JP7090003B2 - How to treat excess water - Google Patents

Description

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

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 stainless steel and Inconel) that make up the system of a nuclear plant are exposed to 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 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.

As such a chemical decontamination method, for example, Patent Document 1 describes a decontamination waste liquid treatment method for removing permanganate 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 for pumping 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, by adding sodium bisulfite, the permanganate ion in the surplus water is decomposed and removed. Excess water from which permanganate ions have been removed is finally concentrated by an evaporator and then discarded.

At the time of concentration, it is preferable that the excess water is weakly alkaline so as not to damage the evaporator or the like. Therefore, in order to make the surplus water weakly alkaline, a chemical for adjusting the pH such as sodium hydroxide is added. At that time, a part of the surplus water is sampled from the tank, and the amount of chemicals to be added in the laboratory or the like is confirmed.

Japanese Unexamined Patent Publication No. 2014-92442

By the way, a large amount of carbonic acid is contained in the surplus water after the addition of sodium bisulfite. This carbonic acid escapes from the sampled excess water during the work of checking the amount of chemicals to be added. Therefore, there is a difference in the amount of carbonic acid contained between the sampled surplus water and the surplus water stored in the tank. Due to the difference in the amount of carbonic acid contained, the excess water in the tank may not reach the target pH even if the amount of chemicals determined based on the sampled excess water is added. In this case, it is necessary to adjust the pH of the excess water in the tank again, which requires additional work. In addition, even if the target pH is the excess water in the tank containing a large amount of carbonic acid due to additional work, the pH of the excess water is targeted by the large amount of carbonic acid vaporized in the excess water during concentration. It will be higher than the value. As a result, the surplus water is no longer weakly alkaline and affects the subsequent treatment. Therefore, it is required to stabilize the pH of excess water at the time of concentration.

The present invention has been made in consideration of such circumstances, and an object of the present invention is to provide a method for treating excess water that can be treated in a state where the pH of the excess water at the time of concentration is stabilized.

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 permanganate ions 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 after the permanganate ion decomposition step of adding sodium hydrogen sulfite to the surplus water to decompose the permanganate ion in the surplus water and the permanganate ion decomposition step. Using a part of the surplus water sampled from the above, the target addition amount of the alkali metal hydroxide required to set the target pH of the surplus water in a state where carbon dioxide has escaped is the target pH. The addition amount determination step is included, and the addition amount determination step includes a hydroxide addition step of adding the hydroxide to the tank based on the target addition amount determined in the addition amount determination step. The step is a carbon dioxide reduction step of reducing the carbon dioxide in the surplus water so that the surplus water contains almost no carbon dioxide, and the surplus water after the carbon dioxide reduction step is set as the target pH. It includes a post-reduction determination step of determining the required addition amount of alkali metal hydroxide as the target addition amount .

According to such a configuration, the addition amount determination step eliminates the influence of carbonic acid in the excess water and determines the amount of the hydroxide of the alkali metal in order to set the excess water as the target pH. Therefore, in the hydroxide addition step, an amount of alkali metal hydroxide assuming a state in which carbon dioxide is removed from the surplus water at the time of concentration is added to the surplus water. This makes it possible to prevent the pH of the surplus water at the time of concentration from becoming higher than expected.

With such a configuration, the carbonic acid in the sampled excess water is actually reduced by the carbonic acid reduction step. Therefore, the state of the surplus water after the carbonic acid has escaped can be easily simulated with the sampled surplus water.

Further, in the method for treating excess water according to the second aspect of the present invention, in the first aspect, the excess water may be boiled in the carbonic acid reduction step.

With such a configuration, carbonic acid in the sampled excess water can be easily reduced by boiling. Therefore, the state of the surplus water after the carbonic acid is removed can be easily simulated with the sampled surplus water without using a complicated device for removing carbonic acid from the surplus water.

Further, the method for treating excess water according to the third aspect of the present invention is the method for treating excess water containing permanganate ions generated by supplying pure water to the equipment in the system during chemical decontamination of the system of a nuclear plant. In the treatment method, the surplus water is stored after the permanganate ion decomposition step of adding sodium hydrogen sulfite to the surplus water to decompose the permanganate ion in the surplus water and the permanganate ion decomposition step. Using a part of the surplus water sampled from the tank, the target is the amount of alkali metal hydroxide added to achieve the target pH, which is the target pH of the surplus water in the state where carbon dioxide has escaped. The addition includes a step of determining the amount of addition, and a step of adding the hydroxide to the tank based on the target addition amount determined in the step of determining the amount of addition . The amount determination step includes a carbon dioxide amount acquisition step for acquiring the carbon dioxide amount in the surplus water, and a post-acquisition determination step for determining the target addition amount based on the carbon dioxide amount in the carbon dioxide amount acquisition step .

With such a configuration, the amount of carbonic acid in the sampled excess water is acquired by the carbonic acid amount acquisition step. Therefore, it is possible to check the state of the surplus water after the carbonic acid is removed without providing a device for removing the carbonic acid from the sampled surplus water. This makes it possible to treat the excess water at the time of concentration in a state where the pH is stabilized with high accuracy.

Further, in the method for treating excess water according to the fourth aspect of the present invention, in the third aspect, in the hydroxide addition step, the target addition amount determined in the post-acquisition determination step is acquired in the carbon dioxide amount acquisition step. The alkali metal hydroxide may actually be added to the tank in an amount obtained by subtracting the amount of carbon dioxide.

With such a configuration, an amount of alkali metal hydroxide assuming a state in which carbonic acid has escaped from the surplus water at the time of concentration is added to the surplus water. This makes it possible to prevent the pH of the surplus water at the time of concentration from becoming higher than expected.

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, the permanganate ion decomposition step is the concentration of the permanganate ion in the excess water. The reference hydrogen sulfite, which is the amount of sodium bisulfite required to decompose the permanganate ion based on the concentration acquisition step of acquiring the concentration and the concentration of the permanganate ion acquired in the concentration acquisition step. The standard sodium bisulfite amount determination step for determining the amount of sodium and the sodium bisulfite amount 1.2 to 1.5 times the standard sodium bisulfite amount determined in the standard sodium bisulfite amount determination step are used. It may include a step of adding sodium bisulfite to be added to the surplus water.

With such a configuration, permanganate ions contained in excess water can be decomposed with high accuracy.

According to the present invention, the treatment can be performed in a state where the pH of the excess water at the time of concentration is stabilized.

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 1st Embodiment. It is a treatment system of surplus water in the first embodiment. It is a flow chart of the treatment method of surplus water in 2nd Embodiment.

<< First 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, will be described. The decontamination target 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 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. This 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. The steam turbine 56 driven by the steam generated in the above, the generator 57 generated by the drive of the steam turbine 56, the water reactor 58 that returns the steam from the steam turbine 56 to water, and the water from the water reactor 58 are steamed. It is equipped with 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 as described above, 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 cooling material 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 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 element constituting the decontamination target Z is 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 component.

The chemical decontamination method S1 relates to system 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 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. As shown in FIG. 2, the chemical decontamination method S1 includes an oxidation step S2, a decontamination step S3, a decomposition step S4, and a purification step S5.

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

The decontamination step S3 is carried out after the oxidation step S2. In the decontamination step S3, oxalic acid is added as an organic acid to the primary treated water. Further, in the decontamination step S3 of the present embodiment, the eluted metal is removed by passing the secondary treated water, which is the primary treated water to which oxalic acid is added, through the adsorbent such as an ion exchange resin. 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 secondary treatment water. Further, when the decontamination object Z is eluted in the secondary treatment water, chromium, iron, nickel, and cobalt are removed by the adsorbent.

The decomposition step S4 is carried out after the decontamination step S3. In the decomposition step S4, oxalic acid in the secondary treatment water is decomposed. In the decomposition step S4, 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 S4, 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 produced.

The purification step S5 is carried out after the decomposition step S4. In the purification 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 purification step S5, 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 S5 are repeatedly carried out as needed. The decontaminated treated water can be reused in the next cycle and thereafter.

Here, a method for treating excess water generated by supplying pure water to the equipment in the system during chemical decontamination of the system of a nuclear power plant will be described. In the above-mentioned chemical decontamination method S1, in order to protect the primary coolant pump 54, the oxidation step S2 and the decontamination step S3 are carried out while pure water is supplied to the primary coolant pump 54. In the oxidation step S2 and the decontamination step S3, the amount of treated water circulating in the decontamination target Z is increased by supplying pure water. Therefore, a part of the treated water (treated water having the same amount as the amount of supplied pure water) 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, various treatments are performed on the surplus water stored in the tank and containing the permanganate ion. Specifically, as shown in FIG. 3, the surplus water treatment method S100 includes a surplus water acquisition step S200, a permanganate ion decomposition step S300, an addition amount determination step S400, and a hydroxide addition step S500. A concentration step S600 and a disposal step 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) 110, a sampling unit 120, a control unit 130, a drug supply unit 140, a waist hold-up tank 150, and a waste liquid evaporator 160.

In the surplus water acquisition step S200, the surplus water after the addition of permanganate is taken into the plurality of hold-up tanks 110. That is, in the surplus water acquisition step S200, the surplus water containing the permanganate ion is recovered in the hold-up tank 110. In the present embodiment, the surplus water generated in the oxidation step S2 is taken in. The pH of the surplus water generated in the oxidation step S2 is about 6.0 to 7.0.

The permanganate ion decomposition step S300 is carried out before the addition amount determination step S400. In the permanganate ion decomposition step S300, sodium bisulfite is added to the excess water in the hold-up tank 110 to decompose the permanganate ions in the excess water. The permanganate ion decomposition step S300 of the present embodiment includes a concentration acquisition step S301, a reference sodium bisulfite amount determination step S302, and a sodium bisulfite addition step S303. In the present embodiment, the hold-up tank 110, the control unit 130, and the drug supply unit 140 are permanganate ion decomposition units.

In the concentration acquisition step S301, a part of the surplus water is sampled by the sampling unit 120 from the hold-up tank 110. The sampling unit 120 is connected to the hold-up tank 110 via a sampling line 121 provided with a valve in the middle. Therefore, in the concentration acquisition step S301, a part of the surplus water containing the permanganate ion is sampled from the hold-up tank 110. In the sampling unit 120, the molar concentration of permanganate ion is measured with respect to the first sampling water which is a part of the sampled surplus water. Information on the measured molar concentration of permanganate ion is sent to the control unit 130.

In the reference sodium bisulfite amount determination step S302, all the permanganate ions in the surplus water stored in the hold-up tank 110 are decomposed based on the molar concentration of the permanganate ion acquired in the concentration acquisition step S301. The amount of sodium bisulfite required for this is calculated and determined as the reference sodium bisulfite amount. The reference sodium bisulfite amount is calculated by the control unit 130 based on the information on the molar concentration of the permanganate ion measured by the sampling unit 120.

Specifically, the reference sodium bisulfite amount is calculated from the following formula 1.
2MnO _4- + ^5NaHSO ₃ → 2MnSO ₄ + 2Na ₂ SO ₄ + NaHSO ₄ + 3H ₂ O ... (Equation 1)
That is, the reference sodium bisulfite amount is based on the total amount of permanganate ions [mmol / L] in the surplus water stored in the hold-up tank 110 calculated from the obtained molar concentration of permanganate ions. The amount is 2.5 times.

In the sodium bisulfite addition step S303, the amount of sodium bisulfite actually added to the surplus water is determined and added to the hold-up tank 110. In the sodium bisulfite addition step S303, the control unit 130 calculates the amount of sodium bisulfite based on the calculated reference sodium bisulfite amount information. Information on the calculated amount of sodium bisulfite is sent from the control unit 130 to the drug supply unit 140.

The drug supply unit 140 is connected to the hold-up tank 110 via a drug injection line 141 provided with a valve in the middle. The chemical supply unit 140 prepares various chemicals to be supplied to the surplus water, and supplies a necessary amount to the hold-up tank 110. In the sodium bisulfite addition step S303, sodium bisulfite is prepared in the drug supply unit 140. The drug supply unit 140 supplies a predetermined amount of sodium bisulfite based on the information from the control unit 130 to the hold-up tank 110 via the drug injection line 141. The drug supply unit 140 is flushed after supplying sodium bisulfite to the hold-up tank 110.

The amount of sodium bisulfite actually added into the hold-up tank 110 in the sodium bisulfite addition step S303 is 1.2 to 1.5 times the standard sodium bisulfite amount. Carbon dioxide, which is a decomposition product of the chemical (oxalic acid) used for decontamination, is dissolved in the solution, and the surplus water becomes a solution with a very high carbon dioxide concentration.

In the addition amount determination step S400, a part of the surplus water to which sodium bisulfite is added is sampled from the stored hold-up tank 110. Therefore, in the addition amount determination step S400, a part of the surplus water containing carbonic acid is sampled by the sampling unit 120. Assuming that the surplus water does not contain carbonic acid using some of the sampled surplus water, the target addition amount, which is the amount of sodium hydroxide required to set the surplus water to the target pH, is It is determined. Therefore, in the addition amount determination step S400 of the present embodiment, the hold-up tank 110, the control unit 130, and the drug supply unit 140 are the addition amount determination units.

Specifically, the target pH is a value such that the excess water shows weak alkalinity, for example, a value of about pH 8.0 to 10.5. The addition amount determination step S400 of the present embodiment includes a carbonic acid reduction step S401 and a post-reduction determination step S402.

In the carbonic acid reduction step S401, a part of the excess water containing carbonic acid is sampled from the hold-up tank 110 to the sampling unit 120. The second sampled water, which is a part of the sampled surplus water, is boiled for a predetermined time (for example, one hour). As a result, most of the carbonic acid in the second sampling water is vaporized and escapes. That is, the second sampling water has a reduced amount of carbonic acid and is in a state of containing almost no carbonic acid.

In the post-reduction determination step S402, the pH of the second sampling water is measured while adding sodium hydroxide to the second sampling water in a state where almost no carbonic acid is contained. When the pH of the second sampling water reaches the target pH, information on the amount of sodium hydroxide added to the second sampling water up to that point is acquired. The acquired information on the amount of sodium hydroxide added is sent to the control unit 130. The control unit 130 is required to set all the surplus water in the hold-up tank 110 as the target pH based on the acquired information on the amount of sodium hydroxide added and the amount of surplus water in the hold-up tank 110. The amount of sodium hydroxide added is calculated. The calculated addition amount of sodium hydroxide is determined as the target addition amount. That is, the addition amount determining unit of this embodiment is the control unit 130.

In the hydroxide addition step S500, the target addition amount of sodium hydroxide defined in the post-reduction determination step S402 is added to the hold-up tank 110. In the hydroxide addition step S500, the calculated target addition amount information is sent from the control unit 130 to the drug supply unit 140. In the hydroxide addition step S500, sodium hydroxide is prepared in the drug supply unit 140. The drug supply unit 140 supplies a predetermined amount of sodium hydroxide to the hold-up tank 110 via the drug injection line 141 based on the information from the control unit 130. That is, the hydroxide addition section of the present embodiment is the drug supply section 140. The drug supply unit 140 is flushed after supplying sodium hydroxide to the hold-up tank 110. Therefore, in the hydroxide addition step S500 of the present embodiment, the hold-up tank 110, the control unit 130, and the drug supply unit 140 are the hydroxide addition units.

In the hydroxide addition step S500, after sodium hydroxide is added to the hold-up tank 110, a part of the surplus water is sampled again from the hold-up tank 110 into the sampling unit 120. The sampled surplus water is boiled again for a predetermined time, and the pH is measured. If the pH of the sampled surplus water after boiling is within the range of 8.0 to 10.5 and it is weakly alkaline, the concentration step S600 is carried out. If the pH of the sampled surplus water after boiling is not within the range of 8.0 to 10.5, the addition amount determination step S400 and the hydroxide addition step S500 are carried out again.

The concentration step S600 is carried out after the hydroxide addition step S500. In the concentration step S600, the excess water in the hold-up tank 110 to which sodium hydroxide is added 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 110 via a transfer line 151 provided with a pump on the way. The waste liquid evaporator 160 is connected to the waist hold-up tank 150 via a waste liquid line 161 provided with a pump on the way. Therefore, in the concentration step S600 of the present embodiment, the waste liquid evaporator 160 is the hydroxide addition portion.

In the concentration step S600, the surplus water sent to the waste liquid evaporator 160 is evaporated and concentrated several tens of times. At this time, the carbonic acid contained in the surplus water vaporizes and escapes from the surplus water. 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 solidified asphalt and discarded.

According to the surplus water treatment method S100 as described above, in the addition amount determination step S400, the target addition amount of sodium hydroxide when it is assumed that the surplus water does not contain carbonic acid is determined. That is, the amount of sodium hydroxide for making the surplus water weakly alkaline is determined by eliminating the influence of carbonic acid in the surplus water. Therefore, in the hydroxide addition step S500, an amount of sodium hydroxide assuming a state in which carbon dioxide has escaped from the surplus water in the concentration step S600 is added to the surplus water. This makes it possible to prevent the pH of the excess water in the concentration step S600 from becoming higher than expected. Therefore, regardless of the content of carbonic acid contained in the surplus water, the treatment can be performed in a state where the pH of the surplus water at the time of concentration is stabilized.

In particular, in the present embodiment, the carbonic acid in the second sampling water is actually reduced by the carbonic acid reduction step S401. Therefore, the state of the surplus water after the carbonic acid has escaped can be easily simulated with the second sampling water. Therefore, it is possible to obtain with high accuracy the amount of sodium hydroxide added in order to make the excess water in the state where carbonic acid has been removed in the concentration step S600 to be weakly alkaline. This makes it possible to treat the excess water at the time of concentration in a more stable pH state.

Furthermore, by boiling, carbonic acid in the second sampling water can be easily reduced. Therefore, it is possible to easily obtain the second sampling water simulating the state of the surplus water after the carbonic acid is removed without using a complicated device for removing the carbonic acid from the second sampling water.

Further, when sodium bisulfite is added to the surplus water, a part of the sodium bisulfite is decomposed by the dissolved oxygen in the surplus water. Therefore, even if an amount of sodium bisulfite equivalent to the amount required for decomposing all of the permanganate ions in the surplus water stored in the hold-up tank 110 is added, some of the permanganate ions are decomposed. It may not be done. However, by adding an amount of sodium bisulfite 1.2 to 1.5 times the standard amount of sodium bisulfite, almost all of the permanganate ions contained in the surplus water in the hold-up tank 110 are high. It can be disassembled with accuracy.

In the carbonic acid reduction step S401 in the first embodiment, a method of boiling excess water was used as a method of reducing the amount of carbonic acid in the second sampling water, but the method is not limited to such a method. In the carbonic acid reduction step S401, it is sufficient that the amount of carbonic acid in the second sampling water can be reduced, for example, bubbling the second sampling water, reducing the pressure of the second sampling water, or acidifying the second sampling water once. You may.

<< Second Embodiment >>
Next, a second embodiment of the method for treating excess water of the present invention will be described with reference to FIG. In the method for treating excess water shown in the second embodiment, the addition amount determination step is different from that in the first embodiment. Therefore, in the description of the second embodiment, the same parts as those of the first embodiment are designated by the same reference numerals, and duplicate description will be omitted. The surplus water treatment method S110 of the second embodiment can also be carried out by the surplus water treatment system 100 of the first embodiment.

As shown in FIG. 5, in the surplus water treatment method S110 of the second embodiment, the addition amount determination step S450 includes a carbonic acid amount acquisition step S451 and a post-acquisition determination step S452.

In the carbonic acid amount acquisition step S451, the surplus water to which sodium bisulfite is added is sampled from the hold-up tank 110. Therefore, in the carbonic acid amount acquisition step S451, a part of the surplus water containing carbonic acid is sampled from the hold-up tank 110 in the sampling unit 120. The amount of carbonic acid contained in the second sampling water, which is a part of the sampled surplus water, is measured and obtained. The acquired carbonic acid amount information is sent to the control unit 130.

As a method for measuring the amount of carbon dioxide, a total organic carbon meter (TOC meter) that measures organic substances in a solution as the total amount of organic carbon (carbon amount), or a quantitative analysis that measures the amount of chemical substances using a chemical reaction. A method such as titration or ion chromatography for analyzing an ionic component in a solution using liquid chromatography is used.

In the post-acquisition determination step S452, the amount of sodium hydroxide added to achieve the target pH is calculated based on the amount of carbonic acid in the acquired second sampling water. Specifically, based on the amount of carbonic acid in the second sampling water acquired in the carbonic acid amount acquisition step S451 and the amount of surplus water in the hold-up tank 110, all the surplus water in the hold-up tank 110 is targeted. The control unit 130 calculates the amount of sodium hydroxide added to obtain the pH. The calculated addition amount of sodium hydroxide is determined as the target addition amount.

In the hydroxide addition step S500, sodium hydroxide in an amount obtained by subtracting the carbonic acid amount acquired in the carbonic acid amount acquisition step S451 from the target addition amount determined in the post-acquisition determination step S452 is actually added to the hold-up tank 110. To. After adding sodium hydroxide to the hold-up tank 110, a part of the surplus water is sampled from the hold-up tank 110 again. If the pH of the sampled excess water is within the range of 8.0 to 10.5 and is weakly alkaline, the concentration step S600 is carried out.

According to the surplus water treatment method S110 of the second embodiment, the carbonic acid amount in the second sampling water is acquired by the carbonic acid amount acquisition step S451. Therefore, it is possible to grasp the state of the surplus water after the carbonic acid is removed without providing a device for removing the carbonic acid from the second sampling water. Therefore, in the hydroxide addition step S500, sodium hydroxide can be added to the surplus water in an amount assuming a state in which carbon dioxide has escaped from the surplus water in the concentration step S600. As a result, the treatment can be performed with the pH of the surplus water at the time of concentration stabilized.

Further, in the hydroxide addition step S500, sodium hydroxide is added in an amount obtained by subtracting the carbonic acid amount acquired in the carbonic acid amount acquisition step S451 from the target addition amount. In the excess water containing carbonic acid, not all of the added sodium hydroxide is used to adjust the pH of the excess water, and part of it is consumed by reacting with carbonic acid as shown in Equation 2 below. It ends up.
H ₂ O + CO ₂ + NaOH → NaHCO ₃ ... (Equation 2)

At this time, the amount of carbonic acid [mol] and the amount of sodium hydroxide [mol] consumed by carbonic acid are equal. Therefore, the amount of sodium hydroxide required to set the target pH of the excess water containing carbonic acid is from the target addition amount, which is the amount of sodium hydroxide required to set the target pH of the excess water containing carbonic acid. , The value is obtained by subtracting the amount of carbonic acid in the excess water in the hold-up tank 110. Therefore, by adding sodium hydroxide in an amount obtained by subtracting the obtained carbon dioxide amount from the target addition amount, the amount of sodium hydroxide assuming a state in which carbon dioxide is removed from the surplus water in the concentration step S600 is added to the surplus water. Is added to. This makes it possible to prevent the pH of the excess water in the concentration step S600 from becoming higher than expected. Therefore, regardless of the content of carbonic acid contained in the surplus water, the treatment can be performed in a state where the pH of the surplus water at the time of concentration is stabilized.

(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 each embodiment are examples, and the configurations may be added or omitted within a range not deviating from the gist of the present invention. , Replacements, and other changes are possible. Further, the present invention is not limited to the embodiments, but only to the scope of claims.

The surplus water taken in in the surplus water acquisition step S200 is not limited to the surplus water generated in the oxidation step S2. The surplus water taken in in the surplus water acquisition step S200 may be any surplus water containing permanganate ions, and may be, for example, surplus water generated in the decontamination step S3. Oxalic acid is added to the surplus water generated in the decontamination step S3. Therefore, the pH of the surplus water generated in the decontamination step S3 is about 2.0 to 3.0.

Further, the chemicals to which the addition amount is determined in the addition amount determination step S450 and added in the hydroxide addition step S500 are not limited to sodium hydroxide. The chemical added in the hydroxide addition step S500 may be an alkali metal hydroxide capable of making excess water alkaline, and may be, for example, potassium hydroxide.

Further, the surplus water treatment methods S100 and S110 are not limited to those implemented in the surplus water treatment system 100 as in the present embodiment. For example, a separate drug supply unit may be provided for each drug to be supplied to the tank. Further, the various calculated and acquired values in the surplus water treatment methods S100 and S110 may be manually calculated by the operator instead of the control unit 130.

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 restorer 59 Water supply pump 55a Primary cooling water piping 55b Primary cooling Water pipe 55c Steam pipe 55d Water supply pipe 100 Excess water treatment system 110 Hold-up tank 120 Sampling unit 121 Sampling line 130 Control unit 140 Chemical supply unit 141 Chemical injection line 150 West hold-up tank 160 Waste liquid evaporator S1 Chemical decontamination method S2 Oxidation Step S3 Decontamination step S4 Decomposition step S5 Purification step S100, S110 Surplus water treatment method S200 Surplus water acquisition step S300 Permanganate ion decomposition step S301 Concentration acquisition step S302 Standard hydrogen sulfite sodium amount determination step S303 Hydrogen sulfite addition step S400 , S450 Addition amount determination process S401 Carbon dioxide reduction process S402 Post-reduction determination process S500 Hydroxide addition process S600 Concentration process S700 Disposal process S451 Carbon dioxide amount acquisition process S452 Post-acquisition determination process

Claims (5)

A method for treating excess water containing permanganate ions generated by supplying pure water to the equipment in the system during chemical decontamination of the system of a nuclear power plant.
A permanganate ion decomposition step of adding sodium bisulfite to excess water to decompose permanganate ions in the excess water, and
After the permanganate ion decomposition step, a part of the surplus water sampled from the tank in which the surplus water is stored is used to set the target pH of the surplus water in a state where the carbon dioxide has escaped. The addition amount determination step for determining the target addition amount, which is the addition amount of the alkali metal hydroxide required for
A hydroxide addition step of adding the hydroxide to the tank based on the target addition amount determined in the addition amount determination step is included.
The addition amount determination step is
A carbonic acid reduction step of reducing carbonic acid in the surplus water so that the surplus water contains almost no carbonic acid.
A method for treating surplus water, which comprises a post-reduction determination step of determining the addition amount of an alkali metal hydroxide required to bring the surplus water after the carbon dioxide reduction step to the target pH as the target addition amount . The method for treating excess water according to claim 1 , wherein in the carbonic acid reduction step, the excess water is boiled. A method for treating excess water containing permanganate ions generated by supplying pure water to the equipment in the system during chemical decontamination of the system of a nuclear power plant.
A permanganate ion decomposition step of adding sodium bisulfite to excess water to decompose permanganate ions in the excess water, and
After the permanganate ion decomposition step, a part of the surplus water sampled from the tank in which the surplus water is stored is used to set the target pH of the surplus water in a state where the carbon dioxide has escaped. The addition amount determination step for determining the target addition amount, which is the addition amount of the alkali metal hydroxide required for
A hydroxide addition step of adding the hydroxide to the tank based on the target addition amount determined in the addition amount determination step is included.
The addition amount determination step is
The carbonic acid amount acquisition step for acquiring the carbonic acid amount in the surplus water and
A method for treating excess water , which comprises a post-acquisition determination step of determining the target addition amount based on the carbonic acid amount in the carbonic acid amount acquisition step . In the hydroxide addition step, the alkali metal hydroxide is actually added to the tank in an amount obtained by subtracting the carbon dioxide amount acquired in the carbon dioxide amount acquisition step from the target addition amount determined in the acquisition post-acquisition determination step. The method for treating excess water according to claim 3 . The permanganate ion decomposition step is
A concentration acquisition step for acquiring the concentration of the permanganate ion in the surplus water, and a concentration acquisition step.
Based on the concentration of the permanganate ion acquired in the concentration acquisition step, the reference hydrogen sulfite that determines the reference sodium bisulfite amount, which is the amount of the sodium bisulfite required for decomposing the permanganate ion. Sodium amount determination process and
It includes a sodium bisulfite addition step of adding the sodium bisulfite in an amount of 1.2 to 1.5 times the reference sodium bisulfite amount determined in the reference sodium bisulfite amount determination step to the surplus water. The method for treating excess water according to any one of claims 1 to 4 .

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