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

Abstract

To provide a chemical decontamination method and chemical decontamination device for a carbon steel member of a nuclear plant that do not form a precipitated film that prevents an oxide film from dissolving during dissolution, and can suppress corrosion of a base material during decomposition of a reduction decontamination agent compared to conventional arts.SOLUTION: A chemical decontamination method comprises a decontamination process of decontaminating a carbon steel member using a decontamination agent containing formic acid and ascorbic acid, and a decomposition process of decomposing the formic acid and ascorbic acid in the decontamination agent used in the decontamination process using a decontamination agent decomposition liquid containing hydrogen peroxide. In the decomposition process, a residual state of hydrogen peroxide in the decontamination agent decomposition liquid is determined based on a residual oxidation-reduction potential of hydrogen peroxide remaining in the decontamination agent decomposition liquid, which is determined based on an iron concentration, and injection of hydrogen peroxide into the decontamination agent decomposition liquid is stopped.SELECTED DRAWING: Figure 7

Description

[Technical Field]

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

[0002] A chemical decontamination method is described which includes a dissolution step of dissolving radioactive insoluble material containing metal oxides attached to an object to be decontaminated, including carbon steel, with a decontamination solution, and a metal ion removal step of removing metal ions by contacting the metal ion-containing decontamination solution produced by the dissolution step with a cation exchange resin, wherein the dissolution step includes a reduction dissolution step with a decontamination solution containing formic acid, ascorbic acid and / or erythorbic acid, and a corrosion inhibitor. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2018-151210 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] For example, a boiling water reactor (hereinafter referred to as a BWR plant) has a reactor with a core housed within a reactor pressure vessel (referred to as an RPV).

[0005] The reactor water supplied to the core by recirculation pumps or internal pumps is heated by the heat generated by the nuclear fission of the nuclear fuel material in the fuel assemblies loaded into the core, and some of it turns into steam. This steam is guided from the RPV to the turbine, which rotates the turbine. The steam discharged from the turbine condenses into water in the condenser. This water is supplied to the reactor as feedwater.

[0006] To suppress the generation of radioactive corrosion products within the RPV, the feedwater is filtered and desalined in the feedwater piping, primarily removing metal impurities. Reactor water refers to the cooling water present within the RPV.

[0007] Furthermore, since corrosion products that are the source of radioactive corrosion products are generated on the surfaces of BWR plant components such as RPVs and recirculation system piping that come into contact with reactor water, corrosion-resistant steels such as stainless steel and nickel-based alloys are used for the main primary system components. In addition, RPVs made of low-alloy steel are reinforced with stainless steel on the inside to prevent the low-alloy steel from coming into direct contact with reactor water. Moreover, a portion of the reactor water is purified by a filtration and demineralization system in the reactor purification system, actively removing small amounts of metallic impurities present in the reactor water.

[0008] However, even with the corrosion countermeasures described above, the presence of minute amounts of metallic impurities in the reactor water is unavoidable, and some of these metallic impurities adhere to the surface of the fuel rods contained in the fuel assembly as metallic oxides. These impurities (e.g., metallic elements) adhering to the fuel rod surface undergo nuclear reactions when irradiated with neutrons released by the nuclear fission of the nuclear fuel material within the fuel rods, becoming radioactive nuclides such as cobalt-60, cobalt-58, chromium-51, and manganese-54.

[0009] These radionuclides mostly remain attached to the fuel rod surface in the form of oxides. However, some radionuclides dissolve into the reactor water as ions, depending on the solubility of the incorporated oxides, or are re-released into the reactor water as an insoluble solid called cladding. Radioactive materials in the reactor water are removed by a reactor cleanup system connected to the RPV (Reactor Propulsion Vehicle).

[0010] Radioactive materials not removed by the reactor purification system accumulate on the surfaces of components of the nuclear plant (e.g., piping) that come into contact with reactor water as they circulate with the reactor water through recirculation systems and other means. As a result, radiation is emitted from the surfaces of these components, causing radiation exposure to workers during routine inspections.

[0011] The radiation doses of these workers are managed to ensure that each individual does not exceed a set limit. In recent years, this limit has been lowered, making it necessary to keep each person's radiation dose as low as possible.

[0012] Therefore, if high radiation exposure is expected during routine inspection work, chemical decontamination may be carried out to dissolve and remove radioactive nuclides attached to the piping. One example of such a technique is the one described in Patent Document 1.

[0013] In the aforementioned Patent Document 1, carbon steel components are decontaminated by reduction using formic acid and ascorbic acid, in addition to an organic corrosion inhibitor. After adjusting the iron ion concentration by passing a cationic resin through the water, hydrogen peroxide is added to decompose the formic acid. Next, ascorbic acid is decomposed by ultraviolet irradiation and hydrogen peroxide. A corrosion inhibitor is added in each decomposition step to suppress corrosion of the base material.

[0014] The chemical decontamination method described in Patent Document 1 does not use oxalic acid, and therefore does not cause the problem of iron(II) oxalate dihydrate film formation when dissolving oxide films containing radioactive materials. Furthermore, when hydrogen peroxide is used during the decomposition of formic acid and ascorbic acid, an organic rust inhibitor is added to suppress corrosion of the base material. As a result, although the amount of corrosion at this time is small compared to the corrosion tolerance of carbon steel piping, it became clear that there is room for further improvement, and the inventors investigated methods to further suppress corrosion of the base material.

[0015] Therefore, the object of the present invention is to provide a chemical decontamination method and apparatus for carbon steel members of a nuclear power plant that, in chemical decontamination using formic acid and ascorbic acid, does not form a precipitated film that hinders the dissolution of the oxide film during the dissolution of the oxide film, and suppresses corrosion of the base material during the decomposition of the reducing decontamination agent compared to conventional methods. [Means for solving the problem]

[0016] The present invention includes a plurality of means for solving the above problems. For example, it is a method for chemical decontamination of carbon steel members in a nuclear power plant, comprising a decontamination step of decontaminating the carbon steel members using a decontamination agent containing formic acid and ascorbic acid, and a decomposition step of decomposing the formic acid and the ascorbic acid in the decontamination agent used in the decontamination step using a decontamination agent decomposition solution containing hydrogen peroxide. In the decomposition step, The oxidation-reduction potential value measured by the oxidation-reduction potential meter installed downstream of the surge tank is Based on the residual oxidation-reduction potential of hydrogen peroxide remaining in the decontamination agent decomposition solution, which is determined based on the iron concentration, the residual state of hydrogen peroxide in the decontamination agent decomposition solution is judged, When the residual oxidation-reduction potential exceeds the aforementioned limit The injection of hydrogen peroxide into the decontamination agent decomposition solution is stopped.

Advantages of the Invention

[0017] According to the present invention, in chemical decontamination using formic acid and ascorbic acid, a precipitation film that hinders the dissolution of the oxide film is not formed during the dissolution of the oxide film, and corrosion of the base material can be suppressed as compared with the conventional method during the decomposition of the reducing decontamination agent. Other problems, configurations, and effects than those described above will be clarified by the description of the following examples.

Brief Description of the Drawings

[0018] [Figure 1] It is a drawing comparing the corrosion test results of carbon steel in the water quality of the reducing decontamination agent decomposition process. [Figure 2] It is a drawing showing the configuration of a test apparatus in which a corrosion test of carbon steel is performed in the water quality of the reducing decontamination agent decomposition process. [Figure 3] It is a drawing showing the results of examining the relationship between the oxidation-reduction potential at which hydrogen peroxide remains and the iron concentration in the decomposition process of the reducing decontamination agent solution due to the addition of hydrogen peroxide. [Figure 4] It is a drawing showing the configuration of the primary cooling water system of a BWR plant, which is one of the suitable objects of the present invention. [Figure 5] It is a detailed configuration drawing of a chemical decontamination apparatus used in the method for chemical decontamination of carbon steel members in a nuclear power plant according to Example 1. [Figure 6]This flowchart shows the procedure performed in the chemical decontamination method for carbon steel components of a nuclear power plant in Example 1. [Figure 7] This flowchart shows the decomposition step of the reducing decontamination agent carried out in the chemical decontamination method for carbon steel components of a nuclear power plant in Example 1. [Figure 8] This is a detailed configuration diagram of the chemical decontamination apparatus used in the chemical decontamination method for carbon steel components of a nuclear power plant in Example 2. [Modes for carrying out the invention]

[0019] Examples 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 below with reference to the drawings. In the drawings used herein, the same or corresponding components are denoted by the same or similar reference numerals, and repeated descriptions of these components may be omitted.

[0020] First, the process leading to the completion of this invention will be explained using Figures 1 to 3. Figure 1 is a diagram comparing the corrosion test results of carbon steel in the water quality of the reducing decontamination agent decomposition process, Figure 2 is a diagram showing the configuration of the test apparatus used for the corrosion test of carbon steel in the water quality of the reducing decontamination agent decomposition process, and Figure 3 is a diagram showing the results of investigating the relationship between the oxidation-reduction potential of residual hydrogen peroxide and iron concentration during the decomposition process of the reducing decontamination agent solution by adding hydrogen peroxide.

[0021] The present inventors investigated a method to suppress corrosion of carbon steel during the formic acid decomposition process in chemical decontamination of carbon steel using an aqueous solution (reducing decontamination solution) containing formic acid, ascorbic acid, and an organic corrosion inhibitor.

[0022] Specifically, the corrosion inhibitor used was an aqueous solution containing 4% thiourea, 1-5% quaternary ammonium salt, and 1-5% organic sulfur compounds. The decontamination agent consisted of 3500 ppm formic acid, 1500 ppm ascorbic acid, and 200 ppm corrosion inhibitor, to which magnetite was added to achieve an iron concentration of 100 ppm. This mixture was dissolved at 90°C, and carbon steel test pieces were then immersed in it. Furthermore, 8000 ppm of hydrogen peroxide, approximately 1.5 times the amount of formic acid decomposition equivalent, was added to create the decontamination agent decomposition solution.

[0023] 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)

[0024] First, we investigated the precipitate film of iron(II) oxalate dihydrate. The iron(II) oxalate film is formed by the reaction of iron(II) ions with oxalic acid, resulting in the formation of iron(II) oxalate dihydrate with low solubility as shown in the following reaction equation (3), which then precipitates. Fe 2+ +(COO)2 2- +2H2O = Fe(COO)2·2H2O ……(3)

[0025] A solution of formic acid, ascorbic acid, and a corrosion inhibitor was mixed with 500 ppm oxalic acid, and carbon steel test pieces were immersed for 2 hours to promote the formation of iron oxalate. As a result, iron(II) oxalate dihydrate was formed, but it only weakly deposited on the surface and peeled off with a stream of water. When the amount of corrosion was determined by adding hydrogen peroxide, the amount of corrosion was 90% of the standard, and the corrosion inhibition effect was about 10%.

[0026] Therefore, the inventors of the present invention examined the reasons why iron(II) oxalate dihydrate did not adhere to the base material. As a result, it was considered that the corrosion inhibitor adsorbed on the carbon steel surface and inhibited the adhesion of iron(II) oxalate dihydrate. Thus, it was decided to remove the corrosion inhibitor adsorbed on the surface with hydrogen peroxide and then provide a period of time for the formation of iron(II) oxalate dihydrate.

[0027] The addition of hydrogen peroxide was carried out in two steps. First, 1 / 10 of the equivalent amount of formic acid was added to remove the corrosion inhibitor from the surface of the carbon steel test piece. Then, iron(II) oxalate dihydrate was formed for 2 hours. After that, the remaining hydrogen peroxide was added, and the corrosion amount of the carbon steel test piece was measured.

[0028] As a result, a precipitation film of iron(II) oxalate dihydrate that does not peel off with water flow was formed on the carbon steel test piece. The corrosion amount of the carbon steel was suppressed to 30% of the reference, and the corrosion inhibition effect of the iron(II) oxalate dihydrate precipitation film during the addition of hydrogen peroxide was confirmed. These results are shown in FIG. 1.

[0029] In FIG. 1, "1" is corrosion inhibitor + H2O2 8000 ppm, "2" is corrosion inhibitor + oxalic acid + H2O2 8000 ppm, "3" is corrosion inhibitor + H2O2 800 ppm + oxalic acid 2 h + H2O2 7200 ppm, and "4" is corrosion inhibitor + formic acid + ascorbic acid adjusted to an ORP of 620 mV.

[0030] From the above results, it was found that when hydrogen peroxide acts on carbon steel, the corrosion inhibitor is affected. Therefore, the inventors of the present invention considered that it is necessary to reduce the amount of hydrogen peroxide supplied to the carbon steel surface in order to maintain the effect of the corrosion inhibitor. Thus, a method of suppressing contact by in-line detection of hydrogen peroxide in the decontamination agent decomposition solution was examined.

[0031] In the decontamination agent decomposition solution, in addition to the oxidation-reduction system of Fe by hydrogen peroxide shown in Reaction Formula (1), there is also an oxidation-reduction system of ascorbic acid as shown in the following Reaction Formula (4). 2+ and Fe 3+ In addition to the oxidation-reduction system of Fe by hydrogen peroxide shown in the following Reaction Formula (1), there is also an oxidation-reduction system of ascorbic acid as shown in Reaction Formula (4) below. 2Fe 2+ +C6H8O6→ 2Fe 3+ +C6H6O6+2H + ...(4)

[0032] Therefore, the oxidation-reduction potential of the decontamination agent decomposition solution is determined by the equilibrium between reaction equation (1) and reaction equation (4). However, it was expected that the oxidation-reduction potential would increase if hydrogen peroxide disappeared, or if the decomposition of the decontamination agent progressed and ascorbic acid disappeared, as one of the reactions would cease to exist.

[0033] Therefore, we investigated the relationship between the oxidation-reduction potential of the decontamination agent decomposition solution and the presence or absence of residual hydrogen peroxide. The method involved preparing a solution by dissolving 3500 ppm formic acid, 1500 ppm ascorbic acid, and 200 ppm corrosion inhibitor with magnetite added to achieve an iron concentration of 100 ppm at 90°C. Hydrogen peroxide was then added, and the oxidation-reduction potential was measured. The presence or absence of hydrogen peroxide at that time was determined using a test strip capable of detecting a hydrogen peroxide concentration of 1 ppm.

[0034] Figure 2 shows a device for evaluating the amount of corrosion of carbon steel using a decontamination agent decomposition solution. The device circulates the decontamination agent decomposition solution in a surge tank 101 through piping 102 using a pump 103, with an oxidation-reduction potential meter 104 placed on the outlet side of the pump 103. Under experimental conditions, hydrogen peroxide was added to the surge tank 101 at a rate of 100 ppm / min, and the oxidation-reduction potential at which hydrogen peroxide began to be detected by sampling was determined.

[0035] As a result, hydrogen peroxide began to be detected at 620mV vs SHE. Therefore, a corrosion test was conducted using the apparatus shown in Figure 2, which can measure oxidation-reduction potential, with a carbon steel specimen mounting section. A carbon steel specimen 106 was set in the specimen mounting section 105, and a decontamination agent decomposition solution was prepared in the same manner as in the oxidation-reduction potential measurement test, and hydrogen peroxide was injected. The test was terminated when the oxidation-reduction potential exceeded 620mV vs SHE, and the specimen was removed. The amount of corrosion was determined from the weight difference before and after the start of the test. As a result, as shown in Figure 1, the amount of corrosion of the carbon steel was suppressed to 20% of the standard.

[0036] Based on the above results, by pre-determining the oxidation-reduction potential of the hydrogen peroxide added to the decontamination agent decomposition solution, and then stopping the supply of hydrogen peroxide or switching the system to a closed loop when the oxidation-reduction potential reaches that value after the decomposition of the decontamination agent begins, the rust inhibitor (corrosion inhibitor) adsorbed on the carbon steel surface is not affected by hydrogen peroxide, thereby suppressing corrosion of the carbon steel base material.

[0037] Next, the inventors also investigated the iron concentration dependence of the oxidation-reduction potential in which hydrogen peroxide remains in the decontamination agent decomposition solution. In the method, when preparing the decontamination agent decomposition solution, the previously described method involved dissolving magnetite equivalent to 100 ppm of iron. However, this time, magnetite with iron concentrations of 50 ppm and 200 ppm was dissolved to prepare the solution. Using these decontamination agent decomposition solutions, the oxidation-reduction potential and the presence or absence of hydrogen peroxide in the sample solution were examined using test paper capable of detecting a hydrogen peroxide concentration of 1 ppm while adding hydrogen peroxide in the same manner as described above.

[0038] As a result, the oxidation-reduction potential at which hydrogen peroxide begins to persist showed a dependence on iron concentration: 545 mV vs SHE at an iron concentration of 50 ppm, and 650 mV vs SHE at an iron concentration of 200 ppm. Based on these findings, the iron concentration dependence is shown in Figure 3.

[0039] Based on the above results, the oxidation-reduction potential of the hydrogen peroxide added to the decontamination agent decomposition solution can be determined based on the relationship shown in Figure 3, using the iron concentration in the decontamination agent decomposition solution. By ensuring that the decontamination agent decomposition solution containing hydrogen peroxide does not come into contact with the carbon steel when this oxidation-reduction potential is reached, the corrosion inhibitor adsorbed on the carbon steel surface is not affected by hydrogen peroxide, thus enabling the decomposition treatment of a reducing decontamination agent that suppresses corrosion of the carbon steel base material.

[0040] <Example 1> An example 1 of the chemical decontamination method and chemical decontamination apparatus for carbon steel components of a nuclear power plant according to the present invention will be described with reference to Figures 4 to 7.

[0041] The chemical decontamination method for carbon steel components in a nuclear power plant described in this embodiment is an example of its application to a boiling water reactor (BWR) plant. First, the schematic configuration of a BWR plant to which the chemical decontamination method for carbon steel components in a nuclear power plant described in this embodiment is applied will be explained using Figure 4. Figure 4 is a diagram showing the configuration of the primary cooling water system of a BWR plant, which is one of the preferred targets of the present invention.

[0042] The BWR plant shown in Figure 4 includes a reactor 49, a turbine 56, a condenser 57, a recirculation system, a reactor purification system, and a feedwater system, among other things.

[0043] The reactor 49, installed within the reactor containment vessel 11, has a reactor pressure vessel 50 (hereinafter referred to as RPV50) containing the reactor core 51, and a jet pump 52 is installed within the RPV50. Multiple fuel assemblies (not shown) are loaded into the reactor core 51. Each nuclear fuel assembly contains multiple fuel rods filled with multiple fuel pellets made of nuclear fuel material. The recirculation system has a recirculation pump 53 and stainless steel recirculation system piping 54, with the recirculation pump 53 installed in the recirculation system piping 54.

[0044] The water supply 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 from the condenser 57 to the RPV 51 in a water supply pipe 58 connecting the condenser 57 and the RPV 50. A hydrogen injection device 66 is connected to the water supply pipe 58 between the condenser 57 and the condensate pump 59. A bypass pipe 65 that bypasses the condensate purification device 60 is also connected to the water supply pipe 58.

[0045] 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 a purification system pipe 67 that connects the recirculation system pipe 54 and the feedwater pipe 58. The purification system pipe 67 is connected to the recirculation system pipe 54 upstream of the recirculation pump 53.

[0046] The cooling water in the RPV50 is pressurized by the recirculation pump 53 and ejected through the recirculation system piping 54 into the bell mouth (not shown) of the jet pump 52 from its nozzle (not shown). The reactor water present around this nozzle is also drawn into the bell mouth by the action of the ejected flow from the nozzle.

[0047] The reactor water discharged from the jet pump 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. A portion of the heated reactor water turns into steam. This steam is led from the RPV 50 through the main steam pipe 55 to the turbine 56, which rotates the turbine 56. A generator (not shown) connected to the turbine 56 rotates, generating electricity. The steam discharged from the turbine 56 is condensed into water in the condenser 57.

[0048] This water is supplied to the RPV 50 as feedwater through the feedwater pipe 58. The feedwater flowing through the feedwater pipe 58 is pressurized by the condensate pump 59, impurities are removed by the condensate purification device 60, it is further pressurized by the feedwater pump 63, and heated by the low-pressure feedwater heater 61 and the high-pressure feedwater heater 62. Extracted steam extracted from the main steam pipe 55 and turbine 56 through the extraction pipe 74 is supplied to the low-pressure feedwater heater 61 and the high-pressure feedwater heater 62, respectively, and serves as a heat source for the feedwater.

[0049] The system connecting the recirculation system and RPV50 includes a residual heat removal (RHR) system that removes residual heat from the reactor core when the reactor is shut down. The RHR system consists of carbon steel RHR piping 82, a heat exchanger (not shown), and a pump 83.

[0050] As shown in Figure 4, one end of the RHR piping 82 is connected to the recirculation system piping 54 upstream of the recirculation pump 53. The other end of the RHR piping 82 is connected to the RPV 50. Thus, the RHR piping 82 is connected to a core spray sparger (not shown) having multiple core spray nozzles (not shown), which is located 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 core spray sparger are part of the components that make up the high-pressure spray system.

[0051] Although the RHR system is shown only to the right of the RPV50 in Figure 3, it is connected to the recirculation system piping 54 upstream of the recirculation pump 53 on the left side, and the other end of the RHR piping 82 is connected to a system that connects to the RPV50 (not shown). These two RHR systems have the same configuration.

[0052] The detailed configuration of the chemical decontamination apparatus 1 used in the chemical decontamination method for carbon steel members of a nuclear power plant in this embodiment will be explained with reference to Figure 5. Figure 5 is a detailed configuration diagram of the chemical decontamination apparatus used in the chemical decontamination method for carbon steel members of a nuclear power plant in Embodiment 1.

[0053] As shown in Figure 5, the chemical decontamination apparatus 1 is a device for chemically decontaminating carbon steel components such as the purification system piping 67 and RHR piping 82 of a nuclear power plant, and includes circulation piping 2A, 2B, a hydrogen peroxide injection device 7 which is an oxidizing agent that decomposes formic acid and ascorbic acid in the decontamination agent, a decontamination agent supply unit 6 which supplies the decontamination agent containing formic acid and ascorbic acid, a surge tank 17 with a heater 19 installed inside, a corrosion inhibitor addition device 12 which adds a corrosion inhibitor to the decomposition liquid of the decontamination agent, circulation pumps 20, 26, a filter 21, an ultraviolet irradiation device 25, a cation exchange resin tower 23, a mixed bed resin tower 24, and a hopper 5, etc.

[0054] The shut-off valve 27, circulation pump 20, and valves 28, 29, 30, and 31 are installed in the circulation pipe 2A in this order from the upstream purification system pipe 67, and the circulation pump 26, valve 32, and shut-off valve 33 are installed in the circulation pipe 2B in this order from the upstream side via the surge tank 17.

[0055] A valve 35 and a filter 21 are installed in pipe 34, which bypasses valve 28 and is connected to circulation pipe 2A.

[0056] A pipe 36 bypassing valve 29 is connected to circulation pipe 2A, and the cooler 22 and valve 37 are installed in pipe 36.

[0057] A cation exchange resin tower 23 and a valve 40 are installed in a pipe 38 that is connected to the circulation pipe 2A at both ends and bypasses the valve 30. A mixed-bed resin tower 24 and a valve 41 are installed in a pipe 39 that is connected to the pipe 38 at both ends and bypasses the cation exchange resin tower 23 and a valve 40. The cation exchange resin tower 23 has a resin layer filled with cation exchange resin inside. The mixed-bed resin tower 24 has a resin layer filled with cation exchange resin and anion exchange resin inside.

[0058] The piping 42, on which valve 43 and ultraviolet irradiation device 25 are installed, bypasses valve 31 and is connected to circulation piping 2A.

[0059] The hydrogen peroxide injection device 7 includes a chemical tank 8, an injection pump 9, and injection piping 44.

[0060] The chemical tank 8 is connected to piping 42 upstream of the ultraviolet irradiation device 25 by injection piping 44, which has an injection pump 9 and a valve 45. The chemical tank 8 is filled with an oxidizing agent (e.g., hydrogen peroxide or ozonated water).

[0061] The ultraviolet irradiation device 25 is primarily used for the decomposition of ascorbic acid. For the reductive decontamination agent used in chemical decontamination, organic acids that can decompose into water and carbon dioxide are used, taking into consideration the reduction of waste volume. Among these, formic acid, a monocarboxylic acid with few CH bonds that is less likely to form complexes with iron and create precipitates, is used. However, other monocarboxylic acids can also be used.

[0062] The surge tank 17 is installed between the valve 31 and the circulation pump 26, downstream of the circulation pipe 2A and upstream of the circulation pipe 2B.

[0063] 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. A hopper 5 is connected to the ejector 4. Formic acid, ascorbic acid, and a corrosion inhibitor are added to the hopper 5, and water is added as needed to open the valve 3, which draws the reagents in the hopper 5 into the water flow from the ejector 4 and supplies them to the surge tank 17.

[0064] The oxidation-reduction potential meter 18, which measures the oxidation-reduction potential in the decontamination agent decomposition solution, is installed between valve 32 and on-off valve 33 on the circulation pipe 2B that connects the surge tank 17 and the purification system pipe 67, which is downstream of the surge tank 17.

[0065] The corrosion inhibitor addition device 12 consists of a chemical tank 13 that holds a chemical solution containing the corrosion inhibitor, an injection pump 14 that delivers the chemical solution containing the corrosion inhibitor, an injection pipe 16, and a valve 15. The corrosion inhibitor is injected into the decontamination wastewater circulating from the circulation pipe 2B via the injection pipe 16.

[0066] The bypass pipe 47, equipped with an on-off valve 46, has both ends connected to the circulation pipe 2B located between the oxidation-reduction potential meter 18 and the on-off valve 33, and to the circulation pipe 2A located between the on-off valve 27 and the circulation pump 20, respectively, and is arranged in parallel with the purification system pipe 67.

[0067] The cooling water in the RPV50 undergoes radiolysis due to radiation generated by the nuclear fission of the nuclear fuel material contained in the fuel assemblies loaded into the core 51, producing oxidizing chemical species such as hydrogen peroxide and oxygen. These oxidizing chemical species increase the corrosion potential of the components of the nuclear power plant that come into contact with the cooling water. For this reason, some BWR plants inject hydrogen into the feedwater from a hydrogen injection unit 66 as an environmental mitigation measure against stress corrosion cracking. By reacting this hydrogen with the oxidizing chemical species such as hydrogen peroxide and oxygen contained in the cooling water using radiation, the concentration of oxidizing chemical species in the cooling water is reduced, thereby lowering the corrosion potential of the components of the nuclear power plant.

[0068] In a BWR plant, operation with hydrogen injected into the feedwater is called Hydrogen Water Chemistry (HWC) operation, while operation without hydrogen injection is called Normal Water Chemistry (NWC) operation. While it is desirable to continue hydrogen injection to reduce the corrosion potential of a BWR plant during operation, there are times when hydrogen injection may be interrupted. In such cases, the BWR plant operates in NWC mode, resulting in a high corrosion potential for the nuclear power plant components.

[0069] A portion of the furnace water flowing through the recirculation system piping 54 flows into the purification system piping 67 by the drive of the purification system pump 68, is cooled by the regenerative heat exchanger 69 and the non-regenerative heat exchanger 70, is purified by the furnace water purification device 71, is heated by the regenerative heat exchanger 69, and then supplied to the feedwater flowing through the feedwater piping 58 and returned to the RPV 50.

[0070] A BWR plant is shut down after completing one operating cycle. Following this shutdown, a periodic inspection is performed on the BWR plant. After this inspection is completed, the BWR plant is restarted. During this periodic inspection, some of the fuel assemblies in the core 51 are replaced with new fuel assemblies. That is, some of the fuel assemblies in the 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 core 51. Before the periodic inspection is performed after the BWR plant has been shut down, chemical decontamination of the piping and other components of the BWR plant may be carried out.

[0071] The chemical decontamination method for carbon steel components of the nuclear power plant in Example 1, which is carried out according to the procedure shown in Figure 6, will be described in detail below. Figure 6 is a flowchart showing the procedure carried out in the chemical decontamination method for carbon steel components of the nuclear power plant in Example 1.

[0072] For example, consider a case where, during a periodic inspection planned for the inspection and maintenance of the purification system pump 68, regenerative heat exchanger 69, and non-regenerative heat exchanger 70 installed in the purification system piping 67 of a reactor purification system, which is made of carbon steel, chemical decontamination is carried out on the purification system piping 67 in order to reduce the radiation exposure of inspection or maintenance workers. In this case, each of the steps S1 to S7 shown in Figure 6 is carried out on the purification system piping 67.

[0073] In BWR plants that have been in operation, an oxide film containing radionuclides is formed on the inner surfaces of the recirculation system piping 54 and the purification system piping 67 through which the cooling water in the RPV 50 flows, and this oxide film is removed by chemical decontamination. The chemical decontamination method in this embodiment is performed on carbon steel components of a BWR plant, and therefore, it is a process to remove the oxide film from the inner surface of carbon steel piping, such as the purification system piping 67.

[0074] For chemical decontamination of the purification system piping 67, the chemical decontamination apparatus 1 shown in Figure 5 is used.

[0075] First, the chemical decontamination device 1 is connected to the piping system of the shut-down nuclear power plant, which is the object to be chemically decontaminated (step S1).

[0076] The upstream end of the circulation pipe 2A and the downstream end of the circulation pipe 2B of the temporary chemical decontamination equipment 1 are connected to the carbon steel purification system pipe 67, which is the target of chemical decontamination. The procedure for connecting these circulation pipes 2A and 2B to the purification system pipe 67 will be described in detail.

[0077] After the BWR plant is shut down, for example, the bonnet of valve 72, which is installed in the purification system piping 67 connected to the recirculation system piping 54, is opened to seal the recirculation system piping 54 side. The end of the circulation piping 2A of the chemical decontamination device 1 is connected to the flange of valve 72. This connects the end of circulation piping 2A to the purification system piping 67 upstream of the purification system pump 68.

[0078] On the other hand, the bonnet of valve 73 installed in the purification piping 67 downstream of the purification pump 68 is opened to seal the side of the regenerative heat exchanger 69. The end of the circulation piping 2B of the chemical decontamination device 1 is connected to the flange of valve 73. As a result, one end of the circulation piping 2B is connected to the purification piping 67 downstream of the purification pump 68, forming a closed loop including the purification piping 67 and circulation pipings A and 2B.

[0079] After the chemical decontamination device 1 is connected to the purification system piping 67, before driving the circulation pumps 20 and 26 in step S2, water is filled into the circulation pipes 2A and 2B, the surge tank 17, and the purification system piping 67 between valves 72 and 73. To do this, first all the valves in Figure 5 are closed, then the surge tank 17 is filled with water, and the on-off valve 46 and valves 32, 28, 29, 30, and 31 are opened to circulate water through the circulation system of the chemical decontamination device 1. Subsequently, the on-off valves 27 and 33 are opened, and the on-off valve 46 is closed to fill the purification system piping 67 with water. During this time, the water supply to the surge tank 17 is continued.

[0080] Next, the temperature of the circulating water is adjusted (step S2). A heater 19 installed in the surge tank 17 heats the circulating water circulating in the circulation pipes 2A and 2B and the purification system pipes 67, adjusting the temperature of the circulating water to approximately 90°C.

[0081] After heating, reductive decontamination is performed on the inner surface of the purification system piping 67 using a reductive decontamination solution. The reductive decontamination in the chemical decontamination method of this embodiment will be described in detail below.

[0082] Formic acid, ascorbic acid, and a corrosion inhibitor are added to generate a reducing decontamination solution (Step S3). This Step S3 is the reducing decontamination process. As the corrosion inhibitor, for example, "30AR" of Ibit® manufactured by Asahi Chemical Industries, Ltd. can be used.

[0083] In step S3, formic acid, ascorbic acid, and a corrosion inhibitor are added to the hopper 5, and water is added as needed to open the valve 3. The water flow from the ejector 4 draws the reagents in the hopper 5 into the hopper 5 and supplies them to the surge tank 17.

[0084] The supplied reagents are delivered into the chemical decontamination system by the operation of circulation pumps 26 and 20, dissolving the oxide film formed on the inner surface of the purification system piping 67, and also dissolving radionuclides such as Co-60 that have been incorporated into the oxide film.

[0085] Here, while higher concentrations of formic acid and ascorbic acid are more effective in dissolving the oxide film, it is not practical to decontaminate at very high concentrations because it puts a strain on the subsequent decontamination agent decomposition process. For example, a concentration of 3500 ppm formic acid and about half that amount, 1750 ppm, for ascorbic acid should be used as a guideline. The concentration ranges for each should be, for example, between 1750 ppm and 7000 ppm for formic acid and between 875 ppm and 3500 ppm for ascorbic acid.

[0086] As the oxide film dissolves, the iron and Co-60 concentrations in the reducing decontamination solution increase. Therefore, the opening of valves 40 and 30 is adjusted to allow the chemical decontamination solution to flow through the 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 removal amount is supplied from hopper 5. The reducing decontamination aqueous solution from which metal cations have been removed in the cation exchange resin tower 23 is mixed with the reducing decontamination aqueous solution that has passed through valve 30 and is led into the surge tank 17.

[0087] The reducing decontamination aqueous solution circulates within the closed loop formed by the purification system piping 67 and the circulation piping 2A and 2B, while performing reducing decontamination of the inner surface of the purification system piping 67.

[0088] During the reduction decontamination process, a portion of the reduction decontamination solution is directed to the cation exchange resin tower 23, where metal cations contained in the reduction decontamination solution are removed.

[0089] A radiation detector 76, positioned on the outside of the purification system piping 67 which is the target of chemical decontamination, detects radiation emitted from the purification system piping 67 and outputs a radiation detection signal. Based on this radiation detection signal, the dose rate of the purification system piping 67 is determined. When the dose rate determined by the removal of radionuclides falls below the dose rate required for post-decontamination work, for example, if the dose rate required for post-decontamination exposure dose management is 0.1 mSv / h, then the reduction decontamination is terminated when the dose rate falls below this rate, or when the downward trend in the dose rate stops decreasing, for example, when the decrease in the dose rate per hour falls below 1% of the initial dose rate. Alternatively, the reduction decontamination is terminated after a predetermined time has elapsed since the start of reduction decontamination.

[0090] After the reduction decontamination is completed, the reduction decontamination agent is decomposed (step S4). Step S4 is the decomposition and purification process of the reduction decontamination agent. This process consists of multiple steps, which are shown in detail in Figure 7. Figure 7 is a flowchart showing the decomposition process of the reduction decontamination agent carried out in the chemical decontamination method for carbon steel components of a nuclear power plant in Example 1.

[0091] First, to determine the oxidation-reduction potential at which hydrogen peroxide begins to remain in the reducing decontamination solution, the iron concentration in the reducing decontamination solution is determined (Step S4-1).

[0092] Here, the oxidation-reduction potential (E) of the reductive decontamination wastewater is given by the iron(II) ion concentration [Fe 2+ ], iron(III) ion concentration [Fe 3+ It can be expressed by the Nernst equation shown in the following formula (5), which includes the concentration of ascorbic acid [C6H8O6] and its oxidized form, dehydroascorbic acid [C6H6O6]. E=E0+(RT / nF)·log([Fe 3+ ][C6H8O6] / {[Fe 2+ ][C6H6O6]}) ……(5)

[0093] Here, E0 is the standard electrode potential, R is the gas constant, T is the temperature of the reducing decontamination solution, n is the valence, and F is the Faraday constant.

[0094] As the decomposition of formic acid and ascorbic acid progresses, the ascorbic acid that reduces iron(III) ions in reaction equation (4) is depleted, so the iron(II) ions decrease and iron(III) ions become dominant, causing the oxidation-reduction potential to increase. At this point, the iron(II) ions that decompose hydrogen peroxide are depleted, and hydrogen peroxide begins to remain. The relationship between the oxidation-reduction potential at which hydrogen peroxide begins to remain and the iron concentration has been confirmed experimentally, as shown in Figure 3. Therefore, using the iron concentration obtained in step S4-1 and Figure 3, the oxidation-reduction potential at which hydrogen peroxide remains in the reducing decontamination solution to be decomposed is determined (step S4-2).

[0095] Next, hydrogen peroxide is injected into the decontamination liquid to be decomposed (step S4-3). A hydrogen peroxide injection device 7 is used to add hydrogen peroxide. The openings of valves 31 and 43 are adjusted to allow the decontamination waste liquid to flow through piping 42. At this time, the ultraviolet light of the ultraviolet irradiation device 25 is kept off. Valve 45 is opened to drive the injection pump 9 and the hydrogen peroxide from the chemical tank 8 is injected through the injection pipe 44 into the decontamination waste liquid flowing through piping 42.

[0096] The amount of hydrogen peroxide injected is less than the equivalent amount required for the decomposition of formic acid and ascorbic acid. For example, assuming 0.5 equivalents, the hydrogen peroxide concentration will be approximately 6000 ppm when the formic acid concentration is 3500 ppm and the ascorbic acid concentration is 1500 ppm.

[0097] The added hydrogen peroxide reacts with iron(II) ions contained in the decontamination wastewater according to equation (1), forming hydroxyl radicals and iron(III) ions. The hydroxyl radicals then decompose formic acid according to equation (2).

[0098] On the other hand, iron(III) ions are reduced to iron(II) ions by ascorbic acid contained in the decontamination wastewater according to reaction equation (4). If hydrogen peroxide remains, it reacts with the iron(II) ions formed in reaction equation (4), further decomposing the formic acid.

[0099] The decomposition of formic acid proceeds in the surge tank 17 through these reaction equations (1), (2), and (4). The hydroxyl radicals used in formic acid decomposition have low reaction selectivity and also decompose the corrosion inhibitor, so adding a corrosion inhibitor to compensate for this (step S4-4) is effective in suppressing corrosion of the downstream carbon steel piping.

[0100] However, the amount of hydrogen peroxide added is 0.5 times the equivalent amount needed to decompose the formic acid and ascorbic acid in the decontamination wastewater flowing through 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. The valve 15 of the corrosion inhibitor addition device 12 is opened, and the injection pump 14 is driven to inject the corrosion inhibitor in the chemical tank 13 into the decontamination wastewater circulating through the injection pipe 16 connected to circulation pipe 2B.

[0101] Since the reductive decontamination wastewater remains in the surge tank 17, the decomposition reactions of formic acid and ascorbic acid proceed during this time. The amount of hydrogen peroxide added is 0.5 times the equivalent amount required to decompose the formic acid and ascorbic acid in the reductive decontamination wastewater flowing through the circulation pipes 2A and 2B, so the hydrogen peroxide disappears first while it remains in the surge tank 17.

[0102] As a result, in the decontamination wastewater discharged from the surge tank 17, ascorbic acid remains, and iron ions are reduced to iron(II) ions according to reaction equation (4). Therefore, the value of the oxidation-reduction potential meter 18 shows a low potential with iron(II) ions being dominant. As the decomposition of formic acid and ascorbic acid progresses, the ascorbic acid that reduces iron(III) ions in reaction equation (4) disappears, so the amount of iron(II) ions decreases and iron(III) ions become dominant, causing the oxidation-reduction potential to increase.

[0103] Therefore, for example, if the iron concentration is 100 ppm, we determine from Figure 3 whether the oxidation-reduction potential exceeds 620 mV vs SHE (step S4-5). If it does not exceed this value, we continue adding hydrogen peroxide; if it does exceed this value, the hydrogen peroxide will start to remain and reach the purification system piping 67, so we stop injecting hydrogen peroxide (step S4-6). By operating in this manner, the reduction decontamination wastewater containing hydrogen peroxide and formic acid does not reach the purification system piping 67, thus suppressing carbon steel corrosion caused by surface contact between hydrogen peroxide, which has a high corrosive effect, and undecomposed formic acid.

[0104] Furthermore, if hydrogen peroxide remains in the decontamination solution undergoing decomposition in the surge tank 17 even after stopping the addition of hydrogen peroxide, one method is to open the shut-off valve 46, close the shut-off valves 33 and 27 to bypass the target of decontamination, and wait for the hydrogen peroxide to decompose by circulating it in the bypass system for a while.

[0105] Furthermore, if residual hydrogen peroxide does not decompose easily, an activated carbon filter can be introduced into filter 21, and water can be passed through it to decompose the residual hydrogen peroxide.

[0106] Next, in order to remove iron(III) ions contained in the reductive decontamination wastewater, ultraviolet light is irradiated onto the reductive decontamination wastewater, which now contains residual hydrogen peroxide, to reduce it to iron(II) ions, and then the wastewater is passed through the cation exchange resin tower 23 (step S4-7).

[0107] Specifically, the opening degrees of valves 31 and 43 are adjusted to allow the reductive decontamination wastewater to flow through the ultraviolet irradiation device 25. Using organic substances such as dehydroascorbic acid, which are decomposition products of ascorbic acid remaining in the reductive decontamination wastewater, as a reducing agent, ultraviolet light reduces iron(III) ions to iron(II) ions. As the wastewater passes through the cation exchange resin tower 23, the iron(II) ions are captured by the cation exchange resin and removed from the reductive decontamination wastewater.

[0108] Here, since the corrosion inhibitor is also removed when the cation exchange resin tower 23 is operated, a corrosion inhibitor is added to compensate for this (step S4-8). The concentration of the added agent is, for example, 200 ppm, the same as during reductive decontamination.

[0109] When the iron ion concentration falls below 5 ppm, the ascorbic acid derivative is decomposed by the next ultraviolet irradiation (Step S4-9).

[0110] Specifically, the ascorbic acid used in the reductive decontamination process is mostly converted to its oxidized form, dehydroascorbic acid, by the subsequent addition of hydrogen peroxide, and some of this is further decomposed by hydroxyl radicals formed in the Fenton reaction (1). To decompose and remove these ascorbic acid derivatives, ultraviolet light is irradiated while hydrogen peroxide is added to the reductive decontamination wastewater.

[0111] The opening degrees of valves 31 and 43 are adjusted to allow a portion of the decontamination waste liquid to flow into the ultraviolet irradiation device 25. Valve 45 is opened to drive the injection pump 9 and add hydrogen peroxide from the chemical tank 8 to the decontamination waste liquid before it flows into the ultraviolet irradiation device 25. The amount added is at a concentration equal to or greater than the equivalent amount required for the decomposition of ascorbic acid, for example, to a concentration of 1 equivalent. The decomposition of ascorbic acid and dehydroascorbic acid by hydrogen peroxide and ultraviolet light is carried out by the following reaction equations (6) and (7).

[0112]

number

[0113]

number

[0114] The decomposition of ascorbic acid derivatives can be monitored by regularly sampling the decontamination wastewater and measuring the TOC (Total Organic Carbon). Decomposition of ascorbic acid is terminated when the decrease in TOC concentration stabilizes or when the TOC falls below 10 ppm.

[0115] Next, to further purify any remaining impurities in the decontamination wastewater, a mixed-bed resin is used for purification (Step S4-10).

[0116] To cool the decontamination wastewater by reduction to a temperature matching the heat resistance temperature of the anion exchange resin contained in the mixed bed resin, the openings of valves 29 and 37 are adjusted, and the decontamination wastewater is passed through the cooler 22. The openings of valves 29 and 37 are adjusted so that the temperature at the confluence of the circulation pipe 2A and the pipe 36 containing the cooler 22 is below the heat resistance temperature of the anion exchange resin, for example, below 60°C. The openings of valves 30 and 41 are adjusted so that the decontamination wastewater, now below 60°C, is passed through the mixed bed resin tower 24. The cation and anion components contained in the decontamination wastewater are adsorbed and removed by the cation exchange resin and anion exchange resin contained in the mixed bed resin. This reduces the conductivity of the decontamination wastewater to 2 μS / cm.

[0117] Next, a determination is made as to whether the chemical decontamination is complete (step S5). A radiation detector 76, positioned outside the purification system piping 67 which is the target of chemical decontamination, detects the radiation emitted from the purification system piping 67 and outputs a radiation detection signal. Based on this radiation detection signal, the dose rate of the purification system piping 67 is determined. If this dose rate has not reached the target, the water temperature is heated to 90°C and the process is repeated from reduction decontamination in step S3. The repetition can be continued until the target dose rate is achieved, but the number of possible repetitions is limited by the amount of reagents and ion exchange resin prepared, so it is usually terminated after about 3 times.

[0118] Once the chemical decontamination is deemed complete, the water used in the chemical decontamination process is discharged (Step S6). The discharge destination is the wastewater treatment system of the nuclear power plant, and the water is discharged only after confirming that it meets wastewater standards such as conductivity and pH. After discharge, the temporarily installed chemical decontamination device 1 is removed from the purification system piping 67, the device is removed, and the chemical decontamination is completed (Step S7).

[0119] Next, the effects of this embodiment will be described.

[0120] The chemical decontamination method for carbon steel members of a nuclear power plant according to Example 1 of the present invention described above comprises a decontamination step of decontaminating the carbon steel members using a decontamination agent containing formic acid and ascorbic acid, and a decomposition step of decomposing the formic acid and ascorbic acid in the decontamination agent used in the decontamination step using a decomposition solution containing hydrogen peroxide. In the decomposition step, the residual state of hydrogen peroxide in the decomposition solution is determined based on the residual oxidation-reduction potential, which is determined based on the iron concentration, and the injection of hydrogen peroxide into the decomposition solution is stopped.

[0121] This strongly suppresses contact between the carbon steel piping base material and the formic acid solution containing highly corrosive hydrogen peroxide, thereby reducing the amount of corrosion of the base material due to decontamination of carbon steel components compared to conventional methods. Furthermore, in this embodiment, an organic corrosion inhibitor is added at each decomposition step of the reducing decontamination agent, and the effect of the corrosion inhibitor can be obtained in all steps except for the final purification by the mixed-bed resin tower in the decomposition step of the reducing decontamination agent, thus keeping the corrosion of the carbon steel base material due to chemical decontamination low. Consequently, the oxide film formed on the carbon steel during the operation of the nuclear power plant can be efficiently dissolved without being covered by a precipitated film, and the corrosion of the carbon steel base material during the decomposition of the decontamination agent can be suppressed compared to conventional methods. Moreover, the impact on the remaining lifespan due to corrosion of carbon steel piping is reduced, and the amount of iron leached is also reduced, thus reducing the amount of ion exchange resin waste.

[0122] Furthermore, in the decomposition process, hydrogen peroxide is injected into the decontamination agent decomposition solution so that its concentration is below the equivalent amount of hydrogen peroxide needed to decompose formic acid and ascorbic acid. This makes it possible to more reliably reduce the amount of corrosion of the base material due to the decontamination of carbon steel components.

[0123] Furthermore, the system is further equipped with a surge tank 17, and an oxidation-reduction potential meter 18 is installed downstream of the surge tank 17. By stopping the injection of hydrogen peroxide when the oxidation-reduction potential value measured by the oxidation-reduction potential meter 18 exceeds the residual oxidation-reduction potential, residual hydrogen peroxide can be detected before it reaches the carbon steel member, thereby suppressing the inflow of hydrogen peroxide into the carbon steel member and inhibiting its corrosion.

[0124] Furthermore, a bypass pipe 47 is provided in parallel with the purification system piping 67. After injecting the decontamination agent decomposition liquid, the bypass pipe 47 circulates it within a closed loop including the surge tank 17. This allows the flow path of the decontamination agent decomposition liquid containing hydrogen peroxide to be switched, further reducing the opportunities for the carbon steel components to be decontaminated to come into contact with the decomposition liquid, thereby further reducing corrosion.

[0125] Furthermore, by further providing a corrosion inhibitor addition device 12 that attaches a corrosion inhibitor to the decontamination agent decomposition solution, corrosion of the base material during the decontamination of carbon steel members can be further reduced.

[0126] Furthermore, the system includes a hydrogen peroxide injection device 7 located in the circulation pipe 2A connecting the purification system piping 67 and the surge tank 17, and an oxidation-reduction potential meter 18 installed in the circulation pipe 2B connecting the surge tank 17 and the purification system piping 67. This allows the residual state of hydrogen peroxide to be determined before it flows into the carbon steel member, thereby further reducing the possibility of the carbon steel member coming into contact with hydrogen peroxide.

[0127] <Example 2> The chemical decontamination method and chemical decontamination apparatus for carbon steel members of a nuclear power plant according to Embodiment 2 of the present invention will be explained with reference to Figure 8. Figure 8 is a detailed configuration diagram of the chemical decontamination apparatus used in the chemical decontamination method for carbon steel members of a nuclear power plant according to Embodiment 2. The chemical decontamination method for carbon steel members of a nuclear power plant according to this embodiment is also an example applied to a boiling water reactor (BWR) nuclear power plant.

[0128] As shown in Figure 8, the chemical decontamination apparatus 1A used in the chemical decontamination method of this embodiment has an additional configuration compared to the chemical decontamination apparatus 1 described above, which includes a control device 110 and a configuration in which the control device 110 is connected to an oxidation-reduction potential meter 18, injection pump 9, injection pump 14, on-off valve 33, on-off valve 46, and on-off valve 27 by an information control cable 111. The other configurations of the chemical decontamination apparatus 1A are the same as those of the chemical decontamination apparatus 1.

[0129] The control device 110 stores information on the residual oxidation-reduction potential of hydrogen peroxide, which is determined in advance based on the iron concentration as shown in Figure 3. Upon receiving the measurement results from the oxidation-reduction potential meter 18, it determines the residual state of hydrogen peroxide in the decontamination agent decomposition solution and controls the injection pump 9 of the hydrogen peroxide injection device 7 to stop the injection of hydrogen peroxide into the decontamination agent decomposition solution. It also receives the measurement results from the oxidation-reduction potential meter 18 and controls the opening and closing of the on-off valve 27 in the circulation pipe 2A, the on-off valve 33 in the circulation pipe 2B, and the on-off valve 46 in the bypass pipe 47 to bypass the purification system pipe 67.

[0130] The chemical decontamination method for carbon steel components of a nuclear power plant in this embodiment is the same as the chemical decontamination method for carbon steel components of a nuclear power plant in Embodiment 1, but the content of the reduction decontamination agent decomposition step (step S4) is received by the control device 110 from the oxidation-reduction potential meter 18, and based on this, the pump is started and stopped and the valves are opened and closed in the same way as in Embodiment 1, and the other procedures are the same as in Embodiment 1.

[0131] The chemical decontamination method for carbon steel components of a nuclear power plant in this embodiment is also carried out after the BWR plant has been shut down, for example, on carbon steel purification system piping 67, similar to the chemical decontamination method for carbon steel components of a nuclear power plant in Embodiment 1.

[0132] In this embodiment, in step S1 of Figure 6, the ends of the circulation pipes 2A and 2B of the chemical decontamination device 1A are connected to the purification system pipe 67, similar to the first embodiment. Subsequently, steps S2 and S3 (see Figure 6) are performed, similar to the first embodiment, to carry out reductive decontamination of the inner surface of the purification system pipe 67 using formic acid, ascorbic acid, and a corrosion inhibitor as reductive decontamination agents.

[0133] When the dose rate in the purification system piping 67 stops decreasing, or when the reduction decontamination time has elapsed for a predetermined period, the reduction decontamination (step S3) is terminated and the process moves to the decomposition of the reduction decontamination agent and the purification process (step S4).

[0134] In the decomposition process of the reducing decontamination agent, the iron concentration of the reducing decontamination solution is first measured, as shown in Figure 7. In Example 2, this value is input to the control device 110 (step S4-1). The control device 110 determines the oxidation-reduction potential of the residual hydrogen peroxide corresponding to the iron concentration (step S4-2). The control device 110 has also continuously input the concentrations of formic acid and ascorbic acid in the preceding reducing decontamination process, and the amount of hydrogen peroxide equivalent required for their decomposition has been calculated.

[0135] The control device 110 sends a signal to the injection pump 9 via the information control cable 111 to control the injection rate of the injection pump 9 so that the concentration of hydrogen peroxide becomes the required equivalent amount, for example, 0.5 equivalents, and the addition of hydrogen peroxide begins (step S4-3). At the same time, it sends a signal to the injection pump 14 to start injection, and the injection of the corrosion inhibitor begins (step S4-4).

[0136] The control device 110 constantly monitors the signal from the oxidation-reduction potential meter 18 and determines the residual hydrogen peroxide in the decontamination solution based on the oxidation-reduction potential at which hydrogen peroxide remains, as determined in step S4-2 (step S4-5).

[0137] A signal is sent from the control device 110 to the injection pump 9 to continue injecting hydrogen peroxide while the measured oxidation-reduction potential is below a reference level.

[0138] In response to this, if the oxidation-reduction potential rises above the standard, a stop signal is sent from the control device 110 to the injection pump 9 (step S4-6). Furthermore, while the oxidation-reduction potential remains above the standard, signals are sent to the on-off valves 33, 46, and 27 to operate the valves to bypass the decontamination target, so that hydrogen peroxide does not flow into the carbon steel piping.

[0139] This allows the hydrogen peroxide to decompose by circulating in the bypass system for a while. If the remaining hydrogen peroxide does not decompose easily, an activated carbon filter can be introduced into filter 21, and the water can be passed through it to decompose the remaining hydrogen peroxide. When the oxidation-reduction potential falls below the standard, a signal is sent to open valves 27 and 33 and close valve 46 to cancel the bypass operation.

[0140] From this point onward, the removal of iron ions by ultraviolet irradiation and cation exchange resin water flow (step S4-7), the removal of residual ion components in the decomposition wastewater of the reducing decontamination agent by water flow through the mixed-bed resin tower (step S4-10), and the determination of completion of decontamination as shown in Figure 6 (step S5) to the removal of the chemical decontamination equipment (step S7) are carried out in the same manner as in Example 1.

[0141] The other configurations and operations are substantially the same as those of the chemical decontamination method and chemical decontamination apparatus for carbon steel components of a nuclear power plant described in Example 1 above, and details are omitted.

[0142] In the chemical decontamination method and chemical decontamination apparatus for carbon steel members of a nuclear power plant according to Embodiment 2 of the present invention, substantially the same effects as those obtained in the chemical decontamination method and chemical decontamination apparatus for carbon steel members of a nuclear power plant according to Embodiment 1 described above can be obtained.

[0143] Furthermore, by providing a control device 110 that receives the measurement results from the oxidation-reduction potential meter 18, determines the residual state of hydrogen peroxide in the decontamination agent decomposition solution, and controls the decomposition solution supply unit to stop the injection of hydrogen peroxide into the decontamination agent decomposition solution, the decision to stop hydrogen peroxide injection based on the oxidation-reduction potential can be automatically made through the control device 110, thus shortening the operation time, thereby suppressing the inflow of hydrogen peroxide into the carbon steel piping, and enabling a reduction in the number of workers.

[0144] Furthermore, the control device 110 receives the measurement results from the oxidation-reduction potential meter 18 and controls the opening and closing of the on-off valves 27, 33, and 46 installed in the circulation pipe 2A, circulation pipe 2B, and bypass pipe 47 to bypass the purification system pipe 67. As a result, the bypass operation of the carbon steel pipes to be decontaminated based on oxidation-reduction potential can be automatically performed through the control device 110, thus shortening the operation time, thereby suppressing the inflow of hydrogen peroxide into the carbon steel pipes, and reducing the number of workers.

[0145] <Other> It should be noted that the present invention is not limited to the embodiments described above, and includes various modifications. The embodiments described above are explained in detail for the purpose of clearly illustrating the present invention, and are not necessarily limited to those having all the configurations described.

[0146] Furthermore, it is possible to replace parts of the configuration of one embodiment with parts of the configuration of another embodiment, and it is also possible to add parts of the configuration of another embodiment to the configuration of one embodiment. In addition, it is possible to add, delete, or replace parts of the configuration of each embodiment with parts of other configurations.

[0147] For example, the chemical decontamination apparatus and chemical decontamination method of Example 1 or Example 2 can also be applied to pressurized water reactor nuclear power plants.

[0148] Furthermore, the process is not limited to "decomposition step, determining the residual state of hydrogen peroxide in the decontamination agent decomposition solution based on the residual oxidation-reduction potential determined based on the iron concentration, and stopping the injection of hydrogen peroxide into the decontamination agent decomposition solution," but rather "determining the residual state of hydrogen peroxide in the decontamination agent decomposition solution based on the residual oxidation-reduction potential determined based on the iron concentration, and circulating the decontamination agent decomposition solution in a closed loop isolated from the carbon steel member," which can also achieve a similar effect. [Explanation of Symbols]

[0149] 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… valve 4…Ejector 5... Hoppa 6…Decontamination agent supply unit 7…Hydrogen peroxide injection device (decomposition liquid supply unit) 8,13…Chemical solution tank 9,14…Injection pump 11…Reactor containment vessel 12…Corrosion inhibitor addition device 16,44… Injection piping 17… Surge Tank 18,104... Oxidation-reduction potential meter 19...heater 20, 26… Circulation pump 21…Filter 22...Cooler 23…Cation exchange resin tower 24…Mixed bed resin tower 25...UV irradiation device 27, 33, 46… Shut-off valves 34, 36, 38, 39, 42, 75, 102… Piping 47…Bypass piping 49...Nuclear reactor 50…Reactor pressure vessel 51…Core 52... Jet pump 53…Recirculation pump 54…Recirculation system piping 55... Main steam piping 56... Turbine 57...Condenser 58...Water supply piping 59...Condensate pump 60...Condensate purification system 61... Low-pressure feedwater heater 62... High-pressure feedwater heater 63...Water pump 65... Bypass piping 66…Hydrogen injection device 67…Purification system piping (carbon steel components) 68... Purification pump 69…Regenerative heat exchanger 70…Non-regenerative heat exchanger 71… Furnace water purification system 74... Extraction piping 76…Radiation detector 82…RHR piping (carbon steel components) 83,103... pumps 101... Surge Tank 105... Test specimen installation section 106…Carbon steel test piece 110...Control device 111... Information control cable

Claims

1. A method for chemically decontaminating carbon steel components of a nuclear power plant, A decontamination step of decontaminating the carbon steel member using a decontamination agent containing formic acid and ascorbic acid, The process includes a decomposition step in which the formic acid and ascorbic acid in the decontamination agent used in the decontamination step are decomposed using a decontamination agent decomposition solution containing hydrogen peroxide, In the decomposition process, the residual state of hydrogen peroxide in the decontamination agent decomposition solution is determined based on the residual oxidation-reduction potential, which is determined based on the iron concentration, measured by an oxidation-reduction potential meter installed downstream of the surge tank. If the residual oxidation-reduction potential exceeds this value, the injection of hydrogen peroxide into the decontamination agent decomposition solution is stopped. Chemical decontamination methods.

2. In the chemical decontamination method described in claim 1, In the decomposition step, hydrogen peroxide is injected into the decontamination agent decomposition solution so that the concentration of hydrogen peroxide is less than or equal to the equivalent amount of hydrogen peroxide necessary to decompose the formic acid and the ascorbic acid. Chemical decontamination methods.

3. In the chemical decontamination method described in claim 1, In the decomposition step, the decontamination agent decomposition liquid is circulated within a closed loop including the surge tank. Chemical decontamination methods.

4. In the chemical decontamination method described in claim 1, In the aforementioned decomposition step, a corrosion inhibitor is added to the decontamination agent decomposition solution. Chemical decontamination methods.

5. A chemical decontamination device for carbon steel components of a nuclear power plant, A decontamination agent supply unit that supplies a decontamination agent containing formic acid and ascorbic acid, A decomposition liquid supply unit supplies a decomposition liquid containing hydrogen peroxide that decomposes the formic acid and ascorbic acid in the decomposition agent, Surge tank and, The surge tank is installed downstream of the aforementioned surge tank and includes an oxidation-reduction potential meter for measuring the oxidation-reduction potential in the decontamination agent decomposition solution, If the oxidation-reduction potential value measured by the oxidation-reduction potential meter exceeds the residual oxidation-reduction potential at which hydrogen peroxide remains in the decontamination agent decomposition solution, which is determined based on the iron concentration, the injection of hydrogen peroxide into the decontamination agent decomposition solution is stopped. Chemical decontamination equipment.

6. In the chemical decontamination apparatus described in claim 5, The system further comprises bypass piping arranged in parallel with the carbon steel member, After injecting the decontamination agent decomposition solution, the solution is circulated through the bypass piping within a closed loop including the surge tank. Chemical decontamination equipment.

7. In the chemical decontamination apparatus described in claim 5, The decontamination agent decomposition solution further comprises a corrosion inhibitor addition section for adding a corrosion inhibitor. Chemical decontamination equipment.

8. In the chemical decontamination apparatus described in claim 5, The system further comprises a hydrogen peroxide injection device arranged in the first piping connecting the carbon steel member and the surge tank, The oxidation-reduction potential meter is installed in the second piping that connects the surge tank and the carbon steel member. Chemical decontamination equipment.

9. In the chemical decontamination apparatus according to claim 6, The system further includes a control device that receives the measurement results from the oxidation-reduction potential meter, determines the residual state of hydrogen peroxide in the decontamination agent decomposition solution, and controls the decomposition solution supply unit to stop injecting the hydrogen peroxide into the decontamination agent decomposition solution. Chemical decontamination equipment.

10. In the chemical decontamination apparatus according to claim 9, A first pipe connecting the carbon steel member and the surge tank, The system further comprises a second pipe connecting the surge tank and the carbon steel member, The control device receives the measurement result from the oxidation-reduction potential meter and controls the opening and closing of the on-off valves provided in the first pipe, the second pipe, and the bypass pipe to bypass the carbon steel member. Chemical decontamination equipment.

Images