JPH0299899A - Decontamination of radioactive metallic waste and its device - Google Patents

Abstract

PURPOSE:To intend an effective removal of radioactive metallic waste to a radioactivity level as low as ordinary wastes by applying ultrasonic vibration for a predetermined duration just before drawing up contaminated objects from a decontamination tank. CONSTITUTION:Metallic wastes of carbon steel in radioactive metallic wastes 14 is simply immersed in a decontamination tank 1. At this process, a part of cruds which stay on surfaces of the metallic wastes and contains major parts of radioactive substances, and a base metal dissolve and, at the same time, the cruds are stripped off accompanied by dissolution of the base metal. To raise an effectiveness of the stripping-off, an ultrasonic wave of around 10 to 40kHz is charged for about 1 to 30 minutes by an ultrasonic wave oscillator element 5, just before drawing up of the base metal 14. With this process, the wastes 14 can be removed to such a radioactivity level as low as that of ordinary wastes.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method and apparatus for decontaminating radioactive metal waste whose surface is contaminated and which is generated from facilities handling radioactive materials such as nuclear power plants, and in particular, The present invention relates to a decontamination method and device for reducing the volume of radioactive waste by decontaminating metal waste to a radioactivity level comparable to that of general waste.

[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. The number of drums in storage is increasing year by year. For this reason, it is desired to reduce the volume of the radioactive metal waste. Current volume reduction technologies that have been developed include electropolishing decontamination methods and blasting decontamination methods, but these methods require the 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 available as 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
The purpose is to dissolve the crud that is firmly attached to the surface of metal waste and contains most of the radioactive materials.

[Problem to be solved by the invention]

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 melting 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 method to solve this problem, a method has been proposed in which the cladding is dissolved or peeled off using a solution in which tetravalent cerium is added to nitric acid or sulfuric acid, and the surface layer of the metal base material is also oxidized and dissolved.

In this method, the metal is melted using the oxidizing power when tetravalent cerium becomes trivalent, so the amount of melted cerium is
The valence depends on the amount of cerium. For example, when iron dissolves as trivalent ions as shown in the following equation, tetravalent cerium accepts three electrons and becomes trivalent cerium, so iron 1
Three moles of tetravalent cerium are consumed to dissolve one mole.

Fe −+Fe34+ 3e 3 Ce4++3 e−− → 3 Ce” Therefore, there is a problem that a large amount of tetravalent cerium is consumed before decontamination is completed.
A method has also been proposed in which valent cerium is electrolytically regenerated into tetravalent cerium, but this method has the drawbacks of complicating the equipment and using a large amount of electricity.

The present invention solves the drawbacks of the prior art and provides a method and apparatus for decontaminating radioactive metal waste, which can efficiently treat radioactive metal waste to a radioactivity level comparable to that of general waste without using oxidizing metal salts. For the purpose of providing.

[Means to solve the problem]

In the present invention, radioactive metal waste carbon steel dissolves simply by immersing it in sulfuric acid, while stainless steel, which is passivated, does not dissolve only by immersing it in sulfuric acid, and part of the passive film is removed. If removed, the sulfuric acid and the base material in this part will form the following equation (1): Fe+Has O<→Fe"+ 3042-+ 2 H
"+ 2e (1), and the electrons (e) generated along with this reaction convert Fe20a, which is the main component of the remaining passive film, to the following formula (2) % formula % (2) The action of the local battery to reduce and melt progresses gradually, and the entire surface of the object to be decontaminated is dissolved in the sulfuric acid solution.
In sulfuric acid, oxides of copper and chromium that are difficult to dissolve are deposited on the surface of the object to be decontaminated, and some of the radioactive materials that have been eluted are incorporated into this deposit, reducing the decontamination effect. As a means to remove this deposit, an ultrasonic oscillator is installed in the decontamination tank, and at the final stage of the sulfuric acid immersion process, that is, just before the object to be decontaminated is lifted from the decontamination tank, It is configured to apply temporal ultrasonic vibrations.

That is, in the decontamination method for radioactive metal waste according to the present invention, sulfuric acid is used as a decontamination liquid, and after immersing the radioactive metal waste in the sulfuric acid, the surface of the metal waste is electrolytically reduced for a predetermined period of time. A method for decontaminating radioactive metal waste by chemically dissolving radioactive materials in sulfuric acid to remove radioactive materials, the method is characterized by applying ultrasonic vibration for a predetermined period of time immediately before the object to be decontaminated is lifted from the decontamination tank.

In order to carry out such a decontamination method, the present invention provides a method for measuring the potential of radioactive metal waste in the decontamination tank at a small (more than one location) of the bottom and side surfaces of the decontamination tank, which is equipped with a potential measuring device for measuring the potential of radioactive metal waste in the tank. The present invention provides a decontamination device for radioactive metal waste characterized by being equipped with a sound wave oscillator.

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

FIG. 1 is a system diagram showing one embodiment of a radioactive metal waste decontamination apparatus according to the present invention.

In Figure 1, the decontamination tank 1 contains 5 to 30% by weight of sulfuric acid.
, preferably 10-20% by weight. Ultrasonic oscillators 5 are provided on the bottom and side surfaces of the decontamination tank 1. In addition, a removable electrode 4 is provided in the decontamination tank 1, and the decontamination tank 1 is connected to a potential measurement tank 2 at the bottom of the tank, and this potential measurement tank 2 has a reference electrode. 8 has been installed. The decontamination tank 1 is further equipped with a heater 6 to adjust the temperature of the decontamination solution, and a stirring pump 12 is used to make the temperature of the decontamination solution uniform in the decontamination tank 1. Circulate the liquid and stir to mix. The temperature of the decontamination liquid is 40~
90°C, preferably 50 to 80°C.

In the washing tank 3, water is used to wash and remove the decontamination liquid remaining on the metal waste after the decontamination is completed. In the washing tank 3, washing water is circulated by a circulation pump 13 in order to improve the washing effect, and a filter 7 is installed in the circulation line to trap solid matter.

According to the present invention, radioactive metal waste is decontaminated as follows.

Among the radioactive metal wastes, carbon steel wastes are simply immersed in the decontamination tank 1. At this time, part of the cladding on the surface of the metal waste, which contains most of the radioactive materials, and the base metal melt, and the cladding peels off as the base metal melts, but the peeling effect is In order to raise the radioactive metal waste 14, immediately before pulling up the radioactive metal waste 14, the ultrasonic oscillator 5
Apply ultrasonic waves of 10 to 40 KHz, preferably about 25 kHz.The decontamination time at this time is 10 to 6 kHz.
The ultrasonic wave application time is approximately 1 to 30 minutes, which is sufficient. The waste taken up from the decontamination tank 1 can be washed with water in the washing tank 3 and then disposed of in the same way as general waste.

Among radioactive metal waste, stainless steel waste is
At the initial stage of immersion in decontamination tank 1, radioactive metal waste 1
The potential of No. 4 is recorded on the recording device 11 through the potential measuring device IO using the reference electrode 8 installed in the potential measuring tank 2.

At this time, if the measured potential is in the passive region, the reference electrode 8
While measuring the potential, a voltage is applied for a certain period of time between the radioactive metal waste 14 and the electrode 4 by the power source 9 so that the potential of the radioactive metal waste 14 is below the active range, thereby electrolytically reducing the waste. After that, after stopping the voltage application, the potential of the radioactive metal waste 14 is measured again using the reference electrode 8,
If it is in the active region, continue dipping while stopping the voltage application. At this time, part of the cladding and the base metal dissolve, and as the base metal melts, the cladding peels off, making it possible to remove most of the radioactive materials. However, in the case of stainless steel, it is difficult to dissolve in sulfuric acid. Copper, chromium oxides, etc. are deposited on the surface of the radioactive metal waste 14 that is the object to be decontaminated, and some of the radioactive materials that have been eluted are incorporated into this deposit, reducing the decontamination effect. In order to remove this deposit, immediately before pulling up the radioactive metal waste 14 after dissolution, an ultrasonic oscillator 5 is used to generate 10 to 40 KHz.
, preferably applying ultrasonic waves of about 25 KHz. The decontamination time at this time is about 30 to 120 minutes, the electrolytic reduction time performed at the initial stage of immersion in the decontamination solution is about 0 to 10 minutes, and the ultrasonic application time is about 1 to 30 minutes. It is enough. The waste taken up from the decontamination tank 1 can be washed with water in the washing tank 3 and then disposed of in the same way as general waste.

〔Example〕

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

Example 1 A stainless steel valve contaminated with radioactive materials was immersed in a decontamination solution with a sulfuric acid concentration of 15% and a temperature of 80°C. At the beginning of the immersion, a voltage of 2 seconds was applied. ,
For samples subjected to electrolytic reduction for 5 minutes, changes over time in GM tube measurement values were measured and compared when ultrasonic waves at a frequency of 25 KHz were applied for 10 minutes immediately before pulling the sample and when not. The results are shown in Figure 2. Curve A is when no ultrasound is applied, curve B is when immersion in decontamination liquid is 60
These are the results of the method of the present invention in which ultrasonic waves were applied for 10 minutes after 10 minutes.

As is clear from Figure 2, in curve A where no ultrasonic waves were applied, some of the radioactive substances that had previously eluted were incorporated into deposits such as copper and chromium oxide, which are difficult to dissolve in sulfuric acid. Therefore, it is possible to decontaminate only up to about 150 cpm, but as shown in curve B, deposits can be sufficiently removed by applying ultrasonic waves, reaching the detection limit (80 cpm).
cpm).

[Effects of the Invention] According to the method of the present invention, ultrasonic vibration is applied at the final stage of the immersion process in the sulfuric acid solution, that is, immediately before the object to be decontaminated is pulled up from the decontamination tank, thereby efficiently disposing of general waste. It is possible to decontaminate radioactive metal waste to a normal level of radioactivity, and it is extremely economical because no oxidizing metal salts are used.

In addition, the decontamination device of the present invention has an ultrasonic oscillator installed at one or more locations on the bottom and side surfaces of a decontamination tank having a potential measuring device, and has a simple configuration and achieves a high decontamination effect. can do.

[Brief explanation of drawings]

FIG. 1 is a schematic system diagram of an apparatus for carrying out the method of the present invention, and FIG. 2 is a graph showing changes over time in GM control measurement values in Example 1. Explanation of symbols 1... Decontamination tank, 2... Potential measurement tank, 3... Washing tank, 4... Electrode, 5... Ultrasonic oscillator, 7... Filter, 8...・Reference electrode, 9... Power supply, 10...
Potential measuring device, 11... Recording device, 14... Radioactive metal waste

Claims (2)

[Claims] (1) Using sulfuric acid as a decontamination solution, at the beginning of immersing radioactive metal waste in this sulfuric acid, after electrolytically reducing it for a predetermined period of time,
In a method of decontaminating radioactive metal waste by chemically dissolving the surface of the metal waste in sulfuric acid to remove radioactive substances, applying ultrasonic vibration for a predetermined period of time immediately before lifting the object to be decontaminated from the decontamination tank. A method for decontaminating radioactive metal waste characterized by: (2) A claim characterized in that an ultrasonic oscillator is provided at at least one of the bottom and side surfaces of a decontamination tank having a potential measuring device for measuring the potential of radioactive metal waste in the decontamination tank. A decontamination device that implements the method described in 1.