JP2000227499A - Chemical decontamination - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lower leakage of hydrogen peroxide to the downstream side of a decomposing device by controlling an injection quantity of hydrogen peroxide in compliance with impurity concentration and pH in a decontaminating agent when decomposing a reducing decontaminating agent mainly consisting of dicarboxylic acid. SOLUTION: For example, chemical decontamination of a nuclear reactor 1 and a nuclear reactor recirculation line 2 is carried out by using a temporarily installed apparatus provided with a reducing agent decomposing device 18 such as a catalytic decomposing device and a ultraviolet irradiator. First of all, an oxidizing agent such as a potassium permanganate solution is used for carrying out oxidizing decontamination, and then, the oxidizing agent is decomposed. Subsequently, reducing decontamination is carried out according to a predetermined way using a reducing agent prepared by adjusting pH of oxalic acid solution with hydrazine. After the reducing decontamination process is finished, the decontaminating agent is introduced to the reducing agent decomposing device 18. In this process, hydrogen peroxide is injected onto the upstream side of the reducing agent decomposing device 18 from a hydrogen peroxide tank 32. An injection quantity is regulated in compliance with impurity concentration representing iron concentration or the like measured in a sampling point 36 and pH measured by a pH meter 37.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water-cooled nuclear power plant. It relates to the method of removing.

[0002]

2. Description of the Related Art As a conventional technique relating to chemical decontamination, as disclosed in JP-A-3-10919, permanganic acid is used as an oxidizing agent and dicarboxylic acid which is a kind of organic acid as a reducing agent. There is a method of chemically decontaminating metallic structural parts of a nuclear reactor. As a method of decomposing organic acids used in this case, Japanese Patent Application Laid-Open No. 9-510784 discloses an iron complex and ultraviolet light using an iron complex and water. A method for disassembling is described. According to this method, the iron complex acts as a catalyst,
Hydrogen peroxide, which is an oxidizing agent, and an organic acid react with each other to generate carbon dioxide and water, thereby preventing the ion exchange resin for removing the organic acid from becoming a large amount of secondary waste.

[0003]

When the above-mentioned chemical decontamination technology is applied to the decontamination of a system of a nuclear reactor to decompose a reducing decontamination agent, the reducing agent depends on the concentration of impurities in the decontamination solution and the value of pH. It is necessary to control the amount of hydrogen peroxide injected into the decomposition apparatus at the inlet side.

[0004] If the injection amount of hydrogen peroxide is appropriately controlled,
Leakage of hydrogen peroxide to the downstream side of the decontamination agent decomposition device is suppressed, and there is no risk of damaging the resin in the ion exchange resin tower installed in the system. If this control is inadequate, chemical decontamination may occur because hydrogen peroxide leaks downstream of the decontamination agent decomposer and destroys the resin in the ion-exchange resin tower installed in the system. The target water quality at the time of final purification at the end (for example, the conductivity is 1 μS / cm or less) cannot be satisfied.

[0005] Therefore, when the decomposition method described in Japanese Patent Application Laid-Open No. 9-510784 is used, the impurity concentration and p
If H fluctuates, water passes through the ion exchange resin tower without sufficient reduction treatment of the reducing agent in the reducing agent decomposition device, so that hydrogen peroxide leaks downstream of the reducing agent decomposition device. It breaks the ion exchange resin.
As a result, the target water quality at the end of chemical decontamination cannot be met.

An object of the present invention is to provide a chemical decontamination method capable of suppressing the leakage of hydrogen peroxide to the downstream side of a reducing agent decomposition apparatus and stably achieving a target water quality at the time of final purification.

[0007]

In order to achieve the above object, the present invention provides a chemical decontamination method for chemically removing a radionuclide from the surface of a metal member contaminated with the radionuclide. A reduction decontamination step using a reduction decontamination agent as a component, and at the time of decomposition of the reduction decontamination agent performed after the step, controlling the hydrogen peroxide injection amount according to the impurity concentration and pH in the decontamination solution, Decomposing.

That is, before the decomposition of the reducing agent is started, the impurity concentration (for example, iron concentration, etc.) in the decontamination solution is measured to determine the amount of hydrogen peroxide to be injected at the beginning of injection.

As an example, FIG. 3 shows a difference in injection amount of hydrogen peroxide depending on a difference in iron (Fe) concentration. When iron is not present, the initial injection amount of hydrogen peroxide can be sufficiently reduced by one equivalent of the reducing agent, but when the iron concentration increases, the initial injection amount of hydrogen peroxide is reduced. Unless more than one equivalent of the agent is injected, the amount of hydrogen peroxide leaking to the outlet of the reducing agent decomposition device cannot be suppressed (to several ppm or less).

The temperature at which the reducing agent is decomposed is normally about 90 ° C., but when decomposing at a lower temperature, the initial injection amount of hydrogen peroxide is reduced by one equivalent of the reducing agent. If not, the amount of hydrogen peroxide leaking to the downstream side of the reducing agent decomposition apparatus cannot be suppressed.

As an example, FIG. 4 shows the ratio of the initial injection amount of hydrogen peroxide when the reducing agent is decomposed at 65 ° C. As can be seen from this figure, the leakage of hydrogen peroxide to the outlet of the reducing agent decomposer cannot be suppressed unless the amount of hydrogen peroxide is at least one equivalent of the reducing agent.

During the decomposition, the amount of hydrogen peroxide injected is controlled while monitoring the pH at the outlet of the reducing agent decomposition apparatus.
As an example, FIG. 5 shows a case where the reducing agent is 2,000 ppm of oxalic acid (adjusted to pH 2.5 with hydrazine). As can be seen from this figure, if the pH value is maintained at 4 or more, the amount of hydrogen peroxide leaked downstream of the reducing agent decomposition apparatus can be suppressed to several ppm or less.

By controlling the amount of hydrogen peroxide injected as described above, the amount of hydrogen peroxide leaking to the outlet of the reducing agent decomposition apparatus can be suppressed, and the water quality during final purification at the end of chemical decontamination can be stabilized. And within the target value.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. FIG. 1 shows an example of a system configuration when performing system chemical decontamination of a nuclear reactor. The object to be decontaminated is the reactor 1 and the reactor recirculation line 2 including the reactor recirculation pump 3.

[0015] As temporary equipment for performing decontamination,
A circulation line 7 connected to the control rod guide tube 4, a circulation pump 5, a heater 6, a purification system pump 8, a purification line 9, a cation resin tower 10, a mixed bed resin tower 11, a cooler 12, a resin tower bypass valve 13, Cationic resin tower inlet valve 14, cationic resin tower outlet valve 15, mixed bed resin tower inlet valve 16, mixed bed resin tower outlet valve 17, reducing agent decomposition unit 18, reducing agent decomposition unit bypass valve 19, reducing agent decomposition unit inlet valve 20, reducing agent decomposition device outlet valve 21, recovery device 22, recovery device bypass valve 2
3, recovery device inlet valve 24, recovery device outlet valve 25, filter 26, filter bypass valve 27, filter inlet valve 2
8, a filter outlet valve 29, a decontamination agent tank 30, a decontamination agent injection pump 31, a hydrogen peroxide tank 32, a hydrogen peroxide injection pump 33, and a tank vent device 34 are used.

As the decontaminating agent, a reducing agent prepared by adding hydrazine to oxalic acid (0.2%) to adjust the pH and an oxidizing agent of potassium permanganate (0.05%) are used.

In the decontamination step, as shown in FIG. 2, after the reactor recirculation pump 3 is started, the temperature is raised by using the circulation pump 5 and the heater 6 to reach a predetermined temperature (90 ± 5 ° C.). At the stage where it reaches, oxidative decontamination is performed first, and then oxidizing agent is decomposed. Thereafter, oxalic acid as a reducing agent and hydrazine as a pH adjuster are injected from the decontamination agent tank 30 into the nuclear reactor 1 using the decontamination agent injection pump 31.

When the oxalic acid concentration reaches a predetermined concentration (0.2%), the decontamination agent injection pump 31 is stopped to stop the injection of oxalic acid, and reductive decontamination is performed. During reduction decontamination, the purifying system pump 8 is started to remove dissolved metal ions and radioactive ions, and the valves 13, 16, 17, and 2 are activated.
0, 21, 24, 25, 28, 29 are closed and valves 14 and 1 are closed.
5, 19, 23 and 27 are opened, and a part of the decontamination liquid is passed through the cationic resin tower 10. At the stage where the decontamination agent is decomposed after the reductive decontamination step (about 4 to 15 hours) is completed, the valves 20, 21, 24, 25, 28 and 29 are opened, and the valves 19, 23 and 27 are closed. And the reducing agent decomposing device 18 and the recovery device 2
2. The flow is changed so as to guide the decontamination liquid to the filter 26.
At this time, in order to decompose oxalic acid and hydrazine,
Injection pump 33 for hydrogen peroxide from hydrogen peroxide tank 32
And injected into the upstream side of the reducing agent decomposer 18.

At this time, the injection amount of hydrogen peroxide is adjusted in accordance with (according to) the concentration of impurities in the decontamination solution and the pH at the outlet of the reducing agent decomposing device 18. The impurity concentration in the decontamination liquid is measured at a sampling point 36 on the inlet side of the reducing agent decomposing device 18. The pH is measured by a pH meter 37 on the outlet side of the reducing agent decomposing device 18.

As the reducing agent decomposing device 18, a catalyst decomposing device, an ultraviolet irradiation device or the like can be applied. The decomposed gas generated in the reducing agent decomposing device 18 is separated by the recovery device 22, guided to the tank vent device 34 through the gas vent line 35, and finally discharged to the exhaust system.

After the completion of the reduction step of the second cycle,
The reducing agent is decomposed according to the same procedure as the reducing agent decomposition step in the first cycle, and after the decomposition is completed, final purification is performed using a mixed bed resin.
In FIG. 2, the process assumes two cycles. However, if a higher decontamination effect is desired, three or more cycles may be used. In the case of three or more cycles, the oxidation step, the oxidizing agent decomposition step, the reducing step, the reducing agent decomposition step, and the purification step may be regarded as one cycle and inserted between the first cycle and the second cycle.

[0022]

According to the decontamination method of the present invention, when the reducing agent is decomposed by the reducing agent decomposition apparatus, the amount of hydrogen peroxide to be injected is controlled according to the impurity concentration and the pH of the decontamination solution. By doing so, it is possible to suppress the leakage of hydrogen peroxide to the downstream side of the reducing agent decomposition device, and to stably achieve the target water quality at the time of final purification.

[Brief description of the drawings]

FIG. 1 is a diagram showing a system configuration when performing systematic chemical decontamination.

FIG. 2 is a diagram showing an example of a decontamination step.

FIG. 3 is a diagram showing the iron concentration dependency of the initial hydrogen peroxide injection amount and the hydrogen peroxide leak amount during decomposition of a reducing agent.

FIG. 4 is a diagram showing temperature dependence of an initial hydrogen peroxide injection amount and a hydrogen peroxide leak amount during decomposition of a reducing agent.

FIG. 5 is a diagram showing a relationship between a reducing agent decomposition device outlet pH and a hydrogen peroxide leak amount at the time of reducing agent decomposition.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor, 2 ... Reactor recirculation line, 3 ... Reactor recirculation pump, 4 ... Control rod guide tube, 5 ... Circulation pump, 6 ... Heater, 7 ... Circulation line, 8 ... Purification system pump, 9 ... Purification line, 10: cationic resin tower, 11: mixed-bed resin tower, 1
2 ... cooler, 13 ... resin tower bypass valve, 14 ... cationic resin tower inlet valve, 15 ... cationic resin tower outlet valve, 16 ... mixed bed resin tower inlet valve, 17 ... mixed bed resin tower outlet valve, 18 ... reducing agent Decomposition device, 19 ... bypass valve for reducing agent decomposition device, 20 ...
Reducing agent decomposition device inlet valve, 21 ... reducing agent decomposition device outlet valve,
Reference numeral 22: recovery device, 23: recovery device bypass valve, 24: recovery device inlet valve, 25: recovery device outlet valve, 26 ... filter, 27 ... filter bypass valve, 28 ... filter inlet valve, 29 ... filter outlet valve, 30 ... Decontamination agent tank, 31
... Decontaminant injection pump, 32 ... Hydrogen peroxide tank, 33 ...
Hydrogen peroxide injection pump, 34 tank vent device, 35
... gas release line, 36 ... sampling point, 37 ... pH
Total.

 ──────────────────────────────────────────────────続 き Continuation of the front page (71) Applicant 390023928 Hitachi Engineering Co., Ltd. 3-2-1, Sachimachi, Hitachi-shi, Ibaraki (72) Inventor Fumito Nakamura 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Stock Company (72) Inventor Makoto Nagase 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Power & Electricity Development Division, Hitachi, Ltd. (72) Motohiro Aizawa Motomachi Aizawa Three-Year Town, Hitachi City, Ibaraki Prefecture Hitachi Engineering Co., Ltd. (2-1) Hiromi Kawachi 2-2-2 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Kurita Engineering Co., Ltd. (72) Inventor Seiji Furukawa Atsugi, Kanagawa Prefecture Ichimori no Sato Wakamiya 7-1 Kurita Industries Co., Ltd.

Claims (5)

[Claims] 1. A chemical decontamination method for chemically removing a radionuclide from the surface of a metal member contaminated with a radionuclide, comprising: a reduction and decontamination step using a reduction and decontamination agent containing a dicarboxylic acid as a main component; At the time of decomposition of the reducing decontamination agent performed after the end of the process,
Controlling the amount of injected hydrogen peroxide according to the impurity concentration or pH in the decontamination solution to perform the decomposition. 2. The chemical decontamination method according to claim 1, wherein the dicarboxylic acid of the reduction decontamination agent is oxalic acid. 3. The chemical decontamination method according to claim 1, wherein hydrazine is added as the reducing decontamination agent. 4. The chemical decontamination method according to claim 1, wherein an iron concentration is used as the impurity concentration, and the hydrogen peroxide injection amount is controlled according to the iron concentration. 5. The method according to claim 1, wherein a value of the pH at the outlet of the reducing agent decomposer for the reducing and decontaminating agent is used as the pH, and the injection amount of the hydrogen peroxide is controlled in accordance with the value. The chemical decontamination method according to any one of the above.