JPH0361900A - Chemical decontamination of stainless steel - Google Patents

Abstract

PURPOSE:To decontaminate radiation metallic waste having a narrow part where liquid is hard to pass readily in a short time by forming a through-hole at the narrow part where the liquid is hard to pass with a mechanical means, thereafter performing decontaminating steps comprising two steps. CONSTITUTION:At first, a through-hole 7 is formed from the outside at a narrow part where liquid is hard to pass in a radioactive metallic waste 5 by using a drill 6. Thereafter, the waste is immersed in sulfuric acid solution which is the first liquid in a first decontaminating tank 1. At this time, 90% or more of radioactive material is removed in the first decontaminating tank 1 by the dissolution of the exposed base material and the peeling of a clad which is fixed to the surface due to the dissolution. Then the deposited material on the surface of the material to be decontaminated is dissolved. For this purpose, the waste is immersed in a second decontaminating tank 3. In this tank, liquid wherein oxidizing meatal salt is added in sulfuric acid is inputted as second liquid. The radioactive waste of stainless steel which is made inoperative in this way is decontaminated to the same radioactive level as that of ordinary waste. The waste can be discarded after washing with water.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for chemically decontaminating radioactive metal waste whose surface is contaminated with radioactive materials, which is generated from facilities handling radioactive materials. This invention relates to a chemical decontamination method for radioactive metal waste made of stainless steel that has narrow parts that are difficult to distribute.

[Conventional technology]

Metal waste contaminated with radioactive materials generated from nuclear power plants, nuclear fuel reprocessing plants, etc. has traditionally been cut into drums and stored in waste buildings. For this reason, it is desired to reduce the volume of the radioactive metal waste.

Current volume reduction techniques such as electrolytic polishing decontamination method and blast decontamination method are known, but these methods require insertion of electrodes or nozzles into the object to be decontaminated, and pulp, pumps, etc. It is difficult to handle objects with complex shapes such as Chemical decontamination methods are decontamination methods that can be applied to objects with such complex shapes, but many of the chemical decontamination methods that have been developed so far adhere firmly to the surface of metal waste and remove radioactive materials. The aim is to dissolve the crud that incorporates most of the

However, pitting corrosion occurs on the surface layer of the metal base material to which this cladding is attached, and it is said that the surface layer of the base material is also contaminated because radioactive materials have penetrated deep into this layer. ing. Therefore, chemical dissolution of only the surface cladding is insufficient to remove this radioactive material, and there is a problem in that it cannot be decontaminated to a level of radioactivity comparable to that of general waste.

As a way to solve this problem, the applicant has proposed
In the specification of No. 2-21608, the first step is to immerse radioactive metal waste in a sulfuric acid solution to dissolve or peel off most of the radioactive materials, and to remove copper and chromium, which are difficult to dissolve in a sulfuric acid solution alone. We propose a decontamination method for radioactive metal waste that consists of a second step of immersion in a solution of sulfuric acid and oxidizing metal salts for the purpose of dissolving substances deposited on the surface of the object to be decontaminated, such as oxides. did.

Furthermore, the applicant has filed a patent application for a method of chemically removing passivated stainless steel radioactive metal waste to a radioactivity level comparable to that of general waste, using the above method.
In the specification of No. 3-172271, it was proposed to mechanically grind a portion of the surface of stainless steel, which is an object to be decontaminated, using a grinder, a blast grinder, or the like to remove a portion of the passive film.

According to these methods, complex-shaped objects such as valves and pumps can be decontaminated to a radioactivity level comparable to that of general waste, and can be applied to non-1*M coatings such as stainless steel.

[Problem to be solved by the invention]

However, if the object to be decontaminated has a narrow part where it is difficult for liquid to flow through it, even if the above method is used, the mixing properties of the decontamination liquid will be poor when decontaminating the inner surface, and decontamination will take an extremely long time. In other words, in the above method, the object to be decontaminated is immersed in the decontamination liquid and the metal surface is dissolved by a chemical reaction. In narrow areas where circulation is difficult, the fluidity of the liquid is poor.
Convection occurs due to slight bubbles on the surface and temperature differences,
The problem is that the liquids are only mixed, and the diffusion of dissolved metal ions, which are reaction products, and the renewal of hydrogen ions, which are reactants, are slow, and decontamination takes a considerable amount of time.

Therefore, an object of the present invention is to provide a decontamination method that overcomes the drawbacks of the prior art and can easily decontaminate narrow areas where liquid flow is difficult in a short time.

[Means to solve the problem]

The present invention solves the above problem by drilling a through hole from the outside in a narrow portion where it is difficult for liquid to flow. That is, the chemical decontamination method for stainless steel according to the present invention includes a first step of immersing metal waste made of stainless steel whose surface is contaminated with radioactive substances and has narrow parts through which liquid can flow into a sulfuric acid solution. In the chemical decontamination method for stainless steel, which consists of a second step of immersing the steel in an aqueous solution containing sulfuric acid and an oxidizing metal salt, through-holes are made by mechanical means in narrow areas where it is difficult for the liquid to flow. After that, a decontamination step consisting of the first and second steps described above is carried out.

When a through-hole is drilled from the outside in a narrow area like this, even if the hole is narrow, the base material will be exposed on the side of the hole, and a local battery will be formed between the oxide film and the oxide film. It is easy for dissolution to occur due to reduction of
Decontamination can be carried out efficiently. Further, if the hole is thin, it is easy to penetrate it by mechanical means, and for example, the through hole can be made using a drill or the like.

Next, each step in the method of the present invention will be explained.

In the method of the present invention, as described above, first, as a pretreatment step, a through hole is drilled in a narrow portion of the object to be decontaminated, where it is difficult for liquid to flow through the object. It is preferable that the through-hole be drilled in the deepest part of the narrow part where it is difficult for liquid to flow.

There are no particular restrictions on the method for making the hole, and for example, a drill or the like may be used. The larger the number of holes, the greater the effect, but it is sufficient if there is at least one hole per narrow portion. Also, the larger the hole diameter, the greater the effect;
When the diameter of the hole becomes large, it takes a lot of effort to penetrate the hole. On the other hand, if it is too thin, there are problems such as the drill breaking.

Therefore, the number and diameter of the holes may be appropriately determined in consideration of the length of the narrow portion, etc.

Next, in the first step, melting with sulfuric acid is performed, and the sulfuric acid solution has a concentration of 5% by weight or more, preferably 5% to 5% by weight.
Using a sulfuric acid solution with a concentration of 30%, the solution was heated at 60°
The object to be decontaminated is heated to a temperature of C or higher and immersed. It is not effective if the sulfuric acid concentration is too low, and it is also not effective if the sulfuric acid concentration is too low.

Furthermore, at high temperatures of 100°C or higher, a pressure vessel is required and the equipment becomes complicated, so the practical decontamination conditions are preferably a sulfuric acid concentration of about 15% by weight and a temperature of about 80'C.

Furthermore, in the second step, decontamination is performed using an aqueous solution prepared by adding an oxidizing metal salt to sulfuric acid. The sulfuric acid used here may have the same concentration as that used in the first step.

Examples of oxidizing metal salts include hexavalent chromates such as potassium chromate, dichromates such as potassium dichromate, tetravalent cerium salts such as ceric sulfate, and permanganate. Acid salts such as potassium permanganate can be used. These oxidizing metal salts are added at a concentration of 1 to 6 weight percent, preferably 2 to 4 weight percent. The solution used in the second step is 30°
It is preferable to use it by heating it to a temperature of C or higher.

Next, the present invention will be explained based on the drawings.

FIG. 1 is a system diagram illustrating the method of the present invention.

In FIG. 1, sulfuric acid is used as the first liquid in the first decontamination tank l. The first liquid is heated to a predetermined temperature by the heater 2. In addition, in the second decontamination tank 3, a liquid added with the above-mentioned oxidizing metal salt is used as the second liquid.
The liquid is also heated to a predetermined temperature by the heater 4.

In FIG. 1, the radioactive metal waste 5 is first drilled into the drill 6.
The through hole 7 is inserted from the outside into a narrow area where it is difficult for liquid to flow.
Open. Note that the mechanical means for making the through hole is not limited to a drill, and may be any other tool.

Thereafter, it is immersed in a sulfuric acid solution, which is the first liquid, in the first decontamination tank 1. At this time, in the first decontamination tank 1, more than 90% of the radioactive substances are removed by dissolving the exposed base material and peeling off the cladding adhered to the surface.

However, when stainless steel is dissolved by a chemical reaction with a sulfuric acid solution, the copper contained in the stainless steel as an impurity and some of the cladding that remains without being peeled off are deposited on the surface of the object to be decontaminated. - Since some of the radioactive substances that have been eluted are taken in, a second solution containing sulfuric acid and an oxidizing metal salt is added as a second solution to dissolve the deposits on the surface of the object to be decontaminated. Immerse in decontamination tank 3. In this way, the passivated stainless steel radioactive waste is decontaminated to a radioactivity level comparable to that of general waste, and can be disposed of after washing with water.

〔Example〕

Next, the present invention will be described in detail based on Examples, but the present invention is not limited thereto.

Example 1 5US304 contaminated with radioactive substances and rendered inactive
A through hole was drilled from the outside in a narrow part of the manufactured valve where it is difficult for liquid to flow.

Furthermore, according to the system diagram shown in FIG. 1, decontamination was performed using the first decontamination liquid and the second decontamination liquid.

In the case of decontamination according to the system diagram shown in Figure 1 (method of the present invention) and the case of decontamination without making through holes (conventional method), the radioactivity was at the detection limit (1 × 10-5 μCi/cn
l) We compared the decontamination time until the following. Decontamination conditions are shown in Table 1, and decontamination results are shown in Table 2.

Table 1 Table 2 Decontamination time As is clear from the above results, in the conventional method, in the second step, the dissolution rate in the second liquid is slow, so the decontamination time is long. In contrast, in the method of the present invention, since the base material is exposed on the wall surface of the through hole, local batteries occur between the base material and the contaminated oxide film on the inner surface of the object to be decontaminated. The reaction is promoted in the liquid, and as a result, the load on the second liquid is reduced and the reaction time in the second liquid is significantly shortened.

In addition, the through holes promote liquid distribution and bubble diffusion, so that the reaction proceeds quickly and the overall reaction time becomes faster.

〔Effect of the invention〕

According to the method of the present invention, it is possible to easily chemically decontaminate radioactive metal waste made of passivated stainless steel and having narrow parts where it is difficult for liquid to flow to a level of radioactivity comparable to that of general waste. The required decontamination time can also be significantly shortened, and efficient decontamination can be achieved even for complex-shaped radioactive metal waste.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing one embodiment of the method of the present invention. Explanation of symbols■...First decontamination tank, 2 and 4...Heater, 3...
Second decontamination tank, 5...Radioactive metal waste, 6...Drill, 7...Through hole

Claims (2)

[Claims] (1) The first step is to immerse stainless steel metal waste, whose surface is contaminated with radioactive substances and has narrow areas where liquid flow is difficult, into a sulfuric acid solution, and the addition of an oxidizing metal salt to the sulfuric acid. In the chemical decontamination method for stainless steel, which consists of a second step of immersing the stainless steel in an aqueous solution, after making a through hole by mechanical means in a narrow part where it is difficult for the liquid to flow, A method for chemical decontamination of stainless steel, characterized by carrying out a decontamination process consisting of: (2) The method for chemical decontamination of stainless steel according to claim 1, wherein the through hole is formed in the deepest part of the narrow portion.