JP2017138139A - Chemical decontamination method, chemical decontamination device, and nuclear power plant using them - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a chemical decontamination method in which removal of iron oxalate (II) formed on the surface of a carbon steel member is promoted and a removal ratio of radioactive substance is improved.SOLUTION: A chemical decontamination method includes: a step (S6) for reducing and decontaminating a carbon steel member by supplying oxalic acid which is a reduction decontaminating agent; and a step (S8) for supplying an oxidizer to the carbon steel member which is reduced and decontaminated. A state of a coating made of iron oxalate (II) formed in the carbon steel member is detected and the decontamination efficiency is improved by measuring change in a potential of a carbon steel electrode provided so as to come into contact with the reduction decontaminating agent and the oxidizer (S9), and decontamination efficiency is improved.SELECTED DRAWING: Figure 1

Description

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

  For example, a boiling water nuclear power plant (hereinafter referred to as “BWR plant”) has a nuclear reactor in which a reactor core is built in a reactor pressure vessel (hereinafter 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 refers to 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. Less stainless steel and nickel-base alloys are used. Further, the RPV made of low alloy steel has a stainless steel overlay on the inner surface, preventing 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 released by fission of 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 nuclear reactor purification system is accumulated on the surface of the nuclear plant component (for example, piping) in contact with 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 prescribed 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.

  Patent Document 1 discloses a first step of reducing and decontaminating a carbon steel member with an oxalic acid aqueous solution, and a reducing decontamination of the carbon steel member with an oxalic acid aqueous solution containing formic acid after the first step. A chemical decontamination method including a second step is disclosed. This chemical decontamination method is performed on piping and the like in a nuclear power plant. Patent Document 1 also describes that after the second step, formic acid is decomposed and oxalic acid is decomposed after the formic acid is decomposed.

  Patent Document 2 discloses a chemical decontamination method for decontaminating carbon steel with an oxalic acid aqueous solution in which oxygen gas is bubbled. This is because iron (II) of iron (II) oxalate formed on the surface of carbon steel is oxidized to Fe (III) by the oxidizability of dissolved oxygen, and iron (II) oxalate is oxidized from the surface of carbon steel. The oxalic acid can easily reach the oxide film containing the radionuclide under the removal. Thereby, since the dissolution of the oxide film containing the radionuclide that has been stopped by the formation of iron (II) oxalate proceeds again, the decontamination efficiency is improved.

JP 2009-109427 A JP 2015-28432 A

  In the chemical decontamination method described in Patent Document 1, 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 the surface of the carbon steel member. It precipitates in. 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.

  In the chemical decontamination method described in Patent Document 2, in order to dissolve iron (II) oxalate, oxygen gas is bubbled into an aqueous oxalic acid solution, and Fe (II) of iron (II) oxalate is dissolved in Fe (II) by dissolved oxygen. Oxidized to III). Depending on the amount of iron (II) oxalate, dissolved oxygen is consumed by the downstream side of the system, and dissolution of iron (II) oxalate may not proceed. In addition, if the system has an open-to-air part, oxygen escapes, and the oxygen concentration decreases downstream from this, and there is a possibility that the dissolution of iron (II) oxalate will not proceed.

  An object of the present invention is to promote the removal of iron (II) oxalate formed on the surface of a carbon steel member when chemical decontamination of the carbon steel member is to improve the removal rate of radioactive substances.

  The chemical decontamination method of the present invention is a method for chemically decontaminating carbon steel members, which are structural members of nuclear power plants, by supplying oxalic acid, which is a reducing decontamination agent, to reduce decontamination of carbon steel members. Measuring the change in the potential of the carbon steel electrode provided in contact with the reducing decontamination agent and the oxidizing agent, including the step of dyeing and the step of supplying the oxidizing agent to the carbon steel member subjected to reduction decontamination. By doing so, the state of the iron (II) oxalate film formed on the carbon steel member is detected.

  According to the present invention, when chemical decontamination of a carbon steel member is performed, the removal of iron (II) oxalate formed on the surface of the carbon steel member can be promoted, and the removal rate of radioactive substances can be improved.

  Moreover, according to this invention, iron (II) oxalate can be removed over the whole decontamination object. Thereby, the oxide film containing the radionuclide covered with iron (II) oxalate can be dissolved, and the decontamination efficiency by chemical decontamination of the carbon steel member can be improved.

It is a flowchart which shows the chemical decontamination method of the carbon steel member of this invention. It is a graph which shows the electrical potential change of the carbon steel electrode immersed in the 90 degreeC aqueous solution containing oxalic acid 2000ppm and hydrazine 500ppm. It is a block diagram which shows the boiling water nuclear power plant to which the chemical decontamination apparatus of Example 1 was connected. It is a detailed block diagram of the chemical decontamination apparatus of FIG. It is a block diagram which shows the connection state to the boiling water nuclear power plant of the chemical decontamination apparatus of Example 2. FIG. It is the schematic which shows the structure regarding control of the chemical decontamination apparatus of Example 3. FIG.

  The feature of the present invention is that a carbon steel electrode is provided in a temporary piping part on the decontamination liquid outlet side of the chemical decontamination target part, and the electric potential during immersion in the oxalic acid decontamination liquid is measured to remove iron (II) oxalate Therefore, it is to confirm that the hydrogen peroxide added to the decontamination solution reaches the carbon steel electrode by measuring the potential.

  A film of iron (II) oxalate is formed on the carbon steel electrode immersed in the oxalic acid aqueous solution, like the carbon steel pipe. When this film becomes thick, the corrosion rate of the base material decreases and the potential becomes stable. Since the iron (II) oxalate film is also formed on the carbon steel pipe side to be decontaminated, dissolution of the oxide film incorporating the radionuclide is suppressed, and the efficiency of decontamination is reduced.

  Therefore, in order to remove iron (II) oxalate, hydrogen peroxide as an oxidizing agent is added to the decontamination solution. In order to cause hydrogen peroxide to act on iron (II) oxalate formed on the carbon steel pipe to be decontaminated, hydrogen peroxide is added upstream of the decontamination object. Since the added hydrogen peroxide reacts with iron (II) oxalate and disappears, it is necessary to confirm that the hydrogen peroxide reaches the downstream side to remove iron (II) oxalate. is there. Although it is possible to sample the downstream decontamination solution and measure the concentration of hydrogen peroxide, it is not known whether iron (II) oxalate has been removed at the measured concentration.

  Therefore, the change in the potential of the carbon steel electrode installed on the downstream side is measured to determine whether or not iron (II) oxalate has been removed. When hydrogen peroxide reaches the carbon steel electrode covered with iron (II) oxalate and iron (II) oxalate is removed, the dissolution rate of the base material increases, causing a change in potential. By measuring this, it can be confirmed that hydrogen peroxide acts to the downstream side and the removal of iron (II) oxalate has reached.

  The present inventor reduced the decontamination efficiency (DF) due to 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. If the deposited iron (II) oxalate covers the oxide film containing the radionuclide on the surface of the carbon steel to be decontaminated, the dissolution of the oxide film is suppressed, which causes a decrease in DF. Then, when the decreasing tendency of the dose rate was settled, this iron (II) oxalate film was considered to be removed by the action of hydrogen peroxide.

Fe ²⁺ in iron (II) oxalate undergoes a Fenton reaction represented by the following formula (1) with hydrogen peroxide, changes to Fe ³⁺ , and dissolves in the aqueous solution.

Fe ²⁺ + H ₂ O ₂ → Fe ³⁺ + OH ⁻ + OH ^* (1)
The hydroxyl radical OH ^* generated by this reaction is consumed by the reaction with the surrounding oxalic acid represented by the following formula (2) or the reaction with Fe ²⁺ represented by the following formula (3).

(COOH) ₂ + 2OH ^* → CO ₂ + 2H ₂ O (2)
Fe ²⁺ + OH ^* → Fe ³⁺ + OH ⁻ (3)
Fe ³⁺ forms a complex with oxalic acid by the reaction represented by the following formulas (4), (5) and (6), and is retained in the aqueous solution as chelate ions.

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)
By such a reaction, iron (II) oxalate is removed from the oxide film containing the radionuclide, and oxalic acid in the reduction decontamination solution can dissolve the oxide film again.

  In this specification, “reduction decontamination” means that an oxide film formed on a carbon steel member, which is a structural material member, is reduced by oxalic acid or the like to be dissolved and removed.

  Since the added hydrogen peroxide is consumed by the above reaction, the concentration decreases from the upstream side to the downstream side of the decontamination target system. For this reason, in the downstream carbon steel member, there is a possibility that hydrogen peroxide is insufficient and dissolution of iron (II) oxalate does not proceed.

  Although it is possible to sample the downstream reductive decontamination solution and measure the hydrogen peroxide concentration, the reaction of the above formula (1) proceeds, the removal of iron (II) oxalate proceeds, and it is below Whether or not the dissolution of the oxide film containing the radionuclide proceeds cannot be judged only by the hydrogen peroxide concentration.

  Therefore, the present inventor, as a method for confirming that iron (II) oxalate is removed by hydrogen peroxide in the entire decontamination target part including the downstream side of the decontamination target part, A carbon steel electrode was installed on the side and the potential was measured continuously.

  When the decontamination apparatus is incorporated into the decontamination target part, a carbon steel electrode is installed in the decontamination apparatus temporary pipe downstream of the decontamination target part so that the potential can be measured. In addition, a dose rate monitor is installed in the decontamination target section so that the decrease in dose due to decontamination can be monitored.

  When the reduction decontamination is started, the dose rate of the carbon steel member to be decontaminated decreases, but at the same time, precipitation of iron (II) oxalate occurs. Similarly, iron (II) oxalate is deposited on the carbon steel electrode. When the deposition of iron (II) oxalate proceeds to form a film, the carbon steel member to be decontaminated slows down the dose rate, and the carbon steel electrode suppresses corrosion and stabilizes the potential. The slowdown of the dose rate is obtained from the continuous measurement result of the dose rate monitor installed in the decontamination target part, and the stabilization of the potential is obtained from the continuous measurement result of the potential of the carbon steel electrode installed in the decontamination equipment installation pipe. be able to.

  Therefore, hydrogen peroxide is added from the upstream side to be decontaminated for the purpose of removing iron (II) oxalate to improve the DF.

The added hydrogen peroxide causes the reaction of the above formula (1) with Fe ²⁺ contained in iron (II) oxalate to remove iron (II) oxalate. Hydrogen peroxide disappears by this reaction.

  In order to confirm that iron (II) oxalate has been removed by this reaction also on the downstream side of the decontamination target, the measurement result of the potential of the carbon steel electrode installed in the decontamination equipment temporary piping is utilized. Before the hydrogen peroxide arrives at the carbon steel electrode, the potential is stable and constant in a state where a coating of iron (II) oxalate is formed and corrosion of the base metal is suppressed. When hydrogen peroxide arrives there, iron (II) oxalate is removed and the carbon steel base material is exposed. Then, since the electric potential changes, it is possible to confirm the removal of iron (II) oxalate. Thereby, it can be judged that the removal of iron (II) oxalate is progressing from the upstream side to the downstream side of the carbon steel member to be decontaminated with the added hydrogen peroxide.

  FIG. 2 shows the measurement results of the potential of the carbon steel electrode performed by the present inventor. The horizontal axis represents time, and the vertical axis represents the carbon steel electrode potential.

  An aqueous solution containing 2,000 ppm of oxalic acid and 500 ppm of hydrazine was heated to 90 ° C., a carbon steel electrode was immersed therein, and the potential was measured using an Ag / AgCl electrode as a reference electrode.

  The carbon steel electrode was polished and then immersed, but a film of iron (II) oxalate was formed, and the potential showed a constant value as shown in the range from 0 to 10 minutes in this figure. . When hydrogen peroxide was added after reaching a certain potential, iron (II) oxalate dissolved in about 1 minute, and the potential decreased. Thereafter, a film of iron (II) oxalate was formed again, and the potential showed a constant value after about 1 hour.

  Since the removal of iron (II) oxalate has also occurred on the upstream side of the carbon steel electrode, the oxide film containing the radionuclide of the carbon steel member can come into contact with the reductive decontamination solution containing oxalic acid, As dissolution proceeds, the DF improves.

  Hereinafter, embodiments of the present invention will be described together.

  The chemical decontamination method of the present invention is a method for chemically decontaminating carbon steel members, which are structural members of nuclear power plants, by supplying oxalic acid, which is a reducing decontamination agent, to reduce decontamination of carbon steel members. Measuring the change in the potential of the carbon steel electrode provided in contact with the reducing decontamination agent and the oxidizing agent, including the step of dyeing and the step of supplying the oxidizing agent to the carbon steel member subjected to reduction decontamination. By doing so, the state of the iron (II) oxalate film formed on the carbon steel member is detected.

  The carbon steel electrode is preferably supplied with a liquid supplied to the carbon steel member and passed through the carbon steel member.

  It is desirable that the oxidizing agent is added to the liquid containing the reductive decontamination agent and then contact the carbon steel member.

  When reducing decontamination and supplying an oxidizing agent, it is desirable to measure the dose rate of carbon steel members.

  When performing reduction decontamination, it is desirable to continue reductive decontamination if the dose rate continues to decrease, and to start supplying the oxidizing agent when the dose rate becomes substantially constant.

  It is desirable to continue the supply of the oxidant until the potential of the carbon steel electrode is lowered, and to stop the supply of the oxidant after the potential of the carbon steel electrode is lowered.

  It is desirable to continue reductive decontamination when the dose rate continues to decrease after the supply of the oxidizing agent is stopped.

  After the reduction of the dose rate is completed, it is desirable to decompose the reductive decontaminant and purify the liquid containing the reductive decontaminant.

  It is desirable to repeatedly stop the supply of the oxidizing agent and continue the reduction decontamination.

  When the dose rate becomes substantially constant, the dose rate decrease rate for 1 hour after the start of reductive decontamination is compared with the subsequent dose rate decrease rate, and the subsequent dose rate decrease rate is 1 hour after the start. It is desirable to set the time when the dose rate decreases to 1/10 or less.

  The oxidizing agent is preferably hydrogen peroxide or ozone.

  The chemical decontamination apparatus of the present invention is an apparatus for chemical decontamination of a carbon steel member that is a structural material member of a nuclear power plant, and a chemical supply unit that supplies oxalic acid that is a reducing decontamination agent to the carbon steel member; An oxidant supply unit for supplying an oxidant to the carbon steel member, and a carbon steel electrode provided in contact with the reducing decontamination solution and the oxidant, and measuring a change in potential of the carbon steel electrode. By this, it has the structure which detects the state of the film of iron (II) oxalate formed on the carbon steel member.

  Furthermore, it is desirable that a circulation pipe that can be attached to the carbon steel member is included, and the chemical supply section and the oxidant supply section are attached to the circulation pipe.

  It is desirable that the carbon steel electrode is disposed at a site to which the liquid that is supplied from the circulation pipe to the carbon steel member and passes through the carbon steel member is sent.

  The carbon steel electrode is preferably configured so that a part of the liquid supplied to the carbon steel member from the circulation pipe and passing through the carbon steel member is sent.

  Furthermore, a radiation detector attached to the carbon steel member, and a control unit, the control unit using at least the potential of the carbon steel electrode and the dose rate of the carbon steel member measured by the radiation detector, It is desirable to control the supply of reductive decontaminants and oxidants.

  It is desirable that the nuclear power plant of the present invention can be provided with a chemical decontamination apparatus.

  The nuclear power plant of the present invention may be provided with a chemical decontamination apparatus.

  Examples of the present invention will be described below.

  A method and apparatus for chemical decontamination of a carbon steel member in Example 1, which is a preferred example of the present invention, will be described with reference to FIG. 1, FIG. 3, and FIG. The present embodiment is an example applied to a boiling water nuclear power plant (BWR plant).

  A schematic configuration of a BWR plant to which the present embodiment is applied will be described with reference to FIG.

  In this figure, the BWR plant includes a nuclear reactor 49, a turbine 56, a condenser 57, a recirculation system, a nuclear reactor purification system, a water supply system, and the like. The reactor 49 installed in the reactor containment vessel 11 has a reactor pressure vessel 50 (hereinafter referred to as “RPV”) in which a core 51 is built, and a jet pump 52 is installed in the RPV 50. A plurality of fuel assemblies (not shown) are loaded on the core 51. 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 53 and a stainless steel recirculation system pipe 54, and the recirculation pump 53 is installed in the recirculation system pipe 54.

  The feed water system includes a condensate pump 59, a condensate purification device 60, a low pressure feed water heater 61, a feed water pump 63, and a high pressure feed water heater 62 connected to a feed water pipe 58 that connects the condenser 57 and the RPV 50. To RPV50 in this order. A hydrogen injection device 66 is connected to a 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 connected to the water supply pipe 58.

  The reactor water purification system includes a purification system pump 70, a regenerative heat exchanger 71, a non-regenerative heat exchanger 72, and a reactor water purification device 73 in a purification system pipe 69 that connects the recirculation system pipe 55 and the feed water pipe 60. Installed and configured. The purification system pipe 69 is connected to the recirculation system pipe 54 upstream from the recirculation pump 53.

  The cooling water in the RPV 50 is pressurized by the recirculation pump 53, and is jetted from the nozzle (not shown) of the jet pump 52 into the bell mouth (not shown) of the jet pump 52 through the recirculation system pipe 54. . 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 52 is supplied to the core 51 and heated by heat generated by nuclear fission of nuclear fuel material in the fuel rods. Part of the heated reactor water becomes steam. This steam is guided from the RPV 50 through the main steam pipe 55 to the turbine 56 to rotate the turbine 56. A generator (not shown) connected to the turbine 56 is rotated to generate electric power. The steam discharged from the turbine 56 is condensed in the condenser 57 to become water.

  This water is supplied into the RPV 50 through the water supply pipe 58 as water supply. The feed water flowing through the feed water pipe 58 is boosted by the condensate pump 59, impurities are removed by the condensate purification device 60, further boosted by the feed water pump 63, and heated by the low pressure feed water heater 61 and the high pressure feed water heater 62. . The extracted steam extracted from the main steam pipe 55 and the turbine 56 in the extracted pipe 64 is supplied to the low-pressure feed water heater 61 and the high-pressure feed water heater 62, 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 adjusting agent injection device 12, a hydrogen peroxide injection device 7 (oxidant supply unit), a surge tank 17 in which a heater 19 is installed, circulation pumps 20 and 26, a filter. 21, a decomposition device 25, a cation exchange resin tower 23, a mixed bed resin tower 24, a hopper 5, and a carbon steel potential measuring device 81.

  The circulation pipe 2 is provided with an on-off valve 27, a circulation pump 20, a valve 28, valves 29, 30, 31, a surge tank 17, a circulation pump 26, a valve 32, and an on-off valve 33 in this order from the upstream.

  A valve 35 and a filter 21 are installed in a pipe 34 that bypasses the valve 28 and is connected to the circulation pipe 2. A pipe 36 that bypasses the valve 29 is connected to the circulation pipe 2. The pipe 36 is provided with a cooler 22 and a valve 37. The circulation pipe 36 is connected to a pipe 80 having both ends connected to the pipe 36 and bypassing the cooler 22 and the valve 37. A carbon steel electrode 81 and a valve 82 are installed in the pipe 80. A cation exchange resin tower 23 and a valve 40 are installed in a pipe 38 having both ends connected to the circulation pipe 2 and bypassing 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 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.

  Furthermore, the circulation pipe 2 is connected to a pipe 42 in which the valve 43 and the decomposition device 25 are installed, bypassing the valve 31. 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. The surge tank 17 is installed in the circulation pipe 2 between the valve 31 and the circulation pump 26.

  The pH adjusting agent injection device 12 includes a chemical liquid tank 13, an injection pump 14, and an injection pipe 16. The chemical liquid tank 13 is connected to the circulation pipe 2 by an injection pipe 16 having an injection pump 14 and a valve 15. The chemical tank 13 stores hydrazine as a pH adjuster. In the circulation pipe 2, a pH meter 83 is installed between the part where the injection pipe 16 is connected to the circulation pipe 2 and the on-off valve 35.

  The hydrogen peroxide injection device 7 (oxidant supply unit) includes a chemical liquid tank 8, an injection pump 9, an injection pipe 44 and an injection pipe 84. The chemical liquid tank 8 is connected to a pipe 42 upstream of the decomposition apparatus 25 by an injection pipe 44 having an injection pump 9 and a valve 45. The chemical tank 8 stores an oxidizing agent (for example, hydrogen peroxide or ozone). The injection pipe 84 has a valve 85 and is connected to the injection pipe 44 between the injection pump 9 and the valve 45, and the other end is connected to the circulation pipe 2 between the valve 32 and the pH meter 83. ing.

  A pipe 75 is connected to the upper end of the surge tank 17. The pipe 75 is connected to the circulation pipe 2 between the circulation pump 26 and the valve 32. A valve 3 and an ejector 4 are installed in the pipe 75. A hopper 5 is connected to the ejector 4. Both ends of a pipe 47 provided with a valve 46 are connected between the pH meter 83 of the circulation pipe 2 and the on-off valve 33 and between the on-off valve 27 and the circulation pump 20. The hopper 5 is a chemical supply unit for adding an oxidative decontamination agent such as potassium permanganate or a reductive decontamination agent such as oxalic acid to the liquid flowing through the circulation pipe 2. Here, when adding an oxidative decontamination agent, it is also called an oxidative decontamination supply part. On the other hand, when a reducing decontaminating agent is added, it is also called a reducing decontaminating agent supply unit.

  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.

  In FIG. 3, the cooling water in the RPV 50 undergoes radiolysis upon receiving radiation generated in conjunction with the nuclear fission of the nuclear fuel material contained in the fuel assembly loaded in the core 51, such as hydrogen peroxide and oxygen. This produces oxidizing species. This oxidizing species increases the corrosion potential of the components of the nuclear plant that come into contact with the cooling water. Therefore, in the BWR plant, as an environmental mitigation measure against stress corrosion cracking, hydrogen is injected into the feed water from the hydrogen injection device 66, and this hydrogen reacts with oxidizing chemical species such as hydrogen peroxide and oxygen contained in the cooling water. By doing so, the operation of reducing the oxidizing chemical concentration of the cooling water and lowering the corrosion potential of the components of the nuclear power plant is performed.

  The operation performed while injecting hydrogen into the feed water in the BWR plant is referred to as hydrogen water chemistry (HWC) operation, and the operation without hydrogen injection in the BWR plant is referred to as normal water quality (NWC). It is called driving. The operation of the BWR plant that lowers the corrosion potential by hydrogen injection is preferably continued during the operation, but the hydrogen injection may be interrupted, and the operation of the BWR plant when this hydrogen injection is interrupted. Is NWC operation, and the corrosion potential of the nuclear plant components becomes high.

  A part of the reactor water flowing in the recirculation system pipe 54 flows into the purification system pipe 67 by driving the purification system pump 68 and is cooled through the regenerative heat exchanger 69 and the non-regenerative heat exchanger 70. Impurities are removed by the water purification device 71. The purified reactor water is heated by the regenerative heat exchanger 69 and returned to the RPV 50 through the purification system pipe 67 and the water supply pipe 58.

  The BWR plant is stopped after the operation of 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 51 are replaced with new fuel assemblies. That is, a part of the fuel assembly in the core 51 is taken out from the RPV 50 as a spent fuel assembly, and a new fuel assembly having a burnup of 0 GWd / t is loaded into the core 51. After the operation of the BWR plant is stopped and before inspection of each device in the periodic inspection is performed, 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 68, the regenerative heat exchanger 69, the non-regenerative heat exchanger 70, etc., provided in the purification system piping 67 of the reactor purification system, which is a pipe made of carbon steel, is planned. This is a case in which chemical decontamination is performed on the purification system pipe 67 in order to reduce radiation exposure of an inspection worker or a maintenance worker. At this time, each process of step S1-S8 shown in FIG. In the following description, the reference numerals in FIGS. 3 and 4 are also used.

  In the BWR plant that has experienced operation, an oxide film containing a radionuclide is formed on the inner surface of the recirculation system pipe 54 and the purification system pipe 67 through which the cooling water in the RPV 50 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. For this reason, it is a process of removing the oxide film from the inner surface of the purification system pipe 67 or the like that is a pipe made of carbon steel. . In the chemical decontamination of the purification system pipe 67, the chemical decontamination apparatus 1 shown in FIG. 3 is used.

  First, the chemical decontamination apparatus 1 is connected to a piping system that is an object of chemical decontamination, that is, a piping system of a nuclear power plant whose operation has been stopped (step S1). The chemical decontamination apparatus 1 is a temporary facility, and both ends of the circulation pipe 2 are connected to a carbon steel purification system pipe 67 that is a chemical decontamination target.

  Here, the operation | work which connects the circulation piping 2 to the purification system piping 67 is demonstrated concretely.

  After the operation of the BWR plant is stopped, for example, the bonnet of the valve 171 installed in the purification system pipe 67 connected to the recirculation system pipe 54 is opened, and the recirculation system pipe 54 side is sealed. One end of the circulation pipe 2 of the chemical decontamination apparatus 1 is connected to the flange of the valve 171. As a result, one end of the circulation pipe 2 is connected to the purification system pipe 67 upstream of the purification system pump 68. On the other hand, on the downstream side of the purification system pump 68, the bonnet of the valve 74 installed in the purification system pipe 67 is opened to seal the regenerative heat exchanger 69 side. The other end of the circulation pipe 2 of the chemical decontamination apparatus 1 is connected to the flange of the valve 74. Thereby, one end of the circulation pipe 2 is connected to the purification system pipe 67 downstream of the purification system pump 68. A closed loop including the purification system pipe 67 and the circulation pipe 2 is formed.

  After the chemical decontamination apparatus 1 is connected to the purification system pipe 67, before the circulation pumps 20 and 26 are driven in step S2, the purification system pipe between the circulation pipe 2, the surge tank 17, and the valve 171 and the valve 74 is provided. 67 is filled with water. This water is injected from the surge tank 17, for example.

  The temperature of circulating water is adjusted (step S2).

  First, the on-off valves 27 and 33 are closed, the valves 28, 29, 30, 31, 32 and 46 are opened, and the other pumps are closed, the circulation pumps 20 and 26 are started, and the surge tank 17 Water is circulated in the circulation pipe 2. Subsequently, the valves other than the on-off valves 27 and 33 are opened and closed, and each pipe connected to the circulation pipe is vented. After the air venting is finished, the valves other than the valves 28, 29, 30, 31, 32, 46 are closed, the open / close valves 27, 33 are opened, and the other pumps are closed, and the circulation pumps 20, 26 are kept running. The valve 46 is closed and the water in the surge tank 17 is circulated in the circulation pipe 2 and the purification system pipe 67. And the circulating water which circulates in the circulation piping 2 and the purification system piping 67 is heated with the heater 19 installed in the surge tank 17, and the temperature of circulating water is adjusted to about 90 degreeC.

  The measurement of the potential is started with the carbon steel electrode incorporated in the chemical decontamination apparatus 1 (step S3).

  Circulating water is passed through the carbon electrode potential measuring device 81 by opening the valve 82 and adjusting the opening degree of the valve 29. A carbon steel electrode and an Ag / AgCl electrode are inserted into the carbon steel potential measuring device 81. The carbon steel electrode and the Ag / AgCl electrode are taken out of the carbon steel electrode and the electrode cable is connected to an electrometer, and the potential difference between the two electrodes is measured.

  The piping system is subjected to oxidative decontamination (step S4).

  The valve 3 is opened, and potassium permanganate (oxidative decontamination agent) charged into the hopper 5 is supplied into the pipe 75 through the ejector 4, and is further introduced into the surge tank 17 by the water flowing through the pipe 75. . Potassium permanganate is dissolved in 90 ° C. water in the surge tank 17 to produce an oxidative decontamination solution (potassium permanganate aqueous solution). This oxidative decontamination liquid is pressurized by the circulation pump 26 and supplied from the surge tank 17 to the purification system pipe 67 through the circulation pipe 2. 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 67 (oxidation decontamination step).

  Next, oxalic acid is added (step S5).

  After the oxidative decontamination (step S4) is completed, the oxalic acid charged into the hopper 5 is supplied from the ejector 4 to the oxidative decontamination liquid flowing in the pipe 75 and guided into the surge tank 17. By this oxalic acid, potassium permanganate contained in the oxidative decontamination solution is decomposed (oxidative decontaminant decomposition step).

  After completion of the oxidative decontamination agent decomposition step, reductive decontamination is performed on the inner surface of the piping system using the reductive decontamination liquid, for example, the purification system pipe 67 that has undergone oxidative decontamination.

  The reduction decontamination in the chemical decontamination method of this example will be described in detail below.

  Addition of oxalic acid (step S6) and steps S7 to S12 for generating a reduction decontamination solution are reduction decontamination steps.

  When the oxalic acid in the hopper 5 is continuously supplied into the pipe 75 in step S <b> 6, the potassium permanganate eventually disappears and an oxalic acid aqueous solution (reduction decontamination solution) is generated in the surge tank 17.

  A pH adjuster is injected into this oxalic acid aqueous solution. A 90 ° C. oxalic acid aqueous solution is supplied from the surge tank 17 to the purification system pipe 67 through the circulation pipe 2. Hydrazine, which is a pH adjusting agent, is injected into the aqueous oxalic acid solution saturated with dissolved oxygen at 90 ° C. flowing through the circulation pipe 2 by the pH adjusting agent injection device 12.

  Specifically, the valve 15 is opened to drive the infusion pump 14. As a result, hydrazine in the chemical tank 13 is injected into the circulation pipe 2 through the injection pipe 16. The pH of the oxalic acid aqueous solution at 90 ° C. is adjusted to a range of 2 to 3, for example, pH 2.5 by injection of hydrazine. By adjusting the pH of the aqueous oxalic acid solution within the range of 2 to 3, excessive dissolution of the carbon steel purification system pipe 67 is suppressed. The injection amount of hydrazine into the circulation pipe 2 is such that the pH measurement value becomes 2.5 based on the pH measurement value of the oxalic acid aqueous solution flowing in the circulation pipe 2 measured by the pH meter 83. It is adjusted by controlling the rotational speed of 14 (or the opening degree of the valve 15).

Since the circulation pumps 20 and 26 are driven, an aqueous oxalic acid solution having a pH of 2.5 at 90 ° C. is supplied into the purification system pipe 67 that is a chemical decontamination target. The oxalic acid concentration of the oxalic acid aqueous solution is 2000 ppm. When this aqueous oxalic acid solution contacts the inner surface of the purification system pipe 67, the oxide film adhering to the inner surface of the purification system pipe 67 is dissolved by the oxalic acid. The oxide film contains corrosion products containing radionuclides. Part of Fe ²⁺ generated by dissolution of Fe (II) contained in the oxide film or dissolution of carbon steel in the purification system pipe combines with oxalate ions to form iron (II) oxalate precipitates. .

  Next, water is passed through the cation exchange resin tower 23.

After the oxidative decontaminant decomposition step is completed, the valve 40 is opened to adjust the opening of the valve 30, and a part of the oxalic acid aqueous solution discharged from the purification system pipe 67 to the circulation pipe 2 is cation exchanged. The resin is supplied to the 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 67 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 30 and guided into the surge tank 17. An aqueous oxalic acid solution having a pH of 2.5 at 90 ° C. is supplied from the circulation pipe 2 to the purification system pipe 67. The oxalic acid aqueous solution circulates in the closed loop formed by the purification system pipe 67 and the circulation pipe 2, thereby acting as a medium for reductive decontamination on the inner surface of the purification system pipe 67. 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.

  Then, the radiation detector 76 (dose rate monitor) disposed outside the purification system pipe 67 that is a chemical decontamination object detects radiation emitted from the purification system pipe 67 that is a chemical decontamination object, Outputs radiation detection signal. Based on this radiation detection signal, the dose rate of the purification system pipe 67 is obtained (step S7).

  Reducing decontamination is continued while the downward trend in dose rate determined by radionuclide removal continues.

When the decreasing tendency is settled, in other words, when the dose rate becomes a substantially constant value, the process proceeds to hydrogen peroxide addition, which is the next step S8. The end of the downward trend is, for example, the time when the rate of decrease in the dose rate becomes 1/10 or less compared to the previous hour. This lowering of the dose rate is due to the fact that the oxide film dissolved by oxalic acid or Fe ²⁺ dissolved from the carbon steel base metal binds to oxalic acid, covers the surface of the pipe, and dissolves the oxide film containing radionuclides. Because it was suppressed.

  When the rate of decrease in the dose rate becomes 1/10 of the rate of decrease in the previous hour, the valve 85 is opened, the injection pump 9 is driven, and hydrogen peroxide in the tank 8 is circulated through the pipe 84. Supply to the pipe 2 (step S8).

At this time, in order to prevent deterioration of the cation exchange resin due to hydrogen peroxide, the valve 40 is closed and the valve 30 is fully opened. The hydrogen peroxide supplied to the circulating water acts on iron (II) oxalate formed on the surface of the carbon steel purification system pipe 67. Then, Fe ²⁺ is oxidized to Fe ³⁺ by the above formulas (1), (3), (4), (5) and (6) and removed from the surface of the pipe. The added hydrogen peroxide is consumed by the reaction with iron (II) oxalate from the upstream side of the purification system pipe 67, but reaches the carbon steel electrode potential measuring device 81 by supplying sufficient hydrogen peroxide. To do. Since the carbon steel electrode of the carbon steel electrode potential measuring device 81 was also exposed to the oxalic acid solution, an iron (II) oxalate film was formed on the surface thereof. As a result, the corrosion rate decreased and the potential showed a constant value.

  When hydrogen peroxide arrives there, removal of iron (II) oxalate occurs as in the carbon steel pipe, and the potential decreases (step S9). If no potential drop is observed, the hydrogen peroxide is consumed before reaching the carbon steel electrode, so the addition of hydrogen peroxide is continued. By detecting this potential drop, it can be confirmed that the removal of iron (II) oxalate by hydrogen peroxide occurs from upstream to downstream of the purification system pipe 67. Stop the addition of hydrogen peroxide when the potential drop of the carbon steel electrode is confirmed.

  As iron (II) oxalate is removed, dissolution of the oxide film containing the radionuclide formed in the purification system pipe 67 with oxalic acid proceeds, and the dose rate begins to decrease.

  Continue reducing decontamination while the dose rate continues to decline. When the dose rate becomes a substantially constant value, for example, reductive decontamination is continued until it falls below the rate of decrease in dose rate before the addition of hydrogen peroxide (step 10). When the reduction decontamination is continued, if the hydrogen peroxide concentration in the reduction decontamination solution is below 1 ppm, the decontamination solution is passed through the cation exchange resin tower 23.

  If the rate of decrease in dose rate before the addition of hydrogen peroxide falls below, hydrogen peroxide is added again. Confirm the potential drop of the carbon steel electrode, confirm that the removal of iron (II) oxalate by hydrogen peroxide has reached the downstream side, confirm the potential drop, and then stop adding hydrogen peroxide . If the dose rate begins to decrease again, continue with decontamination.

  If the dose rate does not decrease due to the addition of hydrogen peroxide, the process proceeds to the decomposition of the reductive decontaminating agent (step S11).

  When decomposing the reducing decontaminating agent, the decontaminating solution is sampled to check for the presence of hydrogen peroxide. If hydrogen peroxide remains, the valve 43 is opened, the valve 31 is closed, and the reducing decontamination solution is passed through the decomposition device 25 to decompose the hydrogen peroxide. After confirming the decomposition of hydrogen peroxide, the valve 40 is opened and the valve 30 is closed, and the reducing decontamination solution is passed through the cation exchange resin tower 23. The circulating water exiting the cation exchange resin tower 23 is guided to the decomposition device 25. The valve 45 is opened to start the injection pump 9, and the hydrogen peroxide in the tank 8 is injected into the circulating water immediately before the decomposition device 25. In the decomposition device 25, oxalic acid and hydrazine remaining in the circulating water are decomposed by the action of hydrogen peroxide and a catalyst. 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 9 is stopped, the valve 45 is closed, and the supply of hydrogen peroxide from the chemical tank 8 to the decomposition device 25 is stopped. Further, the valve 31 is fully opened and the valve 43 is closed.

  Purification is performed (step S12).

  After the decomposition of oxalic acid and hydrazine, the energization to the heater 19 is stopped, the heating of the oxalic acid aqueous solution by the heater 19 is stopped, the valves 35, 37, 41 are opened, and the valves 28, 29, 82, 30 are closed. . The aqueous oxalic acid solution that has flowed into the pump 20 from the circulation pipe 2 is supplied to the filter 21 through the pipe 34. In the filter 21, fine solids contained in the oxalic acid aqueous solution are removed. 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. Thereby, the electrical conductivity of the aqueous solution discharged | emitted from the mixed bed resin tower 24 falls. 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, purification process S12 is complete | finished.

  When the dose rate of the purification system pipe 67 is higher than the target set dose rate after the purification process is completed, the processes in steps S2 to S12 are performed until the dose rate of the purification system pipe 67 becomes equal to or lower than the set dose rate. It is carried out repeatedly.

  When the dose rate of the purification system pipe 67 becomes equal to or lower than the set dose rate, the water remaining in the circulation pipe 2 and the purification system pipe 67 at the end of the final purification process becomes waste liquid and is discharged outside these pipes. . Thereafter, the on-off valves 27 and 33 are closed, the valve 46 is opened, water is filled into the circulation pipe 26 and the pipe 49, and the circulation pumps 20 and 26 are driven. The inside of the closed loop formed by the circulation pipe 2 and the pipe 46 is circulated to clean the inner surface of the circulation pipe 2 and the like.

  After the cleaning is completed, the water in the circulation pipe 2 and the pipe 47 becomes a waste liquid and is discharged outside these pipes. Thereby, the chemical decontamination of the purification system piping 67 in the present embodiment is completed. Each of the waste liquids discharged from the circulation pipe 2, the purification system pipe 67, and the pipe 46 is treated as a radioactive waste liquid. After chemical decontamination is completed, the circulation pipe 2 is removed from the purification system pipe 67, and the purification system pipe 67 is restored to its original state. Then, the BWR plant is activated after the periodic inspection is completed.

  According to this example, when removing the iron (II) oxalate film formed on the carbon steel member during reduction decontamination with hydrogen peroxide, the carbon steel installed on the downstream side of the carbon steel member to be decontaminated. By measuring the potential change of the electrode, it can be confirmed that the removal of iron (II) oxalate by hydrogen peroxide reaches the entire carbon steel member. By removing iron (II) oxalate, oxalic acid in the reductive decontamination solution can act on the oxide film formed on the carbon steel member containing the radionuclide, and DF can be improved. In addition, since the amount of hydrogen peroxide added can be properly controlled, the DF decreases due to iron (II) oxalate residue due to the shortage of hydrogen peroxide, and the excess in the subsequent process due to excessive hydrogen peroxide. The influence of the increase in the decomposition time of hydrogen oxide can be suppressed.

  A method and apparatus for chemical decontamination of a carbon steel member in Example 2, which is another preferred embodiment of the present invention, will be described with reference to FIG. The present embodiment is an example applied to a boiling water nuclear plant (BWR plant).

  In this figure, when chemical decontamination of the purification pipe 67 made of carbon steel is performed, chemical decontamination of the recirculation pipe (stainless steel member) 54 made of stainless steel is also performed. Therefore, a dose rate monitor 77 for measuring the dose rate of the recirculation system pipe 54 is installed. In the chemical decontamination method of this example, the chemical decontamination apparatus 1 used in Example 1 is used.

  In the chemical decontamination method of this embodiment, steps S1 to S12 shown in FIG. 1 are performed. In the following description, the reference numerals in FIG. 5 are also used. Further, description will be made using part of the reference numerals in FIG.

  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 50 (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 54. One end of the circulation pipe 2 is connected to the flange of the valve 74 provided in the purification system pipe 67 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 54 on the downstream side of the recirculation pump 53 and connected to the separated branch pipe. The As a result, a closed loop including the circulation pipe 2, the recirculation system pipe 54 and the purification system pipe 67 is formed.

  Then, each process of step S2-S12 is implemented similarly to Example 1. FIG.

  In step S4, oxidative decontamination of the inner surfaces of the recirculation system pipe 54 and the purification system pipe 67 is performed with an aqueous potassium permanganate solution.

  By performing the steps S5 to S6 after step S4, an oxalic acid aqueous solution having a pH of 2.5 at 90 ° C. is generated as in the first embodiment. This oxalic acid aqueous solution is supplied to the recirculation system pipe 54 and the purification system pipe 67, and reductive decontamination is performed on the inner surfaces of these pipes. While reducing decontamination, the oxalic acid aqueous solution discharged from the purification system pipe 67 to the circulation pipe 2 is supplied to the cation exchange resin tower 23, and the metal cation is removed.

  A dose rate monitor 76 is installed on the recirculation system pipe 54 side, and a dose rate monitor 77 is installed on the reactor water purification system pipe 67 side, and each pipe dose rate is constantly measured. As the reduction decontamination progresses, the formation of iron (II) oxalate proceeds in the reactor water purification system pipe 67, and the reduction in the dose rate is reduced. On the other hand, although the dose rate on the side of the recirculation pipe 54 made of stainless steel continues to decrease, the decrease in dose rate will eventually stop. Hydrogen peroxide is added when the decrease in the dose rate of the two has subsided (step S8).

  The valve 85 is opened to start the pump 9, and hydrogen peroxide in the tank 8 is injected into the reductive decontamination liquid circulating through the circulation pipe 2 through the pipe 84. The added hydrogen peroxide does not act on the recirculation system pipe 54, but reacts with iron (II) oxalate formed on the surface in the reactor water purification system pipe 67 to remove it. Hydrogen peroxide contained in the decontamination solution that has no time to contact with iron (II) oxalate and has flowed out of the reactor water purification system pipe 67 to the circulation pipe 2 reaches the carbon steel electrode potential measuring device 81, The iron (II) oxalate film formed on the steel electrode is removed to lower the potential of the carbon steel electrode.

  It can be confirmed that due to the potential drop of the carbon steel electrode, hydrogen peroxide reaches the downstream side of the reactor water purification system pipe 67 and iron (II) oxalate is removed.

  After confirming the potential drop of the carbon steel electrode, the pump 9 is stopped to stop the addition of hydrogen peroxide, and the valve 85 is closed. The removal of the iron (II) oxalate on the surface of the reactor water purification system pipe 67 causes the dissolution of the oxide film containing the radionuclide by the oxalic acid in the reductive decontamination solution. Dose rate decreases. However, since iron (II) oxalate is formed again to cover the surface of the carbon steel member, the reduction in dose rate converges.

  In Example 1, hydrogen peroxide was added again, but in Example 2, since the system including the stainless steel pipe is decontaminated, there is an opportunity to carry out reductive decontamination at least once again. For this reason, it transfers to the following reductive decontaminant decomposition process here (step S11).

  When decomposing the reducing decontaminating agent, the decontaminating solution is sampled to check for the presence of hydrogen peroxide. When hydrogen peroxide remains, the valve 43 is opened, the valve 31 is closed, and the reducing decontamination solution is passed through the decomposition device 25 to decompose the hydrogen peroxide. After confirming the decomposition of hydrogen peroxide, the valve 40 is opened and the valve 30 is closed, and the reducing decontamination solution is passed through the cation exchange resin tower 23.

  The circulating water exiting the cation exchange resin tower 23 is led to the decomposition device 25. The valve 45 is opened, the injection pump 9 is started, and the hydrogen peroxide in the tank 8 is injected into the circulating water immediately before the decomposition device 25. In the decomposition device 25, oxalic acid and hydrazine remaining in the circulating water are decomposed by the action of hydrogen peroxide and a catalyst. Hydrazine degrades faster than oxalic acid. It is assumed that the decomposition of oxalic acid is completed when the oxalic acid concentration is reduced to 10 ppm.

  After the decomposition of oxalic acid is completed, the injection pump 9 is stopped, the valve 45 is closed, and the supply of hydrogen peroxide from the chemical tank 8 to the decomposition device 25 is stopped. Further, the valve 31 is fully opened and the valve 43 is closed.

  Purification is performed (step S12).

  After the decomposition of oxalic acid and hydrazine, the energization to the heater 19 is stopped, the heating of the oxalic acid aqueous solution by the heater 19 is stopped, the valves 35, 37, 41 are opened, and the valves 28, 29, 82, 30 are closed. . The aqueous oxalic acid solution that has flowed into the pump 20 from the circulation pipe 2 is supplied to the filter 21 through the pipe 34. Fine solids contained in the oxalic acid aqueous solution are removed by the filter 21.

  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. Thereby, the electrical conductivity of the aqueous solution discharged | emitted from the mixed bed resin tower 24 falls. 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 67 is higher than the target set dose rate after the purification process is completed, the processes in steps S2 to S12 are performed until the dose rate of the purification system pipe 67 becomes equal to or lower than the set dose rate. It is carried out repeatedly.

  In the present embodiment, each effect according to the first embodiment can be obtained. In the present embodiment, the oxidation decontamination liquid and the reduction decontamination liquid can be supplied to the stainless steel recirculation system pipe 54 together with the carbon steel purification system pipe 67. It can be carried out.

  A method and apparatus for chemical decontamination of a carbon steel member in Example 3, which is another preferred embodiment of the present invention, will be described with reference to FIG. The present embodiment is an example applied to a boiling water nuclear plant (BWR plant).

  In the chemical decontamination method of this embodiment, steps S1 to S12 shown in FIG. 1 are performed.

  In the chemical decontamination method of this embodiment, as shown in FIG. 6, the operations of the injection pump 9, the valve 45 and the valve 85 of the hydrogen peroxide injection system of the chemical decontamination apparatus 1 (shown in FIG. 3) are controlled. A control device 86 (control unit) for collecting and analyzing data from the radiation monitor 76 and the carbon steel electrode potential measuring device 81 is installed.

  In FIG. 1, steps S1 to S2 are performed in the same manner as in the first embodiment.

  In the potential measurement in step S3, the control device 86 collects the potential difference between the carbon steel electrode from the carbon steel electrode potential measuring device 81 and the Ag / AgCl reference electrode, and then continues to collect potential difference data. From the start of the decontamination with the medicine in step S4, the collection of the pipe dose rate data measured by the radiation monitor 76 is also started by the control device 86.

  When the reduction decontamination of step S6 is started, the pipe dose rate starts to decrease and the potential of the carbon steel electrode also starts to change. If the data of the dose rate is continuously monitored, the rate of decrease of the dose rate is reduced by the production of iron (II) oxalate and the coating on the surface of the oxygen steel member. At the same time, the carbon steel potential also shows a constant value. For example, when the rate of decrease in dose rate (dose rate decrease rate) becomes 1/10 or less than the rate of decrease in dose rate for 1 hour after the start of reductive decontamination, the addition of hydrogen peroxide in step S8 is started. The controller is programmed in advance. The control device 86 detects that the rate of decrease in the dose rate is 1/10 or less than the rate of decrease in the dose rate for 1 hour after the start of reductive decontamination, and outputs an opening signal for the valve 85 and an activation signal for the pump 9. Output. As the injection speed of the pump 9, for example, the rotation speed of the pump at which the hydrogen peroxide concentration at the injection point is 100 ppm is set as an initial value.

  There is a time difference between the start of hydrogen peroxide injection and the arrival of hydrogen peroxide at the carbon steel electrode. The difference is based on the residence time determined from the flow rates of the pump 20 and the pump 26 shown in FIG. short. When the potential of the carbon steel electrode does not decrease even after the residence time has elapsed since the start of hydrogen peroxide addition, the hydrogen peroxide addition rate is increased, for example, by a factor of 2, and the amount of hydrogen peroxide injected is increased. Allow hydrogen peroxide to reach the carbon steel electrode. When the carbon steel electrode potential starts to fall by the end of one residence time after the signal for increasing the rotational speed from the controller 86 to the pump 9 is output, the controller 86 is configured to stop the injection of hydrogen peroxide. To output a signal indicating that the pump 9 is stopped and the valve 85 is closed.

  Thereby, since it can confirm that iron oxalate (II) was removed, melt | dissolution of the oxide film containing a radionuclide by a reductive decontamination liquid advances again, and the fall of a dose rate restarts. This dose rate is measured by the dose rate monitor 76, and the value is collected in the control device 86.

  When iron (II) oxalate is generated again and it is detected that the dose rate declines as in the previous case, hydrogen peroxide injection is commanded from the controller 86, and the carbon steel electrode potential is reduced. Continue until the dose rate does not decrease.

  Next, the reducing decontaminant decomposition step of Step S11 is performed, and the subsequent steps are performed in the same manner as in Example 1.

  In this embodiment, the same effect as in Embodiment 1 can be obtained, and addition of hydrogen peroxide, stop of addition, determination of completion of reductive decontamination and equipment operation can be automated, so that the work time can be shortened and decontamination can be achieved. It is possible to reduce the work man-hours when performing the work.

  1: Chemical decontamination device, 2: Circulation piping, 4: Ejector, 5: Hopper, 7: Hydrogen peroxide injection device, 8, 13: Chemical solution tank, 9, 14: Injection pump, 12: pH adjuster injection device, 16, 44, 84: injection pipe, 17: surge tank, 19: heating device, 20, 26: circulation pump, 21: filter, 22: cooler, 23: cation exchange resin tower, 24: mixed bed resin tower, 25: decomposition device, 11: nuclear reactor, 50: reactor pressure vessel, 51: core, 54: recirculation piping, 58: water supply piping, 67: purification piping, 76, 77: radiation detector, 81: carbon Steel electrode potential measuring device.

Claims (19)

A method for chemical decontamination of a carbon steel member that is a structural material member of a nuclear power plant,
A step of reducing and decontaminating the carbon steel member by supplying oxalic acid which is a reducing decontamination agent;
Supplying an oxidizing agent to the carbon steel member subjected to the reduction decontamination, and
The state of the iron (II) oxalate film formed on the carbon steel member is detected by measuring the change in potential of the carbon steel electrode provided in contact with the reducing decontamination agent and the oxidizing agent. , Chemical decontamination method.   The chemical decontamination method according to claim 1, wherein a liquid that has been supplied to the carbon steel member and passed through the carbon steel member is sent to the carbon steel electrode.   The chemical decontamination method according to claim 1, wherein the oxidizing agent is added to a liquid containing the reductive decontaminating agent and then contacts the carbon steel member.   The chemical decontamination method according to claim 1, wherein a dose rate of the carbon steel member is measured when the reductive decontamination and the oxidizing agent are supplied.   5. When reducing the decontamination, the reduction decontamination is continued if the dose rate continues to decrease, and the supply of the oxidizing agent is started when the dose rate becomes substantially constant. Chemical decontamination method.   The chemical decontamination method according to claim 5, wherein the supply of the oxidizing agent is continued until the potential of the carbon steel electrode is lowered, and the supply of the oxidizing agent is stopped after the potential of the carbon steel electrode is lowered.   The chemical decontamination method according to claim 6, wherein after the supply of the oxidizing agent is stopped, the reduction decontamination is continued when the dose rate continues to decrease.   The chemical decontamination method according to claim 7, wherein after the reduction of the dose rate is completed, the reductive decontaminant is decomposed to purify the liquid containing the reductive decontaminant.   The chemical decontamination method according to claim 7, wherein the supply of the oxidizing agent is stopped and the reductive decontamination is repeated.   The determination at the time when the dose rate becomes substantially constant is made by comparing the dose rate decrease rate for one hour after the start of the reduction decontamination with the subsequent dose rate decrease rate, and the subsequent dose rate decrease rate is the start rate. The chemical decontamination method according to claim 5, wherein the chemical decontamination method is set to a time when the dose rate lowering rate of 1 hour later becomes 1/10 or less.   The chemical decontamination method according to claim 1, wherein the oxidizing agent is hydrogen peroxide or ozone. An apparatus for chemical decontamination of carbon steel members that are structural members of nuclear power plants,
A chemical supply section for supplying oxalic acid as a reductive decontamination agent to the carbon steel member;
An oxidant supply unit for supplying an oxidant to the carbon steel member;
A carbon steel electrode provided in contact with the reductive decontamination solution and the oxidizing agent,
The chemical decontamination apparatus which has the structure which detects the state of the film | membrane of the iron (II) oxalate formed in the said carbon steel member by measuring the change of the electric potential of the said carbon steel electrode. Furthermore, including a circulation pipe that can be attached to the carbon steel member,
The chemical decontamination apparatus according to claim 12, wherein the chemical supply unit and the oxidant supply unit are attached to the circulation pipe.   The chemical decontamination apparatus according to claim 13, wherein the carbon steel electrode is disposed at a site to which a liquid supplied to the carbon steel member from the circulation pipe and passed through the carbon steel member is sent.   The chemical decontamination apparatus according to claim 14, wherein a part of the liquid that is supplied from the circulation pipe to the carbon steel member and passes through the carbon steel member is sent to the carbon steel electrode. Furthermore, a radiation detector attached to the carbon steel member,
A control unit,
The control unit controls the supply of the reducing decontamination agent and the oxidizing agent by using at least the potential of the carbon steel electrode and the dose rate of the carbon steel member measured by the radiation detector. 12. The chemical decontamination apparatus according to 12.   The chemical decontamination apparatus according to claim 12, wherein the oxidizing agent is hydrogen peroxide or ozone.   The nuclear power plant which enabled to attach the chemical decontamination apparatus as described in any one of Claims 12-17.   A nuclear power plant provided with the chemical decontamination device according to any one of claims 12 to 17.

Images