopper 6... Decontamination agent supply unit (carbon steel component decontamination agent supply unit, permanganate ion supply unit, stainless steel component reduction decontamination agent supply unit) 7...Hydrogen peroxide injection device (decomposition liquid supply unit, oxalic acid decomposition liquid supply unit) 8, 13, 90...Chemical tank 9, 14, 91...infusion pump 11...Reactor containment vessel 12...Corrosion inhibitor adding device 16,44...Injection piping 17...Surge tank 18...Oxidation-reduction potentiometer 19...heater 20,26...Circulation pump 21...Filter 22...Cooler 23...Cation exchange resin tower 24…Mixed bed resin tower 25...Catalytic tower (catalytic decomposition unit) 27, 33, 46...Shut-off valves 34, 36, 38, 39, 42, 75, 87, 93...Piping 47...Bypass piping 49...Nuclear reactor 50...Reactor pressure vessel 51...Reactor core 52...Jet pump 53,83...Recirculation pump 54...Recirculation system piping (stainless steel components, parts to be decontaminated) 55...Main steam pipe 56...Turbine 57...Condenser 58...Water supply pipe 59...Condensate pump 60...Condensate purification device 61...Low pressure feedwater heater 62...High pressure feedwater heater 63...Water supply pump 65...Bypass piping 66...Hydrogen injection device 67...Purification system piping (carbon steel components, parts to be decontaminated) 68...Purification system pump 69…Regenerative heat exchanger 70…Non-regenerative heat exchanger 71...Reactor water purification equipment 74...Bleeding pipe 76,86...Radiation detector 82...RHR piping (carbon steel components, decontamination target area) 84...Instrumentation piping 89...Permanganate injection device (permanganate ion supply unit)

Claims

1. A chemical decontamination method for a decontamination target part including a carbon steel member at a nuclear power plant, a carbon steel member decontamination step of supplying formic acid, ascorbic acid, and a corrosion inhibitor to decontaminate the carbon steel member of the decontamination target portion; a formic acid decomposition step of decomposing the formic acid using a decomposition solution, decomposition agent, or decomposition method capable of decomposing the formic acid, after the carbon steel member decontamination step; a residual organic matter decomposition step in which a compound containing permanganate ions is supplied after the formic acid decomposition step to decompose the residual organic matter. Chemical decontamination methods.

2. The chemical decontamination method according to claim 1, In the formic acid decomposition step, hydrogen peroxide is supplied and the formic acid is decomposed using the oxidation-reduction potential as an indicator. A chemical decontamination method characterized by:

3. The chemical decontamination method according to claim 1, The part to be decontaminated includes a stainless steel member in addition to the carbon steel member. Chemical decontamination methods.

4. The chemical decontamination method according to claim 3, After the residual organic matter decomposition step, a reduction decontamination step is further included in which oxalic acid is supplied to decompose the permanganate ions and perform reduction decontamination. Chemical decontamination methods.

5. The chemical decontamination method according to claim 4, The step of supplying the compound containing permanganate ions and the reduction decontamination step are repeated multiple times. Chemical decontamination methods.

6. The chemical decontamination method according to claim 2, In the formic acid decomposition step, the supply of hydrogen peroxide is stopped when the oxidation-reduction potential, which is the indicator, exceeds 200 mV. Chemical decontamination methods.

7. The chemical decontamination method according to claim 2, In the formic acid decomposition step, the supply of hydrogen peroxide is stopped when the oxidation-reduction potential, which is the indicator, exceeds 500 mV. Chemical decontamination methods.

8. The chemical decontamination method according to claim 1, In the residual organic matter decomposition step, the permanganate ions are supplied so that the concentration of the permanganate ions after supply is 100 ppm or more. Chemical decontamination methods.

9. The chemical decontamination method according to claim 1, In the residual organic matter decomposition step, the permanganate ions are supplied so that the concentration of the permanganate ions is 200 ppm or more. Chemical decontamination methods.

10. A chemical decontamination device for decontamination targets including carbon steel members at a nuclear power plant, a carbon steel member decontamination agent supply unit that supplies a decontamination agent containing formic acid and ascorbic acid; a decomposition liquid supply unit that supplies a decontamination agent decomposition liquid that decomposes the formic acid and the ascorbic acid; a permanganate ion supply unit that supplies permanganate ions for decomposing organic matter remaining in the decontamination agent decomposition solution, The formic acid is decomposed using a decomposition solution, decomposition agent, or decomposition method capable of decomposing the formic acid, and the compound containing permanganate ions is supplied to decompose the residual organic matter. Chemical decontamination equipment.

11. The chemical decontamination apparatus according to claim 10, further comprising an oxidation-reduction potentiometer for measuring an oxidation-reduction potential in the decontamination agent decomposition solution; the decomposition liquid supply unit supplies a decontamination agent decomposition liquid containing hydrogen peroxide; The decomposition state of the formic acid in the decontamination agent decomposition solution is determined based on the oxidation-reduction potential of the decontamination agent decomposition solution measured by the oxidation-reduction potentiometer, and it is determined whether or not the hydrogen peroxide is to be supplied by the decomposition solution supply unit. Chemical decontamination equipment.

12. The chemical decontamination apparatus according to claim 11, The part to be decontaminated includes a stainless steel member in addition to the carbon steel member. Chemical decontamination equipment.

13. The chemical decontamination apparatus according to claim 12, a stainless steel component reducing decontamination agent supply unit that supplies a decontamination agent containing oxalic acid; a catalytic decomposition device for decomposing the oxalic acid; an oxalic acid decomposition liquid supply unit that supplies hydrogen peroxide to the catalytic decomposition device. Chemical decontamination equipment.

14. The chemical decontamination apparatus according to claim 13, It is further equipped with a surge tank, the oxidation-reduction potentiometer is installed downstream of the surge tank; When the value of the oxidation-reduction potential measured by the oxidation-reduction potentiometer exceeds a reference value, the injection of hydrogen peroxide is stopped. Chemical decontamination equipment.

15. 15. The chemical decontamination apparatus according to claim 14, Further provided is a bypass pipe arranged in parallel with the carbon steel member and the stainless steel member to be decontaminated, After the decontamination agent decomposition liquid is injected, the decontamination agent decomposition liquid is circulated in a closed loop including the surge tank through the bypass piping. Chemical decontamination equipment.

16. 15. The chemical decontamination apparatus according to claim 14, The apparatus further includes a hydrogen peroxide injection device disposed in a first pipe flowing from the carbon steel member and the stainless steel member to be decontaminated to the surge tank, The oxidation-reduction potentiometer is installed in a second pipe that flows from the surge tank to the carbon steel member and the stainless steel member to be decontaminated. Chemical decontamination equipment.

17. 15. The chemical decontamination apparatus according to claim 14, The permanganate ion supply unit is installed in a second pipe between the surge tank and the carbon steel member and the stainless steel member to be decontaminated. Chemical decontamination equipment.

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