JP2000065989A - Chemical decontamination method for radioactive contaminants - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To efficiently decontaminate metal structures and parts of radioactively contaminated nuclear reactors, simplify the radioactive decontamination process and reduce the amount of secondary waste generated. Decrease. In a method for chemically removing radioactive contaminants, a dicarboxylic acid is used for reductive dissolution of an oxide incorporating radioactivity, a permanganate is used for oxidative dissolution, and reduction and oxidation are repeated. Perform at a liquid temperature of 100 ° C or less. That is, first perform an oxidation treatment with a permanganate solution, and then treat the permanganate by adding dicarboxylic acid,
Chemically decontaminate nuclear reactor metal structures and components contaminated by radioactivity with excess dicarboxylic acid. By adding the dicarboxylic acid directly to the permanganate solution, the conventional purification steps and removal and disposal steps of the permanganate solution can be omitted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chemical decontamination method for radioactive contaminants for chemically decontaminating radioactively contaminated metal structural materials, for example, metal structural materials, parts, piping, etc. of nuclear reactors.

[0002]

2. Description of the Related Art As a method for chemically decontaminating nuclear reactor structural parts contaminated with radioactivity, for example, the technique described in JP-A-52-118200 is known. In this chemical decontamination method,
Approximately 10 alkaline permanganate solutions for oxidation
Use at a temperature of 0 ° C. The solution is then washed with a deionizing agent and then further treated with a citrate-oxalate solution.

FIG. 4 is a flowchart showing a basic radioactive contamination removal process according to a conventional example. That is, in the chemical decontamination method according to the prior art, as shown in FIG. 4, after discharging system water in the decontamination system to be decontaminated, the alkaline potassium permanganate solution is adjusted to pH 11 with ammonia. After the prepared solution is added to the system for oxidative dissolution, the solution in the system is discharged. The drainage is neutralized. Thereafter, an organic acid such as ascorbic acid and a chelating agent such as ethylenediaminetetraacetic acid (EDTA) are added and reduced and dissolved. Next, the liquid is discharged in the same manner as described above, and the discharged liquid is solidified. After that, the system is washed and the washing liquid is discharged. The cleaning liquid is solidified.

[0004]

However, in the prior art, as shown in FIG. 4, starting from a system water discharging step, an alkaline potassium manganate solution preparing step, a liquid discharging step, an organic acid and a chelating agent adding step, Liquid discharge process,
It has a number of liquid discharging steps from the in-system cleaning step to the cleaning liquid discharging step. An alkaline permanganate solution is used at a temperature of about 100 ° C. for the oxidation treatment. This solution is then washed with a deionizing agent, and then further treated with a citrate-oxalate solution, where the solution is adjusted to pH 3.5 with ammonia and contains an inhibitor and EDTA. . The inhibitor is iron (III) -formate. Moreover, treatment with high concentrations of chemicals requires an extremely long time.

According to this conventional method, it is necessary to discharge the decontamination solution in the decontamination system for cleaning purposes so that the decontamination solution is once empty, and after the cleaning treatment, the decontamination solution is again used. Have to be filled. Therefore, the conventional technology is not yet applied to the primary reactor system.

The present invention has been made to solve the above problems, and can be applied to a primary system of a nuclear reactor, and can efficiently reduce a metal structural material contaminated by radioactivity, particularly, a metal structural material and parts of a nuclear reactor. Well, by chemical decontamination,
For example, it is an object of the present invention to provide a method for chemical decontamination of radioactive contaminants that can reduce radiation exposure of workers engaged in inspection and repair of a nuclear reactor and reduce radioactive contamination.

In addition, the present invention simplifies the chemical decontamination process by eliminating the washing steps in the intermediate and final steps in the chemical decontamination, and has an important secondary significance as the chemical decontamination. An object of the present invention is to provide a method for chemical decontamination of radioactive contaminants, which can significantly reduce the generation of secondary waste.

[0008]

According to the first aspect of the present invention, there is provided a method for chemical decontamination of radioactive contaminants using a dicarboxylic acid and a permanganate, in which a radioactive oxide is reduced. It is characterized in that dicarboxylic acid is used for dissolution, a permanganate solution is used for oxidative dissolution, and reduction and oxidation are repeated at a liquid temperature of 100 ° C. or less.

The invention corresponding to claim 2 is characterized in that hydrazine and / or hydrogen peroxide are added as a metal material corrosion inhibitor in removing the chemical contamination.

[0010] The invention corresponding to claim 3 is to decompose and remove the permanganate and the dicarboxylic acid in the system for performing the chemical decontamination, so that the liquid in the system is not discharged. It is characterized by repeatedly performing chemical decontamination. The invention corresponding to claim 4 is characterized in that dicarboxylic acid in the chemical decontamination treatment system is decomposed and removed by ultraviolet irradiation.

According to the present invention, the same effect can be obtained at a low permanganate ion concentration by applying permanganate to the oxidation treatment without using an alkali, and further, the treatment with a dicarboxylic acid can be achieved. Can also be carried out with a smaller amount of dicarboxylic acid, since the transition from near neutral to the acidic side rather than from the alkaline side to the acidic side.

Further, the generation of secondary waste such as sodium ions generated by the addition of alkali can be slightly suppressed. Most of all, this can be done by adding permanganate to the reactor primary coolant.
Therefore, there is no need to discharge the primary coolant. Further, the method of the present invention can be practiced by purifying the primary coolant with an ion exchange resin and leaving it in the reactor for the next operation.

The present invention uses a dicarboxylic acid,
Added directly to the permanganate solution. As a result, the conventional washing step can be omitted, and the removal and disposal of the permanganate solution can be omitted. In addition, waste is greatly reduced by decomposing dicarboxylic acid into water and carbon dioxide using ultraviolet light.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the method for chemically decontaminating radioactive contaminants according to the present invention will be described with reference to FIG. FIG. 1 shows a basic radioactive contamination removal process of the present invention. That is, in FIG. 1, in order to chemically decontaminate the inside of the reactor and the system, after adjusting the reactor water level, the temperature of the coolant held in the system is raised to 90 ° C. Oxalic acid, which is one of the dicarboxylic acids, is added thereto, dissolved and circulated.

When a certain amount of radioactivity is dissolved, oxalic acid is decomposed into carbon dioxide and water, and potassium permanganate, which is one of permanganates, is added, or if necessary, ultraviolet irradiation is performed. To decompose oxalic acid. in this case,
Unlike the known examples, the pH provided by potassium permanganate without the addition of alkali, ie pH = 6 to
8 is maintained near neutrality.

When the dissolution of the chromium oxide dissolved by the oxidation treatment becomes constant, oxalic acid is further added in excess, and potassium permanganate is decomposed into potassium ions and manganese ions by a redox reaction. Potassium ions and manganese ions are removed with a cation exchange resin. Here, since excess oxalic acid remains in the furnace and the system, reduction and dissolution are performed again with this.

This cycle can be repeated to eventually return the radioactivity and the chemical decontamination agent to the purified water. Here, for example, tartronic acid as the dicarboxylic acid,
Similar effects can be obtained by using sodium permanganate as the permanganate.

On the other hand, in the conventional method shown in FIG. 4, the alkaline potassium permanganate solution of the oxidizing agent and the reducing agents of ascorbic acid and ethylenediaminetetraacetic acid (EDTA) do not react with each other to form simple ions. Must. In addition, the amount of secondary waste generated at that time is enormous because it is equivalent to the volume of the system, so that chemical decontamination cannot be repeated.

Next, an example in which the method for chemically decontaminating radioactive contaminants according to the present invention is applied to a temporary cooling water system of a boiling water reactor will be described with reference to FIG. FIG. 2 shows a primary cooling water system 1 of a boiling water reactor containing a decontamination target to be decontaminated, and a radioactively contaminated decontamination target in the primary cooling water system 1. FIG. 2 is a diagram showing equipment and piping systems showing various decontamination facilities and auxiliary facilities. FIG. 3 is a characteristic diagram showing an example of the contamination removal processing according to the present embodiment in relation to the concentration and time.

In FIG. 2, reference numeral 1 denotes a primary cooling water system of a boiling water reactor to be decontaminated.
Includes a reactor pressure vessel 2 and a reactor recirculation pump 3. A reactor recirculation pump 3 draws primary cooling water from the reactor pressure vessel 2 and a reactor recirculation (PLR) pipe 4
To the reactor pressure vessel 2. In order to chemically decontaminate the radioactivity of the primary cooling water system 1, a decontamination loop as shown in FIG. 1 is used.

That is, the primary cooling water is drawn out from the control rod drive mechanism housing and the neutron monitor housing (hereinafter referred to as the housing) 14 penetrating the bottom of the reactor pressure vessel 2 by the pump 5. It is returned from the upper part of the container 2. The decontamination solution is in the chemical tank 13
Is injected directly into the liquid in the reactor pressure vessel 2 by the injection pump 8.

A part of the solution is taken out of the decontamination liquid circulation loop and passed through an ion exchange resin packed tower 10 by a purification pump 9 to remove radioactivity dissolved in the drug. In addition, at the time of decomposition of dicarboxylic acid which is a decontamination agent, a part of the water is passed through the ultraviolet lamp 11,
The carbon dioxide gas generated by decomposition is discharged from the vent skid 12.

Next, the following technical treatment process involving several steps is involved in the decontamination of the primary cooling water system 1.
These steps will be described. In the first step, (1) a primary cooling water system 1 having an operating reactor recirculation pump 3. The temperature is about 90 ° C. (2) 1000-3000mg / of dicarboxylic acid in primary cooling water
kg, hydrogen peroxide 5-50mg / kg, hydrazine 100-1000mg
/ Kg up to the concentration. (3) Decomposition of dicarboxylic acid and hydrazine by hydrogen peroxide injection and ultraviolet irradiation.

In the second step, (1) the permanganate is metered in the primary cooling water to a concentration of 100 to 1000 mg / kg. (2) Recirculation of the solution in the decontamination solution circulation loop and reactor recirculation pump 3
Operated for 5 hours for chemical decontamination. (3) Injection of dicarboxylic acid. Emission of generated carbon dioxide. (4) Removal of generated potassium ions and manganese ions by the ion exchange resin tower 10.
(5) Dicarboxylic acid added, metered to 1000-3000 mg / kg, hydrogen peroxide 5-50 mg / kg, hydrazine 100-1000 mg / kg concentration. (6) The solution is circulated in the decontamination solution circulation loop, and the reactor recirculation pump 3 is operated to chemically remove pollution for 5 hours.
(7) Removal of dissolved and ionized radioactivity and metal by ion exchange resin. (8) Decomposition of dicarboxylic acid and hydrazine by hydrogen peroxide injection and ultraviolet irradiation. (9) If necessary, repeat steps (1) to (8) in the second step (second cycle).
(10) The second step (1) to (8) is repeated as necessary (second cycle). (11) Final purification with ion exchange resin.

FIG. 3 illustrates the chemical agent concentration in ppm for each cycle. The horizontal axis is time. FIG.
In the above, a is oxalic acid, b is hydrazine, c is iron, d is permanganate, and e is chromium.

At time 0 minutes, the iron oxide causing the contamination is dissolved in the decontamination process of oxalic acid concentration of about 2000 ppm, which is started by supplying oxalic acid as dicarboxylic acid into the target system. After 300 minutes, oxalic acid and hydrazine are decomposed by ultraviolet irradiation and hydrogen peroxide injection.

Further, after about 400 minutes, that is, about 700 minutes after the start of decontamination, the potassium permanganate concentration started by supplying potassium permanganate is 500p.
Starting from pm, an oxidation treatment is performed to dissociate the structure of the chromium oxide layer that causes the contamination.

This process is indicated by a decrease in potassium permanganate concentration and an increase in chromic acid. About 200 more
After one minute, that is, about 900 minutes after the start of decontamination, oxalic acid is directly added to the potassium permanganate solution in the system. In this case, after about 1 hour (about 1000 minutes after the start of decontamination), oxalic acid reaches a concentration of 2000 ppm, and permanganate ion is reduced to divalent manganese ion.

The oxalic acid consumed here is oxidized and decomposed into carbon dioxide, and the carbon dioxide is discharged from the vent line.
After the completion of the reaction between potassium permanganate and oxalic acid, manganese ions and potassium ions are removed by introducing the solution to a purification device using a cation exchange resin in which a part of the solution has been decomposed.
When the oxalic acid concentration reaches 2000 ppm, the same process as the first decontamination process is performed.

[0030]

According to the present invention, decontamination can be efficiently achieved with an extremely small amount of chemical agent as compared with the prior art. As a result, the risk of unwanted corrosion of the base material of the structural component to be decontaminated is also reduced.

An important secondary significance is that, when removing chemical contamination, the treatment can be carried out with a lower concentration of the chemical agent, so that the amount of secondary waste generated can be significantly reduced. Furthermore, the decontamination step can be performed repeatedly,
As a result, high decontamination efficiency is obtained. In addition, all intermediate and final cleaning steps can be omitted.

[Brief description of the drawings]

FIG. 1 is a flow chart showing a basic radioactive contamination removal process for explaining an embodiment of a method for chemical decontamination of radioactive contaminants according to the present invention.

FIG. 2 is a system diagram for explaining an example in which nuclear reactor temporary cooling water is applied in the embodiment of the present invention.

FIG. 3 is a characteristic diagram showing an example of a chemical contamination removal process in the process of FIG. 2;

FIG. 4 is a flowchart showing a basic radioactivity removal process for explaining a conventional method of chemical decontamination of radioactive contaminants.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Primary cooling water system, 2 ... Reactor pressure vessel, 3 ... Reactor recirculation pump, 4 ... PLR piping, 5 ... Decontamination liquid transfer pump,
6 ... heater, 7 ... cooler, 8 ... chemical liquid injection pump, 9 ... purification pump, 10 ... ion exchange resin tower, 11 ... ultraviolet lamp,
12 ... vent skid, 13 ... chemical tank, 14 ... housing, a ... oxalic acid, b ... hydrazine, c ... iron, d ... permanganic acid, e ... chromium.

Claims (4)

[Claims] In a method for chemical decontamination of radioactive contaminants using a dicarboxylic acid and a permanganate, a dicarboxylic acid is used for reductive dissolution of an oxide incorporating radioactivity, and a permanganate is used for oxidative dissolution. A method for chemical decontamination of radioactive contaminants, comprising using a salt solution and repeating the reduction and oxidation at a liquid temperature of 100 ° C. or less. 2. The chemical decontamination method for radioactive contaminants according to claim 1, wherein hydrazine and / or hydrogen peroxide are added as a metal material corrosion inhibitor during the chemical contamination removal. 3. The chemical removal of the permanganate and dicarboxylic acid is performed by decomposing and removing the permanganate and the dicarboxylic acid in the system for performing the chemical contamination decontamination without discharging the liquid in the system. The method for chemical decontamination of radioactive contaminants according to claim 1, characterized in that: 4. The method for chemical decontamination of radioactive contaminants according to claim 1, wherein the dicarboxylic acid in the chemical decontamination treatment system is decomposed and removed by ultraviolet irradiation.