JP6901947B2 - Chemical decontamination method - Google Patents

Description

The present invention relates to a chemical decontamination how.

At a nuclear plant, decontamination work is performed to remove radionuclides adhering to equipment, piping, etc. during maintenance work during operation and dismantling work at the time of decommissioning. In particular, the piping members (mainly including stainless steel and Inconel) that make up the system of a nuclear plant are exposed to a high temperature and high pressure environment during operation, so an oxide film is formed on the surface and radionuclides are formed in the film. Is taken in. As a method of removing the oxide film and decontaminating a member, a method called chemical decontamination is known.

The following techniques are known as chemical decontamination methods. The member is brought into contact with an aqueous solution to which an oxidizing agent such as permanganate is added to oxidize and elute chromium in the chromium-based oxide contained in the oxide film adhering to the surface of the member. Next, an organic acid such as oxalic acid is added to the aqueous solution to elute iron and nickel in the iron-based oxide which is the main component of the oxide film. At this time, radionuclides such as cobalt are also eluted at the same time. Then, in the chemical decontamination method, the aqueous solution in which the radionuclide is eluted is brought into contact with the ion exchange resin to remove the radioactive substance together with the oxide film.

Further, as another chemical decontamination method, for example, in the chemical decontamination method described in Patent Document 1, the oxide film is oxidatively dissolved by the oxidizing power of an ozone aqueous solution containing ozone. By using ozone, it is not necessary to use an oxidizing agent such as permanganate, and the amount of secondary waste such as an ion exchange resin caused by the oxidizing agent can be reduced. Further, in this chemical decontamination method, an ozone aqueous solution in which ozone gas is brought into contact with an acidic aqueous solution having a pH of 5 or less is used. As a result, the oxidative dissolution performance of the oxide film is improved.

Japanese Unexamined Patent Publication No. 2000-81498

However, when an acidic aqueous solution having a pH of 5 or less is used, the surface of the base material such as the piping member, which is the object to be decontaminated, is corroded and thinned, and in some cases, the soundness of the equipment may be impaired. Further, when an additive for preventing corrosion thinning of the surface of the base material is added, the amount of used ion exchange resin generated increases. As a result, the amount of secondary waste reduced by not using an oxidant may increase. Therefore, it is desired to carry out chemical decontamination by reducing the amount of secondary waste while suppressing the influence on the decontamination target.

The present invention has been made in order to meet the above demands, and it is possible to carry out chemical decontamination by using ozone while reducing the amount of secondary waste and suppressing the influence on the decontamination target. and to provide a chemical decontamination how.

The present invention employs the following means in order to solve the above problems.
In the chemical decontamination method according to the first aspect of the present invention, the decontamination method is performed by supplying ozone water having an ozone concentration of 30 mg / L or more to the treated water in contact with the decontamination object containing stainless steel and a nickel-based alloy. An oxidation step of oxidatively eluting chromium from the dyeing target, and a decontamination step of eluting iron, nickel, and cobalt from the decontamination target by adding oxalic acid to the treated water after the oxidation step. After the decontamination step, a removal step of removing the radioactive nuclei containing the chromium, the iron, the nickel, and the cobalt eluted in the treated water, and after the decontamination step and before the removal step. the ozone concentration again supplies the ozone water above 30 mg / L, see containing the decomposed organic acid degradation step the oxalic acid contained in the treated water by the ozone water, the oxidation step, the treated water The pressure is 0.5 MPa or more, and the oxygen generated in the treated water is pressure-dissolved and removed by the ozone water.

According to such a configuration, the ozone concentration in the treated water is increased by supplying ozone water having an ozone concentration of 30 mg / L or more. As a result, the oxidative dissolution effect of ozone can be enhanced regardless of the pH of the treated water. Therefore, it is possible to promote the oxidative elution of chromium from the decontamination target without lowering the pH of the treated water. Therefore, it is possible to remove chromium in the oxide film while suppressing the influence on the decontamination target. At this time, in the oxidation step, chromium can be oxidatively eluted with ozone without using an oxidizing agent that must be finally removed from the treated water. Therefore, it is possible to suppress the generation of secondary waste caused by the oxidizing agent.

Further, in the chemical decontamination method according to the second aspect of the present invention, in the first aspect, the pH of the treated water may be 5.8 or more and 7.0 or less in the oxidation step.

With such a configuration, it is possible to surely suppress the influence that the treated water becomes acidic and causes on the decontamination target. In addition, it is possible to suppress the autolysis of ozone and efficiently oxidize and elute chromium.

Further, in the chemical decontamination method according to the third aspect of the present invention, in the first or second aspect, the temperature of the treated water may be 50 ° C. or higher in the oxidation step.

With such a configuration, the solubility of ozone in treated water can be improved. As a result, the oxidative dissolution effect of ozone can be improved. Therefore, it is possible to shorten the decontamination time and reduce the amount of ozone supplied to the treated water.

Further, in the chemical decontamination method according to the fourth aspect of the present invention, in any one of the first to third aspects, ozone water containing ozone as fine bubbles is supplied in the oxidation step. Then, the ozone concentration of the treated water may be adjusted.

With such a configuration, the ozone content in the treated water can be increased. In addition, since ozone is made into a microvalve, it becomes difficult for ozone to escape in the treated water, and the time until the ozone concentration reaches the half-life can be lengthened. Therefore, the oxidative dissolution effect of ozone can be further improved.

Further, in the chemical decontamination method according to the fifth aspect of the present invention, in any one of the first to fourth aspects, it is contained in the treated water after the decontamination step and before the removal step. The organic acid decomposition step of decomposing the organic acid may be included.

With such a configuration, the organic acid remaining in the treated water without being consumed in the decontamination step can be decomposed. Therefore, the amount of organic acid that must be removed in the removal step can be reduced, and the amount of secondary waste generated by the organic acid can be reduced.

According to the present invention, ozone can be used to carry out chemical decontamination while reducing the amount of secondary waste and suppressing the influence on the decontamination target.

It is a figure which shows the example of the decontamination target object which is the target of the chemical decontamination method in this embodiment. It is a schematic diagram of the chemical decontamination system in this embodiment. It is a flow chart of the chemical decontamination method in this embodiment.

Hereinafter, embodiments according to the present invention will be described with reference to FIGS. 1 to 3.
First, the decontamination target Z, which is the target of the present embodiment, will be described. The object Z to be decontaminated, which is the target of the present embodiment, is a component such as a pipe, a container, and various devices constituting the nuclear power plant, and is a component with which the furnace water comes into contact.

Here, as the nuclear power plant, for example, as shown in FIG. 1, there is a nuclear power plant P including a pressurized water reactor 50. The nuclear power plant P has a pressurized water reactor 50 in which a fuel rod 51 and the like are housed, and a pressurizer 52 that pressurizes the primary cooling water in order to suppress boiling of the primary cooling water (light water) in the pressurized water reactor 50. A steam generator 53 that turns the secondary cooling water into steam by the heat of the primary cooling water, a coolant pump 54 that returns the primary cooling water from the steam generator 53 to the pressurized water reactor 50, and a steam generator 53. A steam turbine 56 driven by the generated steam, a generator 57 that generates power by driving the steam turbine 56, a condenser 58 that returns the steam from the steam turbine 56 to water, and a steam generator that generates water from the condenser 58. It is equipped with a water supply pump 59 that returns to the vessel 53.

The pressurized water reactor 50 and the steam generator 53 are connected by primary cooling water pipes 55a and 55b. The steam generator 53 and the steam turbine 56 are connected by a steam pipe 55c. The condenser 58 and the steam turbine 56 are connected by a water supply pipe 55d.

In the nuclear power plant P configured as described above, the decontamination target Z is the reactor water, that is, the primary cooling system member which is a member in contact with the primary cooling water. The decontamination target Z includes a pressurized water reactor 50, a pressurizer 52, a steam generator 53, a coolant pump 54, primary cooling water pipes 55a and 55b connecting these, primary cooling water pipes 55a and 55b, and the like. There are various valves provided. These decontamination objects Z are formed of stainless steel containing iron as a main component and chromium and nickel, inconel which is a nickel-based alloy, and the like.

The metal elements constituting the decontamination target Z are slightly eluted in the reactor water, and a part thereof adheres to the surface of the fuel rod 51 in the pressurized water reactor 50. The metal element adhering to the surface of the fuel rod 51 undergoes a nuclear reaction when irradiated with neutron rays from the fuel, and becomes a radionuclide such as chromium, iron, nickel, and cobalt. Most of these radionuclides remain attached to the surface of the fuel rod 51 in the form of oxides, but some of them are eluted into the furnace water or released as insoluble solids. Radionuclides eluted or released into the furnace water adhere to the furnace water contact surface of the decontamination object Z. Therefore, the worker working in the vicinity of the decontamination target Z is exposed to the radiation from the radionuclide in the oxide film formed on the part.

The chemical decontamination method S1 of the present embodiment relates to the system decontamination of the nuclear power plant P described above. In the chemical decontamination method S1 of the present embodiment, the treated water W, which is a chemical solution, is circulated inside a pipe or the like, which is the decontamination target Z. As a result, the oxide film containing the radionuclide adhering to the inner surface of the decontamination object Z is removed and discarded.

The chemical decontamination method S1 of the present embodiment is carried out using the chemical decontamination system 1 as shown in FIG. The chemical decontamination system 1 of the present embodiment includes a circulation line (supply system) 11, an ozone concentration adjusting unit 12, an organic acid supply unit 13, a removal unit 14, and an exhaust gas treatment unit 15.

The circulation line 11 supplies the treated water W to the decontamination object Z containing the stainless steel and the nickel-based alloy. Specifically, the circulation line 11 of the present embodiment supplies the treated water W to the flow path through which the primary cooling water of the nuclear power plant P flows. The circulation line 11 circulates the treated water W to the nuclear power plant P so that the treated water W discharged from the flow path is supplied to the flow path again. A pump (not shown) is provided on the circulation line 11 to pump the treated water W. Further, the circulation line 11 is provided with a heater 111. The heater 111 can heat the treated water W circulating in the circulation line 11 to bring the treated water W to a high temperature state of 50 ° C. or higher. In the circulation line 11, for example, the treated water W having the amount of water held in the system is circulated so that the circulating treated water W can be replaced in about one hour.

The ozone concentration adjusting unit 12 supplies ozone water having an ozone concentration of 30 mg / L or more to the treated water W flowing through the circulation line 11. As a result, the ozone concentration adjusting unit 12 adjusts the ozone concentration of the treated water W to about 1/10 of the ozone concentration of the ozone water that supplies the ozone concentration. As a result, the ozone concentration adjusting unit 12 oxidizes and elutes chromium from the decontamination target Z into the treated water W. The ozone concentration adjusting unit 12 of the present embodiment generates ozone water containing 30 mg / L (30 ppm) of ozone and supplies it to the treated water W. The ozone concentration adjusting unit 12 of the present embodiment preferably can supply ozone water containing 100 ppm or more of ozone, and more preferably can supply ozone water containing 150 ppm of ozone. The ozone concentration adjusting unit 12 supplies ozone water to the circulation line 11 on the downstream side of the heater 111. The ozone concentration adjusting unit 12 generates ozone water by converting ozone into fine bubbles and supplying them into an aqueous solution. The fine bubbles in the present embodiment include not only bubbles having a diameter of about 10 μm to 100 μm like microbubbles, but also nanobubbles finer than microbubbles. The ozone concentration adjusting unit 12 adjusts the ozone concentration of the treated water W so that the pH of the treated water W is 5.8 or more and 7.0 or less. The ozone concentration adjusting unit 12 of the present embodiment supplies ozone water having a pH of 5.8 or more and 7.0 or less to the treated water W.

The ozone concentration adjusting unit 12 of the present embodiment is not limited to generating ozone water having an ozone concentration of 30 mg / L in advance, and the ozone concentration of the ozone water when it is supplied to the treated water W. Should be 30 mg / L. Therefore, the ozone concentration adjusting unit 12 may have a device that separately generates only ozone instead of ozone water.

The organic acid supply unit 13 supplies the organic acid to the treated water W adjusted by the ozone concentration adjustment unit 12. As a result, the organic acid supply unit 13 elutes iron, nickel, and cobalt from the decontamination object Z into the treated water W. In the organic acid supply unit 13 of the present embodiment, for example, oxalic acid is added as an organic acid to the treated water W flowing through the circulation line 11. The organic acid supplied is not limited to oxalic acid, and may be picolinic acid or citric acid. The organic acid supply unit 13 supplies oxalic acid to the circulation line 11 between the ozone concentration adjustment unit 12 and the decontamination target Z.

The removing unit 14 removes radionuclides in the treated water W using an adsorbent. Examples of the adsorbent used in the removing unit 14 of the present embodiment include an anionic resin, a cationic resin, and an inorganic adsorbent. The removing unit 14 is arranged between the decontamination object Z and the heater 111. The removing unit 14 extracts the treated water W flowing through the circulation line 11 and allows the adsorbent to pass through the treated water W to remove radionuclides from the treated water W.

The exhaust gas treatment unit 15 removes ozone discharged from the treated water W. The exhaust gas treatment unit 15 of the present embodiment is provided so as to communicate with, for example, the primary cooling water pipes 55a and 55b. The exhaust gas treatment unit 15 has a filter, and removes the ozone-containing gas discharged from the treated water W without being consumed while the treated water W circulates.

As shown in FIG. 3, the chemical decontamination method S1 of the present embodiment includes an oxidation step S2, a decontamination step S3, an organic acid decomposition step S4, and a removal step S5.

In the oxidation step S2, ozone water having an ozone concentration of 30 mg / L or more is supplied to the treated water W with which the decontamination target Z containing the stainless steel and the nickel-based alloy comes into contact. In the oxidation step S2 of the present embodiment, the ozone concentration in the treated water W was supplied by supplying the treated water W with high-concentration ozone water having an ozone concentration of 30 mg / L or more from the ozone concentration adjusting unit 12. Ozone concentration is set to about 1/10 of ozone water, and oxidative dissolution is performed. Then, the treated water W having an increased ozone concentration is circulated by circulating in the circulation line 11 and is repeatedly supplied to the inside of the decontamination target Z. In the oxidation step S2, the treated water W whose ozone concentration is adjusted is supplied to the inside of the decontamination target Z, so that the chromium in the oxide film adhering to the decontamination target Z is oxidized and eluted into the treated water W. Will be done.

Further, in the oxidation step S2, the pH of the treated water W is set to 5.8 or more and 7.0 or less. Specifically, in the oxidation step S2 of the present embodiment, ozone water having a pH of 5.8 or more and 7.0 or less is supplied from the ozone concentration adjusting unit 12 to the treated water W. As a result, in the chemical decontamination method S1, it is not necessary to adjust the pH of the treated water W in a separate step. Further, in the oxidation step S2, high-concentration ozone water is supplied while the temperature of the treated water W is maintained at 50 ° C. or higher by the heater 111. Further, in the oxidation step S2, ozone water containing microbubbled ozone is supplied from the ozone concentration adjusting unit 12.

Further, in the oxidation step S2, in the process of circulating the treated water W, ozone in the treated water W becomes bubbles of oxygen decomposed. When the oxygen bubbles stay in the piping or equipment, the contact area between the treated water W and the decontamination target Z decreases. As a result, the decontamination effect may be reduced. Therefore, it is preferable that the oxidation step S2 is carried out while removing oxygen generated in the treated water W by ozone water. Specifically, in the oxidation step S2, the inside of the decontamination system may be pressurized by a pressurizing device such as a pump. For example, the treated water W to be circulated is pressurized by a pump provided in the circulation line 11. As a result, the generated oxygen is pressure-dissolved in the treated water W. At this time, the pressure of the treated water W flowing through the circulation line 11 is preferably 0.5 MPa or more, more preferably 2.5 MPa or more. Further, in the oxidation step S2, a mechanism for removing oxygen may be provided in the middle of the decontamination system instead of the pressurizing device. For example, a device for removing oxygen bubbles may be provided in the middle of the circulation line 11.

In the decontamination step S3, an organic acid is added to the treated water W after the oxidation step S2. In the decontamination step S3 of the present embodiment, oxalic acid is supplied from the organic acid supply unit 13 into the treated water W. Further, in the decontamination step S3 of the present embodiment, the eluted metal is removed by passing treated water through the adsorbent. Specifically, the decontamination step S3 is carried out after the time required for chromium to be eluted by the treated water W whose ozone concentration has increased in the oxidation step S2 has elapsed. The treated water W to which oxalic acid is added is circulated through the circulation line 11 and is repeatedly supplied to the inside of the decontamination target Z. When the treated water W to which oxalic acid is supplied is supplied to the inside of the decontamination target Z, iron, nickel, and cobalt are eluted from the decontamination target Z into the treated water W. Further, by supplying oxalic acid to the treated water W, ozone remaining in the treated water W without being consumed in the oxidation step S2 is decomposed and removed. Further, ozone discharged from the treated water W without being decomposed is recovered by the exhaust gas treatment unit 15. Further, the treated water W to which oxalic acid is added circulates in the circulation line 11 while passing through the removing portion 14. As a result, when the object Z to be decontaminated is eluted with the treated water W, chromium, iron, nickel, and cobalt are removed using an adsorbent.

The organic acid decomposition step S4 is carried out after the decontamination step S3 and before the removal step S5. The organic acid decomposition step S4 decomposes the organic acid contained in the treated water W. In the organic acid decomposition step S4 of the present embodiment, high-concentration ozone water is supplied to the treated water W again from the ozone concentration adjusting unit 12. Specifically, the ozone concentration of the treated water W is set to 30 mg / L or more after the time required for iron, nickel and cobalt to be eluted by the treated water W to which oxalic acid is added in the decontamination step S3 has elapsed. High-concentration ozone water is added to the treated water W so as to do so. The treated water W whose ozone concentration has increased again is circulated by circulating in the circulation line 11, and the oxalic acid remaining in the treated water W without being consumed in the decontamination step S3 is decomposed and removed.

The removal step S5 is carried out after the organic acid decomposition step S4. In the removal step S5 of the present embodiment, chromium, iron, nickel, and cobalt that could not be completely removed in the decontamination step S3 are removed. Specifically, in the removal step S5, for example, using an adsorbent such as an ion exchange resin, the treated water W is contaminated with radionuclides such as chromium, iron, nickel, and cobalt, and these radionuclides. Decontamination is performed by removing residual components (ions such as oxalic acid). After that, the dose of the decontamination target Z is measured, and if the dose is sufficiently reduced, the chemical decontamination method S1 is terminated. Further, when the dose of the decontamination target Z is still high, the oxidation step S2 to the removal step S5 are repeatedly carried out as necessary. The decontaminated treated water W can be reused in the next cycle or later.

According to the chemical decontamination method S1 and the chemical decontamination system 1 as described above, in the oxidation step S2, high-concentration ozone water is supplied to the ozone concentration adjusting unit 12 to increase the ozone concentration of the treated water W to 30 mg / L or more. It is supposed to be. When the ozone concentration of the treated water W is 30 mg / L or more, the oxidative dissolution effect of ozone can be enhanced regardless of the pH of the treated water W. Therefore, it is possible to promote the oxidative elution of chromium from the decontamination target Z without lowering the pH of the treated water W. Therefore, it is possible to remove chromium in the oxide film while suppressing the influence on the decontamination target Z. At this time, in the oxidation step S2 of the present embodiment, chromium can be oxidized and eluted with ozone without using an oxidizing agent that must be finally removed from the treated water W. Therefore, it is possible to suppress the generation of secondary waste caused by the oxidizing agent. Therefore, ozone can be used to decontaminate while suppressing the influence on the decontamination target Z, and the amount of secondary waste can be reduced.

Further, in the decontamination step S3, oxalic acid is supplied to the treated water W from the organic acid supply unit 13. As a result, iron, nickel, and cobalt can be eluted from the decontamination object Z into the treated water W while removing ozone remaining in the treated water W. Therefore, iron, nickel, and cobalt in the oxide film can be eluted while suppressing the excess ozone discharged in the oxidation step S2 into the atmosphere.

After that, in the organic acid decomposition step S4, the ozone water supplied from the ozone concentration adjusting unit 12 can be used to decompose the oxalic acid remaining in the treated water W without being consumed in the decontamination step S3. it can. Residual oxalic acid can be removed at low cost without installing a new device for removing organic acids. Further, the amount of oxalic acid that must be removed in the removal step S5 can be reduced, and the amount of secondary waste generated by oxalic acid can be reduced.

Then, in the removal step S5, the radionuclides such as chromium, iron, nickel, and cobalt can be removed from the treated water W by passing the treated water W through the adsorbent through the removing unit 14.

Further, in the oxidation step S2, the pH of the treated water W is maintained at 5.8 or more and 7.0 or less. Therefore, the treated water W becomes acidic and the surface of the base material of the pipe is corroded and thinned, and in some cases, the influence on the decontamination target Z such as impairing the soundness of the equipment can be surely suppressed. In addition, it is possible to suppress the autolysis of ozone and efficiently oxidize and elute chromium.

Further, in the oxidation step S2, the temperature of the treated water W is set to 50 ° C. or higher by the heater 111. Then, in the present embodiment, high-concentration ozone water is supplied to the treated water W from the ozone concentration adjusting unit 12 on the downstream side of the heater 111. Therefore, ozone water is supplied to the warmed treated water W, and the solubility of ozone in the treated water W can be improved. As a result, the oxidative dissolution effect of ozone can be improved. Therefore, it is possible to shorten the decontamination time and reduce the amount of ozone supplied to the treated water W.

Further, the ozone concentration adjusting unit 12 generates ozone water containing microbubbled ozone. By supplying this ozone water to the treated water W, it is possible to contain more ozone in the treated water W while suppressing the supply amount as the ozone water. Therefore, the ozone content with respect to the treated water W can be increased rather than supplying ozone water containing ozone that has not been microbubbled to the treated water W. Further, since ozone is made into a microvalve, it becomes difficult for ozone in the treated water W to escape, and the time until the ozone concentration reaches the half-life can be lengthened. Therefore, the oxidative dissolution effect of ozone can be further improved.

(Other variants of the embodiment)
Although the embodiments of the present invention have been described in detail with reference to the drawings, the configurations and combinations thereof in the respective embodiments are examples, and the configurations are added or omitted within a range not deviating from the gist of the present invention. , Replacement, and other changes are possible. Further, the present invention is not limited to the embodiments, but only to the scope of claims.

For example, the circulation line 11 is not limited to circulating the treated water W with respect to the decontamination target Z. The circulation line 11 only needs to be able to supply the treated water W to the decontamination target Z. Therefore, for example, when chemical decontamination is performed while the decontamination object Z is immersed in the water tank, the treated water W may be supplied to the water tank without being circulated.

Further, the application target of the chemical decontamination method S1 of the present embodiment is not limited to systematic decontamination, and when the reactor is decommissioned, the dismantled members of the nuclear power plant are used as the decontamination target Z. It may be carried out in a processing facility equipped with necessary equipment such as heating equipment.

Further, the oxidation step S2 is not limited to supplying only ozone water. For example, in the oxidation step S2, another oxidizing agent (potassium permanganate, potassium permanganate, cerium nitrate, etc.) may be supplied to the treated water W together with ozone water.

Z ... Decontamination target P ... Nuclear power plant 51 ... Fuel rod 50 ... Pressurized water reactor 52 ... Pressurizer 53 ... Steam generator 54 ... Cooling material pump 56 ... Steam turbine 57 ... Generator 58 ... Condenser 59 ... Water supply pump 55a ... Primary cooling water pipe 55b ... Primary cooling water pipe 55c ... Steam pipe 55d ... Water supply pipe 1 ... Chemical decontamination system 11 ... Circulation line 111 ... Heater 12 ... Ozone concentration adjustment unit 13 ... Organic acid supply unit 14 ... Removal Part 15 ... Exhaust gas treatment part W ... Treated water S1 ... Chemical decontamination method S2 ... Oxidation step S3 ... Decontamination step S4 ... Organic acid decomposition step S5 ... Removal step

Claims (4)

An oxidation step in which chromium is oxidatively eluted from the decontamination target by supplying ozone water having an ozone concentration of 30 mg / L or more to the treated water in contact with the decontamination target containing stainless steel and a nickel-based alloy.
After the oxidation step, a decontamination step of eluting iron, nickel, and cobalt from the decontamination target by adding oxalic acid to the treated water.
After the decontamination step, a removal step of removing the radionuclide containing the chromium, the iron, the nickel, and the cobalt eluted in the treated water, and
After the decontamination step and before the removal step, the ozone water having an ozone concentration of 30 mg / L or more is supplied again, and the oxalic acid contained in the treated water is decomposed by the ozone water. the decomposition process only contains,
The oxidation step is a chemical decontamination method carried out while setting the pressure of the treated water to 0.5 MPa or more and pressurizing and dissolving oxygen generated in the treated water with the ozone water to remove it. The chemical decontamination method according to claim 1, wherein in the oxidation step, the pH of the treated water is 5.8 or more and 7.0 or less. The chemical decontamination method according to claim 1 or 2, wherein in the oxidation step, the temperature of the treated water is 50 ° C. or higher. The chemical decontamination method according to any one of claims 1 to 3, wherein in the oxidation step, ozone in the form of fine bubbles is supplied to adjust the ozone concentration of the treated water.

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