JPH0450699A - Method and apparatus for decontaminating radioactive solid contaminant - Google Patents

Abstract

PURPOSE:To reduce the generation amount of waste by carrying out washing using a strong alkaline quaternary ammonium compound and thermally decomposing the waste washing solution after washing to reduce the volume thereof. CONSTITUTION:First, contaminant 2 is received in a decontaminating tank 3 and, next, a valve 9 is opened to send tetramethylammonium hydroxide in the decontaminating tank 3 from a detergent tank 1 and the contaminant 2 is immersed in a detergent for several min while stirred by a stirrer 7. After decontamination, valve 10 is opened to send out the waste detergent to a waste detergent tank 4 and the waste detergent tank 4 is heated from the circumfer ence in an electric furnace 5 to reduce the volume of the waste detergent under heating. The waste solution reduced in volume by heating is converted to solid waste, for example, by vitrification to be kept in predetermined place as radio active waste.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method and system for decontaminating radioactive solid contaminants, and in particular, in the decontamination of radioactive solid contaminants generated from nuclear facilities, the present invention relates to a method and system for decontaminating radioactive solid contaminants generated from nuclear facilities. The present invention relates to a decontamination method and system suitable for reducing the amount of waste generated.

[Conventional technology]

Conventional decontamination of radioactive solid contaminants at nuclear facilities includes chemical decontamination methods, mechanical (physical) decontamination methods, and electrical/
Techniques such as magnetic decontamination have been used.

Among these decontamination methods, chemical decontamination methods are suitable for system decontamination of complex systems containing strong contaminants, such as the primary system of a nuclear reactor, and various decontamination methods are used in various countries. Technology is being developed.

In this chemical decontamination method, for example, a method of using an acid, an alkali, an oxidizing agent, a reducing agent, etc. singly or in combination is considered as one method for decontaminating contaminants on a metal surface. For example, as one of the conventional chemical decontamination methods for radioactive solid contaminants, a combination of nitric acid, pure water, sodium hydroxide, citric acid, hydrogen peroxide, and tartaric acid has been reported. For example, "Decontamination Techniques for Nuclear Facilities" (Kenkichi Ishiba's practice, published by NIC), 1932.
9th edition, page 440, Tonray (Doun+ca
y) In the decontamination of radioactive cells at a reprocessing plant, a method involving repeated treatment with high concentration nitric acid, HNO, and high concentration sodium hydroxide, NaOH, was used to reduce the dose from 65 to 500 R/h to 0.1 to 1.2 R/h. It has been reported that the temperature decreases to h.

[Problem to be solved by the invention]

By the way, in the conventional decontamination method described above, a chemical substance containing a metal component such as sodium hydroxide NaOH is used as a decontamination reagent, so this metal component and various reagents react to form inorganic substances such as sodium nitrate NaN0H3. This inorganic salt is contained in the waste cleaning agent generated after decontamination.

- In the chemical decontamination method for boat fishing, the hospital cleaning agent after decontamination is evaporated and concentrated to reduce its volume. Therefore, in the conventional decontamination method described above, when the volume of the hospital cleaning agent is reduced to a certain extent, inorganic salts precipitate. When inorganic salts are precipitated in this manner, subsequent processes such as a filtration step and a dehydration step are required to treat the formed salts. Therefore, in the conventional decontamination method described above, the concentration of the waste hospital cleaning agent is stopped to the extent that inorganic salts do not precipitate. Therefore, the volume of the waste hospital cleaning agent cannot be reduced sufficiently, and a large amount of waste is generated. There was a problem that occurred.

An object of the present invention is to provide a method and system for decontaminating radioactive solid contaminants that can reduce the amount of waste generated.

[Means to solve the problem]

According to the present invention, in order to achieve the above object, a strong alkaline quaternary ammonium compound is used as a cleaning agent to clean the solid metal surface to which radioactive solid contaminants have adhered, and the cleaning agent after cleaning is heated. A method for decontaminating radioactive solid contaminants is provided, which is characterized by decontaminating and reducing the volume of radioactive solid contaminants.

Examples of strongly alkaline quaternary ammonium compounds that can be used as cleaning agents include tetramethylammonium hydroxide and tetraethylammonium hydroxide.

Further, according to the present invention, in order to achieve the above object, a first means for supplying a cleaning agent made of a strong alkaline chemical substance made of a nonmetal, and a cleaning agent from the first means are used to contaminate radioactive solids. a second means for cleaning pollutants to which substances have adhered; a third means for accommodating the waste hospital cleaning agent after cleaning by the second means; and a waste hospital cleaning agent stored in the third means a fourth means for heating to a predetermined temperature;
and a fifth means for discharging gas generated from the hospital cleaning agent as a result of heating by the means described above.

[Effect]

A strongly alkaline quaternary ammonium compound is used as a cleaning agent to decompose organic acidic contaminants adhering to solid metal surfaces in a strongly alkaline atmosphere, thereby liberating metal ions contained in the acidic contaminants. The liberated metal ions react with hydroxide ions in a strongly alkaline atmosphere to cause a complex-forming reaction with the quaternary ammonium compound to produce a complex without changing into a hydroxide precipitate. The formation of this complex cleanses organic acidic contaminants.

On the other hand, at this time, since the quaternary ammonium compound used as a cleaning agent is a non-metal, inorganic salts are not generated in the waste cleaning agent after decontamination, and the salt-containing components contained in the waste cleaning agent are reduced. can be achieved. Therefore, there is no possibility that inorganic salts will precipitate due to concentration, so the volume of the hospital cleaning agent can be significantly reduced, and the amount of waste generated can be significantly reduced.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

In FIG. 1, the decontamination system of this embodiment includes a cleaning agent tank 1 for supplying a cleaning agent, a decontamination tank 3 for cleaning pollutants 2 with the cleaning agent from the cleaning agent tank 1, and a decontamination tank 3 for cleaning contaminants 2 with the cleaning agent from the cleaning agent tank 1. A waste cleaning agent tank 4 that stores the waste cleaning agent after washing in the dyeing tank 3, an electric furnace 5 that heats the waste cleaning agent stored in the waste cleaning agent tank 4 to a predetermined temperature, and heating in the electric furnace 5. Vent line 6 discharges gas generated from waste cleaning agent as a result of
and a stirrer 7 that stirs the decontamination agent in the decontamination tank 3;
It consists of a vent filter 8 for removing radioactive substances in gas discharged from a vent line 6, and valves 9, 10, and 11.

Contaminant 2 is, for example, organic solvent containing radionuclides such as ruthenium Ru, zirconium Zr, and uranium U attached to the solid metal surface, and the cleaning agent is tetramethyl hydroxide as a strong alkaline quaternary ammonium compound. Ammonium N (CH3) and ioH are used.

First, put the pollutant 2 into the decontamination tank, then open the valve 9 to send tetramethylammonium hydroxide from the cleaning agent tank 1 into the decontamination tank 3, and add the pollutant 2 while stirring with the stirrer 7. Let it soak in the cleaning solution for a few minutes.

After decontamination, the valve 10 is opened, the waste cleaning agent is sent to the waste cleaning agent tank 4, and the waste cleaning agent is heated from around the waste cleaning agent tank 4 by the electric furnace 5.
Heat the waste cleaning agent to reduce its volume. At this time, the valve 11 is kept open, and the gas generated by heating and reducing the volume of the waste cleaning agent is discharged from the vent line 6 through the vent filter 8 to the outside of the thread.

The waste liquid after volume reduction by heating is converted into solid waste by, for example, vitrification treatment, and stored in a predetermined location as radioactive waste.

FIG. 2 shows an experimental example of the decontamination effect of this example. In this experimental example, the cleaning agent and the contaminant 2 used were organic solvents containing the above-mentioned radionuclides of tetramethylammonium hydroxide, ruthenium Ru, zirconium Zr1, and uranium U, respectively, or those attached to the metal surface. In addition, the concentration of radionuclides is ruthenium RuO,01mo
A! /I.

Zirconium Z r 0. 1 m oA / n z
Back: /U O.

002 mol/Il. The concentration of tetramethylammonium hydroxide is a parameter and 1. In FIG. 2, the horizontal axis represents the concentration, and the vertical axis represents the cleaning rate.

As shown in FIG. 2, it was revealed that good decontamination performance was obtained for each element, and decontamination efficiency of 98% or more was obtained for each element.

In addition, regarding the amount of waste generated after decontamination, for example, 1
．． If 00 m3 of waste cleaning agent is generated, approximately 160 kg of inorganic salts are produced. Since precipitation of this inorganic salt requires subsequent processes such as filtration and dehydration, evaporative concentration is limited to the extent that the inorganic salt does not precipitate. On the other hand, in this example, tetramethylammonium hydroxide is a non-metal and does not form an inorganic salt, so there is no such restriction and the volume can be significantly reduced by heating.

When tetramethylammonium hydroxide is used as a cleaning agent, methods for heating and reducing the volume of the waste cleaning agent include distillation under normal pressure, thermal decomposition at a predetermined temperature, and platinum-based (
There is a method of thermally decomposing it by oxidation treatment using a catalyst such as Pt (platinum), Pa (palladium), etc., and converting it into an inorganic gas. In this example, when the waste cleaning agent was distilled under normal pressure, it could be concentrated to about 1/12 the volume of the waste liquid after concentration in the conventional method. That is, it is possible to reduce the amount of waste generated to about 1/12 compared to the conventional method.

In addition, when thermal decomposition is performed at approximately 210℃, approximately 1/1
The volume can be reduced to about 300% by an oxidation method using a platinum-based catalyst (Pt (platinum), Pa (palladium), etc.).
It was thermally decomposed at ℃ and turned into an inorganic gas.

As described above, according to this embodiment, 1. Since no inorganic salt is generated, the volume of waste cleaning agent can be significantly reduced, and the amount of waste generated can be significantly reduced. Further, there is no need for subsequent processes such as a filtration process and a dehydration process for treating the generated salt, making it possible to rationalize equipment.

〔Effect of the invention〕

According to the present invention, since a strongly alkaline quaternary ammonium compound made of nonmetal is used as a cleaning agent, inorganic salts are not generated in the waste cleaning agent after decontamination, and solid metal surfaces can be decontaminated. It is possible to significantly reduce the amount of waste generated while maintaining sufficient security.

Furthermore, since the waste does not contain inorganic salts, subsequent processes (filtration process, dehydration process, etc.) associated with the generation of inorganic salts are no longer necessary, and equipment can be rationalized.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a decontamination system according to an embodiment of the present invention, and FIG. 2 is a diagram showing an example of decontamination performed using the decontamination system. Explanation of symbols 1... Cleaning agent tank (first means) 2... Contaminant 3... Decontamination tank (second means) 4... Waste cleaning agent tank (third means) 5... Electric furnace (fourth means) 6... Vent line (fifth means) Applicant a Hitachi, Ltd. 6... Vent line (fifth means)

Claims (3)

[Claims] (1) The method is characterized in that a strong alkaline quaternary ammonium compound is used as a cleaning agent to clean the solid metal surface to which radioactive solid contaminants have adhered, and the waste cleaning agent after cleaning is thermally decomposed to reduce its volume. Decontamination methods for radioactive solid contaminants. (2) The method for decontaminating radioactive solid contaminants according to claim 1, wherein the strongly alkaline quaternary ammonium compound is tetramethylammonium hydroxide. (3) a first means for supplying a cleaning agent made of a strong alkaline chemical substance made of non-metal; and a second means for cleaning contaminants to which radioactive solid contaminants have adhered using the cleaning agent from the first means; a third means for accommodating the waste cleaning agent after cleaning by the second means; and a fourth means for heating the waste cleaning agent contained in the third means to a predetermined temperature; and a fifth means for discharging gas generated from the waste cleaning agent as a result of heating by the fourth means.