JP7411502B2 - Chemical decontamination method for carbon steel parts of nuclear power plants - Google Patents

Description

The present invention relates to a method for chemical decontamination of carbon steel members of a nuclear power plant, and particularly to a nuclear power plant suitable for decontamination of carbon steel members contaminated with radionuclides in a boiling water nuclear power plant before disposal. This article relates to a method for chemical decontamination of carbon steel members of a plant.

For example, a boiling water nuclear power plant (hereinafter referred to as a BWR plant) has a nuclear reactor with a reactor core built into a reactor pressure vessel (RPV). Reactor water supplied to the reactor core by a recirculation pump (or internal pump) is heated by the heat generated by the fission of nuclear fuel material in the fuel assemblies loaded in the reactor core, and a portion of it becomes steam. This steam is guided from the RPV to the turbine and rotates the turbine. Steam discharged from the turbine is condensed into water in a condenser. This water is supplied to the reactor as feed water. In order to suppress the generation of radioactive corrosion products in the RPV, metal impurities are mainly removed from the water supply in a filtration and demineralization device installed in the water supply piping. Reactor water is cooling water present within the RPV.

Corrosion products, which are the source of radioactive corrosion products, occur on the surfaces of BWR plant components such as RPV and recirculation system piping that come into contact with reactor water. Rustless steels such as steel and nickel-based alloys are used. Furthermore, RPVs made of low-alloy steel have stainless steel built-up on their inner surfaces to prevent the low-alloy steel from coming into direct contact with reactor water. Furthermore, a portion of the reactor water is purified by a filtration desalination device in the reactor purification system to actively remove small amounts of metal impurities present in the reactor water.

However, even if the above-mentioned corrosion countermeasures are taken, the presence of extremely small amounts of metal impurities in the reactor water cannot be avoided. Adheres to surfaces. Impurities (e.g., metal elements) attached to the surface of the fuel rods undergo a nuclear reaction when irradiated with neutrons released by nuclear fission of the nuclear fuel material inside the fuel rods, causing radioactive substances such as cobalt-60, cobalt-58, chromium-51, manganese-54, etc. Becomes a nuclide.

These radionuclides remain attached to the fuel rod surface, mostly in the form of oxides. However, some radionuclides are eluted into the reactor water as ions or re-released into the reactor water as insoluble solids called cladding, depending on the solubility of the incorporated oxides. Radioactive substances contained in the reactor water are removed by a reactor purification system connected to the RPV. Radioactive materials that are not removed in the reactor cleanup system are accumulated on surfaces of nuclear plant components (for example, piping) that come into contact with the reactor water while circulating together with the reactor water in a recirculation system or the like. As a result, radiation is emitted from the surface of the component, causing radiation exposure to workers during periodic inspection work.

The exposure doses of these workers are controlled so that they do not exceed the prescribed values for each individual. In recent years, this standard value has been lowered, and it has become necessary to reduce the exposure dose of each person to the lowest possible level.

Therefore, if the exposure dose during regular inspection work is expected to be high, chemical decontamination may be carried out to dissolve and remove radionuclides attached to the pipes. For example, JP-A-2000-105295 discloses reductive dissolution using a reducing decontamination solution containing oxalic acid and hydrazine, decomposition of the reducing decontamination agent (oxalic acid) used for this reductive dissolution, and oxidative dissolution using potassium permanganate. A chemical decontamination method for nuclear power plants that combines the following methods 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 an aqueous formic acid solution. In this chemical decontamination method, carbon steel members of nuclear power plants are chemically decontaminated using an oxalic acid aqueous solution to remove the oxide film containing radionuclides present on the surface of the carbon steel members, and after this decontamination is completed, oxalic acid is Disassemble. Furthermore, after the 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, thereby removing iron oxalate remaining on the surface of the carbon steel member.

JP-A-2002-333498 proposes a chemical decontamination method for decontaminating carbon steel members using an aqueous solution containing an organic acid (eg, 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 when an aqueous solution containing only an organic acid without hydrogen peroxide is used, and radionuclides are removed from the carbon steel surface. Can be removed in a short time.

Japanese Unexamined Patent Publication No. 2015-28432 proposes a chemical decontamination method for decontaminating carbon steel members of a nuclear power plant using an oxalic acid aqueous solution in which oxygen is dissolved. Iron (II) oxalate formed on the surface of carbon steel parts is oxidized by dissolved oxygen and dissolved into iron (III) oxalate complexes, reducing the time required for reductive decontamination of carbon steel parts. can. Further, since oxygen is dissolved in the oxalic acid aqueous solution, deterioration of the cation exchange resin can be suppressed even when the oxalic acid aqueous solution is brought into contact with the cation exchange resin.

In the chemical decontamination method described in JP 2018-159647, the metal oxide film formed on the inner surface of the piping system of a nuclear power plant is dissolved by reductive decontamination using an oxalic acid aqueous solution, and trivalent iron ions are removed. reduce the sources of The oxalic acid aqueous solution is irradiated with ultraviolet rays, and trivalent iron ions contained in the oxalic acid aqueous solution are reduced to divalent iron ions. Divalent iron ions contained in the oxalic acid aqueous solution produced by the reduction are adsorbed by the ion exchange resin and removed.

Japanese Patent Application Publication No. 2000-105295 Japanese Patent Application Publication No. 2003-90897 Japanese Patent Application Publication No. 2002-333498 JP2015-28432A Japanese Patent Application Publication No. 2018-159647 Japanese Patent Application Publication No. 2018-48831

In the chemical decontamination of JP-A No. 2000-105295 and JP-A No. 2004-286471, structural members made of stainless steel are targeted for decontamination, and when this structural member is subjected to reductive decontamination with a reductive decontamination liquid, In this case, the iron concentration in the reducing decontamination solution does not rise to the extent that iron(II) oxalate precipitates.

However, when the target of decontamination is carbon steel parts, the iron concentration in the oxalic acid aqueous solution increases due to decontamination, and iron is supplied by dissolving the base material of the carbon steel parts and the magnetite that is the oxide film on the surface of the carbon steel parts. There is a possibility that Fe ²⁺ formed in the oxalic acid solution forms a complex with oxalic acid contained in the oxalic acid aqueous solution and precipitates as iron(II) oxalate dihydrate. Since this iron (II) oxalate dihydrate has low solubility, it precipitates on the surface of carbon steel members, which are the main source of Fe ²⁺ . When Fe ²⁺ precipitates on the surface of the oxide film, dissolution of the oxide film by oxalic acid is inhibited, and dissolution of radionuclides contained in the oxide film is also inhibited, resulting in a problem that chemical decontamination efficiency is reduced.

In JP-A No. 2003-90897, the aforementioned iron(II) oxalate dihydrate is removed from the surface of the undissolved oxide film present on the surface of the carbon steel member by reductive decontamination of the surface of the carbon steel member with an oxalic acid aqueous solution. may precipitate. In this case, the time required for reductive decontamination of the oxide film becomes longer. Iron(II) oxalate dihydrate deposited on the surface of the undissolved oxide film on the surface of the carbon steel member can be removed with an aqueous formic acid solution. However, during reductive decontamination, the amount of iron (II) oxalate dihydrate precipitated on the surface of carbon steel members increases, so it takes a long time to remove iron (II) oxalate dihydrate with a formic acid aqueous solution. It will require.

The chemical decontamination method described in JP-A-2002-333498 improves the dissolving power of the oxide film on the surface of a carbon steel member by adding hydrogen peroxide to an organic acid that is a reducing decontamination agent. However, since hydrogen peroxide degrades the cation exchange resin, an oxalic acid aqueous solution containing hydrogen peroxide cannot be supplied to the cation exchange resin tower during reductive decontamination. For this reason, radioactive dissolved components and non-radioactive dissolved components generated by reduction decontamination and contained in the reduction decontamination solution cannot be removed.

In JP-A-2015-28432, in order to solve the problem of JP-A-2002-333498, oxygen is supplied to an oxalic acid aqueous solution to generate an oxygen-saturated oxalic acid aqueous solution, and this is used to manufacture carbon steel members. Reduction decontamination is being carried out. As a result, the reaction in which Fe ²⁺ dissolved in the oxalic acid aqueous solution produces iron(II) oxalate dihydrate competes with the reaction in which Fe ²⁺ reacts with dissolved oxygen to form Fe ³⁺ . Dissolved oxygen also acts on iron (II) oxalate dihydrate and oxidizes the Fe ²⁺ contained therein to Fe ³⁺ , thereby suppressing the accumulation of iron (II) oxalate dihydrate and converting it into oxalate. Decrease in decontamination efficiency due to the production of iron (II) dihydrate is suppressed.

In order to improve the decontamination efficiency of carbon steel members, an organic acid other than oxalic acid is injected into an oxalic acid aqueous solution, or hydrogen peroxide or oxygen is injected into an oxalic acid aqueous solution. These injections proceed to dissolve the carbon steel member that is the base material and the oxide film on the surface of the carbon steel member. In these methods, iron ions and radionuclide ions eluted into an oxalic acid aqueous solution are adsorbed onto a cation exchange resin and recovered from the oxalic acid aqueous solution. However, in reductive decontamination of carbon steel members, the amount of iron ions eluted is greater than the amount of radionuclide ions eluted, so a large amount of cation exchange resin is consumed. In particular, in reduction decontamination aiming at clearance, it is necessary to dissolve the carbon steel component side more deeply, which increases the amount of iron ions eluted and inevitably increases the amount of cation exchange resin that adsorbs iron ions. .

An object of the present invention is to provide a method for chemical decontamination of carbon steel members of a nuclear power plant, which can promote reductive decontamination of carbon steel members and reduce the amount of cation exchange resin used.

A feature of the present invention that achieves the above objects is that an oxalic acid aqueous solution is used to perform reductive decontamination of carbon steel members of a nuclear power plant, and when the reductive decontamination is being performed, an oxidizing agent is added to the oxalic acid aqueous solution. The method is to inject iron(II) oxalate dihydrate in the oxalic acid aqueous solution by irradiating the reduced-decontaminated oxalic acid aqueous solution with ultraviolet rays.

In order to inject an oxidizing agent into an oxalic acid aqueous solution, which is a reductive decontamination solution, iron(II) oxalate dihydrate formed on the surface of carbon steel members of a nuclear power plant that has been reductively decontaminated with an oxalic acid aqueous solution is , is dissolved by the action of the oxidizing agent and removed from the surface of the carbon steel member. Therefore, the reduction decontamination of the carbon steel member by the oxalic acid aqueous solution is promoted. Furthermore, since the oxalic acid aqueous solution that has undergone reductive decontamination is irradiated with ultraviolet rays to precipitate iron (II) oxalate dihydrate in the oxalic acid aqueous solution, the cations contained in the oxalic acid aqueous solution (for example, Fe ²⁺ ) decreases, and the amount of cations adsorbed on the cation exchange resin decreases. Therefore, the life of the cation exchange resin can be extended and the amount of cation exchange resin used can be reduced.

Preferably, the precipitated iron (II) oxalate dihydrate is removed from the oxalic acid aqueous solution. Since the radionuclides contained in the oxalic acid aqueous solution are incorporated into the precipitated iron (II) oxalate dihydrate, by removing the iron (II) oxalate dihydrate from the oxalic acid aqueous solution, the oxalate The radionuclide incorporated into the iron(II) acid dihydrate is also removed from the aqueous oxalic acid solution together with the iron(II) oxalate dihydrate. As a result, the concentration of radionuclides contained in the oxalic acid aqueous solution decreases, and the dose of the oxalic acid aqueous solution decreases. Since the radionuclide incorporated into iron(II) oxalate dihydrate is essentially a cation, the amount of radionuclide ions adsorbed to the cation exchange resin also decreases, which shortens the life of the cation exchange resin. Further, it is possible to further reduce the amount of cation exchange resin used.

According to the present invention, reduction decontamination on the surface of a carbon steel member can be promoted and the amount of cation exchange resin used can be reduced.

The steps of a chemical decontamination method for carbon steel members of a nuclear power plant according to Embodiment 1, which is a preferred embodiment of the present invention, are applied to the decontamination of waste carbon steel members generated from a boiling water nuclear power plant. It is a flowchart. 1 is a configuration diagram of a boiling water nuclear power plant that generates waste materials of carbon steel members to which the chemical decontamination method of Example 1 is applied. 1 is a configuration diagram of a chemical decontamination device used in the chemical decontamination method of Example 1. FIG. The test results investigated the transfer rate of Co-60 to iron (II) oxalate dihydrate formed from an oxalic acid aqueous solution with respect to the amount of iron (II) oxalate dihydrate formed on the surface of a carbon steel member. FIG. FIG. 2 is an explanatory diagram showing the test results of examining the concentration of iron eluted from each of iron (II) oxalate dihydrate and Fe ₂ O ₃ with respect to the oxalic acid concentration of an oxalic acid aqueous solution. The procedure of the 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, is applied to the decontamination of waste materials of carbon steel members generated from a boiling water type nuclear power plant. FIG. The procedure of the 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, is applied to the decontamination of waste materials of carbon steel members generated from a boiling water nuclear power plant. FIG. Used in the chemical decontamination method for carbon steel members of a nuclear power plant according to Embodiment 4, which is another preferred embodiment of the present invention, which is applied to the decontamination of waste carbon steel members generated from a boiling water nuclear power plant. FIG. 2 is a configuration diagram of a chemical decontamination device. The procedure of a chemical decontamination method for carbon steel members of a nuclear power plant according to Embodiment 5 applied to carbon steel members of a boiling water nuclear power plant, for example, purification system piping, which is another preferred embodiment of the present invention, is shown. It is a flowchart. FIG. 7 is an explanatory diagram showing a state in which a chemical decontamination device used when carrying out the method for chemical decontamination of carbon steel members of a nuclear power plant according to Example 5 is connected to purification system piping of a boiling water nuclear power plant. 11 is a configuration diagram of the chemical decontamination apparatus shown in FIG. 10. FIG.

The inventors investigated a method that does not use ion exchange resin as a method for separating radionuclide ions and iron ions dissolved in the decontamination solution from the decontamination solution, and found that iron(II) oxalate dihydrate We considered the use of the precipitation reaction.

Iron (II) oxalate dihydrate (Fe(C ₂ O ₄ ).2H ₂ O) is deposited on the surface of carbon steel members during decontamination using oxalic acid. Decontamination efficiency of carbon steel members decreases due to precipitation of iron(II) oxalate dihydrate. Therefore, in order to improve the decontamination efficiency of carbon steel parts, organic acids other than oxalic acid are injected into the oxalic acid aqueous solution in order to dissolve iron(II) oxalate dihydrate formed on the surface of carbon steel parts. Alternatively, an oxidizing agent that oxidizes Fe ²⁺ to Fe ³⁺ is injected into the oxalic acid aqueous solution. The dissolved radionuclide ions and iron ions are removed from the oxalic acid aqueous solution by being adsorbed on the cation exchange resin. However, the inventors thought that it would be possible to reduce dissolved Fe ³⁺ to Fe ²⁺ , form iron (II) oxalate dihydrate on the surface of the carbon steel member, precipitate it, and separate it.

Therefore, in order to confirm whether the dissolved radionuclide ions were incorporated into the precipitated iron(II) oxalate dihydrate, carbon steel was immersed in an oxalic acid aqueous solution to which Co-60 was added. The amount of iron (II) oxalate dihydrate formed and the rate of transfer of Co-60 contained in the aqueous solution to iron (II) oxalate dihydrate were investigated. The results are shown in FIG. According to Figure 4, as iron(II) oxalate dihydrate is formed on the surface of carbon steel, Co-60 is incorporated into iron(II) oxalate dihydrate formed from an aqueous oxalic acid solution. I know I'm going.

The reason why all of the Co-60 contained in the oxalic acid aqueous solution was not removed from the oxalic acid aqueous solution is that under the present test conditions, the formation of iron(II) oxalate dihydrate took place on the carbon steel surface. This is because the formation of iron (II) oxalate dihydrate on the carbon steel surface was suppressed by coating the carbon steel surface with iron (II) oxalate dihydrate. If the formation of iron(II) oxalate dihydrate continues, it is thought that all of the Co-60 contained in the oxalic acid aqueous solution will be removed.

Next, we investigated how much dissolved iron would be removed from the oxalic acid aqueous solution as iron(II) oxalate dihydrate by reducing Fe ³⁺ to Fe ²⁺ . In conducting this study, first, the solubility of Fe ₂ O ₃ and iron (II) oxalate dihydrate in the oxalic acid aqueous solution was investigated with respect to the oxalic acid concentration of the oxalic acid aqueous solution. In tests to investigate these solubility, Fe ₂ O ₃ powder or iron (II) oxalate dihydrate powder was added to water at 90°C that was purged with nitrogen and shielded from light, and the iron concentration in the water was determined. It was measured. After the measurement, oxalic acid was added to the water and dissolved in the water for about 1 hour, and then a portion of the water was sampled to measure the Fe concentration and the oxalic acid concentration. This operation was repeated, and the relationship between oxalic acid concentration and dissolved iron concentration was investigated for each of Fe ₂ O ₃ and iron (II) oxalate dihydrate.

The results are shown in FIG. The concentration of Fe ³⁺ (iron concentration), indicated by Δ, dissolved from Fe ₂ O ₃ increases linearly as the oxalic acid concentration of the oxalic acid aqueous solution increases. On the other hand, the concentration (iron concentration) of Fe ²⁺ dissolved from iron (II) oxalate dihydrate, indicated by ○, increases to about 50 ppm when the oxalic acid concentration of the oxalic acid aqueous solution reaches about 500 ppm, and Even if the acid concentration was increased, the oxalic acid concentration increased only to 80 ppm at 5500 ppm, and showed a tendency to saturation with respect to the oxalic acid concentration. When the oxalic acid concentration is about 500 ppm or more, the concentration of Fe ³⁺ dissolved from Fe ₂ O ₃ is higher than the concentration of Fe ²⁺ dissolved from iron (II) oxalate dihydrate. The difference between the Fe ³⁺ concentration and the Fe ²⁺ concentration at an oxalic acid concentration of 500 ppm or more corresponds to the iron concentration that is precipitated when Fe ³⁺ is reduced to Fe ²⁺ .

For example, an Fe ₂ O ₃ film formed on the surface of a carbon steel member is dissolved by reductive decontamination using an oxalic acid aqueous solution with an oxalic acid concentration of 5500 ppm, and ultraviolet rays are applied to the oxalic acid aqueous solution containing Fe ³⁺ eluted by this dissolution. irradiate. When Fe ³⁺ contained in the oxalic acid aqueous solution is reduced to Fe ²⁺ by this ultraviolet irradiation, about 1200 ppm of iron is precipitated in the oxalic acid aqueous solution as iron (II) oxalate dihydrate. Here, the reduction reaction of Fe ³⁺ contained in the oxalic acid aqueous solution to Fe ²⁺ by ultraviolet irradiation is expressed by equations (1) and (2).

Note that "Fe ^III (C ₂ O ₄ ) ₃ ^3- " present on the left side of formula (1) (left side of "→") is an iron(III) oxalate complex. As mentioned above, the iron(III) oxalate complex is generated by the reaction of formula (3) shown below when dissolving the Fe ₂ O ₃ film formed on the surface of the carbon steel member with an oxalic acid aqueous solution. .

Fe ₂ O ₃ + 3C ₂ O ₄ H ₂ → Fe ³⁺ + 3H ₂ O + 3C ₂ O ₄ ^2-
→ Fe(III)(C ₂ O ₄ ) ₃ ^3- + 3H ₂ O…(3)
By converting Fe ³⁺ dissolved in an oxalic acid aqueous solution into the Fe ²⁺ form, if the Fe ²⁺ concentration exceeds the saturated solubility in the oxalic acid aqueous solution, Fe ²⁺ will precipitate as iron(II) oxalate dihydrate. do. At this time, radionuclides (e.g., Co-60) dissolved in the oxalic acid aqueous solution are incorporated into the iron(II) oxalate dihydrate, so the precipitated oxalate is By removing iron (II) dihydrate from the oxalic acid aqueous solution using a removal device such as a filter, it is possible to remove each of the iron ions and radionuclide ions such as Co-60 contained in the oxalic acid aqueous solution. It is also possible to reduce the amount of ion exchange resin used.

Examples of the present invention that reflect the above study results will be described below.

FIG. 1 shows a method for chemically decontaminating carbon steel members of a nuclear power plant according to Embodiment 1, which is a preferred embodiment of the present invention and is applied to decontaminating waste carbon steel members generated from a boiling water nuclear power plant. , will be explained using FIGS. 2 and 3. The waste material of carbon steel members (for example, the waste material of carbon steel piping) is used to cut existing carbon steel members during decommissioning of boiling water nuclear power plants or replacement work of carbon steel members of boiling water nuclear power plants. Occurs due to removal.

A schematic configuration of a boiling water nuclear power plant that generates waste materials of carbon steel members to which the method for chemically decontaminating carbon steel members of a nuclear power plant of this embodiment is applied will be described with reference to FIG. The BWR plant 1 includes a nuclear reactor 2, a turbine 9, a condenser 10, a recirculation system, a reactor purification system, a residual heat removal system, a water supply system, and the like. The nuclear reactor 2 installed in the reactor containment vessel 72 has a reactor pressure vessel (hereinafter referred to as RPV) 3 containing a reactor core 4, and a plurality of jet pumps 5 are installed in the RPV 3. A plurality of fuel assemblies (not shown) are loaded into the reactor core 4. 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 7 and a recirculation system piping 6 made of stainless steel, and the recirculation pump 7 is installed in the recirculation system piping 6.

The water supply system connects a condensate pump 12, a condensate purification device 13, a low-pressure feed water heater 14, a water pump 15, and a high-pressure feed water heater 16 from the condenser 10 to a water supply pipe 11 that connects the condenser 10 and the RPV 3. They are configured by installing them in this order toward RPV3. A bypass pipe 19 that bypasses the condensate purification device 13 is connected to the water supply pipe 11.

The reactor water purification system includes a purification system pump 21, a regenerative heat exchanger 22, a non-regenerative heat exchanger 23, and a purification device 24 installed in a purification system piping 20 that connects the recirculation system piping 6 and the water supply piping 11. configured. One end of the purification system piping 20 is connected to the recirculation system piping 6 upstream of the recirculation pump 7, and the other end of the purification system piping 20 is connected to the water supply piping 11.

The residual heat removal system is configured by installing a pump 28 and a non-regenerative heat exchanger 29 in residual heat removal system piping 27 that connects purification system piping 20 and RPV 3. One end of the residual heat removal system piping 27 is connected to the purification system piping 20 upstream of the purification system pump 21, and the other end of the residual heat removal system piping 27 is connected to the RPV 3.

The cooling water (reactor water) in the RPV 3 is pressurized by the recirculation pump 7 and passes through the recirculation system piping 6 from the nozzle (not shown) of the jet pump 5 into the bell mouth (not shown) of the jet pump 5. is ejected. Reactor water existing around this nozzle is also sucked into the bell mouth by the action of the jet flow of reactor water ejected from the nozzle. Reactor water discharged from the jet pump 5 is supplied to the reactor core 4 and heated by heat generated by fission of nuclear fuel material within the fuel rods. Some of the heated reactor water turns into steam. This steam is guided from the RPV 3 through the main steam pipe 8 to the turbine 9, and rotates the turbine 9. A generator (not shown) connected to the turbine 9 is rotated to generate electric power. Steam discharged from the turbine 9 is condensed in a condenser 10 and becomes water.

This water is supplied into the RPV 3 through the water supply piping 11 as water supply. The feed water flowing through the water supply pipe 11 is pressurized by the condensate pump 12, impurities are removed by the condensate purifier 13, further pressurized by the feed water pump 15, and heated by the low-pressure feed water heater 14 and the high-pressure feed water heater 16. . Bleed steam extracted from the main steam pipe 8 and the turbine 9 through the bleed pipe 17 is supplied to the low pressure feed water heater 14 and the high pressure feed water heater 16, respectively, and serves as a heating source for the feed water.

A part of the reactor water flowing through the recirculation system piping 6 flows into the purification system piping 20 by the driving of the purification system pump 21, is cooled by the regenerative heat exchanger 22 and the non-regenerative heat exchanger 23, and is transferred to the purification system 24. guided by. The ionic components and crud components contained in the reactor water are removed by the purification device 24. Purified reactor water discharged from the purification device 24 is heated by the regenerative heat exchanger 22, guided to the water supply pipe 11, and supplied to the RPV 3.

The detailed configuration of the chemical decontamination device 31 used in the method for chemical decontamination of carbon steel members of a nuclear power plant according to this embodiment will be described with reference to FIG. The chemical decontamination device 31 includes a decontamination tank 32, a circulation pump 34, a circulation pipe 35, an oxidizing agent injection device 39, an ultraviolet irradiation device 47, a filter 48, a cation resin tower (cation adsorption device) 56, and a mixed bed resin tower 61. Equipped with One end of the circulation pipe 35 is connected to the bottom of the decontamination tank 32, and the other end of the circulation pipe 35 is connected to the top of the decontamination tank 32, for example. A closed loop (hereinafter referred to as a first closed loop) is formed by the decontamination tank 32 and the circulation pipe 35. A heater 33 is attached to the decontamination tank 32.

An on-off valve 36, a circulation pump 34, valves 45 and 37, and an on-off valve 38 are installed in the circulation pipe 35 in this order from upstream to downstream. An oxidizing agent injection device 39 is connected to the circulation pipe 35 downstream of the on-off valve 38. The oxidizing agent injection device 39 used in this embodiment is a hydrogen peroxide injection device. The oxidizer injection device 39 includes a chemical tank 40, an injection pump 41, and an injection pipe 42. An injection pipe 42 having a valve 43 connects the chemical tank 40 and the circulation pipe 35 . An injection pump 41 is provided in injection piping 42 . A chemical tank 40 is filled with hydrogen peroxide, which is an oxidizing agent. As the oxidizing agent, hydrogen peroxide, oxygen, or ozone is used.

A pipe 59 that bypasses the valve 37 is connected to the circulation pipe 35. Specifically, one end of the pipe 59 is connected to the circulation pipe 35 between the valve 45 and the valve 37, and the other end of the pipe 59 is connected to the circulation pipe 35 downstream of the valve 37. The valves 57 and 58 and the cation resin column 56 are installed in the pipe 59 from upstream of the pipe 59 to downstream of the pipe 59 in this order. The cation resin column 56 is filled with a cation exchange resin. A pipe 64 that bypasses the valve 37 and the pipe 59 is connected to the circulation pipe 35. Specifically, one end of the pipe 64 is connected to the circulation pipe 35 between the valve 45 and the connection point on the valve 57 side of the two connection points between the circulation pipe 35 and the pipe 59 . The other end of the pipe 64 is connected to the circulation pipe 35 between the connection point on the cation resin column 56 side of the two connection points in the pipe 59 and the on-off valve 38 . The valve 62, the cooler 60, the valve 63, and the mixed bed resin tower 61 are installed in the pipe 64 in this order from upstream of the pipe 64 to downstream of the pipe 64. The mixed bed resin column 61 is filled with a cation exchange resin and an anion exchange resin. Piping 65 is connected to piping 59 and piping 64. One end of piping 65 is connected to piping 59 between valves 57 and 58, and the other end of piping 65 is connected to piping 64 between cooler 60 and valve 63. On the upstream side of the valve 37, the connection point between the circulation pipe 35 and the pipe 64 is located upstream of the connection point between the circulation pipe 35 and the pipe 59.

One end of an ultraviolet irradiation system piping 49 that bypasses the valve 45 is connected to the circulation piping 35 between the circulation pump 34 and the valve 45 . Further, the other end of the ultraviolet irradiation system piping 49 is connected to the circulation piping 35 between the connection point of the circulation piping 35 and the piping 64 on the valve 62 side and the valve 45 . The valve 50, the ultraviolet irradiation device 47, the filter 48, and the valve 51 are installed in the ultraviolet irradiation system piping 49 in this order from upstream to downstream. Instead of the filter 48, a solid-liquid separator such as a cyclone may be used. The other end of piping 54, which has one end connected to injection piping 42 between injection pump 41 and valve 43, is connected to ultraviolet irradiation system piping 49 between valve 50 and ultraviolet irradiation device 47. A valve 53 is provided in the pipe 54. One end of piping 55 that bypasses filter 48 and valve 51 is connected to ultraviolet irradiation system piping 49 between ultraviolet irradiation device 47 and filter 48, and the other end of piping 55 is located downstream of valve 45. The connection point between the pipe 35 and the ultraviolet irradiation system pipe 49 and the connection point between the circulation pipe 35 and the pipe 64 on the valve 62 side are connected to the circulation pipe 35 .

The BWR plant 1 is stopped after one operation cycle is completed. During the shutdown operation of the BWR plant 1, a residual heat removal system is operated to remove decay heat and residual heat generated in the nuclear fuel material in the fuel assembly loaded in the reactor core 4. Open the valve 30 and drive the pump 28. By driving the pump 28, reactor water in the RPV 3 flows into the residual heat removal system piping 27 via the recirculation system piping 6 and the purification system piping 20. The reactor water introduced into the residual heat removal system is cooled by the non-regenerative heat exchanger 29 and returned to the RPV 3 via the residual heat removal system piping 27 and the water supply piping 11. After cooling of the reactor water by the residual heat removal system is completed, the BWR plant 1 is shut down.

After the operation is stopped, a periodic inspection is performed on the BWR plant 1. After this periodic inspection is completed, the BWR plant 1 is restarted. During this periodic inspection, some fuel assemblies in the core 4 are replaced with new fuel assemblies. That is, some fuel assemblies in the core 4 are taken out from the RPV 3 as spent fuel assemblies, and new fuel assemblies with a burnup of 0 GWd/t are loaded into the core 4.

The operation and periodic inspection of the BWR plant 1 will be repeated, and eventually the BWR plant 1 will be decommissioned. During decommissioning, decontamination of the structures of the BWR plant 1 may be carried out to reduce the exposure of workers performing decommissioning work. After this decontamination, the in-core equipment and piping of the BWR plant 1 are dismantled for removal. Clearance decontamination will be applied to dismantled piping and reactor equipment in order to reduce the amount of radioactive waste. In particular, clearance decontamination of waste materials of carbon steel members, which have a large quantity, will account for the majority. Examples of waste materials of carbon steel members include waste materials of reactor water purification system piping and residual heat removal system piping.

Decontamination for the purpose of clearance of waste carbon steel members is carried out according to the procedure shown in Figure 1. A method for chemically decontaminating carbon steel members of a nuclear power plant according to Example 1, which targets waste materials of carbon steel members (hereinafter simply referred to as carbon steel waste materials), will be specifically described below using FIG. 1.

In the BWR plant 1 that experienced operation, an oxide film containing radionuclides was formed on the inner surfaces of the recirculation system piping 6 and purification system piping 20, etc. through which reactor water flows in the RPV 3. It will be removed through chemical decontamination to reduce radiation exposure associated with demolition work. However, chemical decontamination for the purpose of suppressing radiation exposure cannot remove radionuclides that have been incorporated deep into the base material, so it cannot be treated as cleared waste. In order to enable clearance, it is necessary to dissolve and remove the radionuclides that have been taken deep into the base material of the carbon steel member along with the base metal, and to do this, it is necessary to melt the base material surface by about 20 μm, for example. .

The method of chemically decontaminating carbon steel members of a nuclear power plant according to this embodiment is carried out on waste materials of carbon steel members of a BWR plant. By this chemical decontamination method, the oxide film and the surface of the base material are removed from the inner surface of the waste material 66 of the purification system piping 20, which is carbon steel piping (carbon steel member), for example. For chemical decontamination of the waste material 66 of the purification system piping 20, the aforementioned chemical decontamination device 31 (FIG. 3) is used.

First, waste carbon steel members, which are objects to be chemically decontaminated, are stored in a decontamination tank of a chemical decontamination device (step S1). The purification system piping 20 is cut to a predetermined length and removed from the BWR plant 1 to be decommissioned. A plurality of waste materials 66 (carbon steel waste materials) of the purification system piping 20 cut into a predetermined length are stored in a cage (not shown) made of, for example, a stainless steel wire mesh. A plurality of baskets containing the waste materials 66 are stored from above in the decontamination tank 32 with the lid (not shown) removed. Since the decontamination tank 32 is filled with water, the waste material 66 of each purification system piping 20 is immersed in the water in the decontamination tank 32 while being housed in the cage. After the waste material 66 of a predetermined number of purification system piping 20 is stored in the decontamination tank 32, a lid is attached to the upper end of the decontamination tank 32, and the decontamination tank 32 is sealed. At this time, all valves of the chemical decontamination device 31 are closed. Then, the on-off valve 36, the valves 45 and 37, and the on-off valve 38 are opened to start the circulation pump 34, and the water that becomes the reduced decontamination liquid is circulated in the first closed loop including the decontamination tank 32 and the circulation piping 35. .

After that, the temperature of the circulating water is raised (step S2). While circulating the above-mentioned circulating water within the first closed loop, the circulating water is heated by the heater 33. This heating raises the temperature of the circulating water to 90°C. Then, oxalic acid is added to the circulating water (step S3). The inlet provided in the lid is opened, and oxalic acid is added to the circulating water in the decontamination tank 32 from this inlet. The added oxalic acid is dissolved in the circulating water, and an oxalic acid aqueous solution, which is a reducing decontamination liquid, is generated in the decontamination tank 32. This 90°C aqueous oxalic acid solution is pressurized by the circulation pump 34 and circulates in the first closed loop including the circulation pipe 35, and comes into contact with the inner and outer surfaces of the waste materials 66 of the plurality of purification system pipes 20 in the decontamination tank 32. do. The oxalic acid contained in the oxalic acid aqueous solution dissolves the oxide film (Fe ₂ O ₃ ) containing radionuclides formed on the inner surface of the waste material 66 of each purification system piping 20, and eventually the waste material 66 of the purification system piping 20 dissolves. The surface of the carbon steel base material is also melted. The radionuclides and iron contained in the oxide film and the iron of the base material of the waste material 66 are eluted into the oxalic acid aqueous solution.

An oxidizing agent is injected into the reducing decontamination solution (step S4). Fe ²⁺ is generated by dissolving the oxide film and the inner surface of the base material of the waste material 66 of the purification system piping 20 by the oxalic acid aqueous solution. The outer surface of the base material of the waste material 66 of the purification system piping 20 is also dissolved by the oxalic acid. The Fe ²⁺ reacts with oxalic acid to form sparingly soluble iron(II) oxalate dihydrate. When the formed iron (II) oxalate dihydrate covers the inner and outer surfaces of the base material of the waste material 66 of the purification system piping 20, the dissolution of the waste material 66 of the purification system piping 20 by the oxalic acid aqueous solution progresses. Therefore, reduction decontamination of the waste material 66 of the purification system piping 20 is suppressed.

To overcome this problem, an oxidizing agent, for example hydrogen peroxide, is injected into the aqueous oxalic acid solution. Hydrogen peroxide is injected into the oxalic acid aqueous solution using an oxidizer injector 39. By opening the valve 43 and starting the injection pump 41, hydrogen peroxide filled in the chemical tank 40 is injected into the oxalic acid aqueous solution flowing in the circulation pipe 35 through the injection pipe 42. Valve 53 is closed. The injected 90° C. oxalic acid aqueous solution containing hydrogen peroxide is supplied to the decontamination tank 32 , and the iron oxalate ( II) Contact with dihydrate. Since Fe ²⁺ of the iron(II) oxalate dihydrate is oxidized to Fe ³⁺ by hydrogen peroxide contained in the oxalic acid aqueous solution, the iron(II) oxalate dihydrate is dissolved. Due to the dissolution of iron(II) oxalate dihydrate, the dissolution of the base material of the waste material 66 of each purification system piping 20 proceeds again.

The waste material of carbon steel members is taken out from the decontamination tank (step S5). The iron concentration of the oxalic acid aqueous solution is, for example, 20 μm when the dissolution of the base material of the waste material 66 of each purification system piping 20 progresses and the iron concentration of the oxalic acid aqueous solution that comes into contact with the surface of the waste material 66 of the purification system piping 20 is 20 μm. When the iron concentration reaches the desired iron concentration, the circulation pump 34 is stopped, the lid of the decontamination tank 32 is opened, and the waste material 66 from each purification system piping 20 is poured into the oxalic acid aqueous solution containing hydrogen peroxide injected in step S4. is taken out from the decontamination tank 32 where it exists. The reason why the oxalic acid aqueous solution in the decontamination tank 32 contains an oxidizing agent (for example, hydrogen peroxide) even when the waste material 66 of the purification system piping 20 is taken out from the decontamination tank 32 is because iron(II) oxalate This is to prevent dihydrate from being formed in the waste material 66 of the purification system piping 20.

Note that the surface area (m ² ) of each waste material 66 in the decontamination tank 32 is multiplied by the above 20 μm to determine the volume of the waste material 66 to be dissolved, and this is multiplied by the density of the waste material 66 to calculate the mass of the waste material 66 to be dissolved. Then, by dividing the calculated mass of the waste material 66 by the liquid volume of the oxalic acid aqueous solution (the value obtained by multiplying the flow rate of the oxalic acid aqueous solution by the time during which the waste material 66 is in contact with the oxalic acid aqueous solution), the waste material 66 is It is possible to determine the iron concentration that increases in an oxalic acid aqueous solution due to the dissolution of . This determined iron concentration is the iron concentration of the oxalic acid aqueous solution that comes into contact with the surface of the waste material 66.

When each basket storing a plurality of waste materials 66 placed in the decontamination tank 32 is taken out from the decontamination tank 32 in order, the waste materials 66 of each purification system piping 20 are cleaned with pure water (cleaning liquid), for example. The pure water is then collected in the decontamination tank 32. That is, the lid is removed from the upper end of the decontamination tank 32, and the basket containing the plurality of waste materials 66 of the purification system piping 20 immersed in the oxalic acid aqueous solution in the decontamination tank 32 is sequentially immersed in the oxalic acid aqueous solution. It is pulled above the liquid level. The plurality of waste materials 66 of the purification system piping 20 in the lifted cage are washed with pure water jetted from a shower head placed directly above the decontamination tank 32, and the waste materials 66 The oxalic acid aqueous solution and radionuclides adhering to the surface are washed away. The rinsed oxalic acid aqueous solution and pure water containing radionuclides fall into the decontamination tank 32.

Next, the reducing decontamination solution is irradiated with ultraviolet light (step S6). A lid is attached to the upper end of the decontamination tank 32 again to seal the decontamination tank 32. Valves 45 and 37 are opened to drive circulation pump 34. The oxalic acid aqueous solution (reduced decontamination liquid) in the decontamination tank 32 flows into the circulation pipe 35 and is returned to the decontamination tank 32 through the valves 45 and 37 and the on-off valve 38. This oxalic acid aqueous solution circulates within the first closed loop. After the oxalic acid aqueous solution circulates within this first closed loop for a predetermined period of time, valves 50 and 51 are opened and valve 45 is closed. At this time, valves 52 and 53 are closed.

The oxalic acid aqueous solution discharged from the decontamination tank 32 to the circulation pipe 35 is sequentially supplied to the ultraviolet irradiation device 47 and the filter 48 through the ultraviolet irradiation system pipe 49, and is returned to the circulation pipe 35 downstream of the valve 45. When the oxalic acid aqueous solution passes through the ultraviolet irradiation device 47, the oxalic acid aqueous solution is irradiated with ultraviolet rays from the ultraviolet irradiation device 47.

Iron (II) oxalate dihydrate is produced and the formed iron (II) oxalate dihydrate is removed (step S7). Due to the ultraviolet irradiation, the reactions of formulas (1) and (2) described above occur, and oxalic acid is removed from the iron(III) oxalate complex (Fe ^III (C ₂ O ₄ ) ₃ ^3- ) contained in the oxalic acid aqueous solution. Iron(II) dihydrate is formed. As a result, the concentration of iron (II) oxalate dihydrate in the oxalic acid aqueous solution increases, and iron (II) oxalate dihydrate precipitates in the oxalic acid aqueous solution. The radionuclide (for example, Co-60) contained in the oxalic acid aqueous solution is incorporated into the precipitated iron (II) oxalate dihydrate. When the aqueous oxalic acid solution containing radioactive nuclides and precipitated iron (II) oxalate dihydrate passes through the filter 48, the iron (II) oxalate dihydrate is removed by the filter 48. . The oxalic acid aqueous solution from which the precipitated iron (II) oxalate dihydrate has been removed is discharged from the filter 48 and reaches the decontamination tank 32 through the circulation pipe 35. The irradiation with ultraviolet rays and the removal of precipitated iron (II) oxalate dihydrate are carried out using an aqueous oxalic acid solution in a closed loop (hereinafter referred to as the second closed loop) including the decontamination tank 32, the circulation piping 35, and the ultraviolet irradiation system piping 49. is carried out in circulation. By the above-mentioned ultraviolet irradiation and removal of the precipitated iron (II) oxalate dihydrate, the iron concentration of the oxalic acid aqueous solution circulating in the second closed loop is reduced to the iron (II) oxalate dihydrate shown in FIG. The iron concentration decreases to the saturation solubility of the substance. The iron concentration at the saturation solubility of this iron (II) oxalate dihydrate is the value of the total iron concentration (ppm) on the vertical axis for each oxalate concentration indicated by a circle in the oxalic acid aqueous solution.

The reducing decontamination agent (for example, oxalic acid) is decomposed (step S8). After the reduction in the iron concentration of the oxalic acid aqueous solution, which is the reducing decontamination solution, has ended due to the removal of the precipitated iron (II) oxalate dihydrate, decomposition of the oxalic acid contained in the oxalic acid aqueous solution begins. Since iron (II) oxalate dihydrate is generated in the process of step S7, the hydrogen peroxide (oxidizing agent) injected into the oxalic acid aqueous solution in the process of step S4 disappears by reaction with Fe ²⁺ . Therefore, in the step S8, it is necessary to inject the hydrogen peroxide in the chemical tank 40 into the ultraviolet irradiation system piping 49 upstream of the ultraviolet irradiation device 47.

First, valve 52 is opened and valve 51 is closed. Then, the valve 43 is closed, the valve 53 is opened, and the injection pump 41 is driven. Hydrogen peroxide in the chemical tank 40 passes through the injection pipe 42 and the pipe 54 and is injected into the ultraviolet irradiation system piping 49 between the valve 50 and the ultraviolet irradiation device 47. Specifically, the hydrogen peroxide is injected into the oxalic acid aqueous solution that has reached the ultraviolet irradiation system piping 49 from the decontamination tank 32 via the circulation piping 35 before the ultraviolet irradiation device 47.

The ultraviolet rays from the ultraviolet irradiation device 47 are irradiated onto the oxalic acid aqueous solution containing Fe ²⁺ and hydrogen peroxide. By this ultraviolet irradiation, Fe ²⁺ and hydrogen peroxide contained in the oxalic acid aqueous solution cause a Fenton reaction represented by formula (4), and OH radicals are generated in the oxalic acid aqueous solution. The generated OH radicals react with oxalic acid, as shown in formula (5), and oxidatively decompose the oxalic acid contained in the oxalic acid aqueous solution into carbon dioxide.

In addition, Fe ²⁺ is produced by the fact that when iron (II) oxalate dihydrate formed on the inner surface of the purification system piping 20 is removed, Fe ²⁺ corresponding to its saturated solubility remains in the oxalic acid aqueous solution. It exists. Oxalic acid contained in the oxalic acid aqueous solution is decomposed by the OH radicals generated by the reaction shown in equation (4) between this Fe ²⁺ and the injected hydrogen peroxide, and the oxalic acid concentration of the oxalic acid aqueous solution is reduced. As the oxalic acid concentration decreases, iron ions contained in the oxalic acid aqueous solution begin to precipitate in the oxalic acid aqueous solution in the form of iron hydroxide. Therefore, the injection pump 41 is stopped, the valve 53 is closed, the injection of hydrogen peroxide into the oxalic acid aqueous solution is stopped, and the purification of the iron ions contained in the oxalic acid aqueous solution containing the oxalic acid remaining after the oxalic acid decomposition is started. .

Water is passed through the cation resin tower (step S9). The valves 57 and 58 are opened and the valve 37 is closed, and the aqueous solution flowing in the circulation pipe 35 and the ultraviolet ray irradiation system pipe 49 is supplied to the cation resin column 56 through the pipe 59. Therefore, when the ultraviolet irradiation device 47 irradiates the oxalic acid aqueous solution containing Fe ³⁺ with ultraviolet light, Fe ²⁺ generated from the Fe ³⁺ is adsorbed by the cation exchange resin present in the cation resin column 56 and becomes oxalic acid. removed from the acid solution. While the aqueous solution is circulating in a closed loop (hereinafter referred to as the third closed loop) including the decontamination tank 32, circulation piping 35, ultraviolet irradiation system piping 49, and piping 59, Fe ²⁺ contained in the oxalic acid aqueous solution is transferred to the cation resin column. 56 is removed.

The oxalic acid aqueous solution is purified (step S10). When Fe ²⁺ is removed in the cation resin tower 56 and Fe ²⁺ contained in the oxalic acid aqueous solution is reduced, the reaction of formula (4) no longer occurs even if hydrogen peroxide is injected into the oxalic acid aqueous solution. Therefore, the oxalic acid aqueous solution is supplied to the mixed bed resin column 61 instead of the cation resin column 56 by stopping the power supply to the heater 33, opening the valves 62 and 63, and closing the valves 57 and 58. Before being supplied to the mixed bed resin tower 61, the oxalic acid aqueous solution is cooled in a cooler 60 to lower its temperature. Oxalic acid, cations, and anions remaining in the oxalic acid aqueous solution whose temperature has decreased to a predetermined temperature are adsorbed by the cation exchange resin and anion exchange resin in the mixed bed resin tower 61 and removed. In this way, the oxalic acid aqueous solution remaining in the decontamination tank 32, circulation piping 35, ultraviolet irradiation system piping 49, etc. is purified.

The waste liquid is treated (step S11). After the above purification step is completed, the circulation pipe 35 and the waste liquid treatment device (not shown) are connected by a high pressure hose (not shown) having a pump (not shown). After the purification process is completed, the aqueous radioactive waste liquid remaining in the decontamination tank 32, the circulation piping 35, the ultraviolet irradiation system piping 49, etc. is removed by driving the pump and passing the high-pressure hose from the circulation piping 35 to the waste liquid treatment device ( (not shown) and treated in a waste liquid treatment device. After the aqueous solution in the decontamination tank 32, circulation piping 35, ultraviolet irradiation system piping 49, etc. is discharged, cleaning water is supplied into the decontamination tank 32, circulation piping 35, ultraviolet irradiation system piping 49, etc., and the circulation pump 34 drive to clean the inside of these pipes. After the cleaning is completed, the cleaning water in the decontamination tank 32, circulation piping 35, ultraviolet irradiation system piping 49, etc. is discharged to the above-mentioned waste liquid treatment device.

According to this embodiment, iron(II) oxalate dihydrate formed on the inner and outer surfaces of the waste material 66 of each purification system piping 20 is brought into contact with an oxidizing agent, for example, an oxalic acid aqueous solution containing hydrogen peroxide. By this, Fe ²⁺ contained in iron(II) oxalate dihydrate is oxidized to Fe ³⁺ by hydrogen peroxide, and the iron(II) oxalate dihydrate is converted into iron(III) oxalate complex. Dissolve in aqueous oxalic acid solution. For this purpose, the formed iron(II) oxalate dihydrate is removed from the inner and outer surfaces of the waste material 66 of each purification system piping 20, and the reduction and removal of the inner and outer surfaces of the waste material 66 of each purification system piping 20 by the oxalic acid aqueous solution is performed. In particular, reduction decontamination of the inner surface of the waste material 66 of each purification system piping 20 on which an oxide film containing radionuclides is formed can be promoted.

Furthermore, in this example, before bringing the oxalic acid aqueous solution into contact with the ion exchange resin, by irradiating the oxalic acid aqueous solution containing the iron(III) oxalate complex with ultraviolet rays, the iron(III) oxalate complex was converted into oxalic acid. Iron(II) dihydrate is produced. The concentration of iron (II) oxalate dihydrate in the oxalic acid aqueous solution increases, and iron (II) oxalate dihydrate becomes fine particles and precipitates in the oxalic acid aqueous solution. Precipitation of iron(II) oxalate dihydrate reduces the concentration of cations (eg, Fe ²⁺ ) in the aqueous oxalic acid solution, and reduces the amount of cations adsorbed on the cation exchange resin. Therefore, the life of the cation exchange resin can be extended and the amount of the cation exchange resin used can be reduced. Note that the precipitated iron (II) oxalate dihydrate is not adsorbed by the cation exchange resin.

Since the radionuclides contained in the oxalic acid aqueous solution are incorporated into the precipitated iron(II) oxalate dihydrate, the radionuclides contained in the oxalic acid aqueous solution, which are essentially cations, are incorporated into the precipitated oxalate. It is removed by filter 48 along with the iron(II) acid dihydrate. As a result, the radionuclide concentration of the oxalic acid aqueous solution is reduced, and the dose of the oxalic acid aqueous solution can be reduced. Since the radionuclide incorporated into iron(II) oxalate dihydrate is essentially a cation, the amount of radionuclide ions adsorbed to the cation exchange resin also decreases, which shortens the life of the cation exchange resin. extend. Therefore, the amount of cation exchange resin used can be further reduced.

According to this embodiment, the coexistence of oxalic acid and the oxidizing agent makes it possible to deeply dissolve the base material of the waste material 66 of the purification system piping 20 (for example, 20 μm). This dissolution makes it possible to separate most of the dissolved iron eluted into the oxalic acid aqueous solution from the oxalic acid aqueous solution without using an ion exchange resin, making it possible to suppress the amount of waste produced by the ion exchange resin.

In Example 1, the filter 48 is arranged upstream of the cation resin tower 56, and the oxalic acid aqueous solution from which the precipitated iron(II) oxalate dihydrate has been removed by the filter 48 is supplied to the cation resin tower 56. ing. The amount of precipitated iron(II) oxalate dihydrate removed by the filter 48 disposed upstream of the cation resin column 56 increases, and the oxalic acid flows into the cation resin column 56 together with the oxalic acid aqueous solution. Iron(II) dihydrate is reduced. Since the amount of cations such as Fe ²⁺ contained in the oxalic acid aqueous solution flowing into the cation resin tower 56 is small, the cations (e.g. Fe ²⁺ and radioactive The amount of nuclide cations, etc.) is reduced, extending the life of the cation exchange resin.

In this way, instead of arranging the filter 48 upstream of the cation resin tower 56, the filter 48 may be arranged downstream of the cation resin tower 56. This filter 48 is installed in the circulation pipe 35 , for example, downstream of the cation resin column 56 and between the connection point of the pipe 64 and the circulation pipe 35 and the on-off valve 38 . Even when the filter 48 is disposed downstream of the cation resin column 56, the iron(II) oxalate dihydrate that precipitates upstream of the cation resin column 56 and flows into the cation resin column 56 is contained in the cation resin column 56. Since the cations are not adsorbed by the cation exchange resin and are removed by the filter 48 located downstream of the cation resin column 56, the amount of cations adsorbed by the cation exchange resin decreases, and the amount of cation exchange resin used is reduced. is reduced.

However, when the filter 48 is disposed downstream of the cation resin column 56, when the oxalic acid aqueous solution containing fine particles of iron (II) oxalate dihydrate and Fe ²⁺ is supplied to the cation resin column 56, initially Next, the Fe ²⁺ is ion-exchanged with H ⁺ of the cation exchange resin in the cation exchange resin layer in the cation resin column 56, captured by the cation exchange resin, and removed from the oxalic acid aqueous solution. Scavenging of Fe ²⁺ reduces the Fe ²⁺ concentration of the oxalic acid aqueous solution downstream of the cation exchange resin in the cation exchange resin layer. Then, Fe ²⁺ is supplied to the oxalic acid aqueous solution from the fine particles of iron (II) oxalate dihydrate contained in the oxalic acid aqueous solution. This Fe ²⁺ is also ion-exchanged with H ⁺ of the cation exchange resin located further downstream in the cation exchange resin layer, and is captured by the cation exchange resin. In the cation exchange resin layer, ion exchange occurs between Fe ²⁺ contained in the oxalic acid aqueous solution and H ⁺ of the cation exchange resin, and iron (II) oxalate dihydrate contained in the oxalic acid aqueous solution is transferred to the oxalic acid aqueous solution. The supply of Fe ²⁺ to is repeated, and the ion exchange capacity of the cation exchange resin in the cation exchange resin layer is consumed by Fe ²⁺ adsorbed on the cation exchange resin. Therefore, when the filter 48 is placed downstream of the cation resin column 56, the amount of cation exchange resin used in the cation resin column 56 increases compared to when the filter 48 is placed upstream of the cation resin column 56. do. For this reason, it is desirable that the filter 48 be disposed on the upstream side of the cation resin tower 56 rather than on the downstream side of the cation resin tower 56.

Note that by arranging the filter 48 downstream of the cation resin column 56, the oxalic acid aqueous solution containing the precipitated iron(II) oxalate dihydrate flows into the cation resin column 56 before the filter 48. Since the precipitated iron(II) oxalate dihydrate is not an ion, it is not adsorbed by the cation exchange resin in the cation resin column 56. A part of the precipitated iron(II) oxalate dihydrate may be captured by the cation exchange resin layer in the cation resin tower 56 and removed from the oxalic acid aqueous solution. Most of the precipitated iron(II) oxalate dihydrate passes through the cation exchange resin layer in the cation resin column 56 and is discharged from the cation resin column 56 together with the oxalic acid aqueous solution. The precipitated iron(II) oxalate dihydrate contained in the oxalic acid aqueous solution discharged from the cation resin column 56 is removed by the aforementioned filter 48 installed in the circulation pipe 35.

In this embodiment, in the step S8 (decomposition of reducing decontamination agent (for example, oxalic acid)), oxalic acid is decomposed by "injection of oxidizing agent" and "ultraviolet irradiation." As for "decomposition of a reducing decontamination agent", a method using a catalyst and an oxidizing agent is known (for example, see Japanese Patent Application Publication No. 2018-48831). JP 2018-48831A describes reductive decontamination using an oxalic acid aqueous solution, which is performed on carbon steel purification system piping in paragraphs 0078 to 0091. Radionuclides and Fe ²⁺ whose concentration has increased in the oxalic acid aqueous solution due to the dissolution of the oxide film formed on the inner surface of the purification system piping by the oxalic acid aqueous solution are adsorbed by the cation exchange resin in the cation resin tower and removed. . After the removal of radionuclides and Fe ²⁺ in the cation resin tower is completed, the oxalic acid contained in the oxalic acid aqueous solution is decomposed in the decomposition equipment by the action of a catalyst (e.g., ruthenium) and hydrogen peroxide supplied separately. be done. In the oxalic acid decomposition process, the oxalic acid aqueous solution with a reduced concentration of oxalic acid discharged from the decomposition equipment comes into contact with the inner surface of the purification system piping, forming iron (II) oxalate dihydrate on the inner surface. . To dissolve this iron(II) oxalate dihydrate, the amount of hydrogen peroxide fed to the cracker is increased. Due to the increased amount of hydrogen peroxide in the oxalic acid aqueous solution, the iron(II) oxalate dihydrate formed on the inner surface of the purification system piping becomes an iron(III) oxalate complex and dissolves in the oxalic acid aqueous solution. Since Fe ³⁺ contained in the iron(III) oxalate complex adheres to the catalyst in the decomposition device, the surface area of the catalyst that comes into contact with the oxalic acid aqueous solution decreases, and the decomposition rate of oxalic acid decreases.

However, in this example in which no catalyst is used, the oxalic acid aqueous solution into which an oxidizing agent (for example, hydrogen peroxide) has been injected is irradiated with ultraviolet rays to decompose the oxalic acid contained in the oxalic acid aqueous solution. The problem of a decrease in the decomposition rate of oxalic acid due to the attachment of ³⁺ does not occur. Therefore, the decomposition rate of oxalic acid in this example is higher than the decomposition rate of oxalic acid in JP 2018-48831A using a catalyst.

The chemical decontamination method for carbon steel members of a nuclear power plant in Example 1 is applied not only to the waste of carbon steel members generated due to decommissioning of the BWR plant 1, but also to the waste of carbon steel members removed from the BWR plant 1. applies.

During maintenance inspections during the operation shutdown period of the BWR plant 1, a part of a certain carbon steel pipe (for example, the purification system pipe 20) connected to the reactor pressure vessel 3 is repaired due to damage or thinning. may be deemed necessary. For example, in repair work for a damaged part of the purification system piping 20, a certain range of the purification system piping 20 including the damaged part is cut and removed, and a new pipe is connected to the removed part of the purification system piping 20 by welding. be done. The pipe cut and removed from the purification system piping 20 becomes waste material of carbon steel members, that is, waste material 66 of the purification system piping 20. The steps S1 to S11 in the first embodiment are performed on the waste material 66 of the purification system piping 20 using the chemical decontamination device 31.

Furthermore, during maintenance and inspection during the period when the BWR plant 1 is out of operation, the carbon steel piping system in the BWR plant 1 may be modified. An example of this modification of the piping system is when changing the installation route of the carbon steel piping system in order to install new support for seismic reinforcement at BWR Plant 1, which has experienced operation, and when it becomes a source of radiation exposure. A case may be considered in which the length of a carbon steel piping system is shortened. In each case of changing the installation route of the piping system and shortening the piping length of the piping system, some piping parts are removed in the piping system before the installation route is changed and in the piping system before the piping length is shortened. Those removed pipe portions become waste carbon steel members, that is, waste carbon steel pipes 66. Using the chemical decontamination device 31, each process of steps S1 to S11 in Example 1 is performed on this waste material 66 of carbon steel piping.

Waste material 66 of the purification system piping 20 removed from the BWR plant 1 during repair work, and waste material 66 of carbon steel piping removed from the BWR plant 1 due to changes in the installation route of the piping system, shortening of the piping length, etc. It is possible to apply each of Examples 2 to 4, which will be described later, to waste carbon steel members such as the following.

Embodiment 2 A chemical decontamination method for carbon steel members of a nuclear power plant according to another preferred embodiment of the present invention, which is applied to the decontamination of waste carbon steel members generated from a boiling water nuclear power plant, is shown in FIG. 6 will be used for explanation. The waste material of carbon steel members (for example, the waste material of carbon steel piping) is used to cut existing carbon steel members during decommissioning of boiling water nuclear power plants or replacement work of carbon steel members of boiling water nuclear power plants. Occurs due to removal.

The chemical decontamination device used in the method for chemically decontaminating carbon steel members of this embodiment is the same as the chemical decontamination device 31 (see FIG. 3) used in the first embodiment.

The chemical decontamination method for carbon steel members of a nuclear power plant of this example is a step of removing iron(II) oxalate dihydrate formed in the method of chemical decontamination of carbon steel members of a nuclear power plant of Example 1. An additional determination process for carbon steel waste (step S12) is added between (step S7) and the decomposition process for the reducing decontamination agent (step S8).

As in Example 1, the purification system piping 20 removed from the BWR plant 1 that is subject to decommissioning is cut to a predetermined length and stored in the decontamination tank 32 (step S1). 32 is sealed by a lid. Thereafter, the steps S2 to S5 that are carried out in the first embodiment are also carried out in this embodiment. In step S3, oxalic acid is supplied into the decontamination tank 32 from the input port provided in the lid in order to supplement the oxalic acid removed as iron (II) oxalate dihydrate. In step S4, hydrogen peroxide is injected into the circulation pipe 35 from the oxidizing agent injection device 39. In step S5, the waste material 66 of the purification system piping 20 that has been subjected to reductive decontamination using an oxalic acid aqueous solution is taken out from the decontamination tank 32. After the waste material 66 of the purification system piping 20 is removed from the decontamination tank 32, the decontamination tank 32 is sealed with a lid.

In step S6, the oxalic acid aqueous solution is irradiated with ultraviolet light from the ultraviolet irradiation device 47. In step S7, iron (II) oxalate dihydrate is formed from the iron (III) oxalate complex contained in the oxalic acid aqueous solution by ultraviolet irradiation, and eventually iron (II) oxalate dihydrate is formed in the oxalic acid aqueous solution. Hydrate precipitates. The precipitated iron(II) oxalate dihydrate is removed by a filter 48.

If additional carbon steel waste material exists, the determination in step S12 becomes "yes", and in step S1, the lid of the decontamination tank 32 is removed and the waste material 66 of the plurality of additional purification system pipes 20 is removed from the decontamination tank 32. stored inside. Thereafter, the decontamination tank 32 is sealed with a lid, and the steps S2 to S7 are performed again. If there is no additional carbon steel waste material, the determination in step S12 is "absent", and the steps S8 to S11 are executed.

In this embodiment, each effect produced in the first embodiment can be obtained. In this embodiment, the chemical decontamination equipment 31 is used to store the waste carbon steel members (for example, the waste from the purification system piping 20) in the decontamination tank 32 until the waste carbon steel members generated during the decommissioning of the nuclear power plant are exhausted. While storing the waste material 66), reduction decontamination of the waste material can be continued. Therefore, the oxalic acid used for reductive decontamination of waste carbon steel members stored in the decontamination tank 32 is reused for the next reductive decontamination of carbon steel members newly stored in the decontamination tank 32. This embodiment can reduce the overall amount of oxalic acid used.

Embodiment 3 A method for chemically decontaminating carbon steel members of a nuclear power plant according to another preferred embodiment of the present invention, which is applied to the decontamination of waste carbon steel members generated from a boiling water nuclear power plant, is shown in FIG. This will be explained using 7. The waste material of carbon steel members (for example, the waste material 66 of the purification system piping 20) is used to replace existing carbon steel members during decommissioning of a boiling water nuclear power plant or replacement work of carbon steel members of a boiling water nuclear power plant. Occurs due to cutting and removal.

In the method for chemical decontamination of carbon steel members of a nuclear power plant using the chemical decontamination device 31 of this embodiment, each process of steps S1 to S4 is performed in the same manner as in the first embodiment. After the process of step S4 is completed, each process of steps S6 to S8 implemented in Example 1 is implemented, and each process of steps S13 and S14, which are new processes, is implemented one after another. After completing the step S14, the steps S9 and S10 described above are performed, and then the steps S5 and S11 described above are performed. In this embodiment, even after the process of step S4 (injection of oxidizing agent) is completed, the waste material 66 of the purification system piping 20 remains stored in the decontamination tank 32, and in this state, steps S6 to S8 are carried out. Each process of S13, S14, S9, and S10 is performed.

After the step S8 (decomposition of oxalic acid) is completed, the step S13 (injection of oxidizing agent) is performed. To remove iron(II) oxalate dihydrate precipitated on the surface of the waste material 66 of each purification system piping 20 in the decontamination tank 32 after the decomposition of oxalic acid in step S8 is completed. Next, hydrogen peroxide is injected from the oxidizing agent injection device 39 into the oxalic acid aqueous solution from which the oxalic acid has been decomposed and removed, similarly to the step S4. The injected hydrogen peroxide oxidizes Fe ²⁺ in iron (II) oxalate dihydrate to Fe ³⁺ , and iron (II) oxalate dihydrate is converted into an oxalic acid aqueous solution as iron (III) oxalate complex. dissolve in

In step S13, all valves other than the on-off valves 36, 45, and 37 and the on-off valve 38 are closed, and the circulation pump 34 is driven to transfer the oxalic acid aqueous solution from the decontamination tank 32 to the decontamination tank 32 and the circulation piping 35. circulate in a closed loop containing The valve 43 is opened, the injection pump 41 is driven, and the hydrogen peroxide in the chemical tank 40 is injected into the 90° C. oxalic acid aqueous solution flowing in the circulation pipe 35.

Fe ³⁺ in the iron(III) oxalate complex undergoes hydrolysis to produce iron(III) hydroxide. Iron (III) hydroxide is precipitated in the oxalic acid aqueous solution, and the precipitated iron (III) hydroxide is removed by the filter 48 (step S14). At this time, if iron (II) oxalate dihydrate is attached to the filter 48, Fe ²⁺ is eluted from the iron (II) oxalate dihydrate into the oxalic acid aqueous solution. Therefore, the filter 48 is replaced with a new filter 48.

After the iron(II) oxalate dihydrate formed on the waste material 66 of each purification system piping 20 in the decontamination tank 32 is removed by injection of the oxidizer, the lid of the decontamination tank 32 is removed, and each purification system The waste material 66 of the system piping 20 is taken out from the decontamination tank 32 (step S5). After the removal of the waste material 66 of the purification system piping 20 from the decontamination tank 32 is completed, the aqueous solution remaining in the decontamination tank 32, circulation piping 35, etc. and becoming radioactive waste liquid is discharged in the same manner as in Example 1. (Step S11).

This embodiment can obtain each effect produced in the first embodiment.

Embodiment 4 A chemical decontamination method for carbon steel members of a nuclear power plant, which is another preferred embodiment of the present invention and is applied to the decontamination of waste carbon steel members generated from a boiling water nuclear power plant, is shown in FIG. This will be explained using 8. The waste material of carbon steel members (for example, the waste material 66 of the purification system piping 20) is used to replace existing carbon steel members during decommissioning of a boiling water nuclear power plant or replacement work of carbon steel members of a boiling water nuclear power plant. Occurs due to cutting and removal.

In the method for chemical decontamination of carbon steel members of a nuclear power plant according to this embodiment, each process of steps S1 to S11 (see FIG. 1) carried out in the first embodiment is carried out. A chemical decontamination device 31A used in the method of chemically decontaminating carbon steel members of this embodiment will be described with reference to FIG.

The chemical decontamination device 31A has a configuration in which the chemical tank 40 of the oxidizing agent injection device 39 in the chemical decontamination device 31 and the injection pipe 42 of the circulation pipe 35 are disconnected, and an oxygen injection device 67 is provided. The other configuration of the chemical decontamination device 31A is the same as the chemical decontamination device 31. However, in the chemical decontamination device 31A, the injection pipe 42 of the oxidizing agent injection device 39 connected to the chemical tank 40 is connected to the ultraviolet irradiation system piping 49 between the valve 50 and the ultraviolet irradiation device 47.

The chemical decontamination device 31A used in this embodiment includes an oxygen injection device 67 as the oxidizing agent injection device 39. The oxygen injection device 67 includes an oxygen filling device (for example, an oxygen cylinder) 68, an injection pipe 70, and a diffuser pipe 71. The aeration pipe 71 is arranged in the decontamination tank 32 near the bottom of the decontamination tank 32 . The diffuser pipe 71 is formed with a large number of ejection holes for ejecting oxygen. The oxygen filling device 68 and the aeration pipe 71 are connected by an injection pipe 70 provided with a valve 69.

A method for chemically decontaminating carbon steel members of a nuclear power plant according to this embodiment using the chemical decontamination device 31A will be specifically described below.

Each process of steps S1 to S3 shown in FIG. 1 is performed sequentially. In the next oxidizing agent injection step (step S4), oxygen is used as the oxidizing agent instead of the hydrogen peroxide used in Example 1. The valve 69 of the oxygen injection device 67 is opened to supply oxygen in the oxygen filling device 68 to the aeration pipe 71 through the injection pipe 70. The oxygen that has reached the aeration pipe 71 is injected into the oxalic acid aqueous solution filled in the decontamination tank 32 and in which the waste material 66 of the plurality of purification system piping 20 is immersed, from a large number of jet holes formed in the aeration pipe 71. be done. The injected oxygen dissolves in the oxalic acid aqueous solution in the decontamination tank 32. Oxygen dissolved in the oxalic acid aqueous solution, like the aforementioned hydrogen peroxide, destroys iron(II) oxalate dihydrate formed on the surface of the waste material 66 of the purification system piping 20 due to reductive decontamination with the oxalic acid aqueous solution. Dissolve. When the iron (II) oxalate dihydrate formed on the surface is dissolved and removed, the reductive decontamination of the waste material of the purification system piping 20 by the oxalic acid aqueous solution is promoted.

Then, the steps S5 to S11 are carried out, and the method for chemically decontaminating carbon steel members of this embodiment is completed.

In this embodiment, each effect produced in the first embodiment can be obtained.

As the oxidizing agent injection device 39, an ozone injection device may be used instead of the oxygen injection device 67. The ozone injection device has an ozone filling device in place of the oxygen filling device 68 in the oxygen injection device 67. The other configuration of the ozone injection device is the same as the oxygen injection device 67.

When carrying out a chemical decontamination method for carbon steel members of a nuclear power plant using a chemical decontamination device equipped with an ozone injection device (the configuration other than the ozone injection device is the same as the chemical decontamination device 31), in step S4 , the valve 69 is opened to supply the ozone in the ozone filling device to the aeration pipe 71 through the injection pipe 70, and the ozone is injected into the oxalic acid aqueous solution in the decontamination tank 32 from a number of ejection holes formed in the aeration pipe 71. The ozone that is injected and dissolved in the oxalic acid aqueous solution dissolves iron (II) oxalate dihydrate formed on the surface of the waste material of the purification system piping 20. The dissolution of the iron(II) oxalate dihydrate promotes the reductive decontamination of the waste material of the purification system piping 20 by the oxalic acid aqueous solution.

A method for chemically decontaminating carbon steel members of a nuclear power plant according to Example 5, which is another preferred embodiment of the present invention, will be described with reference to FIGS. 9, 10, and 11. The chemical decontamination method for carbon steel members of a nuclear power plant of this embodiment is applied to carbon steel purification system piping (carbon steel members) of a boiling water nuclear power plant (BWR plant).

The detailed configuration of the chemical decontamination device 31B used for chemical decontamination of, for example, the purification system piping 20 (see FIG. 10) in the BWR plant 1 will be described using FIG. 11.

The chemical decontamination device 31B includes a circulation pipe 76, a surge tank 90, a heater 33, circulation pumps 34 and 34A, an oxidizing agent injection device 39, a cooler 60, a cation resin tower 56, a mixed bed resin tower 61, an ejector 86, and a hopper. 87.

An on-off valve 36, a circulation pump 34, valves 45, 77, and 78, a surge tank 90, a circulation pump 34A, a valve 79, and an on-off valve 38 are provided in the circulation pipe 76 in this order from upstream. Ultraviolet irradiation system piping 49 that bypasses valve 45 is connected to circulation piping 76 . The valve 50, the ultraviolet irradiation device 47, the filter 48, and the valve 51 are installed in the ultraviolet irradiation system piping 49 in this order from upstream to downstream. One end of piping 55 that bypasses filter 48 and valve 51 is connected to ultraviolet irradiation system piping 49 between ultraviolet irradiation device 47 and filter 48, and the other end of piping 55 is connected to circulation piping 76 on the downstream side of valve 45. It is connected to the circulation pipe 76 between the connection point between the part and the ultraviolet irradiation system pipe 49 and the connection point between the circulation pipe 76 and the pipe 81 upstream of the valve 77 .

A cooler 60 and a valve 80 are installed in a pipe 81 that bypasses the valve 77 and is connected to the circulation pipe 76 at both ends. A cation resin column 56 and a valve 82 are installed in a pipe 83 whose both ends are connected to the circulation pipe 76 and bypass the valve 78 . A mixed bed resin tower 61 and a valve 84 are installed in a pipe 85 that is connected at both ends to a pipe 83 and bypasses the cation resin tower 56 and valve 82 . The cation resin column 56 is filled with a cation exchange resin, and the mixed bed resin column 61 is filled with a cation exchange resin and an anion exchange resin.

A surge tank 90 is installed in the circulation pipe 76 between the connection point between the circulation pipe 76 and the pipe 83 and the circulation pump 34A. A heater 33 is disposed within the surge tank 90. One end of piping 88 provided with valve 89 and ejector 86 is connected to circulation piping 76 between valve 79 and circulation pump 34A, and the other end of piping 88 is connected to surge tank 90. A hopper 87 for supplying oxalic acid (reduced decontamination agent) into the surge tank 90 is provided in the ejector 86 .

A chemical tank 40 of an oxidizer injection device 39 is connected to one end of an injection pipe 42 having an injection pump 41 and a valve 43, and the other end of the injection pipe 42 is connected to a circulation pipe 76 between a valve 79 and an on-off valve 38. . The chemical tank 40 is filled with hydrogen peroxide. One end of a pipe 54 having a valve 53 is connected to the injection pipe 42 between the injection pump 41 and the valve 43 . The other end of the pipe 54 is connected to the ultraviolet irradiation system pipe 49 between the ultraviolet irradiation device 47 and the valve 50 .

A pH meter 50A is attached to the circulation pipe 76 between the connection point between the injection pipe 42 and the circulation pipe 76 and the on-off valve 38. One end of the pipe 44 provided with the valve 46 is connected to the circulation pipe 76 between the pH meter 50A and the on-off valve 38. The other end of the piping 44 is connected to the circulation piping 76 between the on-off valve 36 and the circulation pump 34 .

The BWR plant 1 is stopped after one operation cycle is completed. During the operation shutdown period of the BWR plant 1, reduction decontamination is carried out on the inner surface of the purification system piping 20 to which reactor water (cooling water) in the RPV 3 is supplied when the BWR plant 1 is in operation after being contacted by the RPV 3. Implemented. The reduction decontamination of the purification system piping 20 during the operation stop period of the BWR plant 1 will be explained using FIG. 9. In the chemical decontamination method for carbon steel members of a nuclear power plant, which targets the purification system piping 20 in this embodiment, each process of steps S15, S2 to S4, S16, S6 to S11, and S17 shown in FIG. Implemented.

A chemical decontamination device is connected to the carbon steel piping system to be chemically decontaminated (step S1). When the operation of the BWR plant 1 is stopped, the bonnet of the valve 73 installed in, for example, the purification system piping 20 to be chemically decontaminated is opened to close off the recirculation system piping 6 side. One end of the circulation pipe 76 of the chemical decontamination device 31B on the on-off valve 38 side is connected to the flange of the valve 733. Furthermore, the bonnet of the valve 74 installed in the purification system piping 20 between the purification system pump 21 and the regenerative heat exchanger 22 is opened to close off the regenerative heat exchanger 22 side. The other end of the circulation pipe 76 on the on-off valve 36 side is connected to the flange of the valve 74. Both ends of the circulation piping 76 are connected to the purification system piping 20, forming a closed loop (hereinafter referred to as a fifth closed loop) including the purification system piping 20 and the circulation piping 76.

Thereafter, as in the first embodiment, steps S2 to S4 are performed. In step S2, the on-off valve 36, the valves 45, 77, 78 and 79, and the on-off valve 38 are opened to drive the circulation pumps 34 and 34A to circulate water in the fifth closed loop. The water in the surge tank 90 is heated by the heater 33, and the temperature of the water circulating in the fifth closed loop is raised to 90°C. The valve 89 is opened and a portion of the water flowing in the circulation pipe 76 is guided into the pipe 88 and supplied to the surge tank 90.

In step S3, oxalic acid is put into the hopper 87, and the oxalic acid is supplied to the water 90 flowing through the pipe 88 by the action of the ejector 86. Water containing oxalic acid flows into the surge tank 90, and the oxalic acid is dissolved in the water within the surge tank 90. By dissolving the oxalic acid, 90 oxalic acid aqueous solution (reduction decontamination liquid) is generated in the surge tank 90 . This oxalic acid aqueous solution comes into contact with the inner surface of the purification system piping 20, and reductive decontamination is performed on the inner surface. Through this reduction decontamination, the oxide film containing radionuclides formed on the inner surface of the purification system piping 20 is dissolved and removed from the inner surface of the purification system piping 20.

An oxidizing agent is injected into the reducing decontamination solution (step S4). As in Example 1, the valve 43 is opened with the valve 53 closed, and the injection pump is driven. Hydrogen peroxide in the chemical tank 40 is injected into the oxalic acid aqueous solution flowing through the circulation pipe 76 through the injection pipe 42 . A 90° C. oxalic acid aqueous solution containing hydrogen peroxide is supplied into the purification system piping 20 from the circulation piping 76 . Hydrogen peroxide contained in the oxalic acid aqueous solution dissolves iron(II) oxalate dihydrate formed on the inner surface of the purification system piping 20. As the iron(II) oxalate dihydrate dissolves, reductive decontamination of the inner surface of the purification system piping 20 by the oxalic acid aqueous solution is promoted.

Due to the reduction decontamination of the inner surface of the purification system piping 20 with the oxalic acid aqueous solution, the oxide film formed on the inner surface of the purification system piping 20 is dissolved, and radionuclides such as C0-60 contained in this oxide film are also removed. , diffuses throughout the oxalic acid aqueous solution flowing inside the purification system piping 20. A radiation detector 75 (described later) placed outside the purification system piping 20 measures radiation emitted from the purification system piping 20, but mainly detects radiation on the inner surface of the purification system piping 20 closest to the radiation detector 75. The dose rate will be measured. When the radionuclide diffuses into the oxalic acid aqueous solution, the concentration of the radionuclide in the oxalic acid aqueous solution becomes uniform. The dose rate on the inner surface of the purification system piping 20 is reduced by reduction decontamination, and the radionuclide concentration of the oxalic acid aqueous solution near the inner surface of the purification system piping 20 near the radiation detector 75 is reduced due to the above-mentioned diffusion of radionuclides. Radiation from radionuclides present in the acid aqueous solution at a position far from the radiation detector 75 is shielded by water contained in the oxalic acid aqueous solution, so the dose rate detected by the radiation detector 75 is reduced.

The reducing decontamination solution is irradiated with ultraviolet light (step S6). When the dose rate of the reduction decontamination target portion of the purification system piping 20 detected by the radiation detector 75 decreases below the set dose rate, the valves 50 and 51 are opened and the valve 45 is closed. At this time, valves 52 and 53 are closed. The oxalic acid aqueous solution returned from the purification system piping 20 to the circulation piping 76 is guided to the ultraviolet irradiation device 47 by the ultraviolet irradiation system piping 49. The oxalic acid aqueous solution is irradiated with ultraviolet light emitted from the ultraviolet irradiation device 47.

Iron (II) oxalate dihydrate is produced and the formed iron (II) oxalate dihydrate is removed (step S7). By the ultraviolet irradiation, as in Example 1, iron (II) oxalate dihydrate in which radionuclides are incorporated is generated from the iron (III) oxalate complex contained in the oxalic acid aqueous solution. The iron (II) oxalate dihydrate precipitated in the oxalic acid aqueous solution due to the increase in the concentration of iron (II) oxalate dihydrate in the oxalic acid aqueous solution and containing radionuclides is removed by the filter 48 . The oxalic acid aqueous solution in which the concentration of iron (II) oxalate dihydrate has decreased is discharged from the filter 48 and is supplied to the purification system piping 20 through the circulation piping 76. The irradiation with ultraviolet rays and the removal of precipitated iron (II) oxalate dihydrate are carried out using an aqueous oxalic acid solution in a closed loop (hereinafter referred to as the sixth closed loop) including the purification system piping 20, the circulation piping 76, and the ultraviolet irradiation system piping 49. is carried out in circulation. The iron concentration of the oxalic acid aqueous solution circulating within this sixth closed loop decreases to the iron concentration at the saturation solubility of iron (II) oxalate dihydrate described above.

In the method for chemical decontamination of carbon steel members in this embodiment using the chemical decontamination device 31B, reduction decontamination is performed on the inner surface of the system piping connected to the RPV 3, for example, the purification system piping 20. The purpose of the reduction decontamination is to dissolve the oxide film containing radionuclides such as Co-60, which is formed on the inner surface of the purification system piping 20 made of carbon steel, with an oxalic acid aqueous solution. It is to remove it from. When the inner surface of the purification system piping 20 is reductively decontaminated using an oxalic acid aqueous solution, iron(II) oxalate dihydrate is formed on the inner surface of the purification system piping 20 because the purification system piping 20 is made of carbon steel. The inner surface is coated with iron(II) oxalate dihydrate. The formation of iron (II) oxalate dihydrate on the inner surface causes stagnation of reduction decontamination of the inner surface of the purification system piping 20 by the oxalic acid aqueous solution, and the decrease in the dose rate of the purification system piping 20 is suppressed. Ru.

Therefore, in the process of step S4, an oxidizing agent, for example, hydrogen peroxide is injected, and the oxalic acid aqueous solution containing hydrogen peroxide is mixed with iron (II) oxalate dihydrate formed on the inner surface of the purification system piping 20. By dissolving and removing this iron (II) oxalate dihydrate by bringing it into contact with , reductive decontamination of the inner surface of the purification system piping 20 is promoted, and the dose rate of the purification system piping 20 is reduced. After the dose rate of the part of the purification system piping 20 targeted for reductive decontamination has been reduced to the set dose rate or less and the purpose of reductive decontamination of the purification system piping 20 has been achieved, "irradiation of ultraviolet rays to the oxalic acid aqueous solution" (step S6 Step S7) and "generation and removal of iron(II) oxalate dihydrate" (Step S7) are performed.

The reducing decontamination agent (for example, oxalic acid) is decomposed (step S8). After the decrease in the iron concentration of the oxalic acid aqueous solution ends, decomposition of the oxalic acid contained in the oxalic acid aqueous solution starts. First, the valve 52 is opened, the valve 51 is closed, the valve 43 is closed, and the valve 53 is opened, and the injection pump 41 is driven. Hydrogen peroxide in the chemical tank 40 is injected into the ultraviolet irradiation system piping 49 through the injection piping 42 and the piping 54. The hydrogen peroxide is injected into the oxalic acid aqueous solution that has reached the ultraviolet irradiation system piping 49 from the purification system piping 20 via the circulation piping 76.

The ultraviolet rays from the ultraviolet irradiation device 47 are irradiated onto the oxalic acid aqueous solution containing Fe ²⁺ and hydrogen peroxide. As a result, Fe ²⁺ and hydrogen peroxide contained in the oxalic acid aqueous solution cause the Fenton reaction represented by formula (3), and OH radicals are generated in the oxalic acid aqueous solution. The generated OH radicals react with oxalic acid, as shown in formula (4), and oxidatively decompose the oxalic acid contained in the oxalic acid aqueous solution into carbon dioxide.

As described above, the generated OH radicals decompose the oxalic acid contained in the oxalic acid aqueous solution, and the oxalic acid concentration of the oxalic acid aqueous solution decreases. As the oxalic acid concentration decreases, iron ions contained in the oxalic acid aqueous solution begin to precipitate into the oxalic acid aqueous solution in the form of iron hydroxide, so the injection of hydrogen peroxide into the oxalic acid aqueous solution is stopped.

Water is passed through the cation resin tower (step S9). The valve 82 is opened to reduce the opening degree of the valve 78, and the aqueous solution flowing in the ultraviolet irradiation system piping 49 and circulation piping 76 is supplied to the cation resin column 56 through the piping 83. Therefore, Fe ²⁺ generated by ultraviolet irradiation by the ultraviolet irradiation device 47 is adsorbed by the cation exchange resin in the cation resin tower 56 and removed from the oxalic acid aqueous solution. While the aqueous solution is circulating in a closed loop (hereinafter referred to as the seventh closed loop) including the purification system piping 20, the circulation piping 76, and the ultraviolet irradiation system piping 49, the Fe ²⁺ is removed by the cation resin column 56.

The oxalic acid aqueous solution is purified (step S10). When the reaction of formula (3) no longer occurs even if hydrogen peroxide is injected into the oxalic acid aqueous solution, the power supply to the heater 33 is stopped. Then, by opening the valve 84 and closing the valve 82, the oxalic acid aqueous solution is supplied to the mixed bed resin column 61. The oxalic acid aqueous solution is cooled in the cooler 60 to lower its temperature before being supplied to the mixed bed resin tower 61 by opening the valve 80 and reducing the opening degree of the valve 77. Oxalic acid, cations, and anions remaining in the oxalic acid aqueous solution whose temperature has decreased to a predetermined temperature are adsorbed by the cation exchange resin and anion exchange resin in the mixed bed resin tower 61 and removed. In this way, the oxalic acid aqueous solution remaining in the seventh closed loop and the like is purified.

The waste liquid is treated (step S11). After the above purification step is completed, the circulation pipe 76 and the waste liquid treatment device (not shown) are connected by a high pressure hose (not shown) having a pump (not shown). After the purification process is completed, the aqueous radioactive waste liquid present in the seventh closed loop is discharged from the circulation piping 76 through a high-pressure hose to a waste liquid treatment device (not shown) by driving the pump, and is discharged from the circulation piping 76 through a high-pressure hose to a waste liquid treatment device (not shown). It is processed. After the aqueous solution in the seventh closed loop, etc. is discharged, cleaning water is supplied into the seventh closed loop, etc., and the circulation pump 34 is driven to clean the inside of these pipes. After the cleaning is completed, the cleaning water in the seventh closed loop and the like is discharged to the waste liquid treatment device described above.

The chemical decontamination device is removed from the piping system (step S17). After the step S11 is completed, the chemical decontamination device 31B is removed from the purification system piping 20, and the purification system piping 20 is restored. After the purification system piping 20 is restored, the BWR plant 1 is started, and the BWR plant 1 is operated in the next operation cycle.

According to this embodiment, oxalic acid is produced by contacting iron(II) oxalate dihydrate formed on the inner surface of the purification system piping 20 with an oxidizing agent, for example, an oxalic acid aqueous solution containing hydrogen peroxide. Fe ²⁺ contained in iron (II) dihydrate is oxidized to Fe ³⁺ by hydrogen peroxide, and the iron (II) oxalate dihydrate is dissolved in an oxalic acid aqueous solution as an iron (III) oxalate complex. dissolve. For this reason, the formed iron (II) oxalate dihydrate is removed from the inner surface of the purification system piping 20, and the inner surface of the purification system piping 20 is reductively decontaminated with an oxalic acid aqueous solution, in particular, the oxide film containing radionuclides is removed. It is possible to promote reduction decontamination of the inner surface of the purification system piping 20 in which the decontamination is formed.

Furthermore, in this example, before bringing the oxalic acid aqueous solution into contact with the ion exchange resin, by irradiating the oxalic acid aqueous solution containing the iron(III) oxalate complex with ultraviolet rays, the iron(III) oxalate complex was converted into oxalic acid. Iron(II) dihydrate is produced. The concentration of iron(II) oxalate dihydrate in the oxalic acid aqueous solution increases, and iron(II) oxalate dihydrate precipitates in the oxalic acid aqueous solution. Precipitation of iron(II) oxalate dihydrate reduces the concentration of cations (eg, Fe ²⁺ ) in the aqueous oxalic acid solution, and reduces the amount of cations adsorbed on the cation exchange resin. Therefore, the life of the cation exchange resin can be extended and the amount of cation exchange resin used can be reduced.

Since the radionuclides contained in the oxalic acid aqueous solution are incorporated into the precipitated iron(II) oxalate dihydrate, the radionuclides contained in the oxalic acid aqueous solution, which are essentially cations, are incorporated into the precipitated oxalate. It is removed by filter 48 along with the iron(II) acid dihydrate. As a result, the radionuclide concentration of the oxalic acid aqueous solution is reduced, and the dose of the oxalic acid aqueous solution can be reduced. Since the radionuclide incorporated into iron(II) oxalate dihydrate is essentially a cation, the amount of radionuclide ions adsorbed to the cation exchange resin also decreases, which shortens the life of the cation exchange resin. extend. Therefore, the amount of cation exchange resin used can be further reduced.

According to this embodiment, the coexistence of oxalic acid and the oxidizing agent makes it possible to deeply dissolve the base material of the purification system piping 20 (for example, 20 μm). This dissolution makes it possible to separate most of the dissolved iron eluted into the oxalic acid aqueous solution from the oxalic acid aqueous solution without using an ion exchange resin, making it possible to suppress the amount of waste produced by the ion exchange resin.

This embodiment may be applied to the residual heat removal system piping 27, which is a carbon steel member other than the purification system piping 20.

In this embodiment as well, the filter 48 may be disposed downstream of the cation resin column 56 as in the first embodiment. This filter 48 is installed in the circulation pipe 76, for example, between the connection point between the pipe 83 and the circulation pipe 76 and the surge tank 90.

In the chemical decontamination device 31B used in the fifth embodiment, an oxygen injection device 67 may be used as the oxidizing agent injection device 39. When the oxygen injection device 67 is used in the chemical decontamination device 31B, the diffuser pipe 71 is not necessary. The oxygen injection device 67 used in the chemical decontamination device 31B includes an oxygen filling device (for example, an oxygen cylinder) 68 and an injection pipe 70. One end of the injection pipe 70 provided with the valve 69 is connected to the oxygen filling device 68, and the other end of the injection pipe 70 is connected to the circulation pipe 76 between the valve 79 and the on-off valve 38. One end is connected to the ultraviolet irradiation system piping 49 between the ultraviolet irradiation device 47 and the valve 50, and the other end of the piping 54 having the valve 53 is connected to the injection piping 70 between the oxygen filling device 68 and the valve 69. .

In step S4 of the fifth embodiment, gaseous oxygen is injected from the oxygen filling device 68 into the circulation pipe 76 with the valve 69 opened and the valve 53 closed. In step S8 of the fifth embodiment, the oxygen is injected from the oxygen filling device 68 into the ultraviolet irradiation system piping 49 with the valve 53 opened and the valve 69 closed. Note that the oxygen injection device 67 may be an ozone injection device having an ozone filling device, and ozone in the ozone filling device may be injected into the circulation pipe 76 or the ultraviolet irradiation system pipe 49.

In the chemical decontamination method for carbon steel members of a nuclear power plant according to this embodiment, between the step S8 (decomposition of oxalic acid) and the step S9 (water passage to the cation resin tower) (see FIG. 9), The steps carried out in Example 3, that is, the step S13 (injection of oxidizing agent) and the step S14 (removal of iron hydroxide) may be carried out in this order.

1...BWR plant, 3...Reactor pressure vessel, 4...Reactor core, 6...Recirculation system piping, 9...Turbine, 11...Water supply piping, 20...Purification system piping, 31, 31A, 31B...Chemical decontamination equipment, 32 ... Decontamination tank, 33... Heater, 35, 44, 76... Circulation piping, 39... Oxidizing agent injection device, 40... Chemical tank, 47... Ultraviolet irradiation device, 48... Filter, 49... Ultraviolet irradiation system piping, 56... Cation resin tower, 60... Cooler, 61... Mixed bed resin tower, 90... Surge tank.

Claims (15)

A reductive decontamination step of supplying an oxalic acid aqueous solution to carbon steel members of a nuclear power plant to reductively decontaminate the surface of the carbon steel members;
During the reductive decontamination step , an oxidizing agent injection step of injecting an oxidizing agent into the oxalic acid aqueous solution and bringing the oxidizing agent into contact with the carbon steel member to be reductively decontaminated;
After the reduction decontamination step , the reduction decontamination solution containing the oxalic acid aqueous solution used for the reduction decontamination is irradiated with ultraviolet rays to precipitate iron (II) oxalate dihydrate in the reduction decontamination solution. A method for chemical decontamination of carbon steel members of a nuclear power plant , comprising: an ultraviolet irradiation step of 2. The method according to claim 1 , further comprising an iron (II) oxalate dihydrate removal step of removing the precipitated iron (II) oxalate dihydrate from the reducing decontamination solution after the ultraviolet irradiation step. Method for chemical decontamination of carbon steel components of nuclear power plants. The chemical decontamination of carbon steel members of a nuclear power plant according to claim 1 , further comprising, after the ultraviolet irradiation step , a cation removal step of removing cations contained in the reducing decontamination liquid irradiated with the ultraviolet rays. Method. an iron (II) oxalate dihydrate removal step of removing the precipitated iron (II) oxalate dihydrate from the reducing decontamination solution after the ultraviolet irradiation step ;
2. The method according to claim 1 , further comprising a cation removal step of removing cations contained in the reducing decontamination solution irradiated with the ultraviolet rays after the iron (II) oxalate dihydrate removal step. Method for chemical decontamination of carbon steel components of nuclear power plants. After the ultraviolet irradiation step, a cation removal step of removing cations contained in the reducing decontamination liquid irradiated with the ultraviolet rays;
Claim 1 , further comprising an iron (II) oxalate dihydrate removal step of removing the precipitated iron (II) oxalate dihydrate from the reduction decontamination solution after the cation removal step . A method for chemical decontamination of carbon steel members of a nuclear power plant as described in . The method for chemical decontamination of carbon steel members of a nuclear power plant according to claim 5 , wherein a part of the precipitated iron (II) oxalate dihydrate is removed in the cation removal step . a second oxidizing agent injection step of injecting the oxidizing agent into the reducing decontamination solution after the iron (II) oxalate dihydrate removal step ;
The nuclear power plant according to claim 2 or 4 , further comprising an oxalic acid decomposition step of irradiating the reducing decontamination liquid containing the oxidizing agent with the ultraviolet rays to decompose oxalic acid contained in the reducing decontamination liquid. Chemical decontamination method for carbon steel parts. After the oxalic acid decomposition step , the carbon steel member is brought into contact with an oxidizing agent aqueous solution, and the iron(II) oxalate dihydrate is formed on the surface of the carbon steel member by the oxidizing agent contained in the oxidizing agent aqueous solution. 8. The method for chemical decontamination of carbon steel members of a nuclear power plant according to claim 7 , wherein the oxidizing agent is dissolved in the oxidizing agent aqueous solution. The chemical decontamination method for carbon steel members of a nuclear power plant according to claim 1 , which is carried out on carbon steel waste material that is the carbon steel member and has been removed from the nuclear power plant . an iron (II) oxalate dihydrate removal step of removing the precipitated iron (II) oxalate dihydrate from the reducing decontamination solution after the ultraviolet irradiation step;
Further comprising a carbon steel member storage step of storing the carbon steel waste material in a decontamination tank before the reduction decontamination step,
The reduction decontamination step is carried out with the carbon steel waste immersed in the oxalic acid aqueous solution in the decontamination tank,
In the iron (II) oxalate dihydrate removal step, the iron (II) oxalate dihydrate precipitated in the reducing decontamination solution containing the oxalic acid aqueous solution is discharged from the decontamination tank. The method for chemical decontamination of carbon steel members of a nuclear power plant according to claim 9 , wherein the carbon steel members are removed from the reduced decontamination liquid. After the reduction decontamination step , when the oxidizing agent is contained in the oxalic acid aqueous solution, the carbon steel waste material after the reduction decontamination process is taken out from the decontamination tank to the outside. The method for chemically decontaminating carbon steel members of a nuclear power plant according to claim 10 , further comprising the step of: After the carbon steel waste removal step , another carbon steel waste is stored in the decontamination tank, and the iron(II) oxalate is added to the other carbon steel waste in the decontamination tank. 12. The method for chemical decontamination of carbon steel members of a nuclear power plant according to claim 11 , wherein the reductive decontamination is carried out by contacting the oxalic acid aqueous solution from which dihydrates have been removed. a pipe connection step of connecting a second pipe different from the first pipe to a first pipe made of a carbon steel member of the nuclear power plant, which is connected to a reactor pressure vessel of the nuclear power plant;
a reductive decontamination step of supplying an oxalic acid aqueous solution from the second piping to the first piping and performing reductive decontamination on the inner surface of the first piping with the oxalic acid aqueous solution;
During the reductive decontamination step , an oxidizing agent injection step of injecting an oxidizing agent into the oxalic acid aqueous solution supplied by the second piping and bringing the oxidizing agent into contact with the inner surface of the first piping that is to be reductively decontaminated. and,
After the reduction decontamination step , the reduction decontamination solution containing the oxalic acid aqueous solution that has been subjected to the reduction decontamination is irradiated with ultraviolet rays to precipitate iron (II) oxalate dihydrate in the reduction decontamination solution. A method for chemical decontamination of carbon steel members of a nuclear power plant , comprising: an ultraviolet irradiation step of 14. The method for chemical decontamination of carbon steel members of a nuclear power plant according to claim 13 , wherein the reducing decontamination liquid containing the oxalic acid aqueous solution is circulated in a closed loop including the first pipe and the second pipe. 14. The method further comprises an iron (II) oxalate dihydrate removal step of removing the precipitated iron (II) oxalate dihydrate from the reducing decontamination solution after the ultraviolet irradiation step. 15. The method for chemical decontamination of carbon steel members of a nuclear power plant according to 14.

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