JPH0699193A - Chemical decontamination - Google Patents

Abstract

PURPOSE:To provide a chemical decontamination method capable of surely and effectively treating the primary system of a nuclear reactor and a part thereof by chemical decontamination in as a short time as possible. CONSTITUTION:A chemical decontamination method for dissolving and removing the clad of a pipeline and an apparatus in a nuclear power plant comprises the first step in which oxidizing and dissolving treatment is performed by a cleaning liquid comprising persulfuric acid and persulfate, and the second step in which a reducing agent is added to the cleaning waste liquid formed in the first stage in an excessive amount in comparison with the amount sufficient for decomposing and reducing the residual persulfuric acid and persulfate to carry out reducing and dissolving treatment. At least one kind selected from among ascorbic acid, erysorbic acid, oxalic acid, salts and derivaties thereof is used as the reducing agent.

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, and more particularly to a chemical decontamination method for dissolving and removing a clad (which means a metal oxide) of a nuclear power plant.

[0002]

2. Description of the Related Art In a piping equipment system of a nuclear power generation facility, a clad containing a radioactive material adheres to an inner surface of a pipe or the like by its operation. As a result, the radiation dose around the pipes and equipment is high, and the radiation dose is increased during the periodic inspection work, causing a problem.

[0003] As a countermeasure, in some cases, chemical decontamination is carried out to obtain a considerable effect. However, although various chemical decontamination methods have been proposed, when decontaminating an object consisting of a clad with a high content of chromium oxide,
Alkaline permanganate (referred to as AP method) is mainly used as an oxidizing and dissolving agent, and then it is often carried out in combination with various reducing and dissolving methods. However, there is a demand for a chemical decontamination method that has a higher decontamination effect than the current state and is easy to treat waste liquid. If the decontamination effect increases, the total dose equivalent will be reduced accordingly. The exposure dose in the decontamination work is also reduced by simplifying the process, which is also an important improvement item.

[0004]

Therefore, an object of the present invention is to perform chemical decontamination on the primary system of a nuclear reactor and a part thereof so that chemical decontamination can be performed reliably and effectively in as short a time as possible. It is to provide a dyeing method.

[0005]

In order to solve the above problems, in the present invention, in a chemical decontamination method for dissolving and removing the piping of a nuclear power plant and the cladding of equipment, a cleaning solution using persulfuric acid and persulfate. And a second step in which a reducing agent is added to the cleaning waste liquid in the first step in an amount larger than that for decomposing and reducing the residual persulfuric acid and persulfate, and a reducing and dissolving treatment is performed. It is supposed to consist of. In the above method, as the reducing agent, one or more selected from ascorbic acid, erythorbic acid, oxalic acid, salts thereof and derivatives thereof can be used, and in the second step, excess persulfate Sulfuric acid is generated by reductive decomposition, but sulfuric acid or sulfamic acid may be additionally added if necessary.

Next, the present invention will be described in more detail. According to the invention, persulfuric acid and persulfates are used for the oxidation treatment.
As a result, the chromium oxide (trivalent) in the clad is oxidized and dissolved by the persulfate into soluble chromic acid (hexavalent). Accordingly, persulfuric acid and persulfate are decomposed to generate sulfuric acid and sulfuric acid and sulfate. Aqueous solutions of persulfuric acid and persulfate start reacting rapidly when the temperature exceeds 70 ° C.
It causes the above decomposition. By this decomposition, sulfuric acid is produced as a by-product.
When the dissolution of chromium stops and it is confirmed that the persulfate remains, the oxidation treatment step is completed. Persulfate must always be present during this step. By doing so, corrosion of stainless steel can be suppressed.

Next, after the oxidation treatment step, the process proceeds to the acid washing step. However, adding a reducing agent in the acid washing accelerates the dissolution of iron oxide, and this method is usually used. It is said. According to this method, the reducing agent is added so as to have a concentration of about 0.3 to 1% in excess of the equivalent amount of the remaining amount of persulfuric acid and persulfate.
By this treatment, all persulfuric acid and persulfate become sulfuric acid and sulfuric acid and sulfate. The sulfuric acid generated here is used as a clad dissolving agent. That is, the conditions for reduction cleaning are set. In this process, chromium oxide is dissolved and removed first,
The structure of the clad is altered and changes to one that is easy to melt. Iron and nickel and oxides of other heavy metals are dissolved by reducing and washing with sulfuric acid to dissolve the clad.

For example, this 1% sodium persulfate decomposes into 0.41% sulfuric acid and 0.59% sodium sulfate. When 3% sodium persulfate is used in the oxidation treatment step, the sulfuric acid finally produced is 1.23%. If necessary, sulfuric acid can be added to this to increase the concentration, which is a good measure for increasing the dissolving power of the clad.

[0009]

According to the decontamination method of the present invention, the dissolution of the hardly soluble chromium oxide is almost the same as that of the AP method, and it was confirmed that it is effective as an oxidative dissolution treatment method. Since it is not necessary to use potassium permanganate, which is a metal acid, the solubility of this salt in water is low and the increase in exposure dose due to the time required for on-site work is reduced by this method. To be done. Further, in general, when the concentration of heavy metal ions eluted in decontamination is about 200 to 500 mg / liter, in the case of a decontamination method using permanganate, for example, potassium permanganate 0.
A concentration of 1 mg of manganese ions of 350 mg / liter occupies a large specific gravity in the total concentration of the waste liquid, resulting in an increase in waste. Therefore, this method also produces a remarkable action and effect in reducing the volume of waste materials in this respect as well.

[0010]

EXAMPLES The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples. Example 1 Next, an example of the present invention will be described. It is generally very difficult to actually cut out the water system piping of the nuclear power generation facility and investigate the clad. In other words, the pipes to which the clad containing radioactivity is attached are under strict control even if they exist, so it is generally extremely difficult to obtain the pipes themselves.

Therefore, it is the actual situation that a simulated clad is prepared, various solubility tests using this clad are carried out, and comparative examinations are conducted. The composition of the prepared simulated clad is as shown in Table 1.

[0012]

[Table 1]

The test procedure for the solubility of the simulated cladding is as follows. Into a 500 ml glass container with a stopper, 500 ml of a cleaning agent is placed, and 0.500 g of a simulated clad is precisely weighed and put into this. In order to keep the dissolution conditions constant, a magnetic stirrer is used and stirring is applied at a constant rotation speed. The temperature adjustment varies depending on the condition of each cleaning agent, but using a constant temperature water bath that can be adjusted to each temperature ± 1 of the set temperature
Controlled at ° C. When the simulated cladding is completely melted, Table 2
The concentration will be as shown in.

[0014]

[Table 2]

Taking 0.500 g of the simulated clad, Table 3
Solubility was tested under the detergent and treatment conditions shown in.

[Table 3]

The results are shown in Table 4 below.

[Table 4] Since the concentration in the reduction dissolution step is continuous in the treatment step, it is the total elution concentration including the concentration of metal ions dissolved in the oxidation step. In actual decontamination, based on these selection results,
If there is an actual product, it is desirable to be able to actually use the product itself and perform a solubility confirmation test of the clad in the hot laboratory.

Comparative Example 1 0.500 g of the simulated clad was taken and the solubility of hydrochloric acid was tested at a concentration of 5% at 60 ° C. and 95 ° C. The results are shown in Table 5.
Shown in.

[Table 5] Even with the treatment of 5% concentration of hydrochloric acid, which is a typical chemical cleaning, the dissolved concentration of the treatment for 6 hours is about 30% even at 95 ° C., and it can be judged that the simulated cladding is quite insoluble.

Comparative Example 2 The alkaline potassium permanganate (referred to as AP method) method was used as an oxidative dissolution treatment step which is usually carried out as a decontamination method.
A treatment was carried out in which a step of treating at 0 ° C. for 6 hours and a reducing dissolution method of treating at 5 ° C. for 6 hours as a 5% solution of sulfuric acid containing a reducing agent were combined. The conditions at that time are as shown in Table 6.

[0019]

[Table 6]

The results are shown in Table 7.

[Table 7]

[0021]

EFFECTS OF THE INVENTION According to the method described above, it is possible to dissolve even the insoluble clad better than the conventional method, and the treatment time is shortened, so that the exposure dose of the person engaged in the work is reduced. There is a great advantage over the conventional method in that In addition, the waste solution discharged by decontamination can be oxidized and reduced in the same solution, so the volume of the generated waste can be reduced and metal acid salts such as permanganate should not be used. As a result, the concentration of heavy metal ions in the decontamination waste liquid can be reduced, and waste can be reduced. The treated waste liquid can be treated in the same way as the ion exchange regeneration waste liquid of the existing waste treatment facility of the nuclear power plant, and the volume of the waste can be greatly reduced.

Claims (3)

[Claims] 1. A chemical decontamination method for dissolving and removing piping of a nuclear power plant and a clad of equipment, wherein a first step of performing an oxidative dissolution treatment with a cleaning solution using persulfuric acid and a persulfate, and a first step A chemical decontamination method comprising a second step of performing a reducing dissolution treatment by adding a reducing agent to the cleaning waste liquid in an amount larger than that for decomposing and reducing the residual persulfuric acid and persulfate. 2. The reducing agent is ascorbic acid,
The chemical decontamination method according to claim 1, wherein at least one selected from erythorbic acid, oxalic acid, salts thereof and derivatives thereof is used. 3. The chemical decontamination method according to claim 1, wherein sulfuric acid or sulfamic acid is further added in the second step.