JP2020148728A - Processing method of excess water and processing system of excess water - Google Patents

Abstract

To remove oxalic acid included in excess water with high accuracy.SOLUTION: A processing method S100 of excess water is a processing method of excess water including oxalic acid generated by supplying pure water to an apparatus in a system during chemical decontamination of the system of a nuclear power plant. The processing method S100 of excess water includes a pH adjustment step S500 of adding a pH adjusting agent to a tank storing the excess water and adjusting pH of the excess water to 3.0 or more and 7.0 or less, and a concentration step S600 of concentrating the excess water after the pH adjustment step S500.SELECTED DRAWING: Figure 3

Description

The present invention relates to a method for treating excess water and a system for treating excess water.

In a nuclear plant, decontamination work is performed to remove radionuclides adhering to equipment and piping 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 power 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 this oxide film and decontaminating a member, a method called chemical decontamination is known.

In the chemical decontamination method, first, the member is brought into contact with an aqueous solution to which an oxidizing agent such as permanganic acid 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.

As such a chemical decontamination method, for example, Patent Document 1 describes a decontamination waste liquid treatment method for removing permanganic acid as a precipitate by adding a reducing agent for precipitating permanganate to the treated water. Has been done. In this decontamination waste liquid treatment method, the amount of ion exchange resin used is reduced by reducing manganese in the treated water.

Further, in systematic decontamination in which the inside of a nuclear power plant is decontaminated by a chemical decontamination method, treated water as a chemical solution is circulated in the nuclear power plant like primary cooling water. In order to circulate the treated water, a primary coolant pump provided to pump the primary cooling water is used. At this time, if various fine particles contained in the treated water flow into the seal portion of the primary coolant pump, it causes damage to the primary coolant pump. Therefore, by supplying pure water to the primary coolant pump, system decontamination is performed while protecting the primary coolant pump. By supplying pure water, the flow rate of the treated water flowing in the system increases, and unnecessary surplus water is generated.

Like the treated water, such surplus water contains permanganate ions and therefore needs to be treated separately. As a specific treatment method, surplus water is recovered from the nuclear power plant and stored in the tank. After that, sodium hydrogen sulfite is added to decompose and remove permanganate ions in the excess water. The excess water from which permanganate ions have been removed is finally concentrated by an evaporator and then discarded.

Japanese Unexamined Patent Publication No. 2014-92442

By the way, such surplus water is generated even after oxalic acid is added to the treated water. Therefore, oxalic acid may be contained in the excess water. Excess water is asphalt-solidified and discarded after being concentrated, but if oxalic acid is contained, it may have an adverse effect on asphalt solidification. Therefore, it is desired to remove oxalic acid contained in excess water with high accuracy.

The present invention has been made in consideration of such circumstances, and provides a method for treating excess water and a treatment system for excess water capable of removing oxalic acid contained in excess water with high accuracy. With the goal.

The present invention employs the following means in order to solve the above problems.
The method for treating excess water according to the first aspect of the present invention is a method for treating excess water containing oxalic acid generated by supplying pure water to the equipment in the system during chemical decontamination of the system of a nuclear plant. Then, a pH adjusting agent is added to the tank in which the surplus water is stored to adjust the pH of the surplus water to 3.0 or more and 7.0 or less, and after the pH adjusting step, the surplus water is added. Containing, including a concentration step.

According to such a configuration, since the pH of the surplus water is 7.0 or less, the reaction rate when oxalic acid is decomposed by manganese dioxide in the concentration step can be improved. As a result, oxalic acid remaining in the excess water can be removed with high accuracy. Further, since the pH of the surplus water is 3.0 or more, the amount of corrosion of the equipment used for treating the surplus water can be suppressed.

Further, in the method for treating excess water according to the second aspect of the present invention, in the first aspect, the pH of the excess water may be adjusted to 5.0 or more and 7.0 or less in the pH adjusting step.

With such a configuration, the surplus water approaches weak acidity. Therefore, the amount of corrosion of the equipment used to treat the surplus water can be suppressed.

Further, in the method for treating excess water according to the third aspect of the present invention, in the first aspect, the pH of the excess water may be adjusted to 5.0 or more and 6.0 or less in the pH adjusting step.

With such a configuration, since the pH of the surplus water is 6.0 or less, the surplus water becomes weakly acidic, and the reaction rate at the time of decomposing oxalic acid can be further improved.

Further, in the method for treating excess water according to the fourth aspect of the present invention, boric acid may be added as the pH adjusting agent in the pH adjusting step in any one of the first to third aspects. ..

With such a configuration, boric acid itself has a buffering action, so that the pH is less likely to decrease even if excess water is concentrated. As a result, the surplus water can be concentrated at the planned pH in the concentration step.

Further, in the method for treating excess water according to the fifth aspect of the present invention, in any one of the first to fourth aspects, sodium bisulfite is added to the excess water to add permanganate ions in the excess water. The pH adjusting step may be carried out after the permanganate ion decomposition step, including a permanganate ion decomposition step of decomposing.

With such a configuration, when the amount of oxalic acid contained in the surplus water is large, the oxalic acid in the surplus water is added at a timing that does not affect the surplus water after the pH is adjusted in the pH adjusting step. Many can be removed.

Further, in the method for treating excess water according to the sixth aspect of the present invention, in the fifth aspect, in the permanganate ion decomposition step, 0.5 with respect to the residual amount of the permanganate ion in the excess water. The sodium bisulfite may be added in an equivalent amount or more and 2.0 equivalents or less.

Further, in the method for treating excess water according to the seventh aspect of the present invention, in the fifth aspect, in the permanganate ion decomposition step, 1.0 with respect to the residual amount of the permanganate ion in the excess water. The sodium bisulfite may be added in an equivalent amount or more and 1.5 equivalents or less.

With such a configuration, an appropriate amount of manganese dioxide can be secured for decomposing oxalic acid in the concentration step.

Further, the surplus water treatment system according to the eighth aspect of the present invention is a treatment system for surplus water containing oxalic acid generated by supplying pure water to the equipment in the system at the time of chemical decontamination of the system of a nuclear plant. A tank in which the surplus water is stored, a pH adjusting unit for adjusting the pH of the surplus water to 3.0 or more and 7.0 or less by adding a pH adjusting agent to the tank, and the pH adjusting unit. It is provided with a concentrating section for concentrating the excess water whose pH has been adjusted with.

According to the present invention, oxalic acid contained in excess water can be quickly removed.

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

Hereinafter, embodiments according to the present invention will be described with reference to FIGS. 1 to 4.
First, the decontamination target Z, which is the target of the chemical decontamination method S1 in which excess water is generated, will be described. The decontamination object Z, which is the target of chemical decontamination, is a component such as a pipe, a container, and various devices constituting the system of a nuclear power plant, and is a component with which 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 primary 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 steam generated in the above, a generator 57 generated by driving the steam turbine 56, a water reactor 58 that returns the steam from the steam turbine 56 to water, and steam from the water reactor 58. It includes a water supply pump 59 that returns to the generator 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 in this way, the furnace water, that is, the primary cooling system member which is a member in contact with the primary cooling water is the decontamination target Z. The decontamination target Z includes a pressurized water reactor 50, a pressurizer 52, a steam generator 53, a primary coolant pump 54, primary cooling water pipes 55a and 55b connecting these, and the primary cooling water pipes 55a and 55b. There are various valves etc. provided in. 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 rods 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 rods 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, workers working in the vicinity of the decontamination object Z are exposed to radiation from radionuclides in the oxide film formed on the part.

The chemical decontamination method S1 relates to systematic decontamination performed when the nuclear power plant P described above is abolished. In the chemical decontamination method S1, treated water as a chemical solution is circulated inside a pipe or the like which is a decontamination object Z. As a result, the oxide film containing the radionuclide adhering to the inner surface of the decontamination object Z is removed and discarded. As shown in FIG. 2, the chemical decontamination method S1 includes an oxidation step S2, a reduction step S3, a decontamination step S4, a decomposition step S5, and a purification step S6.

In the oxidation step S2, treated water to which an oxidizing agent is added is supplied and circulated inside the decontamination object Z. Specifically, permanganic acid is added as an oxidizing agent. In the oxidation step S2, by circulating treated water to which permanganate is added inside the decontamination target Z, chromium in the oxide film adhering to the decontamination target Z containing stainless steel or a nickel-based alloy is Cr. ₆ ⁺ as oxidizing elution. As a result, in the oxidation step S2, primary treated water, which is treated water containing chromium, which is a radionuclide, is produced.

The reduction step S3 is carried out after the oxidation step S2. In the reduction step S3, a small amount of oxalic acid is added to the primary treated water as a reducing agent. In the reduction step S3, by adding permanganate, the permanganate ion contained in the primary treated water and the precipitated manganese dioxide are decomposed by oxalic acid. That is, in the reduction step S3, permanganate ions contained in the primary treated water and oxalic acid in an amount capable of decomposing the precipitated manganese dioxide are added. Further, by adding a small amount of oxalic acid, nickel is eluted from the decontamination target Z into the primary treatment water.

The decontamination step S4 is carried out after the reduction step S3. In the decontamination step S4, a larger amount of oxalic acid is added to the primary treated water as an organic acid than in the reduction step S3. Further, in the decontamination step S4 of the present embodiment, the eluted metal is removed by passing the secondary treated water, which is the primary treated water to which a large amount of oxalic acid is added, through the adsorbent such as an ion exchange resin. To. Specifically, the secondary treated water is circulated and supplied to the inside of the decontamination object Z many times. By supplying the secondary treated water to the inside of the decontamination target Z, iron, nickel, and cobalt are eluted from the decontamination target Z into the treated water. Further, when the decontamination object Z is eluted in the secondary treatment water, chromium, iron, nickel, and cobalt are removed by using an adsorbent.

The decomposition step S5 is carried out after the decontamination step S4. In the decomposition step S5, oxalic acid in the secondary treatment water is decomposed. In the decomposition step S5, for example, by irradiating the secondary treated water with ultraviolet rays, oxalic acid in the secondary treated water is decomposed into water and carbon dioxide. In the decomposition step S5, oxalic acid may be decomposed by another method. As a result, tertiary treated water in which oxalic acid is decomposed and removed from the secondary treated water is generated.

The purification step S6 is carried out after the decomposition step S5. In the purification step S6 of the present embodiment, chromium, iron, nickel, and cobalt that could not be completely removed in the decontamination step S4 are removed. Specifically, in the purification step S6, for example, using an adsorbent such as an ion exchange resin, radionuclides such as chromium, iron, nickel, and cobalt contained in the treated water, and residues contaminated with these radionuclides. Decontamination is performed by removing 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. If the dose of the decontamination target Z is still high, the oxidation step S2 to the purification step S6 are repeatedly carried out as needed. The decontaminated treated water can be reused in the next cycle and thereafter.

Here, a method S100 for treating excess water generated by supplying pure water to equipment in the system during chemical decontamination of the system of a nuclear plant will be described. In the chemical decontamination method S1 described above, in order to protect the primary coolant pump 54, the oxidation step S2, the reduction step S3, and the decontamination step S4 are carried out while pure water is supplied to the primary coolant pump 54. .. In the oxidation step S2, the reduction step S3, and the decontamination step S4, pure water is supplied to increase the amount of treated water circulating in the decontamination target Z. Therefore, a part of the treated water (the same amount of treated water as the amount of pure water supplied) is recovered from the inside of the decontamination target Z as surplus water and stored in a tank (not shown). In the surplus water treatment method S100 of the present embodiment, there are various types of surplus water recovered in the reduction step S3 and the decontamination step S4 with respect to the surplus water containing oxalic acid (H ₂ C ₂ O ₄ ). Processing is done.

The surplus water treated in the present embodiment is obtained by mixing the surplus water recovered in the oxidation step S2, which does not contain oxalic acid, with the surplus water recovered in the reduction step S3 and the decontamination step S4. Is also included. The surplus water generated in the oxidation step S2 contains not oxalic acid but permanganate (HMnO ₄ ), manganese dioxide (MnO ₂ ), sodium hydroxide (NaOH), and potassium permanganate (KMnO ₄ ). ing.

Specifically, as shown in FIG. 3, the surplus water treatment method S100 concentrates the surplus water acquisition step S200, the oxalic acid decomposition step S300, the permanganate ion decomposition step S400, and the pH adjusting step S500. The process S600 and the disposal process S700 are included.

Further, the surplus water treatment method S100 is carried out by the surplus water treatment system 100 as shown in FIG. The surplus water treatment system 100 includes a hold-up tank (tank) 111, a chemical supply unit 145, a waist hold-up tank 150, and a waste liquid evaporator 160.

As shown in FIGS. 3 and 4, in the surplus water acquisition step S200, the surplus water recovered in the reduction step S3 and the decontamination step S4 is taken into the hold-up tank 111. In the surplus water acquisition step S200, the surplus water recovered in the oxidation step S2 may be taken into the hold-up tank 111 together with the surplus water recovered in the reduction step S3 and the decontamination step S4. Therefore, excess water containing oxalic acid is stored in the hold-up tank 111.

The oxalic acid decomposition step S300 is carried out after the surplus water acquisition step S200. In the oxalic acid decomposition step S300, potassium permanganate is added as an oxidizing agent to the surplus water in the hold-up tank 111 by the chemical supply unit 145.

The drug supply unit 145 is connected to the hold-up tank 111 via a drug injection line 141 provided with a valve in the middle. In the drug supply unit 145, various drugs to be supplied to the surplus water are prepared, and the required amount is supplied to the hold-up tank 111. In the oxalic acid decomposition step S300, potassium permanganate is prepared in the drug supply unit 145. The drug supply unit 145 supplies potassium permanganate to the hold-up tank 111 via the drug injection line 141. As a result, a part of oxalic acid contained in the surplus water is decomposed. Therefore, in the oxalic acid decomposition step S300, the hold-up tank 111 and the drug supply unit 145 serve as an oxalic acid decomposition unit that decomposes a part of oxalic acid. The drug supply unit 145 is flushed after supplying potassium permanganate to the hold-up tank 111.

The permanganate ion decomposition step S400 is carried out after the oxalic acid decomposition step S300. In the permanganate ion decomposition step S400, sodium bisulfite is added to the surplus water in the hold-up tank 111 by the drug supply unit 145. That is, in the permanganate ion decomposition step S400, sodium bisulfite is prepared in the drug supply unit 145. The drug supply unit 145 supplies sodium bisulfite to the hold-up tank 111 via the drug injection line 141. As a result, a part of the permanganate ion contained in the surplus water after the oxalic acid is decomposed is decomposed. Therefore, in the permanganate ion decomposition step S400, the hold-up tank 111 and the drug supply unit 145 decompose a part of the permanganate ion in the surplus water after potassium permanganate is added. Become a department. The drug supply unit 145 is flushed after supplying sodium bisulfite to the hold-up tank 111.

Further, in the permanganate ion decomposition step S400, an amount of sodium bisulfite determined based on the residual amount of permanganate ions remaining in the surplus water is added to the surplus water. Specifically, sodium bisulfite in an amount of 0.5 equivalent or more and 2.0 equivalent or less with respect to the residual amount of permanganate ion in the surplus water is added to the surplus water. Here, the equivalent is an ion equivalent, which is the amount of sodium bisulfite required for reacting divalent sulfite ion with pentavalent permanganate ion. Therefore, the amount of sodium bisulfite added, which can add 2.5 times the amount of permanganate ion, is 1.0 equivalent. In the permanganate ion decomposition step S400, sodium bisulfite in an amount of 1.0 equivalent or more and 1.5 equivalent or less with respect to the residual amount of permanganate ion in the surplus water may be added to the surplus water. preferable.

The pH adjustment step S500 is carried out after the permanganate ion decomposition step S400 and before the concentration step S600. In the pH adjusting step S500, boric acid is added as a pH adjusting agent to the hold-up tank 111 in which excess water in which a part of permanganate ions has already been decomposed is stored. In the pH adjusting step S500, boric acid is added by the drug supply unit 145. Therefore, in the pH adjusting step S500, the hold-up tank 111 and the drug supply section 145 serve as a pH adjusting section for adjusting the pH of excess water. In the pH adjusting step S500, boric acid is prepared in the drug supply unit 145. The drug supply unit 145 supplies boric acid to the hold-up tank 111 via the drug injection line 141. The drug supply unit 145 is flushed after supplying boric acid to the hold-up tank 111.

In the pH adjusting step S500, boric acid is added until the pH of the excess water at the time of concentration in the concentration step S600 becomes 3.0 or more and 7.0 or less. At this time, the pH of the surplus water is preferably adjusted to 5.0 or more and 7.0 or less, and more preferably 5.0 or more and 6.0 or less. Further, in the pH adjusting step S500, boric acid is added while measuring and confirming the pH of excess water at regular time intervals. At that time, the pH is adjusted so that the pH of the excess water in the waste liquid evaporator 160 is 3.0 or more and 7.0 or less. That is, it is preferable that the pH adjusting step S500 is performed immediately before the concentration step S600, and after the pH adjusting step S500, the concentration step S600 is performed without performing another step.

The concentration step S600 is carried out after the pH adjustment step S500. In the concentration step S600, the excess water in the hold-up tank 111 whose pH has been adjusted in the pH adjustment step S500 is treated. Specifically, the excess water in the hold-up tank 111 is sent to the waste liquid evaporator 160 via the waist hold-up tank 150.

The waist hold-up tank 150 is connected to the hold-up tank 111 via a transfer line 151 provided with a pump and a valve in the middle. The waste liquid evaporator 160 is connected to the waist hold-up tank 150 via a waste liquid line 161 provided with a pump in the middle.

In the concentration step S600, the surplus water sent to the waste liquid evaporator 160 is evaporated by being heated by the evaporator, and is concentrated several tens to several hundred times. Therefore, in the concentration step S600, the waste liquid evaporator 160 serves as a concentration unit for concentrating excess water. In the process of concentrating excess water, manganese dioxide is produced as a decomposition product of permanganate ions remaining in the excess water. As a result, in the waste liquid evaporator 160, the fact that the excess water is heated also has an effect, and the following reaction occurs.
MnO ₂ + H ₂ C ₂ O ₄ → Mn (OH) ₂ + H ₂ O + 2CO ₂
That is, oxalic acid is decomposed by manganese dioxide. Therefore, most of the oxalic acid remaining in the excess water is decomposed in the concentration step S600. As a result, the surplus water is separated into concentrated water in which the residue is concentrated and distilled water in which the residue is scarcely contained.

In the disposal step S700, the concentrated water generated in the concentration step S600 is asphalt-solidified and discarded.

According to the surplus water treatment method S100 and the surplus water treatment system 100 as described above, the pH of the surplus water concentrated in the concentration step S600 is adjusted by the pH adjustment step S500. As a result, the pH of the excess water in the waste liquid evaporator 160 is 3.0 or more and 7.0 or less. When the pH of the excess water is 7.0 or less, the reaction rate when oxalic acid is decomposed by manganese dioxide in the waste liquid evaporator 160 can be improved. As a result, most of the oxalic acid remaining in the excess water can be removed with high accuracy. In particular, when the pH of the surplus water is 6.0 or less, the surplus water is surely weakly acidic rather than neutral, and the reaction rate when decomposing oxalic acid can be further improved.

Further, when the pH of the surplus water is set to 3.0 or more, the amount of corrosion of the equipment used in the surplus water treatment system 100 such as the waste liquid line 161 and the waste liquid evaporator 160 can be suppressed. In particular, when the pH of excess water is 5.0 or higher, the amount of corrosion can be greatly suppressed. Further, since the pH of the surplus water is 5.0 or higher, the surplus water becomes weakly acidic. Therefore, even if the sodium bisulfite added in the permanganate ion decomposition step S400 remains in the excess water even in the concentration step S600, it is possible to suppress the generation of sulfurous acid gas. Therefore, the oxalic acid contained in the excess water can be removed with high accuracy without causing any trouble.

Further, in the pH adjusting step S500, the pH of excess water is adjusted by adding boric acid. Boric acid is a weak acid, and even if a large amount of boron (for example, 20000 ppm or more) is contained when the concentrated water is solidified as asphalt, there is no adverse effect. In addition, sodium hydroxide or sulfuric acid may be used as a pH adjuster used when adjusting the pH of excess water. However, with sodium hydroxide or sulfuric acid, the pH of excess water changes significantly even if a small amount is added. Therefore, it is difficult to adjust the pH. Further, when the pH of the surplus water is adjusted by adding sodium hydroxide or sulfuric acid, the surplus water is concentrated and the pH is likely to decrease. On the other hand, when boric acid is added, since boric acid itself has a buffering action, the pH is less likely to decrease even if excess water is concentrated. As a result, in the concentration step S600, the surplus water can be concentrated at the planned pH. Therefore, the oxalic acid remaining in the excess water can be efficiently removed.

Further, the oxalic acid decomposition step S300 is carried out before the pH adjusting step S500. That is, potassium permanganate is added as an oxidizing agent to the excess water. As a result, oxalic acid in excess water can be decomposed before the concentration step S600. Therefore, when the amount of oxalic acid contained in the surplus water is large, most of the oxalic acid in the surplus water can be removed at a timing that does not affect the surplus water after the pH is adjusted in the pH adjusting step S500.

Further, the permanganate ion decomposition step S400 is carried out after the oxalic acid decomposition step S300 and before the pH adjustment step S500. That is, sodium bisulfite is added to the surplus water. As a result, when the amount of permanganate ions contained in the surplus water is large in the oxalic acid decomposition step S300, the amount of permanganate ions in the surplus water can be reduced in advance. Thus, in the waste evaporator 160, MnO ₄ at concentration of excess water ^- are suppressed by the negative effects.

Further, by setting the addition amount of sodium bisulfite to be added to the surplus water to 2.0 equivalents or less, it is possible to suppress the addition of excess sodium bisulfite to the surplus water. If excess sodium bisulfite is added, the reduction of excess water proceeds, and the possibility that most of the permanganate ions are reduced to manganese hydroxide increases. As a result, manganese dioxide required for decomposing oxalic acid may not be secured in the concentration step S600. Furthermore, since a large amount of sodium bisulfite is added to the surplus water, the possibility of generating sulfurous acid gas increases. Therefore, by setting the amount of sodium bisulfite added to the surplus water to 2.0 equivalents or less, preferably 1.5 equivalents or less, such a problem can be effectively suppressed.

Further, by setting the amount of sodium bisulfite added to the surplus water to 0.5 equivalent or more, the permanganate ion in the surplus water can be appropriately decomposed. When sodium bisulfite is added to excess water, part of sodium bisulfite is decomposed by reacting with dissolved oxygen in excess water and oxygen in the atmosphere. Therefore, if the amount of sodium bisulfite added is 0.5 equivalent or less, permanganate ions may remain more than expected. Therefore, the permanganate ion can be appropriately treated by setting the amount of sodium bisulfite added to the surplus water to 0.5 equivalent or more, preferably 1.0 equivalent or more.

Therefore, by setting the amount of sodium bisulfite added to the surplus water to 0.5 equivalents or more and 2.0 equivalents or less, an appropriate amount of manganese dioxide can be secured for decomposing oxalic acid in the concentration step S600. .. In particular, by setting the amount of sodium bisulfite added to the surplus water to 1.0 equivalent or more and 1.5 equivalent or less, an appropriate amount of manganese dioxide is secured with high accuracy in order to decompose oxalic acid in the concentration step S600. be able to.

(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. Moreover, the present invention is not limited by the embodiment, but only by the scope of claims.

In the surplus water treatment method S100, the surplus water generated in the reduction step S3 and the surplus water generated in the decontamination step S4 are limited to being collectively concentrated and treated as in the present embodiment. It's not something. That is, the surplus water generated in the reduction step S3 and the surplus water generated in the decontamination step S4 may be separately concentrated and discarded.

Further, although the hold-up tank 111 is used as the tank for storing the surplus water, the number of tanks is not limited to one, and a plurality of tanks may be provided.

Further, a drug supply unit 145 is provided for the hold-up tank 111 to supply various drugs, and the drug is flushed after changing the drug to be supplied, but the configuration is not limited to this. A separate drug supply unit 145 may be provided for each drug to be supplied to the tank.

Further, when the residual amount of oxalic acid in the surplus water is small, the oxalic acid decomposition step S300 may not be carried out. In addition, when the amount of permanganate ion in the surplus water is small, such as when the oxalic acid decomposition step S300 is not carried out, the permanganate ion decomposition step S400 may not be carried out.

Z Decontamination target P Nuclear power plant 51 Fuel rod 50 Pressurized water reactor 52 Pressurizer 53 Steam generator 54 Primary coolant pump 56 Steam turbine 57 Generator 58 Water recovery device 59 Water supply pump 55a Primary cooling water piping 55b Primary cooling Water pipe 55c Steam pipe 55d Water supply pipe S1 Chemical decontamination method S2 Oxidation process S3 Reduction process S4 Decontamination process S5 Decomposition process S6 Purification process S100 Excess water treatment method S200 Excess water acquisition process S300 Sulfuric acid decomposition process S400 Permanganate ion Decomposition process S500 pH adjustment process S600 Concentration process S700 Disposal process 100 Excess water treatment system 111 Hold-up tank 145 Chemical supply unit 141 Chemical injection line 150 West hold-up tank 151 Transfer line 160 Waste liquid evaporator 161 Waste liquid line

Claims (8)

A method for treating excess water containing oxalic acid generated by supplying pure water to the equipment in the system during chemical decontamination of the system of a nuclear power plant.
A pH adjusting step of adding a pH adjusting agent to the tank in which the surplus water is stored to adjust the pH of the surplus water to 3.0 or more and 7.0 or less.
A method for treating excess water, which comprises a concentration step of concentrating the excess water after the pH adjustment step. The method for treating excess water according to claim 1, wherein in the pH adjusting step, the pH of the excess water is adjusted to 5.0 or more and 7.0 or less. The method for treating excess water according to claim 1, wherein in the pH adjusting step, the pH of the excess water is adjusted to 5.0 or more and 6.0 or less. The method for treating excess water according to any one of claims 1 to 3, wherein boric acid is added as the pH adjusting agent in the pH adjusting step. A permanganate ion decomposition step of adding sodium bisulfite to the surplus water to decompose permanganate ions in the surplus water is included.
The method for treating excess water according to any one of claims 1 to 4, wherein the pH adjusting step is carried out after the permanganate ion decomposition step. The fifth aspect of claim 5, wherein in the permanganate ion decomposition step, 0.5 equivalent or more and 2.0 equivalent or less of the sodium bisulfite is added to the residual amount of the permanganate ion in the surplus water. How to treat excess water. The fifth aspect of the present invention, wherein in the permanganate ion decomposition step, the sodium bisulfite is added in an amount of 1.0 equivalent or more and 1.5 equivalents or less with respect to the residual amount of the permanganate ion in the excess water. How to treat excess water. A treatment system for excess water containing oxalic acid generated by supplying pure water to the equipment in the system during chemical decontamination of the system of a nuclear plant.
The tank in which the surplus water is stored and
A pH adjuster that adds a pH adjuster to the tank to adjust the pH of the excess water to 3.0 or more and 7.0 or less, and a pH adjuster.
A system for treating excess water, comprising a concentration unit for concentrating the excess water whose pH has been adjusted by the pH adjustment unit.

Images