JP6059106B2 - Chemical decontamination method for carbon steel components in nuclear power plant - Google Patents

Description

  The present invention relates to a chemical decontamination method for a carbon steel member of a nuclear power plant, and more particularly to a chemical decontamination method for a carbon steel member of a nuclear power plant suitable for application to a carbon steel member of a boiling water nuclear power plant.

  For example, a boiling water nuclear power plant (hereinafter referred to as a BWR plant) has a nuclear reactor with a core built in a reactor pressure vessel (referred to as RPV). The reactor water supplied to the core by the recirculation pump (or internal pump) is heated by the heat generated by the fission of nuclear fuel material in the fuel assembly loaded in the core, and a part thereof becomes steam. This steam is led from the RPV to the turbine and rotates the turbine. The steam exhausted from the turbine is condensed in a condenser to become water. This water is supplied to the reactor as feed water. In the feed water, in order to suppress the generation of radioactive corrosion products in the RPV, metal impurities are mainly removed by a filtration demineralizer provided in the feed water pipe. Reactor water is cooling water present in the RPV.

  In addition, corrosion products that are the source of radioactive corrosion products are generated on the surface of the BWR plant components such as RPV and recirculation piping that come into contact with the reactor water. Fewer stainless steels and stainless steels such as nickel-base alloys are used. In addition, the low alloy steel RPV has a stainless steel overlay on the inner surface to prevent the low alloy steel from coming into direct contact with the reactor water. Furthermore, a part of the reactor water is purified by a filter demineralizer of the reactor purification system to positively remove metal impurities that are slightly present in the reactor water.

  However, even if the above-described corrosion countermeasures are taken, the presence of very few metal impurities in the reactor water is inevitable, so some metal impurities are converted into metal oxides to the fuel rods contained in the fuel assembly. Adhere to the surface. Impurities (for example, metal elements) adhering to the surface of the fuel rod cause a nuclear reaction by irradiation of neutrons emitted by nuclear fission of the nuclear fuel material in the fuel rod, and radioactive such as cobalt 60, cobalt 58, chromium 51, manganese 54, etc. Become a nuclide.

  These radionuclides remain mostly attached to the fuel rod surface in the form of oxides. However, some radionuclides are eluted as ions in the reactor water depending on the solubility of the incorporated oxide, or re-released into the reactor water as an insoluble solid called a clad. The radioactive material contained in the reactor water is removed by the reactor purification system communicated with the RPV. The radioactive material that has not been removed by the reactor purification system is accumulated on the surface of the nuclear plant component (for example, piping) that contacts the reactor water while circulating in the recirculation system together with the reactor water. As a result, radiation is emitted from the surface of the component member, which causes radiation exposure of workers during regular inspection work.

  The exposure dose of the worker is managed so that it does not exceed the prescribed value for each person. In recent years, this specified value has been reduced, and it has become necessary to reduce the exposure dose of each person as much as possible.

  Therefore, when it is expected that the exposure dose in the regular inspection work is high, chemical decontamination may be performed to dissolve and remove the radionuclide adhering to the pipe. For example, in Japanese Patent Application Laid-Open No. 2000-105295, reductive dissolution with a reductive decontamination solution containing oxalic acid and hydrazine, decomposition of a reductive decontamination agent (oxalic acid) used for this reductive dissolution, and oxidative dissolution with potassium permanganate Chemical decontamination methods for nuclear power plants that combine these are proposed. JP 2004-286471 A discloses chemical decontamination in which reductive decontamination is performed with a reductive decontamination solution containing formic acid and oxalic acid having a concentration ratio of, for example, about formic acid 0.9 and oxalic acid 0.1. A method has been proposed.

  In the chemical decontamination method for carbon steel members described in JP-A-2003-90897, an oxalic acid aqueous solution and a formic acid aqueous solution are used. It is also described that an aqueous solution containing formic acid and hydrogen peroxide (or ozone) is used instead of the formic acid aqueous solution. In this chemical decontamination method, the carbon steel member of the nuclear power plant is chemically decontaminated with an aqueous oxalic acid solution to remove the oxide film containing the radionuclide present on the surface of the carbon steel member. Decompose. Further, after oxalic acid decomposition, a formic acid aqueous solution or an aqueous solution containing formic acid and hydrogen peroxide (or ozone) is brought into contact with the carbon steel member from which the oxide film has been removed to remove iron oxalate remaining on the surface of the carbon steel member.

  JP-A-2002-333498 proposes a chemical decontamination method for decontaminating carbon steel using an aqueous solution containing an organic acid (for example, oxalic acid or formic acid) and hydrogen peroxide. When an aqueous solution containing formic acid and hydrogen peroxide is used, the elution rate of the oxide film increases compared to the case where an aqueous solution containing only an organic acid without hydrogen peroxide is used, and the radionuclide is removed from the carbon steel surface. It can be removed in a short time.

  JP 2009-109427 A discloses a first step of reducing decontamination of a carbon steel member with an oxalic acid aqueous solution, and a reduction decontamination of the carbon steel member with an oxalic acid aqueous solution containing formic acid after the first step. The chemical decontamination method including the 2nd process of performing is proposed. This chemical decontamination method is performed on piping and the like in a nuclear power plant. Japanese Patent Application Laid-Open No. 2009-109427 also describes that formic acid is decomposed after the end of the second step, and oxalic acid is decomposed after the decomposition of formic acid.

JP 2000-105295 A JP 2004-286471 A JP 2003-90897 A JP 2002-333498 A JP 2009-109427 A

  In the chemical decontamination disclosed in Japanese Patent Application Laid-Open Nos. 2000-105295 and 2004-286471, a structural member made of stainless steel is a decontamination object, and this structural member is subjected to reductive decontamination using a reductive decontamination solution. In this case, the iron concentration in the reductive decontamination solution does not rise so high that iron (II) oxalate is deposited.

However, when the decontamination target is a carbon steel member, the iron concentration in the reductive decontamination solution is increased by decontamination, and the base material of the carbon steel member and the dissolution of magnetite that is an oxide film on the surface of the carbon steel member are dissolved. There is a possibility that Fe ²⁺ ions supplied by the above-mentioned form a complex with oxalic acid contained in the reductive decontamination solution and precipitate as iron (II) oxalate. Since this iron (II) oxalate has low solubility, it precipitates on the surface of the carbon steel member, which is the main source of Fe ²⁺ ions, and when this precipitates on the oxide film, dissolution of the oxide film is inhibited, Since dissolution of the contained radionuclide is also suppressed, there is a problem that the efficiency of chemical decontamination is lowered.

  In JP 2003-90897 A, the surface of a carbon steel member is reductively decontaminated with an aqueous oxalic acid solution. By this reductive decontamination, iron oxalate is generated, and this iron oxalate may be deposited on the undissolved oxide film present on the surface of the carbon steel member. In this case, the time required for reductive decontamination of the oxide film becomes long. The iron oxalate deposited on the surface of the carbon steel member is removed using a formic acid aqueous solution. However, since the amount of iron oxalate deposited on the surface of the carbon steel member during reductive decontamination increases, it takes a long time to remove the iron oxalate with the formic acid aqueous solution.

  The chemical decontamination method described in JP-A-2002-333498 improves the dissolving power of the oxide film by adding hydrogen peroxide to an organic acid. However, since hydrogen peroxide degrades the cation exchange resin, it is not possible to supply a reductive decontamination solution containing hydrogen peroxide to the cation exchange resin tower during reductive decontamination. For this reason, it becomes impossible to remove radioactive dissolved components and non-radioactive dissolved components that are generated by reductive decontamination and are contained in the reductive decontamination solution. Before these dissolved components are removed by the cation exchange resin tower, it is necessary to decompose hydrogen peroxide contained in the reductive decontamination solution. As a result, the time required for chemical decontamination becomes longer. In addition, decomposition of hydrogen peroxide requires addition of a decomposition reagent such as iron (II) ion or catalase to the reduction decontamination solution, which is disadvantageous for purification of the waste solution after completion of the reduction decontamination. When a reductive decontamination solution containing hydrogen peroxide is used, the radioactive concentration and metal concentration of the reductive decontamination solution cannot be reduced during reductive decontamination. Efficiency will be reduced.

  In JP 2009-109427 A, since reduction decontamination of a carbon steel member is performed using an oxalic acid aqueous solution in the first step, iron oxalate is generated, and this iron oxalate is deposited on the surface of the carbon steel member. . For this reason, melt | dissolution of the oxide film on the surface of a carbon steel member is inhibited, and the efficiency of chemical decontamination falls. In the second step, reductive decontamination is performed with an oxalic acid aqueous solution containing formic acid, so iron oxalate is dissolved by formic acid, but iron oxalate is newly generated because oxalic acid is present. Furthermore, after completion of the second step, formic acid is first decomposed, and then oxalic acid is decomposed, so that iron oxalate is generated even after completion of the decomposition of formic acid.

  The objective of this invention is providing the chemical decontamination method of the carbon steel member of the nuclear power plant which can suppress precipitation of iron (II) oxalate and can suppress deterioration of a cation exchange resin.

A feature of the present invention that achieves the above-described object is that oxygen gas is supplied to the oxalic acid aqueous solution to increase the dissolved oxygen concentration in the oxalic acid aqueous solution until it is saturated. The object of the present invention is to reduce the decontamination of the carbon steel member, which is a structural member of the plant, and bring the aqueous oxalic acid solution subjected to the reduction decontamination of the carbon steel member into contact with the cation exchange resin.

Oxygen gas is supplied to the oxalic acid aqueous solution to increase the dissolved oxygen concentration in the oxalic acid aqueous solution until it is saturated, and the oxalic acid aqueous solution in which the dissolved oxygen concentration is saturated is used to reduce and remove carbon steel members that are structural members of nuclear power plants. As the dyeing is carried out, the amount of iron (II) oxalate produced is reduced, and the iron (II) oxalate formed over the surface of the oxide film containing radionuclides formed on the surface of the carbon steel member. The film thickness decreases. For this reason, melt | dissolution by the oxalic acid aqueous solution of the oxide film which covers the surface of a carbon steel member is accelerated | stimulated, and the time required for reductive decontamination of a carbon steel member can be shortened. Further, oxygen dissolved in the oxalic acid aqueous solution does not deteriorate the cation exchange resin that removes the metal cation contained in the oxalic acid aqueous solution.

  According to the present invention, precipitation of iron (II) oxalate can be suppressed and deterioration of the cation exchange resin can be suppressed. For this reason, the time required for the chemical decontamination of the carbon steel member can be shortened.

It is a flowchart which shows the procedure implemented in the chemical decontamination method of the carbon steel member of the nuclear power plant of Example 1 which is one suitable Example of this invention. It is a block diagram of the boiling water nuclear power plant to which the chemical decontamination apparatus used for the chemical decontamination method of the carbon steel member of the nuclear power plant of Example 1 was connected. It is a detailed block diagram of the chemical decontamination apparatus shown in FIG. It is a characteristic view which shows the weight change of the test piece of the carbon steel immersed in each of the oxalic acid aqueous solution which bubbled oxygen gas, and the oxalic acid aqueous solution which does not bubble oxygen gas. In the appearance photograph of the test piece of each carbon steel after the dissolution test immersed in each of the oxalic acid aqueous solution bubbling oxygen gas and the oxalic acid aqueous solution not bubbling oxygen gas, and a schematic cross-sectional view of each carbon steel test piece is there. It is a flowchart which shows the procedure implemented in the chemical decontamination method of the carbon steel member of the nuclear power plant of Example 2 which is another suitable Example of this invention. It is a detailed block diagram of the chemical decontamination apparatus used for the chemical decontamination method of the carbon steel member of the nuclear power plant of Example 2. FIG. It is a flowchart which shows the procedure implemented in the chemical decontamination method of the carbon steel member of the nuclear power plant of Example 3 which is another suitable Example of this invention. It is explanatory drawing which shows the connection permanent connection to the boiling water type | mold nuclear power plan of a chemical decontamination apparatus in the chemical decontamination method of the carbon steel member of the nuclear power plant of Example 3. FIG. It is a detailed system block diagram of the chemical decontamination apparatus used for the chemical decontamination method of the carbon steel member of the nuclear power plant of Example 4 which is another suitable Example of this invention.

The inventors have reduced the decontamination factor (DF) due to the precipitation of iron (II) oxalate in chemical decontamination of carbon steel using an aqueous solution containing oxalic acid (reduction decontamination solution). We studied how to prevent it. Oxalic acid, which is a dicarboxylic acid, has the property of easily forming a complex, and forms a complex with iron (II) ions (Fe ²⁺ ) generated by oxidizing iron of carbon steel by acid protons. At that time, (COO) ₂ ²⁻ and Fe ²⁺ in which two protons are dissociated from oxalic acid are just neutralized, so that precipitation occurs at a low concentration. In order to prevent this, it was considered effective to oxidize Fe ²⁺ to iron (III) ions (Fe ³⁺ ).

Fe ³⁺ also forms a complex with oxalic acid, but since the charge is not neutralized, the precipitation concentration of the complex increases. Therefore, the addition of hydrogen peroxide was considered as a method for oxidizing Fe ²⁺ . Fe ²⁺ undergoes a Fenton reaction as shown in Formula (1) with hydrogen peroxide and changes to Fe ³⁺ .

Fe ²⁺ + H ₂ O ₂ → Fe ³⁺ + OH ⁻ + OH ^* (1)
The hydroxyl radical OH ^* generated by this reaction reacts with the surrounding oxalic acid as in the formula (2), or reacts with Fe ²⁺ as in the formula (3) and consumed.

(COOH) ₂ + 2OH ^* → CO ₂ + 2H ₂ O (2)
Fe ²⁺ + OH ^* → Fe ³⁺ + OH ⁻ (3)
Fe ³⁺ forms a complex with oxalic acid as in formula (4), formula (5), and formula (6), and is held in the aqueous solution as a chelate ion.

Fe ³⁺ + (COO) ₂ ^2- = Fe (COO) ₂ ⁺ (4)
Fe (COO) ₂ ⁺ + (COO) ₂ ²⁻ = Fe [(COO) ₂ ] ₂ ⁻ (5)
Fe [(COO) ₂ ] ₂ ⁻ + (COO) ₂ ²⁻ = Fe [(COO) ₂ ] ₃ ³⁻ (6)
However, when hydrogen peroxide is added to the reductive decontamination solution, the cation exchange resin in the cation exchange resin tower that has been passed through in order to capture radioactive Co ions that have been dissolved by dissolution of the oxide film is converted into hydrogen peroxide. Oxidized and deteriorated, the resin component as a decomposition product is eluted in the reducing decontamination solution. Since the eluted resin component is a polymer ion component and is difficult to remove with an ion exchange resin, when hydrogen peroxide is added to the reduction decontamination solution, the reduction decontamination solution is converted to a cation. It becomes impossible to supply to the exchange resin tower. For this reason, the radioactive Co concentration in the reductive decontamination solution is increased, the atmospheric dose during the decontamination work is increased, and the iron concentration is also increased, so that the dissolution efficiency of the oxide film is lowered.

Therefore, the inventors considered using oxygen gas as an oxidizing agent that oxidizes Fe ²⁺ to Fe ³⁺ and at the same time does not deteriorate the cation exchange resin even when water is passed through the cation exchange resin tower. The cation exchange resin hardly degrades and elutes even when it comes into contact with a reductive decontamination solution containing oxygen gas saturated in the atmosphere, and Fe ²⁺ is oxygen gas by the reaction shown in formula (7). ^Can also be oxidized to Fe ³⁺ .

2Fe ²⁺ + O ₂ + 2H ₂ O = 2Fe ³⁺ + 4OH ⁻ (7)
The inventors immersed carbon steel test pieces in an oxalic acid aqueous solution (reduction decontamination solution) in which oxygen gas was bubbled and an oxalic acid aqueous solution in which oxygen gas was not bubbled, and performed a dissolution test on these test pieces. . About the test piece of each carbon steel, each weight before a dissolution test and after a dissolution test was measured, and the change of the weight before a dissolution test and after a dissolution test was calculated | required. The change in the weight of each test piece is shown in FIG. The weight of the carbon steel specimen immersed in the oxalic acid aqueous solution in which oxygen gas was bubbled was less than that of the carbon steel specimen immersed in the oxalic acid aqueous solution in which oxygen gas was not bubbled.

  When the appearance of a carbon steel test piece immersed in an oxalic acid aqueous solution in which oxygen gas was not bubbled and a carbon steel test piece immersed in an oxalic acid aqueous solution in which oxygen gas was bubbled was observed, the former test piece was shown in the lower part of FIG. As shown in the figure, the sample was white, and the latter test piece was black as shown in the upper part of FIG. In the test piece of carbon steel immersed in an oxalic acid aqueous solution with little dissolved oxygen without bubbling oxygen gas, a thick iron (II) oxalate film is formed on the surface of the carbon steel as shown in FIG. Carbon steel is hardly dissolved by oxalic acid. As a result, as shown in FIG. 4, the weight reduction due to melting of the carbon steel stagnate. On the other hand, in the test piece of carbon steel immersed in an oxalic acid aqueous solution in which dissolved oxygen is increased by bubbling oxygen, the iron (II) oxalate film formed on the surface of the carbon steel is thin as shown in FIG. Become. For this reason, as shown in FIG. 4, the weight reduction due to the melting of the carbon steel becomes large. This indicates that the iron (II) oxalate film formed on the surface of the carbon steel by the oxalic acid aqueous solution in which dissolved oxygen is increased by bubbling oxygen gas does not hinder the corrosion elution of the carbon steel by oxalic acid. ing.

A case is assumed in which the new knowledge described above found by the inventors is applied to chemical decontamination of carbon steel members of a nuclear power plant. When an oxalic acid aqueous solution with little dissolved oxygen is used as a reductive decontamination solution, the Fe dissolved in the oxalic acid aqueous solution from the oxide film covering the surface of the carbon steel member and the base material of the carbon steel member Iron (II) oxalate by ²⁺ and oxalic acid is deposited thickly on the oxide film on the surface of the carbon steel member. For this reason, dissolution of the oxide film by the oxalic acid aqueous solution is inhibited. On the other hand, when an oxalic acid aqueous solution in which oxygen gas is bubbled and a large amount of dissolved oxygen is used as a reductive decontamination solution, the thickness deposited on the oxide film of Fe ²⁺ and iron (II) oxalate is reduced. getting thin. For this reason, dissolution of the oxide film by the oxalic acid aqueous solution is not hindered.

In addition, oxygen dissolved in the oxalic acid aqueous solution by supplying oxygen gas does not deteriorate the cation exchange resin. As a result, while carrying out reductive decontamination of the inner surface of a carbon steel member of a nuclear power plant, for example, a carbon steel piping system connected to a reactor pressure vessel, using an oxalic acid aqueous solution with an increased dissolved oxygen concentration, An aqueous oxalic acid solution is supplied to the cation exchange resin tower, and metal cations (eg, Co-60 ions, Fe ^2+, Fe ^3+, etc.) contained in the oxalic acid aqueous solution are exchanged into the cation in the cation exchange resin tower. It can be removed with an exchange resin. Since the metal cation contained in the oxalic acid aqueous solution can be removed by the cation exchange resin tower, the time required for reductive decontamination can be shortened. As a result, the time required for chemical decontamination can be further shortened.

  By carrying out reductive decontamination of carbon steel components in nuclear power plants using an oxalic acid aqueous solution with an increased dissolved oxygen concentration by supplying oxygen gas, it is possible to suppress the precipitation of iron (II) oxalate. It is possible to suppress the deterioration of the cation exchange resin that removes the metal cation generated by the dyeing. As a result, the time required for reductive decontamination can be further shortened.

  Examples of the present invention reflecting the above examination results will be described below.

  A chemical decontamination method for a carbon steel member of a nuclear power plant according to Example 1, which is a preferred example of the present invention, will be described with reference to FIGS. The chemical decontamination method for the carbon steel member of the nuclear power plant of this embodiment is an example applied to a boiling water nuclear power plant (BWR plant).

  A schematic configuration of a BWR plant to which the method for chemical decontamination of carbon steel members of a nuclear power plant according to the present embodiment is applied will be described with reference to FIG. The BWR plant includes a nuclear reactor 50, a turbine 58, a condenser 59, a recirculation system, a nuclear reactor purification system, a water supply system, and the like. The nuclear reactor 50 installed in the nuclear reactor containment vessel 56 has a nuclear reactor pressure vessel (hereinafter referred to as RPV) 51 containing a core 52, and a jet pump 53 is installed in the RPV 51. A plurality of fuel assemblies (not shown) are loaded in the core 52. Each fuel assembly includes a plurality of fuel rods filled with a plurality of fuel pellets made of nuclear fuel material. The recirculation system has a recirculation pump 54 and a stainless steel recirculation system pipe 55, and the recirculation pump 54 is installed in the recirculation system pipe 55.

  The feed water system includes a condensate pump 61, a condensate purification device 62, a low pressure feed water heater 63, a feed water pump 64, and a high pressure feed water heater 65 from the condenser 59 to a feed water pipe 60 that connects the condenser 59 and the RPV 51. It is installed and configured in this order toward the RPV 51. A hydrogen injection device 68 is connected to the water supply pipe 60 between the condenser 59 and the condensate pump 61. A bypass pipe 66 that bypasses the condensate purification device 62 is connected to the water supply pipe 60.

  In the reactor water purification system, a purification system pump 70, a regenerative heat exchanger 71, a non-regenerative heat exchanger 72, and a reactor water purification device 73 are installed in a purification system pipe 69 that connects the recirculation system pipe 55 and the feed water pipe 60. Configured. The purification system pipe 69 is connected to the recirculation system pipe 55 upstream from the recirculation pump 54.

  The cooling water in the RPV 51 is boosted by the recirculation pump 54, and jetted from the nozzle (not shown) of the jet pump 53 into the bell mouth (not shown) of the jet pump 53 through the recirculation system pipe 55. . The reactor water existing around the nozzle is also sucked into the bell mouth by the action of the jet flow jetted from the nozzle. Reactor water discharged from the jet pump 53 is supplied to the core 52 and heated by heat generated by fission of nuclear fuel material in the fuel rods. Part of the heated reactor water becomes steam. This steam is guided from the RPV 51 through the main steam pipe 57 to the turbine 58 to rotate the turbine 58. A generator (not shown) connected to the turbine 58 is rotated to generate electric power. The steam discharged from the turbine 58 is condensed by the condenser 59 to become water.

  This water is supplied into the RPV 51 through the water supply pipe 60 as water supply. The feed water flowing through the feed water pipe 60 is boosted by a condensate pump 61, impurities are removed by a condensate purification device 62, further boosted by a feed water pump 64, and heated by a low pressure feed water heater 63 and a high pressure feed water heater 65. . Extracted steam extracted from the main steam pipe 57 and the turbine 58 by the extracted pipe 67 is supplied to the low-pressure feed water heater 63 and the high-pressure feed water heater 65, respectively, and serves as a heating source for the feed water.

The detailed structure of the chemical decontamination apparatus 1 used for the chemical decontamination method of the carbon steel member of the nuclear power plant of a present Example is demonstrated using FIG. The chemical decontamination apparatus 1 includes a circulation pipe 2, a pH adjuster injection device 3, a hydrogen peroxide injection device (oxidizer injection device) 8, a surge tank 13 having a heating device 14 installed therein, an oxygen gas supply device 16, a circulation Pumps 20 and 26, a filter 21, a decomposition device 25, a cation exchange resin tower 23, a mixed bed resin tower 24, and a hopper 28 are provided.

  The on-off valve 29, the circulation pump 20, the valve 30, the valves 31, 32, and 33, the surge tank 13, the circulation pump 26, the valve 34, and the on-off valve 35 are provided in the circulation pipe 2 in this order from the upstream. A valve 36 and a filter 21 are installed in a pipe 37 that bypasses the valve 30 and is connected to the circulation pipe 2. A pipe 39 that bypasses the valve 31 is connected to the circulation pipe 2, and the cooler 22 and the valve 38 are installed in the pipe 39. A cation exchange resin tower 23 and a valve 40 are installed in a pipe 41 having both ends connected to the circulation pipe 2 and bypassing the valve 32. The mixed bed resin tower 24 and the valve 42 are installed in a pipe 43 that is connected to the pipe 41 at both ends and bypasses the cation exchange resin tower 23 and the valve 40. The cation exchange resin tower 23 has a resin layer filled therein with a cation exchange resin. The mixed bed resin tower 24 has a resin layer filled therein with a cation exchange resin and an anion exchange resin.

The oxygen gas supply device 16 includes an oxygen gas cylinder 17, a valve 18, and an oxygen gas supply pipe 19. An oxygen gas supply pipe 19 connected to a bubbler 15 installed in the surge tank 13 is connected to the oxygen gas cylinder 17. A valve 18 is provided in the oxygen gas supply pipe 19. A bubbler 15 is disposed above the heating device 14.

  A pipe 45 in which the valve 44 and the decomposition device 25 are installed bypasses the valve 33 and is connected to the circulation pipe 2. The decomposition apparatus 25 is filled with, for example, an activated carbon catalyst in which ruthenium is impregnated on the surface of the activated carbon. A surge tank 13 is installed in the circulation pipe 2 between the valve 33 and the circulation pump 26.

The pH adjuster injection device 3 includes a chemical liquid tank 4, an injection pump 5, and an injection pipe 7. The chemical tank 4 is connected to the circulation pipe 2 by an injection pipe 7 having an injection pump 5 and a valve 6. The chemical tank 4 is filled with hydrazine which is a pH adjusting agent. A pH meter 50 A is installed in the circulation pipe 2 between the connection point between the injection pipe 7 and the circulation pipe 2 and the on-off valve 35.

The hydrogen peroxide injection device 8 includes a chemical liquid tank 9, an injection pump 10, and an injection pipe 12. Chemical liquid tank 9 is connected to the pipe 4 5 upstream of the decomposition device 25 by the injection pipe 12 having an infusion pump 10 and a valve 11. The chemical tank 9 is filled with an oxidizing agent (for example, hydrogen peroxide or ozone).

  A pipe 47 is connected to the upper end of the surge tank 13 and further connected to the circulation pipe 2 between the circulation pump 26 and the valve 34. A valve 46 and an ejector 27 are installed in the pipe 47. A hopper 28 is connected to the ejector 27.

Both ends of the pipe 49 provided with the valve 48 are respectively connected to the circulation pipe 2 existing between the pH meter 50 A and the on-off valve 35 and the circulation pipe 2 existing between the on-off valve 29 and the circulation pump 20. . The decomposition apparatus 25 can decompose oxalic acid and hydrazine as a pH adjusting agent. That is, as a chemical used for chemical decontamination, hydrazine that can be released as an organic acid that can be decomposed into water and carbon dioxide or a harmless gas and does not increase waste is used in consideration of reduction of the amount of waste.

  The cooling water in the RPV 51 undergoes radiation decomposition upon irradiation with radiation generated along with nuclear fission of the nuclear fuel material contained in the fuel assembly loaded in the reactor core 52, and oxidizing chemical species such as hydrogen peroxide and oxygen. Is produced. This oxidizing species increases the corrosion potential of the components of the nuclear power plant that come into contact with the cooling water. For this reason, in the BWR plant, hydrogen is injected from the hydrogen injection device 68 into the feed water as an environmental mitigation measure against stress corrosion cracking, and this hydrogen reacts with oxidizing chemical species such as hydrogen peroxide and oxygen contained in the cooling water. Accordingly, an operation is performed in which the oxidizing chemical species concentration of the cooling water is reduced to lower the corrosion potential of the components of the nuclear power plant.

  In the BWR plant, the operation performed while injecting hydrogen into the feed water is referred to as hydrogen injection water quality (HWC) operation, and the operation in which no hydrogen injection is performed in the BWR plant is referred to as normal water quality (NWC: Normal Water Chemistry). Say driving. The operation of the BWR plant that lowers the corrosion potential by hydrogen injection is preferably continued during the operation. However, the hydrogen injection may be interrupted, and the operation of the BWR plant when the hydrogen injection is interrupted is the NWC operation. Thus, the corrosion potential of the nuclear plant components becomes high.

A part of the reactor water flowing in the recirculation system pipe 55 flows into the purification system pipe 69 by the drive of the purification system pump 70, and is cooled by the regenerative heat exchanger 71 and the non-regenerative heat exchanger 72, thereby purifying the reactor water. It is purified by the reactor water purification device 73 and then heated by the regenerative heat exchanger 71 and returned to the RPV 51 through the purification system pipe 69 and the water supply pipe 60.

  The BWR plant is stopped after the operation in one operation cycle is completed. After this shutdown, a periodic inspection is performed on the BWR plant. After this periodic inspection is completed, the BWR plant is started again. During this periodical inspection, some fuel assemblies in the core 52 are replaced with new fuel assemblies. That is, a part of the fuel assembly in the core 52 is taken out from the RPV 51 as a spent fuel assembly, and a new fuel assembly having a burnup of 0 GWd / t is loaded into the core 52. Before the periodic inspection is performed after the operation of the BWR plant is stopped, chemical decontamination is performed on the piping of the BWR plant.

  The chemical decontamination method of the carbon steel member of the nuclear power plant of Example 1 implemented by the procedure shown in FIG. 1 will be specifically described below.

  For example, inspection and maintenance work of the purification system pump 70, the regenerative heat exchanger 71, the non-regenerative heat exchanger 72, etc. provided in the purification system piping 69 of the reactor purification system, which is a pipe made of carbon steel, is planned. Consider a case where chemical decontamination is performed on the purification system pipe 69 in order to reduce the radiation exposure of an inspection worker or maintenance worker in a periodic inspection. At this time, the steps S1 to S8 shown in FIG.

  In the BWR plant that has experienced operation, an oxide film containing radionuclides is formed on the inner surfaces of the recirculation system pipe 55 and the purification system pipe 69 through which the cooling water in the RPV 51 flows, and this oxide film is formed by chemical decontamination. Removed. The chemical decontamination method of the present embodiment is performed on the carbon steel member of the BWR plant, and for this reason, it is a process of removing the oxide film from the inner surface of, for example, the purification system pipe 69 which is a pipe made of carbon steel. . In the chemical decontamination of the purification system pipe 69, the chemical decontamination apparatus 1 shown in FIG. 3 is used.

First, the chemical decontamination apparatus is connected to a piping system of a nuclear power plant that has been stopped, which is a chemical decontamination object (step S1). Both ends of the circulation pipe 2 of the chemical decontamination apparatus 1 which is temporary equipment are connected to a carbon steel purification system pipe 69 which is a chemical decontamination object. The operation of connecting the circulation pipe 2 to the purification system pipe 69 will be specifically described. After the operation of the BWR plant is stopped, for example, the bonnet of the valve 74 installed in the purification system pipe 69 connected to the recirculation system pipe 55 is opened to block the recirculation system pipe 55 side. One end of the circulation pipe 2 of the chemical decontamination apparatus 1 is connected to the flange of the valve 74. As a result, one end of the circulation pipe 2 is connected to the purification system pipe 69 upstream of the purification system pump 70. On the other hand, on the downstream side of the purification system pump 70 , the bonnet of the valve 75 installed in the purification system pipe 69 is opened to seal the regenerative heat exchanger 71 side. The other end of the circulation pipe 2 of the chemical decontamination apparatus 1 is connected to the flange of the valve 75. As a result, one end of the circulation pipe 2 is connected to the purification system pipe 69 downstream of the purification system pump 70. A closed loop including the purification system pipe 69 and the circulation pipe 2 is formed. Before the circulation pumps 20 and 26 are driven in step S2 after the chemical decontamination apparatus 1 is connected to the purification system pipe 69, the purification system pipe 69 between the circulation pipe 2, the surge tank 13, and the valve 74 and the valve 75 is used. Fill with water. This water is injected from the surge tank 13, for example.

The temperature of circulating water is adjusted (step S2). First, the on-off valve 29, the valves 30, 31, 32, 33, 34, and the on-off valve 35 are opened, the other pumps are closed, the circulation pumps 20 and 26 are started, and the water in the surge tank 13 is drained. Circulation is performed in the circulation pipe 2 and the purification system pipe 69. And the circulating water which circulates the inside of the circulation piping 2 and the purification system piping 69 is heated with the heating apparatus 14 installed in the surge tank 13, and the temperature of circulating water is adjusted to about 90 degreeC.

  The reductive decontamination in the chemical decontamination method of this example will be described in detail below. Steps S3 to S6 are reductive decontamination steps, and reductive decontamination is performed on the inner surface of a piping system using a reducing decontamination solution, for example, a purification system piping 69 that has undergone oxidative decontamination.

  Oxalic acid is added (step S3). When the water temperature rises to about 90 ° C., the valve 46 is opened. Oxalic acid introduced into the hopper 28 is supplied from the ejector 27 to the water flowing through the pipe 47 and guided into the surge tank 13. As a result, an oxalic acid aqueous solution (reductive decontamination solution) is generated in the surge tank 13.

  Oxygen gas is supplied (step S4). The valve 18 is opened and the oxygen gas in the oxygen gas cylinder 17 is supplied to the bubbler 15 through the oxygen gas supply pipe 19. Oxygen gas is bubbled into the oxalic acid aqueous solution in the surge tank 13 from a large number of pores formed in the bubbler 15. The oxygen gas ejected from each pore is dissolved in the oxalic acid aqueous solution, and the dissolved oxygen concentration of the oxalic acid aqueous solution becomes high. Eventually, the dissolved oxygen concentration of the oxalic acid aqueous solution becomes saturated.

A pH adjuster is injected (step S5). The 90 ° C. oxalic acid aqueous solution whose dissolved oxygen concentration is increased by the supply of oxygen gas is supplied from the surge tank 13 to the purification system pipe 69 through the circulation pipe 2 by the circulation pump 26. Hydrazine, which is a pH adjuster, is injected into the oxalic acid aqueous solution saturated with dissolved oxygen at 90 ° C. flowing through the circulation pipe 2 by the pH adjuster injection device 3. Specifically, the valve 6 is opened and the infusion pump 5 is driven. As a result, hydrazine in the chemical tank 4 is injected into the circulation pipe 2 through the injection pipe 7. The pH of the oxalic acid aqueous solution in which dissolved oxygen is saturated at 90 ° C. is adjusted within a range of 2 to 3, for example, 2.5 by hydrazine injection. By adjusting the pH of the aqueous oxalic acid solution within the range of 2 to 3, excessive dissolution of the purification pipe 69 made of carbon steel is suppressed. Injection volume into the circulation pipe 2 of hydrazine, based on the measured pH of the aqueous oxalic acid vessel through a pH meter 50 A measured circulating pipe 2, as this pH measurements, of 2.5 It is adjusted by controlling the rotational speed of the infusion pump 5 (or the opening degree of the valve 6).

Since the circulation pumps 20 and 26 are driven, an aqueous solution of oxalic acid saturated with dissolved oxygen having a pH of 2.5 at 90 ° C. is supplied into the purification system pipe 69 that is a chemical decontamination target. . The oxalic acid concentration of the oxalic acid aqueous solution is 2000 ppm. When this oxalic acid aqueous solution comes into contact with the inner surface of the purification system pipe 69, the corrosion product containing radionuclides contained in the oxide film adhering to the inner surface of the purification system pipe 69 is dissolved by the oxalic acid. Part of Fe ²⁺ generated by dissolution of Fe (II) contained in the oxide film or carbon steel in the purification system piping forms a complex salt with oxalate ions, and precipitates of iron (II) oxalate. Form. However, since the oxalic acid aqueous solution, which is the reducing decontamination solution used in this example, has a dissolved oxygen concentration increased by bubbling oxygen gas, a part of Fe ²⁺ eluted in this oxalic acid aqueous solution is expressed by the formula ( 7) is oxidized to Fe ³⁺ , and this Fe ³⁺ generates a complex ion by causing each reaction of Formula (4), Formula (5) and Formula (6) with oxalate ion. Therefore, when oxygen gas is not bubbled into the oxalic acid aqueous solution, that is, compared with the case where the dissolved oxygen concentration of the oxalic acid aqueous solution is extremely low, the precipitation of iron (II) oxalate becomes very small, and the inner surface of the purification system pipe 69 Suppression of dissolution of the oxide film due to the formation of the iron (II) oxalate film on the surface of the oxide film formed on the surface becomes difficult. In other words, dissolution of the oxide film is promoted by using an oxalic acid aqueous solution having a pH of 2.5 at 90 ° C. and saturated with dissolved oxygen.

Water is passed through the cation exchange resin tower (step S6). After the oxidative decontaminant decomposition step is completed, the valve 40 is opened to adjust the opening degree of the valve 32, and a part of the returned aqueous oxalic acid solution discharged from the purification system pipe 69 to the circulation pipe 2 becomes a cation. It is supplied to the exchange resin tower 23 and comes into contact with the cation exchange resin in the cation exchange resin tower 23. Radionuclide (for example, Co-60) and metal cations such as Fe ²⁺ eluted in the oxalic acid aqueous solution by reductive decontamination of the purification system pipe 69 are adsorbed by the cation exchange resin in the cation exchange resin tower 23. Removed.

The oxalic acid aqueous solution from which the metal cation has been removed by the cation exchange resin tower 23 is mixed with the oxalic acid aqueous solution that has passed through the valve 32 and guided into the surge tank 13. The temperature of the oxalic acid aqueous solution flowing into the surge tank 13 is measured by a thermometer (not shown) provided in the circulation pipe 2 between the connection point of the pipe 45 and the circulation pipe 2 and the surge tank 13. A control device (not shown) inputs the measured temperature of the oxalic acid aqueous solution, and when this temperature falls below the target temperature of 90 ° C., the current supplied to the heating device 14 is increased to increase the oxalic acid aqueous solution. Is adjusted to 90 ° C. Oxygen gas is ejected from the bubbler 15. An aqueous oxalic acid solution having a dissolved oxygen concentration increased by supplying oxygen gas and having a pH of 2.5 at 90 ° C. is supplied from the circulation pipe 2 to the purification system pipe 69. This aqueous oxalic acid solution in which dissolved oxygen is saturated performs reductive decontamination of the inner surface of the purification system pipe 69 while circulating in the closed loop formed by the purification system pipe 69 and the circulation pipe 2. During the reduction decontamination, a part of the oxalic acid aqueous solution is guided to the cation exchange resin tower 23, and the metal cation contained in the oxalic acid aqueous solution is removed by the cation exchange resin tower 23. A radiation detector 76 disposed outside the purification system pipe 69 that is a chemical decontamination target detects the radiation emitted from the purification system pipe 69 that is a chemical decontamination target, and outputs a radiation detection signal. Based on this radiation detection signal, the dose rate of the purification system pipe 69 is obtained. The reduction decontamination is completed when the decreasing trend of the dose rate determined by the removal of the radionuclide has stopped decreasing. The valve 18 of the oxygen gas supply device 16 is closed to stop the supply of oxygen gas to the bubbler 15.

  Oxalic acid is decomposed (step S7). The process of step S7 is a decomposition process of the reducing decontamination agent. After the reduction decontamination is completed, the valve 44 is opened to adjust the opening of the valve 33, and a part of the oxalic acid aqueous solution flowing in the circulation pipe 2 is supplied to the decomposition device 25. Hydrogen peroxide is supplied from the hydrogen peroxide injection device 8 to the decomposition device 25. Specifically, the valve 11 is opened and the infusion pump 10 is driven. Hydrogen peroxide in the chemical tank 9 is supplied into the pipe 45 through the injection pipe 12 and supplied to the decomposition device 25. Oxalic acid and hydrazine contained in the oxalic acid aqueous solution supplied to the decomposition apparatus 25 are decomposed by the action of the activated carbon catalyst and hydrogen peroxide in the decomposition apparatus 25. Hydrazine degrades faster than oxalic acid. When the oxalic acid concentration is reduced to 10 ppm, the decomposition of oxalic acid is completed. After the decomposition of oxalic acid is completed, the injection pump 10 is stopped, the valve 11 is closed, and the supply of hydrogen peroxide from the chemical tank 9 to the decomposition device 25 is stopped. Further, the valve 33 is fully opened and the valve 44 is closed.

Purification is performed (step S8). After decomposition of the oxalic acid and hydrazine, and stops energizing the heating device 19, and stops the heating of the oxalic acid aqueous solution by the heating device 19, closing the valve 30, 31, 32 by opening the valve 36, 38, 42. The aqueous oxalic acid solution flowing into the circulation pump 20 from the circulation pipe 2 is supplied to the filter 21 through the pipe 37. The filter 21 removes fine solids contained in the oxalic acid aqueous solution. The oxalic acid aqueous solution discharged from the filter 21 is cooled by the cooler 22, and the temperature of the oxalic acid aqueous solution is reduced to 60 ° C., for example. The aqueous oxalic acid solution at 60 ° C. is supplied to the mixed bed resin tower 24. The mixed bed resin tower 24 removes impurities (such as anions) contained in the oxalic acid aqueous solution and the remaining oxalic acid. The conductivity of the aqueous solution discharged from the mixed bed resin tower 24 decreases. The oxalic acid concentration and hydrazine concentration of this aqueous solution are analyzed, and after confirming that the respective concentrations have fallen below a predetermined value, the circulation pumps 20 and 26 are stopped. Thereby, the purification process ends.

  When the dose rate of the purification system pipe 69 is higher than the target set dose rate after completion of the purification process, the processes in steps S3 to S8 are performed until the dose rate of the purification system pipe 69 becomes equal to or lower than the set dose rate. It is carried out repeatedly.

When the dose rate of the purification system pipe 69 becomes equal to or lower than the set dose rate, the water remaining in the circulation pipe 2 and the purification system pipe 69 at the end of the last purification process becomes waste liquid and is discharged outside these pipes. . Thereafter, by opening the valve 48 closes the on-off valve 29, 35, the water is filled in the circulating pipe 2及 beauty pipe 49, drives the circulating pump 20 and 26. In a closed loop formed by the circulation pipe 2及 beauty pipe 49 circulates, to clean the inner surface of such circulation pipe 2. After washing, the water in the circulation pipe 2及 beauty pipe 49 becomes a waste, is discharged to the outside of these pipe. Thereby, the chemical decontamination of the purification system pipe 69 in this embodiment is completed. Each of the waste liquids discharged from the circulation pipe 2, the purification system pipe 69, and the pipe 49 is treated as a radioactive waste liquid. After completion of chemical decontamination, the circulation pipe 2 is removed from the purification system pipe 69, and the purification system pipe 69 is restored to its original state. Then, the BWR plant is activated after the periodic inspection is completed.

  According to the present embodiment, oxygen gas is supplied to the oxalic acid aqueous solution that is the reductive decontamination solution at the time of reductive decontamination, and the oxalic acid aqueous solution having a higher dissolved oxygen concentration is supplied to the purification pipe 69 made of carbon steel. Since reductive decontamination is performed in contact with the inner surface of the purification system pipe 69, the production amount of iron (II) oxalate is reduced, and the oxide film containing the radionuclide formed on the inner surface of the purification system pipe 69 is difficult. Thick coating with soluble iron (II) oxalate can be prevented. For this reason, dissolution of the oxide film formed on the inner surface of the purification system pipe 69 by the oxalic acid aqueous solution proceeds, and the dose rate of the purification system pipe 69 can be efficiently reduced. As a result, the time required for chemical decontamination of the purification system pipe 69 can be shortened.

In addition, since oxygen gas is supplied to the oxalic acid aqueous solution, the oxalic acid aqueous solution with an increased dissolved oxygen concentration can be supplied to the cation exchange resin tower 23 during reductive decontamination. Cations of radionuclides eluted from the oxide film and the purification system pipe 69 by dyeing into the oxalic acid aqueous solution, metal cations such as Fe ²⁺ and Fe ³⁺ are removed by the cation exchange resin in the cation exchange resin tower 23. be able to. This is because oxygen dissolved in the oxalic acid aqueous solution does not deteriorate the cation exchange resin. When hydrogen peroxide or ozone is supplied to the oxalic acid aqueous solution instead of oxygen gas, the oxalic acid aqueous solution containing hydrogen peroxide or ozone is converted into hydrogen peroxide in the cation exchange resin tower 23. Or it is deteriorated by ozone. In this embodiment in which oxygen gas is supplied to the oxalic acid aqueous solution, such a problem can be improved.

  In this embodiment, since oxygen gas is supplied to the oxalic acid aqueous solution in the surge tank 13, the oxygen gas is efficiently dissolved in the oxalic acid aqueous solution. Also, oxygen gas bubbles that have not dissolved in the oxalic acid aqueous solution rise in the oxalic acid aqueous solution in the surge tank 13 and reach a gas reservoir formed at the upper end of the surge tank 13. The oxygen gas that has reached the gas reservoir can be easily discharged outside the surge tank 13.

  A chemical decontamination method for carbon steel members of a nuclear power plant according to embodiment 2, which is another preferred embodiment of the present invention, will be described with reference to FIGS. The chemical decontamination method for a carbon steel member of a nuclear power plant according to this embodiment is an example applied to a boiling water nuclear power plant (BWR plant).

  As shown in FIG. 7, the chemical decontamination apparatus 1A used in the chemical decontamination method of the present embodiment has a configuration in which a formic acid injection device 80 is added to the chemical decontamination apparatus 1 described above. Other configurations of the chemical decontamination apparatus 1A are the same as those of the chemical decontamination apparatus 1. The formic acid injection device 80 includes a chemical tank 81, an injection pump 82, and an injection pipe 84. The chemical tank 81 is connected to the circulation pipe 2 by an injection pipe 84 having an injection pump 82 and a valve 83. The chemical tank 81 is filled with a formic acid aqueous solution. The injection pipe 84 is connected to the circulation pipe 2 between the connection point between the injection pipe 7 and the circulation pipe 2 and the valve 34.

  The chemical decontamination method for the carbon steel member of the nuclear power plant according to the present embodiment is the same as the chemical decontamination method for the carbon steel member of the nuclear power plant according to the first embodiment. In the meantime, the formic acid injection step (step S9) and the formic acid decomposition step (step S10) are added. Other steps in the chemical decontamination method for carbon steel members of the nuclear power plant of the present embodiment are the same as the steps of the chemical decontamination method for carbon steel members of the nuclear power plant of the first embodiment.

  Similarly to the chemical decontamination method for the carbon steel member of the nuclear power plant according to the first embodiment, the chemical decontamination method for the carbon steel member of the nuclear power plant according to the present embodiment is, for example, made of carbon steel after the operation of the BWR plant is stopped. This is performed for the purification system pipe 69. In step S1 of the present embodiment, both ends of the circulation pipe 2 of the chemical decontamination apparatus 1A are connected to the purification system pipe 69 as in the first embodiment. Thereafter, in the same manner as in the first embodiment, steps S2 to S7 (see FIG. 6) are performed. Also in this embodiment, reductive decontamination is performed on the inner surface of the purification system pipe 69 using an oxalic acid aqueous solution having a dissolved oxygen concentration increased by supplying oxygen gas and having a pH of 2.5 at 90 ° C.

  When the decomposition process (step S7) of oxalic acid contained in the oxalic acid aqueous solution is completed, a thin film of iron (II) oxalate is formed on the inner surface of the purification system pipe 69, which is a chemical decontamination object. When the purification process (step S8) is carried out as it is and the chemical decontamination is completed, the iron (II) oxalate film remains on the inner surface of the purification system pipe 69 and the BWR plant in the next operation cycle after the periodic inspection is completed. Operation starts.

If a film of iron (II) oxalate remains on the inner surface of the purification system pipe 69, this iron (II) oxalate comes into contact with high-temperature (for example, 280 ° C.) cooling water during operation of the BWR plant. Each reaction of the following formula (8) and formula (9) is generated, and iron oxide particles (Fe ₂ O ₃ and Fe ₃ O ₄ particles) are generated on the inner surface of the purification system pipe 69.

4Fe (COO) ₂ + 3O ₂ → 2Fe ₂ O ₃ + 8CO ₂ (8)
3Fe (COO) ₂ + 4H ₂ O → Fe ₃ O ₄ + 6CO ₂ + 4H ₂ (9)
When producing | generating each iron oxide particle | grain by said each reaction, there exists a possibility of taking in the radionuclide contained in cooling water in each iron oxide particle | grain. Such a phenomenon occurs not only in the purification system pipe 69 but also in a carbon steel pipe connected to the RPV 51. Radionuclides taken into the iron oxide adhering to the inner surface of the purification system pipe 69 cause radiation exposure of workers at the next periodic inspection. In order to prevent this, it is effective not to leave iron (II) oxalate on the inner surface of the purification pipe 69 made of carbon steel that contacts the cooling water at the end of chemical decontamination, for example.

  In Example 1 described above, since oxygen gas is supplied to the oxalic acid aqueous solution (reduction decontamination solution), a thin iron (II) oxalate film that does not inhibit dissolution of the oxide film is formed on the inner surface of the purification system pipe 69. Is done. In this example, for the purpose of removing this thin iron (II) oxalate film, a formic acid injection step (step S9) and a decomposition step of the injected formic acid (step S10) were added.

  In this embodiment, after the oxalic acid decomposition step of Step S7 is completed, a formic acid aqueous solution is injected (Step S9). The valve 31 is opened and the valve 38 is closed to stop the cooling of the oxalic acid aqueous solution by the cooler 22, and the heating device 14 is energized to heat the oxalic acid aqueous solution by the heating device 14. By this heating, the temperature of the oxalic acid aqueous solution rises to 90 ° C. The valve 83 is opened to drive the infusion pump 82. The formic acid aqueous solution in the chemical tank 81 is injected into the 90 ° C. oxalic acid aqueous solution (oxalic acid concentration 10 ppm) flowing through the circulation pipe 2 through the injection pipe 84. The 90 ° C. oxalic acid aqueous solution whose pH is increased by the injection of the formic acid aqueous solution into the oxalic acid aqueous solution is supplied from the circulation pipe 2 into the purification system pipe 69. The thin iron (II) oxalate formed on the inner surface of the purification system pipe 69 is dissolved by the reaction with formic acid and is removed from the inner surface of the purification system pipe 69 as represented by the following formula (10). .

Fe (COO) ₂ + HCOOH → Fe ²⁺ + (COOH) COO ⁻
+ HCOO ^- ...... (10)
The dissolution of iron (II) oxalate from the inner surface of the purification system pipe 69 cannot be confirmed because the inner surface of the purification system pipe 69 cannot be observed directly. For example, a small amount of oxalic acid aqueous solution is added to the circulation pipe 2. A bypass pipe (not shown) through which the water flows is installed, and a part of this bypass pipe is used as a flow cell so that the oxalic acid aqueous solution flowing inside can be seen. In the flow cell, a carbon steel test piece is placed in contact with the oxalic acid aqueous solution, and the surface of the test piece is observed to confirm the dissolution of iron (II) oxalate from the test piece surface. .

  Formic acid contained in the oxalic acid aqueous solution is decomposed (step S10). After the dissolution of the iron (II) oxalate present on the inner surface of the purification system pipe 69 is completed, when the valve 44 is opened to reduce the opening of the valve 33, the aqueous oxalic acid solution containing formic acid flowing through the circulation pipe 2 is decomposed. Supplied to the device 25. Similarly to step S7, hydrogen peroxide is supplied from the hydrogen peroxide injection device 8 to the decomposition device 25. Formic acid and oxalic acid contained in the oxalic acid aqueous solution are decomposed in the decomposition apparatus 25 by the action of the activated carbon catalyst and hydrogen peroxide.

After the decomposition of formic acid and oxalic acid in step S10 is completed, the purification step in step S8 is performed as in the first embodiment. After the purification step, water existing in the circulation pipe 2 and the purification system pipe 69 is discharged as a waste liquid, as in the first embodiment. Thereby, the chemical decontamination method of a present Example is complete | finished.
In the present embodiment, each effect produced in the first embodiment can be obtained. Further, in this embodiment, formic acid is injected into the oxalic acid aqueous solution, so that the thin iron (II) oxalate film formed on the inner surface of the purification system pipe 69 is dissolved when the oxalic acid aqueous solution supplied with oxygen gas is decontaminated. Of the iron oxide that takes in the radionuclide from the iron (II) oxalate remaining on the inner surface of the purification system pipe 69 by the contact of the cooling water that has become hot due to the operation of the BWR in the next operation cycle. Particles can be prevented from being generated. As a result, the dose rate of the purification system pipe 69 can be effectively reduced.

  A chemical decontamination method for carbon steel members of a nuclear power plant according to embodiment 3, which is another preferred embodiment of the present invention, will be described with reference to FIGS. The chemical decontamination method for a carbon steel member of a nuclear power plant according to this embodiment is an example applied to a boiling water nuclear power plant (BWR plant).

  In the chemical decontamination method for the carbon steel member of the nuclear power plant according to the present embodiment, when the chemical decontamination of the carbon steel purification system pipe 69 is performed, a stainless steel recirculation system pipe (stainless steel member) 55 is also used. Chemical decontamination is also performed. In the chemical decontamination method of this example, the chemical decontamination apparatus 1 used in Example 1 is used. Instead of the chemical decontamination apparatus 1, the chemical decontamination apparatus 1A used in Example 2 may be used.

  The chemical decontamination method of the carbon steel member of the nuclear power plant according to the present embodiment is the same as the chemical decontamination method of the carbon steel member of the nuclear power plant according to the first embodiment, and the oxidative decontamination step (step S11) and the dose rate determination step (step S12). ) Is added. Other steps in the chemical decontamination method for carbon steel members of the nuclear power plant of the present embodiment are the same as the steps of the chemical decontamination method for carbon steel members of the nuclear power plant of the first embodiment.

  In the chemical decontamination method for carbon steel members of the nuclear power plant according to the present embodiment, steps S1 to S8, S11 and S12 shown in FIG. 8 are performed. When the operation of the BWR plant is stopped, first, both end portions of the circulation piping 2 of the chemical decontamination apparatus 1 are connected to a piping system communicated with the RPV 51 (step S1). Specifically, one end of the circulation pipe 2 is connected to the purification system pipe, and the other end of the circulation pipe 2 is connected to the recirculation pipe 55. One end of the circulation pipe 2 is connected to a flange of a valve 75 provided in the purification system pipe 69 as in the first embodiment. The other end of the circulation pipe 2 is disconnected from a branch pipe such as a drain pipe (or instrumentation pipe) connected to the recirculation system pipe 55 on the downstream side of the recirculation pump 54 and connected to the separated branch pipe. The As a result, a closed loop including the circulation pipe 2, the recirculation system pipe 55, and the purification system pipe 69 is formed. Then, each process of step S2, S11, S3-S8, and S12 is implemented.

  The piping system is subjected to oxidative decontamination (step S11). When the water temperature rises to about 90 ° C., the valve 46 is opened. Potassium permanganate (oxidative decontamination agent) charged into the hopper 28 is supplied into the pipe 47 through the ejector 27, and further introduced into the surge tank 13 by the water flowing through the pipe 47. Potassium permanganate is dissolved in 90 ° C. water in the surge tank 13 to produce an oxidative decontamination solution (potassium permanganate aqueous solution). The oxidative decontamination liquid is pressurized by the circulation pump 26 and supplied from the surge tank 13 through the circulation pipe 2 into the purification system pipe 69. The oxidative decontamination solution oxidizes and dissolves contaminants (including radionuclides) such as an oxide film formed on the inner surface of the purification system pipe 69 (oxidation decontamination step).

  Oxalic acid is added (step S3). After the oxidative decontamination is completed, the oxalic acid charged into the hopper 28 is supplied from the ejector 27 to the oxidative decontamination liquid flowing in the pipe 47 and guided into the surge tank 13. This oxalic acid decomposes potassium permanganate contained in the oxidative decontamination solution (oxidative decontamination agent decomposition step).

When the supply of oxalic acid in the hopper 28 into the pipe 47 is continued in step S3, the potassium permanganate eventually disappears, and the aqueous oxalic acid solution is stored in the surge tank 13 in the same manner as in step S3 of the first embodiment. Generated.
By performing steps S4 to S5 after step S3, an oxalic acid aqueous solution having a dissolved oxygen concentration increased at 90 ° C. and a pH of 2.5 is generated as in the first embodiment. This oxalic acid aqueous solution is supplied to the recirculation system pipe 55 and the purification system pipe 69, and reductive decontamination is performed on the inner surfaces of these pipes. While carrying out reductive decontamination, the oxalic acid aqueous solution discharged from the purification system pipe 69 to the circulation pipe 2 is supplied to the cation exchange resin tower 23, and the metal cation is removed in step S6.

  After the reductive decontamination of the recirculation system pipe 55 and the purification system pipe 69 is completed, the oxalic acid and hydrazine decomposition process in step S7 is performed, and then the purification process in step S8 is performed. After the purification process is completed, the dose rate is determined (step S12). A radiation detector 76 disposed outside the purification system pipe 69 detects the radiation emitted from the purification system pipe 69 and outputs a radiation detection signal. In addition, the radiation detector 76A arranged outside the recirculation system pipe 55 detects the radiation emitted from the recirculation system pipe 55 and outputs a radiation detection signal. The dose rate of the purification system pipe 69 is obtained based on the radiation detection signal output from the radiation detector 76, and the dose rate of the recirculation system pipe 55 is obtained based on the radiation detection signal output from the radiation detector 76A. When both dose rates are equal to or lower than the set dose rate, chemical decontamination for each of the recirculation system pipe 55 and the purification system pipe 69 ends. When the dose rate of any one of the recirculation system pipe 55 and the purification system pipe 69 exceeds the set dose rate, each process of steps S11 and S3 to S12 is performed in step S12 as “recirculation system pipe 55 and purification system. The process is repeated until it is determined that the dose rate of the pipe 69 is equal to or lower than the set dose rate.

  When it is determined in step S12 that “the dose rate of the recirculation system pipe 55 and the purification system pipe 69 is equal to or less than the set dose rate”, the presence in the circulation pipe 2 and the purification system pipe 69 is the same as in the first embodiment. Water to be discharged as waste liquid. Thereby, the chemical decontamination method of a present Example is complete | finished.

  In the present embodiment, each effect produced in the first embodiment can be obtained. In this embodiment, the oxidation decontamination liquid and the reduction decontamination liquid can be supplied to the stainless steel recirculation system pipe together with the carbon steel purification system pipe 69, so that chemical decontamination of pipes of different materials is performed together. be able to.

  The chemical decontamination method of the present embodiment can also be performed using the chemical decontamination apparatus 1A. Furthermore, this embodiment can also be performed by using the chemical decontamination apparatus 1B used in Embodiment 4 described later.

  A method for chemically decontaminating carbon steel members of a nuclear power plant according to embodiment 4 which is another preferred embodiment of the present invention will be described with reference to FIG. The chemical decontamination method for a carbon steel member of a nuclear power plant according to this embodiment is an example applied to a boiling water nuclear power plant (BWR plant).

  In the chemical decontamination method for carbon steel members of the nuclear power plant of this embodiment, a chemical decontamination apparatus 1B shown in FIG. 10 is used. The chemical decontamination apparatus 1 </ b> B has a configuration in which a microbubble generation pump (microbubble generation apparatus) 85 is provided instead of the bubbler 15 installed in the surge tank 13 in the chemical decontamination apparatus 1. The microbubble generation pump 85 is installed in a pipe 87 connected to the circulation pipe 2 between the connection point between the pipe 47 and the circulation pipe 2 and the valve 34. The pipe 87 is provided with a valve 86. The pipe 87 is connected to a microbubble jet pipe 88 installed in the surge tank 13. The oxygen gas supply pipe 19 of the oxygen gas supply device 16 is connected to the microbubble generation pump 85. Other configurations of the chemical decontamination apparatus 1B are the same as those of the chemical decontamination apparatus 1.

  Similarly to the chemical decontamination method for the carbon steel member of the nuclear power plant according to the first embodiment, the chemical decontamination method for the carbon steel member of the nuclear power plant according to the present embodiment is, for example, made of carbon steel after the operation of the BWR plant is stopped. This is performed for the purification system pipe 69. Both ends of the circulation pipe 2 of the chemical decontamination apparatus 1B are connected to the purification system pipe 69 as in the first embodiment. Then, each process of step S2-S8 of Example 1 is implemented. The supply of oxygen gas to the oxalic acid aqueous solution in the step S4 in the present embodiment will be specifically described with reference to FIG.

  The microbubble generation pump 85 is driven. The oxalic acid aqueous solution flowing in the circulation pipe 2 is supplied to the microbubble generating pump 85 through the pipe 87, and the oxalic acid aqueous solution discharged from the microbubble generating pump 85 is guided into the surge tank 13. The valve 18 is opened to supply the oxygen gas in the oxygen gas cylinder 17 to the microbubble generating pump 85 driven through the oxygen gas supply pipe 19. By supplying oxygen gas to the microbubble generation pump 85, oxygen gas bicro bubbles are generated in the oxalic acid aqueous solution supplied to the microbubble generation pump 85. The oxalic acid aqueous solution containing a large number of microbubbles is guided to the microbubble jet pipe 88 provided in the surge tank 13 through the pipe 87, and the microbubble jet pipe 88 enters the oxalic acid aqueous solution in the surge tank 13. Erupted.

  Unlike the bubbles of oxygen gas generated by the bubbler 15 in the first embodiment, the oxygen gas microbubbles immediately rise in the oxalic acid aqueous solution in the surge tank 13 as a part of the blown oxygen gas bubbles. It does not reach the upper surface of the acid aqueous solution. For this reason, many micro bubbles of oxygen gas can stay in the oxalic acid aqueous solution in the surge tank 13. An aqueous oxalic acid solution containing a large number of microbubbles of oxygen gas having a pH of 2 and 5 at 90 ° C. is supplied from the circulation pipe 2 to the purification system pipe 69 by the circulation pumps 26 and 20.

Oxygen gas microbubbles are formed by reductive decontamination using an oxalic acid aqueous solution and an oxide film formed on the inner surface of the purification system pipe 69 and Fe ²⁺ eluted from the base material of the purification system pipe 69 into the oxalic acid aqueous solution. Dissolved oxygen that disappears in the oxalic acid aqueous solution by the oxidation reaction can be replenished. When the dissolved oxygen concentration of the oxalic acid aqueous solution decreases, the oxygen in the microbubbles dissolves in the oxalic acid aqueous solution. Therefore, microbubbles oxygen gas, in generating a large amount of dissolved oxygen in the oxalic acid aqueous solution is consumed easily clean piping within 69 Fe ^2+, it enhances the dissolved oxygen concentration in the oxalic acid aqueous Fe ²⁺ Has the effect of promoting the oxidation of. Thus, generation of iron oxalate in the aqueous solution of oxalic acid (II) is suppressed, precipitation of oxalate to the inner surface of the purification of piping 69 (II) is suppressed. As a result, oxide film containing radioactive nuclides formed purification of piping 6 9, likely to be dissolved by oxalic acid solution.

Since the concentration of the metal cation such as radionuclide, Fe ²⁺ and Fe ^{3+ in the} oxalic acid aqueous solution increases due to dissolution of the oxide film, the oxalic acid aqueous solution is converted into a cation in this example as well as in Example 1. It supplies to the exchange resin tower 23 (step S6), and the metal cation contained in the oxalic acid is removed by the cation exchange resin tower 23.

In the present embodiment, each effect produced in the first embodiment can be obtained. Furthermore, in this embodiment, since oxygen gas bicyclo bubbles are injected into the oxalic acid aqueous solution, the oxygen gas can be efficiently dissolved in the oxalic acid aqueous solution, and the amount of dissolved oxygen in the oxalic acid aqueous solution increases. . In particular, when the dissolved oxygen in the oxalic acid aqueous solution is consumed by the oxidation of the eluted Fe ²⁺ , oxygen is converted into the oxalic acid aqueous solution from the oxygen gas bicro bubbles contained in the oxalic acid aqueous solution in the purification system pipe 69. Can be replenished as dissolved oxygen.

  Similarly to the chemical decontamination apparatus 1A, the formic acid injection apparatus 80 is connected to the circulation pipe 2 of the chemical decontamination apparatus 1B, thereby using the chemical decontamination apparatus 1B having the formic acid injection apparatus 80, and the nuclear power plant of Example 2. The chemical decontamination method for carbon steel members can be performed.

  Each chemical decontamination method of Examples 1 to 4 can also be applied to a pressurized water nuclear plant.

  DESCRIPTION OF SYMBOLS 1,1A, 1B ... Chemical decontamination apparatus, 2 ... Circulation piping, 3 ... pH adjuster injection apparatus, 4, 9, 81 ... Chemical tank, 5, 10, 82 ... Injection pump, 7, 12, 84 ... Injection piping 8 ... hydrogen peroxide injection device, 13 ... surge tank, 14 ... heating device, 15 ... bubbler, 16 ... oxygen gas supply device, 20, 26 ... circulation pump, 22 ... cooler, 23 ... cation exchange resin tower, 24 ... Mixed bed resin tower, 25 ... Decomposition unit, 50 ... Reactor, 51 ... Reactor pressure vessel, 52 ... Core, 55 ... Recirculation system piping, 60 ... Feed water piping, 69 ... Purification system piping, 76, 76A ... Radiation detector, 80 ... formic acid injection device, 85 ... microbubble generating pump.

Claims (10)

Supply oxygen gas to the oxalic acid aqueous solution to increase the dissolved oxygen concentration of the oxalic acid aqueous solution until saturation ,
Reducing decontamination of carbon steel members that are structural members of nuclear power plants using the oxalic acid aqueous solution saturated with the dissolved oxygen concentration,
A chemical decontamination method for a carbon steel member of a nuclear power plant, wherein the oxalic acid aqueous solution subjected to the reduction decontamination of the carbon steel member is brought into contact with a cation exchange resin.   The method for chemical decontamination of carbon steel members for a nuclear power plant according to claim 1, wherein the oxygen gas is injected into the oxalic acid aqueous solution as microbubbles of oxygen gas.   The iron (II) oxalate present on the surface of the carbon steel member is dissolved using an aqueous solution containing formic acid after the oxalic acid decomposition step for decomposing oxalic acid contained in the oxalic acid aqueous solution is completed. Or the chemical decontamination method of the carbon steel member of the nuclear power plant of 2. The first pipe of the chemical decontamination apparatus having a surge tank provided in the first pipe and the first pipe is connected to a second pipe that is a carbon steel member of a nuclear power plant that is communicated with a reactor pressure vessel. ,
Supply oxalic acid into the surge tank to produce an oxalic acid aqueous solution,
Supplying oxygen gas to the oxalic acid aqueous solution to increase the dissolved oxygen concentration of the oxalic acid aqueous solution until saturation ,
Supplying the oxalic acid aqueous solution saturated with the dissolved oxygen concentration to the second pipe through the first pipe, and performing reductive decontamination of the inner surface of the second pipe;
A chemical decontamination method for a carbon steel member of a nuclear power plant, wherein the oxalic acid aqueous solution discharged from the second pipe is brought into contact with a cation exchange resin.   Both ends of the first pipe are connected to the second pipe, a closed loop including the first pipe and the second pipe is formed, and the contact of the aqueous oxalic acid solution with the cation exchange resin is caused by the oxalic acid. 5. The nuclear plant carbon according to claim 4, wherein the aqueous solution is supplied to a cation exchange resin tower provided in the first pipe upstream of the surge tank and having the cation exchange resin therein. Chemical decontamination method for steel members.   The chemical decontamination method of the carbon steel member of the nuclear power plant of Claim 4 which decomposes | disassembles the oxalic acid contained in the said oxalic acid aqueous solution discharged | emitted from the said 2nd piping. After completion of the oxalic acid decomposition step for decomposing oxalic acid contained in the oxalic acid aqueous solution, formic acid is injected into the first pipe,
The method for chemical decontamination of carbon steel members of a nuclear power plant according to claim 6, wherein iron (II) oxalate existing on the inner surface of the second pipe is dissolved. One end of the first pipe of the chemical decontamination apparatus is connected to a third pipe that is a stainless steel member of a nuclear power plant connected to the reactor pressure vessel, and the third pipe is connected to the third pipe. Connecting the other end of the first pipe of the chemical decontamination device to a second pipe to form a closed loop including the first pipe, the second pipe and the third pipe;
An oxidative decontamination solution is supplied to the second pipe and the third pipe through the first pipe, and oxidative decontamination of the inner surface of the third pipe is performed,
After the oxidative decontamination of the third pipe with the oxidative decontamination liquid is completed, the oxalic acid aqueous solution with the increased dissolved oxygen concentration is supplied to the second pipe and the third pipe through the first pipe. 5. The nuclear power according to claim 4, wherein reductive decontamination of the inner surface of the third pipe using the oxalic acid aqueous solution is performed when reductive decontamination of the inner face of the second pipe using the oxalic acid aqueous solution is performed. A chemical decontamination method for carbon steel components in a plant.   The method of chemical decontamination of carbon steel members for a nuclear power plant according to claim 4 or 8, wherein the oxygen gas is injected into the oxalic acid aqueous solution in the surge tank as microbubbles of oxygen gas.   The iron (II) oxalate present on the inner surface of the second pipe is dissolved using an aqueous solution containing formic acid after the oxalic acid decomposition step for decomposing oxalic acid contained in the oxalic acid aqueous solution is completed. The chemical decontamination method of the carbon steel member of the nuclear power plant as described in 2.

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