Chemistry of inorganic compounds – Extracting – leaching – or dissolving
Reexamination Certificate
1999-12-22
2003-10-21
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Extracting, leaching, or dissolving
C376S310000, C134S003000, C205S626000, C204S158200, C204S158210
Reexamination Certificate
active
06635232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of chemical decontamination for the components of a radioactive material handling facility, such as a nuclear power station, and a system for carrying out the method of removing metal oxides containing radioactive nuclides and adhering to the components of the radioactive material handling facilities from the surfaces of the contaminated components by chemical dissolution.
2. Description of the Related Art
Oxide films containing radioactive nuclides are deposited or formed on the surfaces of components of a nuclear power station in contact with fluids containing radioactive nuclides during operation and subject to radioactive contamination, such as pipes, pieces of equipment and structural members. Consequently, the dose rate around those component members increases and the radiation exposure of workers engaged in work for periodic inspection or dismantlement of a nuclear reactor for decommissioning.
In order to remove the oxide film, a decontaminating solution is supplied the oxide film or a metal forming a contaminated object so as to dissolve them, thereby the oxide film is dissolved in the solution or peeled off into the solution. Aforementioned chemical decontamination method, which dissolves or removes the oxide film chemically, has practically been applied to the decontamination of the components of some nuclear plants and has produced satisfactory results in reducing mediation exposure.
Various chemical decontamination methods intended for the decontamination of stainless steel components of atomic energy plants have been proposed. One of those chemical decontamination methods comprises, in combination, a step of dissolving chromium oxides contained in an oxide film through oxidation using an oxidizing agent, and a step of dissolving ferrous oxides, which are principal components of the oxide film, through reduction a reducing agent.
A chemical decontamination method disclosed in JP B No. Hei 3-10919 employs permanganic acid as an oxidizing agent, and dicarboxylic acid as a reducing agent. The chemical decontaminating method using permanganic acid which has a high oxidizing effect in a low concentration and dicarboxylic acid which can be decomposed into CO
2
and H
2
O produces less secondary wastes as compared with hitherto known chemical decontamination methods and has practically been applied to decontamination work in nuclear power plants.
A decontamination method disclosed in JP A No. Sho 55-135800 uses, in combination, an ozone solution prepared by dissolving ozone in water as an oxidizing agent, and a decontaminating liquid containing an organic acid and a complexing agent. A decontamination method disclosed in JP A No. Hei 9-151798 prepares a foamed decontaminating liquid by blowing ozone gas into a solution containing a foaming agent, and feeds the foamed decontaminating liquid into a contaminated object for decontamination.
When decontaminating contaminated objects by the chemical decontamination method using permanganic acid and dicarboxylic acid as decontaminating agents, the decontaminating agents produce secondary wastes in recovering manganese ion from the permanganic acid solution by means of an ion-exchange resin.
As generally known, ozone is a highly oxidative gas, reacts with water and is decomposed to produce various oxidative active oxygen species. The decontamination method will be a very effective method producing the least amount of secondary wastes attributable to an oxidizing agent if the oxide film can be dissolved in an ozone solution prepared by efficiently dissolving ozone gas in water. However, the ozone concentration of ozone gas produced by a conventional silent discharge ozonizer is low (in general, lower than 1% by volume), and the ozone concentration of ozone solution prepared by supplying the ozone gas in an acid solution is several parts per million or less.
An oxidation process using an ozone solution having such a low ozone concentration, as compared with a conventional chemical decontamination method using permanganic acid, has an inferior oxide film removing ability. To make matters worse, ozone decomposes in water and the ozone concentration of the ozone solution decreases. When the temperature of the ozone solution is high, the half life of dissolved ozone is short and it is possible that ozone disappears in a few minutes. The higher the temperature of the decontaminating liquid for the chemical decontamination method, the higher is the rate of dissolution of the oxide film and the higher is the decontaminating effect. Therefore, the chemical decontamination method must be carried out at temperatures as high as possible to reduce overall time necessary for decontamination work.
Although it is possible to hold ozone gas in foams produced in the decontaminating liquid by a foaming agent thereby holding ozone in the decontaminating liquid for a long time, the foaming agent produces secondary wastes.
The known chemical decontamination method using oxidation and reduction is applied mainly to decontaminating stainless steel components and cannot be applied to decontaminating components made of metallic materials susceptible to the corrosion by chemicals, such as carbon steels.
SUMMARY OF THE INVENTION
The present invention has been made to solve the foregoing problems and it is therefore an object of the present invention to provide a chemical decontamination method and a system for carrying out the same capable of chemically decontaminating components of radioactive material handling facilities and of efficiently dissolving oxide films through oxidation, and producing only a small amount of secondary wastes.
Another object of the present invention is to provide a chemical decontamination method and a system for carrying out the same capable of decomposing organic acid used as a decontaminating agent, such as oxalic acid, and exhaust ozone gas.
To achieve the objects, according to a first aspect of the present invention, a method of removing an oxide film containing radioactive nuclides and adhering to a component of a radioactive material handling facility is provided. The method includes an oxidative dissolving process for dissolving the oxide film through oxidation using an ozone solution prepared by bringing ozone gas into contact with an acid solution.
Preferably, the ozone solution has a pH value of 6 or below, more preferably, 5 or below.
Oxide films deposited or formed on the surfaces of contaminated components, such as pipes and pieces of equipment of a radioactive material handling facility, can effectively dissolve and removed by using a solution prepared by dissolving ozone, i.e. an oxidative gas, in water of a desired quality.
Preferably, the working temperature of the ozone solution for the oxidative dissolving process is in the range of 50 to 90° C.
Preferably, the ozone gas is produced by an electrolytic ozonizer that has an anode chamber formed on one side of a solid electrolyte and a cathode chamber formed on the other side of the solid electrolyte, and generates ozone in the anode chamber by a solid electrolyte electrolytic process in which pure water is subjected to electrolysis using an anode of a catalytic metal disposed in the anode chamber.
The method may further include a monitoring process for measuring the oxidation-reduction potential of the ozone solution to monitor the oxidative dissolving ability of the zone solution.
The method may further include a reductive dissolving process in which a decontaminating solution, such as an organic acid solution, is supplied to the contaminated object for the reductive dissolution of the oxide film. The amount of secondary wastes originating in decontaminating agents can be reduced by using ozone in the oxidative dissolving process and using an reductive organic acid capable of being decomposed into CO
2
and H
2
O in the reductive dissolving process.
The method may further include a reducing agent decomposing process for decomposing an organic acid remaining in the org
Enda Masami
Sakai Hitoshi
Yaita Yumi
Bos Steven
Foley & Lardner
Kabushiki Kaisha Toshiba
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