JP2019191075A - Chemical decontamination method and chemical decontamination apparatus - Google Patents

Abstract

To provide a chemical decontamination method capable of further improving removal of radionuclides from a stainless-steel member of a radioactive substance handling facility.SOLUTION: A chemical decontamination apparatus is connected to, for example, a recirculation pipe made of stainless steel being a chemical decontamination object in a BWR plant (S1), and an aqueous solution of permanganic acid is brought into contact with the inner surface of recirculation system piping to carry out oxidative decontamination (S3). Thereafter, an aqueous solution of oxalic acid is brought into contact with the recirculation system piping, and an oxide film containing iron oxide and radionuclides is dissolved (S5A), and an aqueous solution of oxalic acid containing hydrochloric acid is brought into contact with the recirculation system piping to dissolve the base material of the recirculation system piping (S5B). The oxalic acid contained in the aqueous solution of oxalic acid containing hydrochloric acid is decomposed (S6), and cation and anion contained in the aqueous solution are removed by a mixed bed resin tower (S7). Since the base material of the recirculation system piping is dissolved by the aqueous solution of the oxalic acid containing the hydrochloric acid, it is possible to further improve the removal of radionuclides from a stainless-steel member.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a chemical decontamination method and a chemical decontamination apparatus, and more particularly to a chemical decontamination method and a chemical decontamination apparatus suitable for elongation applied to radioactive material handling facilities such as nuclear power plants.

  Reducing the exposure dose of workers during periodic inspection work and dismantling of nuclear power plants in nuclear power plants such as boiling water nuclear power plants (BWR plants) and pressurized water nuclear power plants (PWR plants) In order to reduce the amount of radioactive waste generated by the dismantling of plants during decommissioning, work to remove radionuclides with chemicals from equipment, pipes and equipment contaminated with radionuclides, and the metal member surfaces of systems that contain pipes That is, chemical decontamination is performed.

  Examples of conventional chemical decontamination techniques are described in JP-A No. 2000-105295, JP-A No. 60-235099 and JP-A No. 9-510784.

  In the chemical decontamination method described in Japanese Patent Application Laid-Open No. 2000-105295, oxalic acid is used as a reducing decontamination agent, and potassium permanganate is used as an oxidative decontamination agent. Reductive decontamination using a reductive decontamination solution containing a decontamination agent and oxidative decontamination using an oxidative decontamination solution containing an oxidative decontamination agent are alternately repeated several times. By reductive decontamination and oxidative decontamination, the oxide film containing iron oxide, chromium oxide and radionuclide formed on the inner surface of the pipe is dissolved and removed. During reductive decontamination, the reduced decontamination solution is passed through a cation exchange resin tower packed with a cation exchange resin, so that dissolved metal ions (iron ions, chromium ions, etc.) and radionuclides It is adsorbed and removed by the cation exchange resin in the ion exchange resin tower. By repeating the oxidative decontamination and the reductive decontamination to completely dissolve the oxide film, most of the radioactive substance adhering to the surface of the metal member can be removed.

  Japanese Patent Application Laid-Open No. 60-235099 describes chemical decontamination using an oxidative decontamination solution containing permanganic acid as an oxidative decontamination agent.

  Japanese National Publication No. 9-510784 describes a method for discarding a solution containing an organic acid. In this disposal method, after reductive decontamination using an oxalic acid aqueous solution that is a reductive decontamination solution is completed, an oxidizing agent (for example, hydrogen peroxide) is added to the oxalic acid aqueous solution, and the oxalic acid aqueous solution containing the oxidizing agent is added. Is described in which oxalic acid is decomposed by irradiating it with ultraviolet rays, and metal cations such as iron ions contained in the aqueous solution are removed by a cation or the like.

  Japanese Patent Application Laid-Open No. 2014-173905 describes a method for elution of radioactive cesium. In this elution method, by contacting an incinerated ash containing radioactive cesium with an oxalic acid aqueous solution containing hydrochloric acid, calcium oxalate is precipitated in the aqueous oxalic acid solution. Also become highly soluble calcium hydrochloride and oxalic acid. Radiocesium present in calcium oxalate is eluted.

  HOBertholdt et al. “HP / CORD D UV A New Decontamination Process for Decommissioning of Nuclear Station”, Proceeding of International Conference on Decommissioning and Nuclear and Hazardous Waste Mnagement, Volume 1, p.116 (1998) Even if the oxide film formed on the surface is removed by chemical decontamination, some radionuclide remains in the crystal grain boundary of the metal member, so the surface of the base material of the metal member is about tens of μm. It is described that the radionuclide existing in the grain boundary can be removed by dissolution.

JP 2000-105295 A Japanese Patent Laid-Open No. 60-235099 JP-T 9-510784 JP 2014-173905 A

H.O.Bertholdt et al. “HP / CORD D UV A New Decontamination Process for Decommissioning of Nuclear Station”, Proceeding of International Conference on Decommissioning and Decontamination and Nuclear and Hazardous Waste Mnagement, Volume 1, p.116 (1998)

  Even if an oxide film formed on a metal member (structural member) of a nuclear power plant that is a decontamination target is dissolved and removed by chemical decontamination, it is included in the oxide film formed on the metal member. Several percent of the radionuclides present before chemical decontamination can be left unremoved by chemical decontamination. HOBertholdt et al. “HP / CORD D UV A New Decontamination Process for Decommissioning of Nuclear Station”, Proceeding of International Conference on Decommissioning and Nuclear and Hazardous Waste Mnagement, Volume 1, p.116 (1998) Thus, even if the surface of the base material of the metal member is melted by about several tens of μm, only a part of several percent of radionuclides is removed.

  In order to reduce the exposure of workers, it is necessary to further remove radionuclides from the structural members that are decontamination objects of radioactive material handling facilities.

  An object of the present invention is to provide a chemical decontamination method and a chemical decontamination apparatus that can further improve the removal of radionuclides from stainless steel members of a radioactive material handling facility.

A feature of the present invention that achieves the above-described object of the present invention is that the first aqueous solution containing oxalic acid and not containing hydrochloric acid is brought into contact with the surface of the stainless steel member of the radioactive material handling facility, thereby including the radionuclide on the surface. Dissolve and remove the oxide film,
The second aqueous solution containing oxalic acid and hydrochloric acid is brought into contact with the surface of the stainless steel member from which the oxide film has been removed to dissolve the base material of the stainless steel member.

  Since the second aqueous solution containing oxalic acid and hydrochloric acid is brought into contact with the surface of the stainless steel member from which the oxide film has been removed, hydrochloric acid alone cannot dissolve the base material of the stainless steel member. Also, the base material of the stainless steel member can be dissolved. For this reason, the radionuclide can be further removed from the stainless steel member from which the oxide film containing the radionuclide has been removed, and the dose rate of the stainless steel member can be further reduced.

  The chemical decontamination apparatus is characterized by a pipe for sequentially leading the first aqueous solution containing oxalic acid and not containing hydrochloric acid, and the second aqueous solution containing oxalic acid and hydrochloric acid, and an oxidative decontamination agent injection device connected to the pipe. And a reducing decontamination agent injection device connected to the pipe and a hydrochloric acid injection device connected to the pipe.

  ADVANTAGE OF THE INVENTION According to this invention, the removal of the radionuclide from the stainless steel member of a radioactive substance handling facility can be improved further.

It is a flowchart which shows the procedure of the chemical decontamination method applied to the recirculation system piping of the boiling water nuclear power plant of Example 1 which is one suitable Example of this invention. It is explanatory drawing which shows the state which connected the chemical decontamination apparatus used when implementing the chemical decontamination method of Example 1 to the recirculation system piping of a boiling water nuclear power plant. It is a detailed block diagram of the chemical decontamination apparatus shown in FIG. It is a characteristic view which shows the relationship between each hydrochloric acid concentration of the oxalic acid aqueous solution and hydrochloric acid aqueous solution containing hydrochloric acid, and the amount of dissolution of the stainless steel test piece with respect to each aqueous solution. It is a characteristic view which shows the relationship between the oxalic acid concentration of an oxalic acid aqueous solution, and the dissolution amount of a stainless steel test piece. It is a characteristic view which shows the relationship between each pH of the oxalic acid aqueous solution which does not contain hydrochloric acid, the oxalic acid aqueous solution containing hydrochloric acid, and hydrochloric acid aqueous solution, and the dissolution amount of a stainless steel test piece. It is a characteristic view which shows the relationship between the Fe ion concentration of the oxalic acid aqueous solution containing hydrochloric acid, and the amount of dissolution of the stainless steel test piece. It is a detailed block diagram of the chemical decontamination apparatus which has a decontamination tank which arrange | positions the structural member of the boiling water type nuclear power plant of Example 2 which is another suitable Example of this invention inside.

  The inventors conducted various studies including experiments on chemical decontamination that can further improve the removal of radionuclides from the structural members of radioactive material handling facilities. In the chemical decontamination of the structural member, particularly a stainless steel structural member (hereinafter referred to as a stainless steel member), oxidation including chromium oxide, iron oxide and a radionuclide formed on the surface of the stainless steel member. Oxidative decontamination to remove chromium oxide from the film (oxidative decontamination solution, for example, using an aqueous solution of permanganate or potassium permanganate), and reduction to remove an oxide film containing iron oxide and radionuclide Decontamination (reduction decontamination solution, for example, using an oxalic acid aqueous solution (an aqueous solution containing oxalic acid and no hydrochloric acid)) is performed. As a result of the examination described above, the inventors conducted oxidative decontamination on a stainless steel member using an oxidative decontamination solution and then dissolved the oxide film on the stainless steel member using an oxalic acid aqueous solution in chemical decontamination. After that, the aqueous oxalic acid solution containing hydrochloric acid produced by injecting hydrochloric acid into the oxalic acid aqueous solution is brought into contact with the stainless steel member from which the oxide film has been removed, and the surface portion of the stainless steel member is dissolved. It was newly found that the removal of radionuclides from steel members can be further improved. That is, in reductive decontamination, the dissolution of the oxide film with the oxalic acid aqueous solution, and the dissolution of the surface portion of the stainless steel member with the oxalic acid aqueous solution containing hydrochloric acid after this dissolution removes the radionuclide from the stainless steel member. It led to further improvement.

  The above examination results will be described in detail below.

The inventors filled 500 mL of a 90 ° C. aqueous solution containing only hydrochloric acid in one container (for example, a beaker), and immersed a test piece made of SUS304 stainless steel having a surface area of 17.2 cm ^{2 in} this aqueous solution. 500 mL of 90 ° C. aqueous solution (decontamination aqueous solution) produced by mixing hydrochloric acid and 0.022 mol / L oxalic acid was filled in another container (for example, a beaker), and this aqueous solution was filled with SUS304 having a surface area of 17.2 cm ² . Another test piece made of stainless steel was immersed, and the dissolution amount of each test piece was determined. Each test piece was immersed in each aqueous solution for 6 hours. The dissolution amount of each test piece is obtained by subtracting the weight of the test piece per unit surface area before being immersed in the aqueous solution from the weight of the test piece per unit surface area after being immersed in the corresponding aqueous solution for 6 hours. Value (weight).

  FIG. 4 shows changes in the dissolution amount of each test piece with respect to the hydrochloric acid concentration of each aqueous solution. In FIG. 4, the smaller the negative value of the dissolution amount of the test piece, that is, the lower the vertical axis in FIG. 4, the greater the degree of dissolution of the test piece (the dissolution amount increases). Show.

  In the 90 ° C. aqueous solution containing only hydrochloric acid, the base material of the immersed test piece did not dissolve until the hydrochloric acid concentration reached 0.075 mol / L, and dissolved when the hydrochloric acid concentration reached 0.078 mol / L or more ( (See the circles in FIG. 4). On the other hand, in the 90 ° C. aqueous solution containing hydrochloric acid and 0.022 mol / L oxalic acid, the base material of the immersed test piece is dissolved when the hydrochloric acid concentration becomes 0.006 mol / L or more, and the hydrochloric acid concentration is 0.009 mol / L. Above L, the amount of dissolution became almost constant (see Δ in FIG. 4). In FIG. 4, “M” represents mol / L, and FIG. 4 is expressed in units of mM. From the experimental results shown in FIG. 4, when an aqueous solution containing oxalic acid and hydrochloric acid was brought into contact with a stainless steel test piece, that is, a stainless steel member, the base material of the stainless steel member was formed using hydrochloric acid alone. It became clear that the base material of the stainless steel member can be dissolved even when it cannot be dissolved.

The inventors filled 500 mL of a 90 ° C. aqueous solution containing only oxalic acid in one container (for example, a beaker), and immersed a test piece made of SUS304 stainless steel having a surface area of 17.2 cm ^{2 in} this aqueous solution for 6 hours. It was. When 6 hours passed after the test piece was immersed in an aqueous solution of 90 ° C. containing only oxalic acid, the test piece was taken out of the aqueous solution, and the amount of dissolution of the test piece was determined. This dissolved amount is obtained by subtracting the weight of the test piece per unit surface area before being immersed in the oxalic acid aqueous solution from the weight of the test piece per unit surface area after being immersed in the oxalic acid aqueous solution for 6 hours. Value (weight).

  FIG. 5 shows the change in the dissolution amount of the test piece with respect to the oxalic acid concentration of the oxalic acid aqueous solution. In FIG. 5, the smaller the negative value of the dissolution amount of the test piece, that is, the lower the vertical axis in FIG. 5, the higher the degree of dissolution of the test piece (the more the dissolution amount increases). Show.

  In the oxalic acid aqueous solution at 90 ° C., the base material of the immersed test piece hardly dissolves until the oxalic acid concentration becomes 0.076 mol / L, and dissolves when the oxalic acid concentration becomes 0.092 mol / L or more. Furthermore, when the oxalic acid concentration was 0.150 mol / L or more, the dissolved amount became almost constant.

  Based on the results shown in FIG. 4 and FIG. 5, the inventors used SUS304 by using an aqueous solution containing oxalic acid and hydrochloric acid, unlike using an aqueous solution containing only oxalic acid and an aqueous solution containing only hydrochloric acid. It was newly found that a stainless steel test piece, that is, a base material of a base material of a stainless steel member can be melted. In particular, by using an aqueous solution containing oxalic acid at a concentration in the range of 0.076 mol / L or less and hydrochloric acid at a concentration in the range of 0.075 mol / L or less, the base material of the stainless steel member is further dissolved. Can be made.

  The inventors arranged the experimental results shown in FIG. 4 and FIG. 5 according to the relationship between the room temperature pH and the dissolution amount of the test piece, and summarized the change in the dissolution amount of the test piece with respect to the room temperature pH in FIG. In FIG. 6, the horizontal axis represents the room temperature pH of the aqueous solution, and the vertical axis represents the amount of dissolution of the test piece. The pH (@RT) described on the horizontal axis in FIG. 6 means a pH at room temperature, that is, a room temperature pH. The relationship between the room temperature pH and the amount of dissolution of the test piece was as follows: 90 ° C. aqueous solution containing only oxalic acid (◯ mark), 90 ° C. aqueous solution containing 0.022 mol / L oxalic acid (Δ mark), and hydrochloric acid It showed about each of 90 degreeC aqueous solution containing only.

  In the aqueous solution containing only oxalic acid, the pH of this aqueous solution was 1.32 or less, and in the aqueous solution containing only hydrochloric acid, the pH of this aqueous solution was 1.16 or less. In an aqueous solution containing an acid, the pH of the aqueous solution is 1.69 or less, and the base material of the test piece is dissolved. The pH of the aqueous solution containing hydrochloric acid and oxalic acid should be within the range of 1.7 or less, which increases the dissolution amount of the stainless steel test piece. It is desirable to set the pH within the range of 0.7 (1.4 or more and 1.7 or less). From this result, not only simply reducing the pH of the aqueous solution by mixing hydrochloric acid with the oxalic acid aqueous solution, but also the aqueous solution containing only oxalic acid and the mixed action of oxalate ions and chloride ions in the oxalic acid aqueous solution and It was found that the base material of the stainless steel member can be dissolved when an aqueous solution containing oxalic acid and hydrochloric acid (decontamination aqueous solution) having a pH higher than that of each aqueous solution containing only hydrochloric acid is used.

Furthermore, a surface area of 17.22 ° C. in an aqueous solution (decontamination solution) at 90 ° C. containing 0.022 mol / L oxalic acid and hydrochloric acid and mixed with iron chloride so that chloride ions are constant at 0.023 mol / L. A test piece made of 2 cm ² of SUS304 stainless steel was immersed for 6 hours. When 6 hours had passed since the immersion, the test piece was taken out from the aqueous solution, and the dissolution amount of the test piece was determined. This dissolved amount is a value obtained by subtracting the weight of the test piece per unit surface area before being immersed in the aqueous solution from the weight of the test piece per unit surface area after being immersed in the aqueous solution for 6 hours (weight). ).

  FIG. 7 shows the change in the dissolution amount of the test piece with respect to the iron ion concentration of the aqueous solution. In the aqueous solution containing the oxalic acid, hydrochloric acid, and chloride ions, dissolution of the base material of the immersed specimen is suppressed when the iron ion concentration exceeds 0.007 mol / L (0.7 mM), and the iron ion concentration is reduced. It turned out that it will not melt | dissolve remarkably when it will be 0.010 mol / L or more. In order to promote dissolution of the base material of the immersed test piece, it is desirable that the iron ion concentration of the aqueous solution containing oxalic acid and chlorine be 0.007 mol / L or less.

  Each characteristic described above regarding SUS304 also occurs in other stainless steels such as austenitic stainless steel (for example, SUS316L), martensitic stainless steel, and ferritic stainless steel other than SUS304.

  From the above examination results, the inventors contacted an aqueous solution containing oxalic acid and hydrochloric acid with a stainless steel member from which a radionuclide-containing oxide film has been removed, which is a target for chemical decontamination of a radioactive material handling facility. Further, it has been newly found that the base material of the stainless steel member can be dissolved even when each of oxalic acid alone and hydrochloric acid alone cannot dissolve the stainless steel member. For this reason, the radionuclide can be further removed from the stainless steel member.

  In particular, the hydrochloric acid concentration of the aqueous solution containing hydrochloric acid and oxalic acid is in the range of 0.006 mol / L to 0.075 mol / L, preferably in the range of 0.009 mol / L to 0.075 mol / L. Thus, the base material of SUS304 stainless steel can be dissolved at a concentration that cannot be dissolved by each of oxalic acid alone and hydrochloric acid alone. Further, by setting the oxalic acid concentration of the aqueous solution containing hydrochloric acid and oxalic acid to a range of 0.010 mol / L or more and 0.076 mol / L or less, the SUS304 stainless steel base material can be formed at a concentration that cannot be dissolved by oxalic acid alone. Can be dissolved. Furthermore, by making the iron ion concentration of the aqueous solution containing hydrochloric acid and oxalic acid 0.007 mol / L or less, dissolution of the base material of S304 stainless steel can be promoted, and the iron ion concentration is reduced to 0.007 mol / L. By increasing the amount, the melting of the S304 stainless steel base material can be suppressed. Furthermore, by making the iron ion concentration of the aqueous solution containing hydrochloric acid and oxalic acid 0.010 mol / L or more, it is possible to realize a state in which dissolution of the base material of SUS304 stainless steel is substantially eliminated.

  Preferably, when the base material of the stainless steel member is dissolved using an aqueous solution containing hydrochloric acid and oxalic acid, the hydrochloric acid concentration of the aqueous solution is set in the range of 0.009 mol / L to 0.075 mol / L, and oxalic acid It is desirable that the concentration is in the range of 0.010 mol / L or more and 0.076 mol / L or less, and the iron ion concentration of the aqueous solution is 0.007 mol / L or less.

  When the oxalic acid concentration is increased even in an aqueous solution containing only oxalic acid, the base material of SUS304 stainless steel can be dissolved as shown in FIG. However, in order to reduce secondary radioactive waste, it is necessary to decompose oxalic acid after reductive decontamination, and the higher the oxalic acid concentration, the longer the time required to decompose oxalic acid. Therefore, the upper limit of the oxalic acid concentration of the aqueous solution containing oxalic acid and hydrochloric acid (reducing decontamination aqueous solution) used for reductive decontamination is preferably 0.076 mol / L.

  Furthermore, when the hydrochloric acid concentration of the aqueous solution containing only hydrochloric acid is increased, the base material of SUS304 stainless steel can be dissolved as shown in FIG. However, in order to reduce secondary radioactive waste, it is necessary to recover the added hydrochloric acid. In order to reduce the amount of hydrochloric acid recovered as secondary radioactive waste, the upper limit of the hydrochloric acid concentration of the aqueous solution containing oxalic acid and hydrochloric acid used for reductive decontamination is preferably 0.075 mol / L.

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

  A chemical decontamination method applied to a stainless steel member, which is a structural member of the nuclear power plant of Example 1, which is a preferred embodiment of the present invention, will be described with reference to FIGS. The chemical decontamination method of the present embodiment is applied to a stainless steel recirculation piping that is a stainless steel member of a boiling water nuclear power plant (BWR plant).

  A schematic configuration of the BWR plant 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 nuclear reactor purification system, a water supply system, and the like. The reactor 2 is a steam generator, and has a reactor pressure vessel (hereinafter referred to as RPV) 3 in which a reactor core 4 is built, and an outer surface of a reactor core shroud (not shown) surrounding the reactor core 4 in the RPV 3 and the RPV 3 A plurality of jet pumps 5 are installed in an annular downcomer formed between the inner surface and the inner surface. A large number of fuel assemblies (not shown) are loaded on the core 4. The fuel assembly includes a plurality of fuel rods filled with a plurality of fuel pellets made of nuclear fuel material.

  The recirculation system includes a stainless steel recirculation pipe (first pipe) 6 and a recirculation pump 7 installed in the recirculation pipe 6. The water supply system includes a condensate pump 12, a condensate purification device (for example, a condensate demineralizer) 13, a low-pressure feed water heater 14, a feed water pump 15, and a high-pressure feed water. The heater 16 is installed in this order from the condenser 10 toward the RPV 3. A drain water recovery pipe 26 connected to the high pressure feed water heater 16 and the low pressure feed water heater 14 is connected to the condenser 10. The reactor purification system includes a purification pipe 18 made of carbon steel that connects the recirculation pipe 6 and the feed water pipe 11, a purification pump 19, a regenerative heat exchanger 20, a non-regenerative heat exchanger 21, and a reactor water purification device. 22 are installed in this order. The purification system pipe 18 is connected to the recirculation system pipe 6 upstream of the recirculation pump 7. The valve 23 is provided in the purification system pipe 18 at a position near the connection point between the connection point of the recirculation system pipe 6 and the purification system pipe 18 and the purification system pump 19, and the valve 24 is not regenerated with the regenerative heat exchanger 20. It is provided in the purification system pipe 18 at a position close to the non-regenerative heat exchanger 21 between the heat exchangers 21. The nuclear reactor 2 is installed in a nuclear reactor containment vessel 25 arranged in a nuclear reactor building (not shown).

  Cooling water (hereinafter referred to as “reactor water”) in the RPV 3 is boosted by the recirculation pump 7 and jetted into the jet pump 5 through the recirculation system pipe 6. The reactor water existing around the nozzle of the jet pump 5 in the downcomer is also sucked into the jet pump 5 and supplied to the reactor core 4. The reactor water supplied to the core 4 is heated by the heat generated by the nuclear fission of the nuclear fuel material in the fuel rods, and a part thereof becomes steam. This steam is guided from the RPV 3 through the main steam pipe 8 to the turbine 9 to rotate the turbine 9. A generator (not shown) connected to the turbine 9 rotates to generate electric power. The steam discharged from the turbine 9 is condensed by the condenser 10 to become water. This water is supplied into the RPV 3 through the water supply pipe 11 as water supply. The feed water flowing through the feed water pipe 11 is boosted by the condensate pump 12, impurities are removed by the condensate purification device 13, and further boosted by the feed water pump 15. The feed water is heated by the low pressure feed water heater 14 and the high pressure feed water heater 16 and guided into the RPV 3. The extraction steam extracted from the turbine 9 by the extraction pipe 17 is supplied to the low-pressure feed water heater 14 and the high-pressure feed water heater 16 respectively, and becomes a heating source of the feed water.

  A part of the reactor water flowing in the recirculation system pipe 6 flows into the purification system pipe 18 by the drive of the purification system pump 19 and is cooled by the regenerative heat exchanger 20 and the non-regenerative heat exchanger 21. It is purified by the water purification device 22. The purified reactor water is heated by the regenerative heat exchanger 20 and returned to the RPV 3 through the purification system pipe 18 and the water supply pipe 11.

  In the chemical decontamination method of this embodiment, a chemical decontamination apparatus 30 is used, and this chemical decontamination apparatus 30 is connected to the recirculation piping 6 of the BWR plant as shown in FIG.

  A detailed configuration of the chemical decontamination apparatus 30 will be described with reference to FIG.

  The chemical decontamination apparatus 30 includes a circulation pipe (second pipe) 31, a surge tank 32, a heater 33, circulation pumps 34 and 35, an oxidative decontamination agent injection device 36, a reductive decontamination agent injection device 41, and a pH adjusting agent injection. The apparatus 46, the hydrochloric acid injection | pouring apparatus 51, the oxidizing agent supply apparatus 56, the cooler 61, the cation exchange resin tower (cation exchange resin tower) 62, the mixed bed resin tower 63, and the decomposition | disassembly apparatus 64 are provided.

  The on-off valve 65, the circulation pump 35, the valves 66, 69 and 74, the surge tank 32, the circulation pump 34, the valve 77 and the on-off valve 78 are provided in the circulation pipe 31 in this order from the upstream. A cooler 61 and a valve 67 are installed in a pipe 68 that bypasses the valve 66 and is connected to the circulation pipe 31 at both ends. A cation exchange resin tower 62 and a valve 70 are installed in a pipe 71 having both ends connected to the circulation pipe 31 and bypassing the valve 69. A mixed bed resin tower 63 and a valve 72 are installed in a pipe 73 having both ends connected to the pipe 71 and bypassing the cation exchange resin tower 62 and the valve 70. The cation exchange resin tower 62 is filled with a cation exchange resin, and the mixed bed resin tower 63 is filled with a cation exchange resin and an anion exchange resin.

  A pipe 76 in which a disassembling device 64 positioned downstream of the valve 75 and the valve 75 is installed bypasses the valve 74 and is connected to the circulation pipe 31. The decomposition device 64 is filled with, for example, a 0.5% Ru-supported activated carbon catalyst in which ruthenium is impregnated on the surface of the activated carbon. A surge tank 32 is installed in the circulation pipe 31 between the valve 74 and the circulation pump 34. A heater 33 is disposed in the surge tank 32. One end of the pipe 80 provided with the valve 79 is connected to the circulation pipe 31 between the valve 77 and the circulation pump 34, and the other end of the pipe 80 is connected to the surge tank 32.

  The oxidative decontamination agent injection device 36 includes a chemical tank 37, an injection pump 38, and an injection pipe 39. The chemical tank 37 is connected to a pipe 80 by an injection pipe 39 having an injection pump 38 and a valve 40. An aqueous solution of permanganic acid, which is an oxidative decontamination agent, is filled in the chemical tank 37.

  The reductive decontamination agent injection device 41 includes a chemical tank 42, an injection pump 43, and an injection pipe 44. The chemical tank 42 is connected to a pipe 80 by an injection pipe 44 having an injection pump 43 and a valve 45. An aqueous solution of oxalic acid, which is a reducing decontamination agent, is filled in the chemical tank 42.

  Injection pipes 39, 44 and 54 are connected to the pipe 80 between the surge tank 32 and the valve 79 in that order from the surge tank 32 to the valve 79.

  The hydrochloric acid injection device 51 has a chemical tank 52, an injection pump 53, and an injection pipe 54. The chemical tank 52 is connected to a pipe 80 by an injection pipe 54 having an injection pump 53 and a valve 55. A hydrochloric acid aqueous solution is filled in the chemical tank 52.

  The pH adjuster injection device 46 includes a chemical tank 47, an injection pump 48 and an injection pipe 49. The chemical tank 47 is connected to the circulation pipe 31 by an injection pipe 49 having an injection pump 48 and a valve 50. The injection pipe 49 is connected to the circulation pipe 31 between the valve 77 and the on-off valve 78. A chemical solution tank 47 is filled with an aqueous solution of hydrazine (hydrazine aqueous solution) which is a pH adjuster.

  The oxidant supply device 56 includes a chemical liquid tank 57, a supply pump 58, and a supply pipe 59. The chemical tank 57 is connected to a pipe 76 upstream of the valve 75 by a supply pipe 59 having a supply pump 58 and a valve 60. The chemical liquid tank 57 is filled with hydrogen peroxide as an oxidant. As the oxidizing agent, an aqueous solution in which ozone is dissolved may be used.

  A pH meter 81 is attached to the circulation pipe 31 between the connection point between the injection pipe 49 and the circulation pipe 31 and the on-off valve 78.

  The BWR plant 1 is stopped after the operation in one operation cycle is completed. After the shutdown, a part of the fuel assembly loaded in the core 4 is taken out as a spent fuel assembly, and a new fuel assembly having a burnup of 0 GWd / t is loaded in the core 4. After such a fuel change is completed, the BWR plant 1 is restarted for operation in the next operation cycle. Maintenance inspection of the BWR plant 1 is performed using a period during which the BWR plant 1 is stopped for fuel replacement.

  During the period when the operation of the BWR plant 1 is stopped as described above, a stainless steel piping system connected to the RPV 12, which is one of the stainless steel members in the BWR plant 1, for example, the recirculation piping 6 The chemical decontamination method of the present example for the above is performed.

  The chemical decontamination method of this example will be described below based on the procedure shown in FIG. In the chemical decontamination method of this embodiment, a chemical decontamination apparatus 30 is used.

  First, a chemical decontamination apparatus is connected to the piping system (step S1). When the operation of the BWR plant 1 is stopped, the chemical decontamination apparatus 30 is connected to a stainless steel piping system, for example, the recirculation piping 6, which is a chemical decontamination target. Specifically, one end of the circulation pipe 31 on the on-off valve 65 side is downstream of the recirculation pump 7 and is connected to the recirculation system pipe 6, for example, the recirculation system pipe 6 (for example, instrumentation). It is connected to a branch pipe from which piping is cut off. Further, the bonnet of the valve 23 installed in the purification system pipe 18 is opened to block the purification system pump 19 side. The other end of the circulation pipe 31 of the chemical decontamination apparatus 30 on the open / close valve 78 side is connected to the flange of the valve 23. In order to prevent the participation decontamination liquid and the reductive decontamination liquid from flowing into the RPV 3, both ends of the recirculation system pipe 6 are respectively sealed with plugs (not shown). For this reason, a closed loop including the recirculation system pipe 6 and the circulation pipe 31 is formed.

  The temperature of water present in the chemical decontamination target piping system and chemical decontamination apparatus is increased (step S2). First, among the valves provided in the chemical decontamination apparatus 30, the valves 40, 45, 50, 55, and 60 are closed, all the valves other than these valves are opened, and the chemical decontamination is performed through the circulation pipe 31 of the chemical decontamination apparatus 30. Pure water is supplied into the recirculation pipe 6 to be dyed, and the chemical decontamination apparatus 30 and the recirculation pipe 6 are filled with pure water. After these interiors are filled with pure water, the on-off valve 65, the valves 66, 69, 74 and 77, and the on-off valve 78 are opened and the other valves are closed, and the circulation pumps 34 and 35 are operated. To drive. Then, the water in the surge tank 32 is heated by the heater 33. When the pure water circulating in the closed loop reaches the temperature range of 80 ° C. or higher and lower than 100 ° C., for example, 90 ° C. due to heating by the heater 33, the valve 79 is opened to flow through the circulation pipe 31. Part of the pure water is introduced into the pipe 80.

  Oxidative decontamination is performed (step S3). The valve 40 of the oxidative decontamination agent injection device 36 is opened and the injection pump 38 is driven to inject the permanganic acid aqueous solution in the chemical tank 37 into 90 ° C. pure water flowing through the pipe 80 through the injection pipe 39. The permanganic acid aqueous solution is guided into the surge tank 32 from the pipe 80 and is supplied from the circulation pipe 31 to the recirculation pipe 6 as an oxidative decontamination aqueous solution (oxidative decontamination liquid). The permanganic acid concentration of the permanganic acid aqueous solution supplied to the recirculation system pipe 6 is within a range of 0.001 mol / L to 0.004 mol / L (0.001 mol / L to 0.004 mol / L), for example. The amount of permanganic acid aqueous solution injected from the injection pipe 39 to the pipe 80 is adjusted by controlling the rotational speed of the injection pump 38 (or the opening degree of the valve 40) so as to be 0.003 mol / L. The

  In the recirculation system pipe 6, the oxide film formed on the inner surface of the recirculation system pipe 6 by the action of permanganate ions contained in the permanganate aqueous solution at a permanganate concentration of 0.003 mol / L and 90 ° C. The contained chromium oxide is dissolved. Furthermore, a recirculation pump 7 (not shown) which is a device provided in the recirculation system pipe 6 includes a stainless steel casing and a stainless steel impeller (not shown) arranged in the casing. It is out. The casing and impeller of the recirculation pump 7 are also stainless steel members. The permanganic acid aqueous solution flowing in the recirculation system pipe 6 dissolves chromium oxide contained in the oxide film formed on the inner surface of the casing in contact with the reactor water and on the surface of the impeller in contact with the reactor water. While the permanganic acid aqueous solution circulates in the closed loop including the circulation pipe 31 and the recirculation pipe 6, the oxidative decontamination of the inner face of the recirculation pipe 6 and the inner surface of the casing and the impeller of the recirculation pump 7 are performed. Perform oxidative decontamination on the surface. The water passage time of the permanganic acid aqueous solution to the recirculation system pipe 6 is in the range of 4 to 6 hours, and oxidative decontamination is performed on the inner surface of the recirculation system pipe 6 within this time range. For example, when 6 hours have passed since the supply of the permanganic acid aqueous solution to the recirculation pipe 6 was started, the oxidative decontamination of the inner surface of the recirculation pipe 6 is completed.

  After the oxidative decontamination is completed, the oxidative decontamination agent is decomposed (step S4). When the oxidative decontamination of the inner surface of the recirculation pipe 6 is completed, the injection pump 38 is stopped and the valve 40 is closed. Then, the valve 45 of the reducing decontamination agent injection device 41 is opened to drive the injection pump 43. The oxalic acid aqueous solution in the chemical tank 42 is injected into the 90 ° C. permanganate aqueous solution flowing through the pipe 80 through the injection pipe 44. The oxalic acid aqueous solution is injected from the chemical tank 42 into the pipe 80 so that the oxalic acid concentration of the permanganic acid aqueous solution flowing in the circulation pipe 31 is seven times the permanganic acid concentration of the permanganic acid aqueous solution. The permanganate aqueous solution containing oxalic acid is led into the surge tank 32 from the pipe 80 and circulates in a closed loop including the circulation pipe 31 and the recirculation system pipe 6. The permanganate ions contained in the permanganate aqueous solution containing oxalic acid are decomposed into manganese ions and carbon dioxide by oxalic acid. A permanganic acid aqueous solution containing oxalic acid flowing in the circulation pipe 31 is appropriately collected from a water sampling device (not shown) provided in the circulation pipe 31. When the color of the collected aqueous solution changes from purple to transparent or light yellow, the decomposition of permanganic acid (oxidative decontamination agent) contained in the aqueous solution is completed.

  In the reduction decontamination, the oxide film is dissolved (step S5). After the decomposition process of the oxidative decontamination agent is completed, the valve 70 is opened to partially reduce the opening of the valve 69, and the aqueous solution flowing in the circulation pipe 31 is supplied to the cation exchange resin tower 62 through the pipe 71. The aqueous solution discharged from the cation exchange resin tower 62 is returned to the circulation pipe 31 and reaches the surge tank 32. The supply of the oxalic acid aqueous solution to the pipe 80 in the step S4 (oxidation decontamination agent decomposition step) is also continuously performed in the oxide film dissolution step (step S5), which is a reduction decontamination step. The injection of the oxalic acid aqueous solution from the chemical tank 42 of the reducing decontamination agent injection device 41 to the pipe 80 is supplied as the reducing decontamination aqueous solution (reducing decontamination solution) from the surge tank 32 and the circulation pipe 31 to the recirculation pipe 6. The oxalic acid concentration of the aqueous oxalic acid solution (aqueous solution containing oxalic acid and not containing hydrochloric acid) is in the range of 0.010 mol / L to 0.076 mol / L, for example, 0.030 mol / L. The amount of oxalic acid aqueous solution injected from the injection pipe 44 to the pipe 80 is adjusted by controlling the rotation speed of the injection pump 43 (or the opening degree of the valve 45).

  A pH adjuster injection device so that the pH of an oxalic acid aqueous solution (aqueous solution containing oxalic acid and not containing hydrochloric acid) flowing through the circulation pipe 31 and having an oxalic acid concentration of 0.030 mol / L and 90 ° C. becomes 2.5. The aqueous hydrazine solution in the 46 chemical tank 47 is injected into the circulation pipe 31 through the injection pipe 49. The pH of the oxalic acid aqueous solution is adjusted to 2.5 by controlling the rotational speed of the injection pump 48 (or the opening of the valve 50) based on the pH value of the oxalic acid aqueous solution measured by the pH meter 81. This is done by adjusting the injection amount of the hydrazine aqueous solution into the pipe 31.

  Since the circulation pumps 34 and 35 are driven, a 90 ° C. oxalic acid aqueous solution having an oxalic acid concentration of 0.030 mol / L and containing hydrazine and having a pH of 2.5 is supplied from the circulation pipe 31 to the recirculation pipe 6. It is supplied and passes through the recirculation pump 7 when flowing through the recirculation system pipe 6. By contacting the oxalic acid aqueous solution with the oxide film formed on the inner surface of the recirculation pipe 6, the inner surface of the casing in contact with the reactor water, and the oxide film formed on the surface of the impeller in contact with the reactor water. Those oxide films containing iron oxide and radionuclide are dissolved and removed from the inner surface of the recirculation piping 6, the inner surface of the casing, and the surface of the impeller. Such reductive decontamination is performed by circulating the oxalic acid solution through a closed loop including the circulation pipe 31 and the recirculation system pipe 6.

  The aqueous oxalic acid solution returned from the recirculation system pipe 6 to the circulation pipe 31 contains Cr ions dissolved by oxidative decontamination, iron ions dissolved by reductive decontamination, and metal cations such as radionuclides. Since the oxalic acid aqueous solution is supplied to the cation exchange resin tower 62 through the pipe 71, those metal cations are adsorbed and removed by the cation exchange resin in the cation exchange resin tower 62. For this reason, an increase in the iron ion concentration of the oxalic acid aqueous solution during reductive decontamination is suppressed, and reductive decontamination of the recirculation piping 6 is promoted.

  Radiation emitted from the recirculation system pipe 6 is detected by a radiation detector installed outside the recirculation system pipe 6, and the dose rate determined based on the output signal of the radiation detector is the first set dose rate. When it falls below, dissolution (reduction decontamination) of the oxide film of the recirculation piping 6 is completed. When 6 hours have passed since the supply of the oxalic acid aqueous solution to the recirculation pipe 6 was started, the reductive decontamination may be terminated.

  A decomposition process of the reductive decontaminant is performed (step S6). After dissolution of the base material of the recirculation system pipe 6 is completed, decomposition of oxalic acid and hydrazine (pH adjusting agent) contained in the oxalic acid aqueous solution containing hydrazine is performed as follows. The oxalic acid aqueous solution containing hydrazine passing through the valve 69 and the valve 70 with the valve 75 opened to partially reduce the opening degree of the valve 74 is supplied to the decomposition device 64 through the pipe 76. At this time, the hydrogen peroxide in the chemical tank 57 is supplied to the decomposition device 64 through the supply pipe 59 and the pipe 76 by driving the supply pump 58. Oxalic acid and hydrazine contained in the oxalic acid aqueous solution are decomposed in the decomposition apparatus 64 by the action of the activated carbon catalyst and the supplied hydrogen peroxide. The decomposition reaction of oxalic acid and hydrazine in the decomposition apparatus 64 is expressed by the formulas (1) and (2).

(COOH) ₂ + H ₂ O ₂ → 2CO ₂ + 2H ₂ O (1)
N ₂ H ₄ + 2H ₂ O ₂ → N ₂ + 4H ₂ O (2)
The decomposition of the oxalic acid and hydrazine in the decomposition device 64 is performed while circulating the oxalic acid aqueous solution containing hydrazine in the closed loop including the circulation pipe 31 and the recirculation system pipe 6. Supply amount of hydrogen peroxide from the chemical tank 57 to the decomposition device 64 so that the supplied hydrogen peroxide is not completely consumed by the decomposition device 64 for decomposition of oxalic acid and hydrazine and does not flow out of the decomposition device 64 Is adjusted by controlling the rotational speed of the supply pump 58. While the oxalic acid and hydrazine are decomposed without the decomposition device 64, the oxalic acid aqueous solution containing hydrazine is supplied to the cation exchange resin tower 62, and the cation contained in the aqueous solution is converted to the cation in the cation exchange resin tower 62. It is removed by ion exchange resin.

  The oxalic acid aqueous solution containing hydrazine flowing in the circulation pipe 31 is collected from the circulation pipe 31 by the water sampling device described above. The oxalic acid concentration of the collected aqueous solution is analyzed by an analyzer such as an ion chromatograph. When the oxalic acid concentration obtained as a result of the analysis becomes a set concentration (for example, 0.0001 mol / L) or less, it is determined that the decomposition of oxalic acid and hydrazine has ended. When the oxalic acid concentration becomes 0.0001 mol / L or less, the entire amount of hydrazine contained in the aqueous solution has already been decomposed.

Purification of the aqueous solution in which the reducing decontamination agent and the pH adjusting agent are decomposed is performed (step S7). After the decomposition of oxalic acid and hydrazine is completed, the valve 67 is opened and the valve 66 is closed, and the valve 72 is opened and the valves 69 and 70 are closed. The heating of the aqueous solution in which oxalic acid and hydrazine are decomposed, that is, the aqueous solution containing hydrochloric acid and a small amount of oxalic acid, is stopped. This aqueous solution is cooled by the cooler 61, the temperature is adjusted to 60 ° C., for example, and supplied to the mixed bed resin tower 63. Anions, metal ions of the anions, chromate ions (CrO ₄ ⁻ ), and oxalate ions ((COO) ₂ ⁻² ) remaining in the aqueous solution are transferred to the anion exchange resin in the mixed bed resin tower 63. Adsorbed and removed. Furthermore, the cation remaining in the aqueous solution is adsorbed and removed by the cation exchange resin in the mixed bed resin tower 63. The electrical conductivity of the aqueous solution collected from the circulation pipe 31 with the water sampling device is measured, and when the measured electrical conductivity is equal to or lower than a predetermined electrical conductivity (for example, 1 mS / m), the purification process in step S7 Exit.

  If the reduction of the dose rate of the recirculation system pipe 6 is insufficient after the purification process (step S7) is completed, the processes of steps S3 to S7 are performed again in order.

  The base material of the piping system to be chemically decontaminated is dissolved (step S8). After the purification step (step S7) is completed, 90 ° C. water substantially flows in the closed loop including the circulation pipe 31 and the recirculation system pipe 6 and the pipe 80. After the purification process, the valves 66 and 69 are opened and the valves 67 and 72 are closed. An aqueous hydrochloric acid solution is injected from the chemical solution tank 52 of the hydrochloric acid injection device 51 into the pipe 80. Further, the injection of the oxalic acid aqueous solution from the chemical solution tank 42 of the reducing decontamination agent injection device 41 to the pipe 80 is performed by opening the valve 45 and driving the injection pump 43.

  The valve 55 of the hydrochloric acid injection device 51 is opened to drive the injection pump 53, and the aqueous hydrochloric acid solution in the chemical solution tank 52 is injected into 90 ° C. water flowing in the pipe 80 through the injection pipe 54. The injected oxalic acid aqueous solution and hydrochloric acid aqueous solution are guided to the surge tank 31 from the pipe 80 and mixed in the surge tank 31 to generate an oxalic acid aqueous solution containing hydrochloric acid. The hydrochloric acid concentration of the 90 ° C. oxalic acid aqueous solution containing the generated hydrochloric acid is, for example, 0.020 mol / L within the range of 0.006 mol / L or more and 0.075 mol / L or less. The acid concentration is 0.030 mol / L. Oxalic acid contained in the oxalic acid aqueous solution containing hydrochloric acid is a reducing decontamination agent. Since the oxalic acid aqueous solution containing hydrochloric acid does not inject hydrazine (pH adjusting agent), it does not contain this hydrazine, and the pH is in the range of 1.4 to 1.7. The aqueous oxalic acid solution containing hydrochloric acid in the surge tank 32 is supplied to the recirculation system pipe 6 through the circulation pipe 31. In the recirculation system pipe 6, the 90 ° C. oxalic acid aqueous solution containing hydrochloric acid is the inner surface of the recirculation system pipe 6, the inner surface of the casing, and the surface of the impeller from which the oxide film has been removed in the above-described oxide film dissolution step. Contact each of the. Due to the action of oxalic acid and hydrochloric acid contained in the aqueous solution, they exist at the grain boundaries of stainless steel on the inner surface of the recirculation pipe 6 and on the inner surface of the casing of the recirculation pump 7 and the surface of the impeller, respectively. Radionuclides are removed. While circulating the oxalic acid aqueous solution containing hydrochloric acid through the closed loop including the circulation pipe 31 and the recirculation system pipe 6, the surface of the base material of the recirculation system pipe 6, the inner surface of the base material of the casing, and the surface of the base material of the impeller, respectively. Is continuously dissolved.

  The concentration of iron ions contained in the collected aqueous solution is appropriately measured from the circulation pipe 31 by the water sampling device with an analyzer such as an atomic absorption photometer, and the iron ion concentration in the aqueous solution is monitored. When the dose rate of the recirculation system pipe 6 is high and the dissolution of the base material of the recirculation system pipe 6 is promoted, the valve 70 is opened and the opening degree of the valve 69 is decreased to supply to the cation exchange resin tower 62. The flow rate of the oxalic acid aqueous solution containing hydrochloric acid is increased, the amount of iron removed by the cation exchange resin tower 62 is increased, and the iron ion concentration of the oxalic acid aqueous solution is decreased to 0.007 mol / L or less. As the iron ion concentration of the oxalic acid aqueous solution decreases below 0.007 mol / L, the degree of dissolution of the base material of the recirculation pipe 6 increases. Further, when the dissolution of the base material in the recirculation system pipe 6 is reduced, the opening of the valve 69 is increased, and in some cases, the opening of the valve 70 is decreased to supply to the cation exchange resin tower 62. Reduce the flow rate of the oxalic acid aqueous solution containing hydrochloric acid. For this reason, with the dissolution of the base material of the recirculation system pipe 6 by the oxalic acid aqueous solution containing hydrochloric acid, the iron concentration of the aqueous solution increases, and when the iron ion concentration exceeds 0.007 mol / L, Dissolution is suppressed. When the iron ion concentration is 0.010 mol / L or more, dissolution of the base material is substantially eliminated. Thus, the flow rate of the oxalic acid aqueous solution containing hydrochloric acid supplied to the cation exchange resin tower 62 can be increased or decreased by increasing or decreasing the opening degree of the valve 69 and increasing or decreasing the opening degree of the valve 70. It is possible to control the melting of the respective base materials of the casing and the impeller.

  Radiation emitted from the recirculation pipe 6 is detected by the radiation detector, and the dose rate obtained based on the output signal of the radiation detector is lower than the second set dose rate lower than the first set dose rate. Is also reduced, the melting of the base material of the recirculation pipe 6 is completed. When 6 hours have passed since the supply of the oxalic acid aqueous solution containing hydrochloric acid to the recirculation pipe 6 was started, the dissolution of the base material may be terminated.

  A decomposition process of the reductive decontaminant is performed (step S9). After the dissolution process of the base material in step S8 is completed, the oxalic acid aqueous solution containing hydrochloric acid is supplied to the decomposition apparatus 64 and included in the oxalic acid aqueous solution containing hydrochloric acid, as in the decomposition process of the reducing decontamination agent in step S6. Oxalic acid is decomposed in the decomposition apparatus 64 by the action of the activated carbon catalyst and hydrogen peroxide. Also in the process of step S <b> 9, an aqueous oxalic acid solution containing hydrochloric acid is supplied to the cation exchange resin tower 62, and cations contained in this aqueous solution are removed by the cation exchange resin tower 62. When the oxalic acid concentration of the oxalic acid aqueous solution containing hydrochloric acid is 0.0001 mol / L or less in the step S9, the decomposition process of the reducing decontaminant in step S9 is completed.

Purification of the aqueous solution in which the reducing decontamination agent is decomposed is performed (step S10). After the decontamination process (step S9) of the reductive decontamination agent, the 90 ° C. oxalic acid aqueous solution containing hydrochloric acid is substantially changed to a 60 ° C. aqueous solution containing hydrochloric acid and a small amount of oxalic acid. A 60 ° C. aqueous solution containing oxalic acid is supplied to the mixed bed resin tower 63 as in step S7. Chloride ions, anionic metal ions, chlorine ions (Cl ⁻ ), and oxalate ions ((COO) ₂ ⁻² ) contained in the aqueous solution are adsorbed by the anion exchange resin in the mixed bed resin tower 63. Removed. Cations remaining in the aqueous solution are adsorbed and removed by the cation exchange resin in the mixed bed resin tower 63. When the electrical conductivity of the aqueous solution becomes equal to or lower than a predetermined electrical conductivity (for example, 1 mS / m), the purification process in step S10 is finished.

  The aqueous solution in the chemical decontamination apparatus is cooled (step S11). After the purification step (step S10) is completed, the aqueous solution existing inside the recirculation system pipe 6 and the circulation pipe 31 of the chemical decontamination apparatus 30 is driven into the circulation pipe 31 and the recycle by driving the circulation pumps 34 and 35. While circulating the closed loop including the circulation system pipe 6, the cooler 61 cools it to room temperature. When the temperature of the circulating aqueous solution reaches room temperature, the driving of the circulation pumps 34 and 35 is stopped, and the circulation pipe 31 and the waste liquid treatment apparatus (not shown) are connected by a high-pressure hose (not shown) having a pump (not shown). Connect). The room temperature aqueous solution that is radioactive waste liquid remaining in the recirculation system pipe 6 and the circulation pipe 31 is discharged from the circulation pipe 31 through a high-pressure hose to a waste liquid treatment device (not shown), and discharged into the waste liquid treatment. Processed by the device. After the aqueous solution in the recirculation pipe 6 and the circulation pipe 31 is discharged, cleaning water is supplied into the recirculation pipe 6 and the circulation pipe 31, and the circulation pumps 34 and 35 are driven to clean the inside of these pipes. To do. After completion of the cleaning, the cleaning water in the recirculation system pipe 6 and the circulation pipe 31 is discharged to the waste liquid treatment apparatus.

  The chemical decontamination device is removed from the piping system (step S12). After each process of step S1-S8 is implemented, the chemical decontamination apparatus 30 is removed from the recirculation system piping 6, and the recirculation system piping 6 is restored. The plugs respectively sealing both ends of the recirculation system pipe 6 are also removed. After the replacement work of the fuel assembly in the core 4 and the maintenance inspection of the BWR plant 1 are completed, the BWR plant 1 is started and the BWR plant 1 is operated in the next operation cycle.

  According to this example, oxalic acid alone and an aqueous solution containing hydrochloric acid and hydrochloric acid are contacted with the inner surface of a recirculation system pipe made of stainless steel, which is a chemical decontamination target, for example, Even if each of hydrochloric acid alone cannot dissolve the base material of the recirculation system pipe 6, the base material of the recirculation system pipe can be dissolved. For this reason, the radionuclide can be further removed from the recirculation system pipe 6 in which the oxide film containing the iron oxide and the radionuclide is removed by the reduction decontamination, and the dose rate is lowered. The rate can be further reduced.

  Since the hydrochloric acid concentration (0.020 mol / L) of the aqueous solution containing oxalic acid and hydrochloric acid used in this example is in the range of 0.006 mol / L to 0.075 mol / L, each of oxalic acid alone and hydrochloric acid alone Then, even in a hydrochloric acid concentration region where the base material of the recirculation system pipe 6 cannot be dissolved, the base material of the recirculation system pipe 6 can be further dissolved, and the dose rate of the recirculation system pipe 6 can be further reduced. Furthermore, since the hydrochloric acid concentration of the aqueous solution containing oxalic acid and hydrochloric acid is in the range of 0.009 mol / L or more and 0.075 mol / L or less, the amount of dissolution of the base material of the recirculation system pipe 6 can be further increased. The dose rate of the recirculation piping 6 can be further reduced.

  Recirculation piping using an aqueous solution containing oxalic acid having a concentration in the range of 0.010 mol / L to 0.076 mol / L and hydrochloric acid having a concentration in the range of 0.009 mol / L to 0.075 mol / L. Since the base material 6 is dissolved, the base material of the recirculation system pipe 6 can be removed even in the oxalic acid concentration and hydrochloric acid concentration regions where the base material of the recirculation system pipe 6 cannot be dissolved by oxalic acid alone and hydrochloric acid alone, respectively. Furthermore, it can melt | dissolve and the dose rate of the recirculation piping 6 can further be reduced.

  When the base material of the recirculation system pipe 6 is dissolved using an aqueous solution containing oxalic acid and hydrochloric acid having a concentration in the range of 0.006 mol / L to 0.075 mol / L, the iron ion concentration of the aqueous solution is set to 0.00. By setting the concentration within the range of 007 mol / L or less, dissolution of the base material of the recirculation system pipe 6 can be promoted. The iron ion concentration can be adjusted by controlling the flow rate of the oxalic acid aqueous solution containing hydrochloric acid supplied to the cation exchange resin tower 62.

  In this embodiment, the oxalic acid solution containing hydrochloric acid is used while maintaining the temperature of the oxalic acid aqueous solution used when dissolving the oxide film containing iron oxide and radionuclide formed on the inner surface of the recirculation pipe 6. Therefore, the temperature raising operation and the temperature lowering operation of the oxalic acid solution containing hydrochloric acid used for dissolving the base material of the recirculation system pipe 6 are no longer necessary. And the time which reduction decontamination including each process of S5B requires can be shortened.

  In the present embodiment, the dissolution of the oxide film containing iron oxide and radionuclide using the aqueous oxalic acid solution and the dissolution of the base material of the recirculation system pipe 6 are performed using one chemical decontamination apparatus 30. be able to.

  In this embodiment, radioactive material handling such as stainless steel members such as stainless steel pipes connected to the reactor vessel of the pressurized water nuclear power plant (PWR plant) and stainless steel members such as nuclear fuel reprocessing facility It can be applied to stainless steel members such as facilities.

  The chemical decontamination method of Example 2, which is another preferred embodiment of the present invention, will be described with reference to FIGS. The chemical decontamination method of the present embodiment is applied to, for example, a cut piece of a recirculation system pipe made of stainless steel generated by dismantling of a nuclear power plant that is the subject of decommissioning.

  The chemical decontamination apparatus 30 used in this embodiment has the same structure as the chemical decontamination apparatus 30 used in Embodiment 1, and has a decontamination container 82. In the chemical decontamination apparatus 30 used in this embodiment, both ends of the circulation pipe 31 are connected to a decontamination container 82 as shown in FIG. A plurality of cut pieces 83 of the recirculation pipe 6 are arranged in the decontamination container 82, and then the decontamination container 82 is sealed.

  In this embodiment, steps S2 to S8 performed in the first embodiment are performed.

  The chemical decontamination apparatus 30 including the decontamination container 82 is filled with pure water, and this pure water is heated by the heater 33 until, for example, 90 ° C. (step S2). Specifically, the pure water is filled into the circulation pipe 31 and the decontamination container 82. A permanganic acid aqueous solution is injected from the oxidative decontamination agent injection device 36 into the pipe 80, and a 90 ° C. permanganic acid aqueous solution is supplied from the circulation pipe 31 to the decontamination container 82. The oxidative decontamination 83 is performed (step S3). After the oxidative decontamination is completed, permanganic acid (oxidative decontamination agent) is decomposed (step S4), and after the decomposition of permanganic acid is completed, a plurality of cut pieces 83 in the decontamination container 82 are formed. The oxide film containing iron oxide and radionuclide is dissolved by an oxalic acid aqueous solution containing hydrazine (step S5A). Note that the permanganate aqueous solution circulates in the closed loop including the decontamination container 82 and the circulation pipe 31 in the step S3 and the oxalic acid aqueous solution in the step S5A. After the step S5A is completed, an aqueous oxalic acid solution containing hydrochloric acid circulates in the closed loop to dissolve the surfaces, particularly the inner surfaces of the plurality of cut pieces 83 in the decontamination container 82 (step S5B). As an aqueous oxalic acid solution containing hydrochloric acid, as in Example 1, a 90 ° C. aqueous oxalic acid solution containing 0.020 mol / L hydrochloric acid, 0.030 mol / L oxalic acid, and hydrazine and having a pH of 2.5. Used.

  In the same manner as in Example 1, after the process of step S5B is completed, the decontamination process (step S6) of the reducing decontaminant, the purification process of the aqueous solution (step S7), and the cooling process of the aqueous solution (step S8) are sequentially performed. Then, the chemical decontamination of the plurality of cut pieces 83 of the recirculation pipe 6 is completed. After the chemical decontamination is completed, the lid (not shown) of the decontamination container 82 is opened, and a plurality of cut pieces 83 of the recirculation pipe 6 that have been subjected to the chemical decontamination are taken out from the decontamination container 82. It is.

  In the present embodiment, each effect produced in the first embodiment can be obtained. Further, in this embodiment, chemical decontamination including dissolution of a base material can be performed on a stainless steel member such as a stainless steel pipe removed from the BWR plant 1.

  As the chemical decontamination device 30 used in the first and second embodiments, the pipe 80 having the valve 79 is removed, and each of the oxidative decontamination agent injection device 36, the reductive decontamination agent injection device 41, and the hydrochloric acid injection device 51 is injected. You may use the chemical decontamination apparatus which connected piping to the circulation piping 31 between the circulation pump 34 and the valve 77 in the same order as the chemical decontamination apparatus 30. FIG.

  This example is a stainless steel member generated by the dismantling of the PWR plant that is the subject of decommissioning, a stainless steel member such as a stainless steel pipe removed from each plant during the repair work of the BWR plant and the PWR plant, It can also be applied to stainless steel members such as stainless steel pipes removed from the facilities at the time of repairing radioactive material handling facilities such as stainless steel pipes and equipment of nuclear fuel reprocessing facilities.

  DESCRIPTION OF SYMBOLS 1 ... Boiling water nuclear power plant, 2 ... Reactor, 3 ... Reactor pressure vessel, 4 ... Reactor core, 6 ... Recirculation system piping, 9 ... Turbine, 11 ... Feed water piping, 18 ... Purification system piping, 30 ... Chemistry Decontamination device, 31 ... circulation piping, 33 ... heater, 34,35 ... circulation pump, 36 ... oxidation decontamination agent injection device, 37,42,47,52,57 ... chemical tank, 38,43,48,53 DESCRIPTION OF SYMBOLS ... Injection pump, 41 ... Reductive decontamination agent injection device, 46 ... pH adjuster injection device, 51 ... Hydrochloric acid injection device, 56 ... Oxidant supply device, 58 ... Supply pump, 61 ... Cooler, 62 ... Cation exchange resin tower (Cation exchange resin tower), 63 ... mixed bed resin tower, 64 ... decomposition apparatus, 56 ... oxidizer supply apparatus, 58 ... supply pump, 82 ... decontamination container.

Claims (15)

The first aqueous solution containing oxalic acid and no hydrochloric acid is brought into contact with the surface of the stainless steel member of the radioactive material handling facility to dissolve and remove the oxide film containing the radionuclide on the surface,
A chemical decontamination method, wherein a second aqueous solution containing oxalic acid and hydrochloric acid is brought into contact with the surface of the stainless steel member from which the oxide film has been removed.   2. The chemical decontamination method according to claim 1, wherein an aqueous solution containing oxalic acid and hydrochloric acid having a concentration in the range of 0.006 mol / L to 0.075 mol / L is used as the second aqueous solution.   The oxalic acid concentration of the second aqueous solution containing oxalic acid and hydrochloric acid having a concentration in the range of 0.006 mol / L to 0.075 mol / L is in the range of 0.010 mol / L to 0.076 mol / L. The chemical decontamination method according to claim 2, which is a concentration.   The chemical decontamination method according to claim 2, wherein an aqueous solution containing oxalic acid and hydrochloric acid having a concentration in the range of 0.009 mol / L to 0.075 mol / L is used as the second aqueous solution.   The oxalic acid concentration of the second aqueous solution containing oxalic acid and hydrochloric acid having a concentration in the range of 0.009 mol / L to 0.075 mol / L is in the range of 0.010 mol / L to 0.076 mol / L. The chemical decontamination method according to claim 4, which is a concentration.   The chemical decontamination method according to any one of claims 2 to 5, wherein the iron ion concentration of the second aqueous solution is adjusted to a concentration within a range of 0.007 mol / L or less.   The adjustment of the iron ion concentration of the second aqueous solution to a concentration within the range of 0.007 mol / L or less is achieved by controlling the flow rate of the aqueous solution containing oxalic acid and hydrochloric acid supplied to the cation exchange resin tower. The chemical decontamination method of Claim 6 performed.   The first aqueous solution is supplied to the first pipe, which is the stainless steel member of the radioactive material handling facility, through the second pipe, the first aqueous solution is brought into contact with the inner surface of the second pipe, and the first pipe is passed through the second pipe. The chemical decontamination method according to any one of claims 1 to 7, wherein two aqueous solutions are supplied to the first pipe and brought into contact with the surface of the first pipe from which the oxide film has been removed.   The chemical decontamination method according to claim 8, wherein the second aqueous solution supplied to the first piping through the second piping is generated by injecting hydrochloric acid into the first aqueous solution in the first piping.   The chemical decontamination method according to claim 8 or 9, wherein the first aqueous solution and the second aqueous solution circulate in a closed loop including the first pipe and the second pipe.   The first aqueous solution is supplied through a pipe into the decontamination container in which the stainless steel member removed from the radioactive material handling facility is stored, and the first aqueous solution is brought into contact with the surface of the stainless steel member, and the pipe The chemical solution decontamination according to any one of claims 1 to 7, wherein the second aqueous solution is supplied to the decontamination container through the surface and brought into contact with the surface of the stainless steel member from which the oxide film has been removed. Method.   The chemical decontamination method according to claim 11, wherein the second aqueous solution supplied to the decontamination container through the pipe is generated by injecting hydrochloric acid into the first aqueous solution in the pipe.   The chemical decontamination method according to claim 11 or 12, wherein the first aqueous solution and the second aqueous solution circulate in a closed loop including the decontamination container and the pipe.   Piping that sequentially leads each of the first aqueous solution containing oxalic acid and not containing hydrochloric acid and the second aqueous solution containing oxalic acid and hydrochloric acid, the oxidative decontamination agent injection device connected to the piping, and the reduction connected to the piping A chemical decontamination device comprising a decontamination agent injection device and a hydrochloric acid injection device connected to the pipe.   15. The decontamination container according to claim 14, further comprising a decontamination container in which each of the first aqueous solution and the second aqueous solution is sequentially supplied and is connected to the pipe and accommodates a stainless steel member removed from the radioactive material handling facility. Chemical decontamination equipment.

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