JPS6050320B2 - Decontamination method for equipment handling radioactive materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for decontaminating radioactive material handling equipment such as equipment and piping of a nuclear reactor facility.

Generally, in the primary cooling system of a nuclear reactor facility, as shown in Figure 1, the radiation dose rate increases from 100 to 150 over the course of operation.
It increases at about mR/re/EFPY.

An increase in the radiation dose rate increases the exposure dose of workers and employees during maintenance and inspection of nuclear reactor facilities, and also impedes the maintenance of nuclear reactor facilities. This increase in radiation dose rate is due to an increase in the amount of corrosion products containing radioactive substances adhering to the surfaces of equipment, piping, and the like. For this reason, corrosion products are decontaminated during maintenance. This decontamination method can be broadly divided into mechanical decontamination and chemical decontamination.
The decontamination efficiency, expressed as a decontamination factor (DF), is approximately 1.5 and 1.5 to 1000, respectively.

Mechanical decontamination methods include methods that use brushing, ultrasound, and jet water, but brushing and ultrasound methods alone cannot be expected to have a sufficient decontamination effect. In addition, the method using jet water can be sufficiently effective by increasing the water pressure sufficiently and spraying it onto the object to be decontaminated. On the other hand, chemical decontamination can be roughly divided into methods that use strong decontamination solutions, methods that use weak decontamination solutions, and methods that use electrolysis. You can expect it.

As an example of pipe decontamination, Table 1 shows the residual radioactivity percentage when oxalic acid cleaning is performed after ultrasonic cleaning J, and then ultrasonic cleaning is performed again. This shows that a large decontamination effect can be obtained, and that an even more sufficient decontamination effect can be obtained by combining ultrasonic cleaning, which is a mechanical decontamination method.

Table 1 also means the following:

When the object to be decontaminated, such as equipment or piping, is made of copper material, corrosion products are formed on the surface of the object to be decontaminated 1 in two layers, a deep layer 2 and a surface layer 3, as shown in Figure 2. , CO-60, Mn-
In the surface layer 3, radioactive substances 4 such as No. 54 adhere to cladding of triiron tetroxide (Fe3O4) and iron sesquioxide (Fe2O3), and this surface layer is in a form that is relatively easy to remove. It can also be easily decontaminated by mechanical decontamination methods such as sonic cleaning. On the other hand, the deep layer 2, which contains most of the total radioactivity (~85%), is the oxide film of the decontaminated object 1, and the radioactive material 4 is firmly held in a stable form as a spinel compound (e.g. COFe2O4). Therefore, the deep layer 2 can be removed by eluting it by chemical decontamination such as oxalic acid cleaning shown in Table 1, or by peeling it off with high-pressure jet water. However, the above-mentioned conventional technology causes the following problems.

When deep layer 2 is removed by chemical decontamination etc. as mentioned above,
Decontaminate surfaces locally. It becomes very rough and may cause craters. When this was returned to the reactor facility and started operation, the surface of the object to be decontaminated was rough and the adhesion area was very large, so as shown in Figure 3, the surface of the object to be decontaminated was is rapidly oxidized to form a film 5, and this oxidation! Since the radioactive substances 4 in the cooling water are incorporated into the coating 5, the radiation dose rate on the surface of the object to be decontaminated rapidly increases. This rate of increase was approximately twice that of the case without decontamination, and in the end, the radiation dose rate was sometimes higher in the case of decontamination. In this way, conventional technology has the effect of temporarily reducing the exposure dose during decontamination maintenance, but in the long term it increases the dose level, so radioactive There is a risk that the integrity of the material handling facility may be reduced, making it difficult to maintain the facility. In view of the problems of the prior art described above, the present invention makes it possible to reduce as much as possible the amount of radioactive substances that adhere to objects to be decontaminated after returning them to the facility, thereby increasing the radiation dose rate. The purpose of this study is to provide a decontamination method for equipment handling radioactive materials that can suppress

In order to achieve this objective, the present invention ``After removing the radioactive substances attached to the object to be decontaminated, the exposed metal surface of the object to be decontaminated is forcibly oxidized in advance. This system is characterized by preventing oxidation while incorporating radioactive materials when the decontamination target is incorporated into a facility.

The present invention will be explained below using detailed experimental examples.

In the experiment, the test pieces were made of carbon steel and had a width, length, and thickness of 10T each! N,5OTTr! Using N.3 Gourd, the following chemical decontamination treatment and oxidation treatment were performed.
[Chemical decontamination treatment] (1) Alkaline treatment As the alkaline treatment liquid, use Turco Decon 4502 (trade name, manufactured by Turco Co., Ltd.), put it in a beaker, and heat it for 80~
The test piece was heated to 90°C and placed in it, immersed for about 4 hours, and then taken out using a beaker.
It was washed with water until it no longer showed alkalinity.

(2) Acid treatment As an acid treatment solution, Turco Decon 45 (trade name) manufactured by Turco Co., Ltd.
21, put it in a beaker and heat it to 80-90℃
The test piece after the alkali treatment was placed therein and immersed for about 6 hours, and then the test piece was taken out from the beaker and washed with water until it no longer showed acidity.

[Generation of oxide film]

(1) Place a part of the test piece that has been chemically decontaminated as described above into an autoclave equipped with a water quality maintenance device, fill the autoclave with degassed pure water, and raise the temperature to about 120°C for about 20 seconds. By holding the sample for a while, an oxide film (Fe3O4) having a thickness of several microns was formed.

(2) Next, oxygen is injected into the autoclave to bring the oxygen concentration in the pure water to about 50 ppm, and the water is kept for about 50 hours to make the surface of Fe3O4 more chemically stable Fe2O3, and it is added to the liquid containing radioactive substances. To reduce the amount of radioactive material taken in when exposed to

Next, in order to confirm the effect of the oxide film formation, the test piece that had undergone the chemical decontamination treatment and the test piece that had an oxide film formed as before after the chemical decontamination treatment were treated with water containing radioactive cobalt. The reactor was placed in a filled autoclave, and the temperature was raised to approximately 280'C, which is the reactor water temperature, and maintained.

After about 5 months, the degree of radioactive contamination of the test piece was as shown in Table 2 below. As described above, the product on which an oxide film is formed on the surface according to the present invention has a significantly lower degree of radioactive contamination than the conventional product on which no oxide film is formed.

The reason for this is thought to be as follows. Those that did not form an oxide film were converted to Fe(0H) during the oxidation process under reactor water conditions.
2, Fe(0H)3 is produced, which adsorbs cobalt well to form a compound and becomes radioactive.On the other hand, the material that forms an oxide film 6 on the metal surface as shown in Fig. 4 according to the present invention is radioactive. Fe(0H)2 in the absence of cobalt
, Fe(0H)3 to generate Fe3O4, and this oxide film (Fe3O4) has strong adhesion to the Fe surface and has a large cohesive force such as crystallization, and forms a dense film to form a test piece. This is thought to be due to the fact that the corrosion rate is rate-limited even when immersed in reactor water conditions, and the amount of corrosion is reduced, and since the oxide film is generated from within, the amount of surface layer 3 formed is reduced. Note that the above-mentioned embodiments are merely examples, and various treatment liquids and treatment methods can be selected for the chemical decontamination treatment.

In addition to chemical decontamination, this decontamination process can also employ mechanical decontamination using high-pressure jet water. Furthermore, by performing both chemical and mechanical decontamination, efficient decontamination can be achieved. can be done. In addition to the above examples, forced oxidation of the surface of the object to be decontaminated can be carried out by, for example, immersing the object in a solution containing an oxidizing agent such as hydrogen peroxide, or by oxidizing the object at low temperatures in oxygen or clean air. For example, any method that can form an oxide film in advance may be used, such as heating at 100° C. or anodic polarization in a solution containing an oxidizing agent. As described above, the decontamination method of the present invention is to forcibly generate an oxide film after decontamination, and compared to conventional methods that do not generate an oxide film, recontamination is significantly reduced. I can do it.

Therefore, according to the present invention, the radiation dose during maintenance and inspection in a facility that handles radioactive materials such as a nuclear reactor is reduced, maintainability is improved, and maintenance of the facility is facilitated. Furthermore, since the interval between decontamination can be extended, maintenance costs for the facility can be reduced.・Brief explanation of the drawings Figure 1 shows the change in radiation dose rate over time in the reactor primary cooling system, Figure 2 is a cross-sectional view showing the oxide layer formed in piping, etc., and Figure 3 shows the conventional removal method. FIG. 4 is a cross-sectional view showing the formation of an oxide layer after decontamination according to the present invention.

1... Body to be decontaminated, 2... Deep layer, 3...
... Surface layer, 4 ... Radioactive substance, 6 ... Oxide film.

Claims (1)

[Claims] 1. Decontaminating steel members of equipment that handles radioactive materials by at least one of chemical means and mechanical means using high-pressure jet water to remove radioactive materials on their surfaces. 1. A method for decontaminating radioactive material handling equipment, which comprises forcibly forming an oxide film on the surface of the member in a state in which there is substantially no radioactive material after removing the radioactive material. 2 The operation of forcibly forming the oxide film is to form an oxide film made of triiron tetroxide on the surface of the steel member, and then oxidize the surface of the oxide film to more stable iron sesquioxide. A method for decontaminating radioactive material equipment according to claim 1, characterized in that: