JPH05223988A - Chemical decontamination method of radioactive metal waste - Google Patents

Abstract

PURPOSE:To quickly and efficiently cool radioactive metal waste down to a radiation level of general in simple operation and a simple device by rolling the radioactive metal in the solution of sulfuric acid together with ceramic abrasive bodies or metal-made abrasive bodies. CONSTITUTION:Radioactive metal waste 1 is put in a rotation case 3 together with abrasive bodies 2 and rolled by the use of a drive device 4 in a decontamination tank 6 containing sulfuric acid solution 5. In addition, the solution 5 is circulated through a filter 8 by the use of a circulation pump 7 and insoluble crud components are removed. And with crud scaled off and removed with the rolling of the abrasive bodies 2, metal base material is exposed in activity and, with the acid dissolved, decontamination proceeds quickly. Thereby easy and quick cooling can be performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for chemical decontamination of radioactive metal waste generated from a radioactive material handling facility whose surface is contaminated with radioactive material. The present invention relates to a chemical decontamination method for radioactive waste metal made of stainless steel that has penetrated not only to the clad (polluted oxide film) that has adhered but also to the inside of the surface layer of the metal base material.

[0002]

2. Description of the Related Art Metal waste contaminated with radioactive materials generated from nuclear power plants, nuclear fuel reprocessing plants, etc.
It is cut into drums and stored in a waste building, and the accumulated amount is increasing year by year. Therefore, it is desired to reduce the volume of radioactive metal waste. As the current volume reduction technology, high compression volume reduction, melt volume reduction, etc. have been developed, but since these not only treat the contaminated portion of the surface but also most of the uncontaminated portion, a large volume reduction effect is obtained. I can't expect.

On the other hand, the decontamination / volume reduction treatment, which removes the surface contaminated part and decontaminates most of the uncontaminated part until it can be disposed of as much as general waste, is regarded as a promising technique for reducing the volume of metal waste. ing. As such decontamination / volume reduction techniques, electro-decontamination method, blast decontamination method, etc. have been developed. However, in these methods, it is necessary to insert electrodes or nozzles into the decontamination target. It is difficult to deal with complicated shapes such as valves, pumps, etc. As a decontamination method applicable to such a complicated shape, there is a chemical decontamination method in which a decontamination target is immersed in a chemical solution to dissolve the surface of the metal waste.

Further, the radioactive contamination is not limited to the cladding,
In order to decontaminate radioactive metal waste that is also present in the metal base material to a level of radioactivity equivalent to general waste, a chemical decontamination method that efficiently dissolves and removes both the clad and the metal base material is required. In general, stainless steel metal waste has a dense passivation film, and therefore acid dissolution hardly progresses by merely immersing it in a strong acid solution such as sulfuric acid. Therefore, a decontamination method has been developed in which a metal base material made of stainless steel is oxidatively dissolved by using a solution in which an oxidizing agent such as tetravalent cerium is added to nitric acid or sulfuric acid, and the clad is peeled and removed. However, in this method, a large amount of expensive tetravalent cerium is consumed, and therefore it is necessary to regenerate the cerium by an electrolytic operation, which causes a problem that the apparatus and the operation are complicated.

Further, a method of reducing and dissolving the clad and the passive film on the surface by an electrochemical method using an external power source, and then performing acid dissolution (JP-A-2-22597).
A method of mechanically grinding a part of metal or metal waste such as blasting, and then dissolving the acid (JP-A-2-22596)
Is proposed. But even with these methods,
Since the pretreatment operation is performed before the acid dissolution, the operation and the apparatus become complicated. Further, in particular, after a mechanical pretreatment operation such as blasting, if there is some time before the acid dissolution step, a dense passivation film is formed by oxygen in the air,
Subsequent acid dissolution cannot proceed reliably.

[0006]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art, and enables radioactive metal waste made of stainless steel to be quickly and efficiently operated by a simple operation and device at a level of radioactivity equivalent to that of general waste. An object is to provide a chemical decontamination method capable of decontaminating up to.

[0007]

The present invention has achieved the above object by carrying out acid dissolution simultaneously with mechanical grinding. That is, the method for chemical decontamination of radioactive metal waste according to the present invention is characterized in that the radioactive metal waste is rolled in a sulfuric acid solution together with the grinding body. In order to carry out the method of the present invention, the radioactive metal waste may be put in a container such as a barrel together with the grinding body in a sulfuric acid solution, and rotated several tens of minutes per minute.

According to the present invention, the radioactive metal waste is rolled together with the grinding body in the sulfuric acid solution to mechanically remove the clad and the passivation film, and the exposed metal base material is uniformly contacted with sulfuric acid. Further, the metal base material (when the carbon steel ground body is used, the carbon steel ground body) also reacts as in the following formula (1), and Fe + H ₂ SO ₄ → Fe ²⁺ + SO ₄ ^{2 - +} 2H + ⁺ 2e - reaction with, Fe ₂ O ₃ as the main component of the passive film and the cladding remaining is by the following equation (2) ^- (1) electrons generated by the reaction (e) Then, Fe ₂ O ₃ + 6H ⁺ + 2e ⁻ → 2Fe ²⁺ + 3H ₂ O (2) Due to the reduction and dissolution, the cladding and the passive film are chemically dissolved and removed by the action of a so-called local battery. The present invention is configured to achieve rapid decontamination by the above-mentioned interactions (1) to (3).

In the present invention, as the grinding body, any material made of various materials can be used as long as it has a hardness and strength capable of peeling the clad and grinding the passivation film. Alternatively, those made of metal are preferable. The use of the ceramic grinding body has the effect of extending the life of the decontamination liquid because the grinding body is not dissolved in the decontamination liquid. Further, when the metal grinding body is used for the stainless steel metal waste, it is preferably made of a metal having a base potential lower than that of the stainless steel metal waste. As the grinding body, a grinding body made of carbon steel, which is a material easily corroded by sulfuric acid, is used, and the reduction dissolution shown in the formulas (1) and (2) is promoted by imparting the effect of a sacrificial anode by contact with different metals. be able to.

The shape of the grinding body may be any shape as long as it has an acute-angled portion or a protrusion that can peel off the clad and grind the passivation film when it collides with the substance to be decontaminated. For example, a triangular pyramid or a cube is preferable.

In the method of the present invention, a sulfuric acid solution having a concentration of 5% by weight or more is used as the sulfuric acid solution. If the sulfuric acid concentration is too low, it will not be effective and the sulfuric acid concentration will be 5
It is preferably 30% by weight and most preferably 10 to 20% by weight. Further, it is preferable that the sulfuric acid solution is generally heated to 60 ° C. or higher to promote the above dissolution reaction. However, if the liquid temperature exceeds 100 ° C., a high-pressure container is required and the apparatus becomes complicated, so a temperature of about 80 ° C. is desirable.

Next, the method of the present invention will be described more specifically with reference to the drawings. FIG. 1 is a system diagram showing an example of an apparatus for carrying out the method of the present invention. In FIG. 1, the radioactive metal waste 1 is a rotating basket (barrel) 3 together with the grinding body 2.
It is placed inside and is rolled in the decontamination tank 6 containing the sulfuric acid decontamination solution 5 by the drive unit 4. The decontamination liquid 5 is circulated through the filter 8 by the circulation pump 7 to remove the insoluble clad component. Further, the decontamination liquid 5 can be heated by the heater 9.

As described above, by rolling the radioactive metal waste 1 and the grinding body 2 in the sulfuric acid decontaminating solution 5, the grinding body 2 peels and removes the clad to expose the metal base material. Since the metal base material after being ground is in an active state and is acid-dissolved by sulfuric acid, the clad is peeled and removed together with the removal effect of the grinding body, and decontamination proceeds rapidly. That is, in the case of stainless steel, in many cases, a strong passive film is formed under the clad, but this film is also mechanically ground by the grinding body to reveal the metal base material, and the above-mentioned formula (1) is used. The electrons (e ⁻ ) promote the reduction and dissolution of the formula (2), and the decontamination proceeds.

After the chemical decontamination is completed in this way, the metal waste 1 is sent to the next step such as water washing together with the barrel 3 by the crane 10.

[0015]

EXAMPLES The present invention will now be described in detail based on examples, but the present invention is not limited thereto.

Example 1 A 15% by weight sulfuric acid solution was used as a decontamination solution, and this was used at a liquid temperature of 8
Figure 1 shows a barrel that is heated to 0 ° C and contains a ceramics grinding body with a triangular cross section and stainless steel radioactive metal waste.
Decontamination was performed by introducing the device into the device shown in. At this time, rolling was performed 30 revolutions per minute. When tumbling started, dissolution proceeded rapidly, and decontamination could be completed in about 20 minutes. The relationship between the decontamination time and the relative radioactivity is shown as a curve a in FIG.

For comparison, decontamination was performed under the same conditions, except that the metal waste was allowed to stand in the decontamination solution without rolling and the solution was simply circulated. Was measured, and the result shown as a curve b in FIG. 2 was obtained. As is clear from the curve b, the metal waste was hardly dissolved and decontamination hardly proceeded.

[0018]

According to the method of the present invention, decontamination of stainless steel metal waste having a passivation film or a clad with extremely simple operation and equipment to a level of radioactivity equivalent to that of general waste quickly and efficiently. can do.

[Brief description of drawings]

FIG. 1 is a system diagram showing an embodiment of an apparatus for carrying out the method of the present invention.

FIG. 2 is a graph showing the time course of the relative radioactivity obtained in Example 1.

[Explanation of symbols]

1 ... Radioactive metal waste, 2 ... Grinding body, 3 ...
Barrel, 4 ... Driving device, 5 ... Decontamination liquid, 6 ...
Decontamination tank, 8 ... Filter, 9 ... Heater

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Haruki Miura 1-1-14, Kanda, Chiyoda-ku, Tokyo Inside Hirit Plant Construction Co., Ltd.

Claims (2)

[Claims] 1. A method for chemical decontamination of radioactive metal waste, which comprises rolling the radioactive metal waste together with a grinding body in a sulfuric acid solution. 2. The method for chemical decontamination of radioactive metal waste according to claim 1, wherein the grinding body is made of ceramics or a metal exhibiting a natural potential lower than the natural potential of radioactive metal waste.