JPH0720291A - Decontamination method for metal waste - Google Patents

Abstract

PURPOSE:To allow decontamination on the surface of a metal waste without heating the entirety of a decontamination tank by coating the surface of metal waste with a decontamination paste, starting dissolution on the surface of metal waste by partially cutting or heating the coated surface, and then dissolving the remainder of the coated surface using the dissolution heat thereof. CONSTITUTION:Nitric acid, hydrochloric acid and hydrofluoric acid are mixed at a weight ratio of 20:2:3 to produce a mixture acid which is then admixed with a paste promoter thus producing a decontamination paste. The decontamination paste is applied uniformly to the surface of a radioactive metal waste formed with an oxide film. The coated surface is then partially cut to remove the oxide film or it is partially heated at about 40 deg.C) or above, preferable at 70 deg.C or above. Consequently, the g decontamination paste begins to dissolve the surface of metal waste and produces dissolution heat. Since the decontamination paste serves as a thermal insulation layer, the region adjacent to the metal waste is heated and the metal waste is also dissolved in that region. Consequently, the entire surface of metal waste is dissolved and the radioactive substance is removed therefrom.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decontaminating a metal waste contaminated with radioactive substances generated from a nuclear power plant or the like.

[0002]

2. Description of the Related Art A large amount of metal waste such as pipes contaminated with radioactive substances is generated during periodical repairs of nuclear power plants. Conventionally, such metal waste is directly packed in drums. Since it was stored as a storage device, there was a problem that a huge storage space was required. Therefore, the inventors of the present invention focused on the fact that only the surface portion of the metal waste is contaminated with radioactive substances, and developed a method of dissolving only the surface layer with a decontamination solution in advance. 1-227902
The patent has been applied for as an issue. By dissolving only the surface layer of the metal waste in this manner, there is an advantage that the remaining portion can be handled as general waste.

However, since the relationship between the properties of the decontamination solution and the preferable decontamination conditions has not been clarified so far, it takes a long time to dissolve the oxide film of the metal waste, and a desired dissolution thickness is required. There was a problem in that it had to be immersed in the decontamination solution for an extremely long time to obtain it. Moreover, it can be easily predicted that the dissolution rate of the metal waste can be increased by heating the decontamination solution, but if the entire decontamination tank is heated, the decontamination tank itself may be eroded and extra There were problems such as requiring energy.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned conventional problems and quickly dissolves the surface layer of metal waste having an oxide film formed thereon without heating the entire decontamination tank. The purpose of the present invention is to provide a decontamination method for metal waste.

[0005]

The present invention, which has been made to solve the above problems, is to apply a decontamination paste in which a mixed acid is mixed with a paste auxiliary agent to the surface of a metal waste having an oxide film formed thereon, It is characterized in that the melting of the surface of the metal waste is started by shaving or heating a part of the coated surface, and the heat of fusion generated by this is used to proceed to the rest of the coated surface. .

[0006]

In order to decontaminate a metal waste by the method of the present invention, first, a decontamination paste prepared by mixing a mixed acid with a paste aid is applied to the surface of the metal waste, and a part of the applied surface is scraped off. The oxide film on the surface is removed, or only a part of the coated surface is heated to 40 ° C or higher, preferably 70 ° C or higher. As a result, the decontamination paste begins to dissolve on the surface of the metal waste,
At the same time, heat of dissolution is generated. The heat of dissolution heats the adjacent area of the metal waste because the decontamination paste acts as a heat insulating layer, and the dissolution of the metal waste proceeds in the adjacent area. In this way, the entire surface of the metal waste is dissolved and the radioactive substances adhering to the surface are removed.

[0007]

EXAMPLES The present invention will be described in more detail below with reference to examples. First, a mixed acid prepared by mixing 2 parts by weight of hydrochloric acid and 3 parts by weight of hydrofluoric acid with 20 parts by weight of nitric acid is mixed with a paste aid to prepare a decontamination paste. As the paste aid, for example, BaSO ₄ powder and polyvinyl chloride powder are mixed in a ratio of 1: 4 to
A mixture of 4: 1 ratio can be used.
This decontamination paste is evenly applied to the surface of metal waste made of stainless steel (SUS304). The surface area of the metal waste of the example is 150 × 150 mm, and the relationship between the coating amount of the decontamination paste and the molten metal dissolution thickness is as shown in FIG. Based on this relationship, the amount of decontamination paste applied may be selected in accordance with the target dissolution thickness. Further, the relationship between the decontamination temperature of the metal waste having the oxide film and the film dissolution time is as shown in FIG. From this relationship, the higher the melting time is, the shorter the film melting time is, and the melting speed shifts to the ingot melting that generates a large amount of heat of melting. Therefore, if the film melting temperature and time are set based on this relationship, Good.

On the basis of this result, the decontamination paste is applied to the metal waste having the oxide film formed thereon, and a part of the applied surface is heated to a temperature of, for example, about 80 ° C. The surface of the melt begins to melt, but heat of dissolution is generated during this melt. When a normal decontamination solution is used, this heat of dissolution is rapidly dissipated from the surface of the metal waste, but the decontamination paste used in the present invention has excellent heat insulation properties, so Emissions are prevented and heat the surface of adjacent metal waste. As a result, the melting progresses even in the adjacent portion of the first heated position, and the heat of dissolution further heats the adjacent portion repeatedly, whereby the entire surface of the metal waste is dissolved. .

FIG. 3 shows this relationship. When the decontamination paste is used, the heating by the heat of dissolution proceeds as described above, and therefore regardless of the temperature of the metal waste at the start of melting. The melt thickness is almost constant. On the other hand, when a gel-like decontamination solution is used, it can be seen that the dissolution thickness greatly changes depending on the temperature of the metal waste at the start of dissolution. Therefore, according to the present invention, the entire surface of the metal waste can be melted to a desired thickness without increasing the temperature of the entire metal waste at the start of melting.

After the dissolution of the entire surface of the metal waste is completed in this way, the decontamination paste is washed off by jetting water, and the radioactive substances adhering to the surface are also collected together with the decontamination paste. Moreover, since the remaining metal waste does not contain radioactive substances, it can be disposed of as general waste after the radiation dose is measured.

In order to start the dissolution, a method of removing the oxide film by scraping the surface of the metal waste may be used in addition to the method of heating a part of the coated surface. When the dissolution is started on the surface from which the oxide film has been removed, the heat of dissolution generated thereby heats the surroundings and sequentially melts the surface of the metal waste having the oxide film as described above.

[0012]

As described above, according to the method for decontaminating metal waste of the present invention, the decontamination paste is applied to the surface of the metal waste, and a part of the applied surface is scraped or heated. To start the dissolution of the metal waste surface with the decontamination paste,
Since the rest of the coated surface is melted by utilizing the heat of dissolution generated by this, it is possible to decontaminate the metal waste without heating the entire decontamination tank as in the conventional case. Therefore, there is an advantage that the decontamination tank itself is not eroded and extra energy is not required.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a coating amount of a decontamination paste and a dissolution thickness.

FIG. 2 is a graph showing a relationship between a melting temperature of a metal waste and a dissolution time of an oxide film.

FIG. 3 is a graph showing the relationship between the melting temperature and the melting thickness of metal waste.

Claims (1)

[Claims] 1. A surface of a metal waste on which an oxide film is formed,
Apply decontamination paste with mixed acid mixed with paste aid,
Metal waste characterized by starting melting of the surface of the metal waste by shaving or heating a part of the coating surface, and using the heat of fusion generated thereby to proceed to the rest of the coating surface Decontamination method.