JPH0815490A - Method and device for decontaminating metal waste - Google Patents

Abstract

PURPOSE:To increase the reaction speed, prevent a decontaminating liquid from evaporating and economically removing contaminants sticking on the surface of metal wastes by attaching a foamy decontaminating liquid containing acid on the surface of the metal wastes and keeping it for a prescribed time. CONSTITUTION:After injecting a decontaminating liquid containing acid such as hydrochloric acid and nitric acid and a surface active agent from a decontaminating liquid supplying tank 2 to a foam generator 5 through a decontaminating liquid circulation tank 3, and blowing compressed air into the decontaminating liquid to foam it, the decontaminating liquid is supplied to a solution tank 1. Then, the soaking of a contaminated metal in the foamed decontaminating liquid progresses the dissolving reaction, and the generated solution heat is insulated with the foamy decontaminating liquid and accumulates in the solution tank 1 to boost the reaction. In this case, it is preferable that the diameter of a foam is approximately 0.1 to 2mm, and the ratio of liquid to a foam is 1 to 10vol.%. If the diameter of a foam and the ratio of liquid to a foam are too large, the quantity of acid becomes insufficient. On the other hand, if they are too small, adiathermancy becomes insufficient and the dissolving efficiency declines. Subsequently, after a defoamer 8 defoams the generated gas, it is released to the air through a scrubber 9, a HEPA filter 10 and a blower 11.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art A large amount of metal waste contaminated with radioactive materials is generated at the time of periodic repairs of a nuclear power plant, but in the past it was stored in drums as it is, so a huge storage space was required. There was a problem of cost. Therefore, the present inventors are paying attention to the fact that only the surface of metal waste is contaminated with radioactive substances, and are developing a method of dissolving only the surface layer with a decontamination solution.

[0003] In this case, it is easily assumed that increasing the temperature of the decontamination liquid will increase the dissolution rate of the surface of the metal waste by the decontamination liquid. Since a mixed acid mixed with hydrofluoric acid is used, there is a problem that if the liquid temperature is increased, the dissolution tank for metal waste itself is eroded. Further, there is a problem that evaporation of the decontamination solution is promoted and 30 to 50% of the decontamination solution evaporates in 60 minutes. Further, there is a problem that running cost for heating is also required.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned conventional problems and enhances the dissolution rate of the surface of metal waste without increasing the temperature of the decontamination solution that comes into contact with the dissolution tank. The present invention has been completed to provide a method and an apparatus for decontaminating metal waste, which enables economical decontamination.

[0005]

The method for decontaminating a metal waste of the present invention, which has been made to solve the above-mentioned problems, is a method for depositing a foamy decontamination solution containing an acid on the surface of a metal waste. By holding the metal waste for a predetermined time, the contaminants adhering to the surface of the metal waste are removed. The metal waste decontamination apparatus of the present invention is characterized in that a foam generator is attached to a metal waste dissolution tank in which a decontamination solution containing an acid is circulated.

[0006]

According to the decontamination method of the present invention, the dissolution reaction of the metal waste proceeds inside the foamy decontamination solution, but the heat of dissolution generated at that time depends on the foamy decontamination solution around it. It is insulated and does not transmit to the surroundings. For this reason, the surface of the metal waste is sufficiently heated by the heat of dissolution to accelerate the dissolution reaction without external heating. In addition, heat transfer to the dissolution tank is prevented by the heat insulation effect of the foamy decontamination solution, which prevents erosion of the dissolution tank and increases the service life, and also prevents evaporation because the surface of the decontamination solution itself does not heat up. To be done.

In this case, the diameter of the bubbles is 0.1-2 mm,
The liquid / foam ratio is preferably 1 to 10% by volume. If the diameter of the foam or the liquid / foam ratio is smaller than these ranges, sufficient heat insulation cannot be obtained and the dissolution performance will be deteriorated. On the contrary, when the diameter of the foam or the liquid / foam ratio is larger than these ranges, the dissolution performance is also deteriorated because the amount of the acid contacting the surface of the metal waste is insufficient. The bubbles can be formed by blowing a gas into a pasty substance such as CMC or by compressing the gas and blowing the bubbles, in addition to the formation of a surfactant.

It is more efficient to change the type and mixing ratio of the acid depending on the material of the metal waste to be decontaminated. In this case, the acid used is 10-30% by weight nitric acid, 0-10 hydrochloric acid.
It is preferable that the content is 0 to 10% by weight of hydrofluoric acid. The surfactant added to the acid is a nonionic or anionic surfactant, and the concentration thereof is preferably 1000 to 10000 ppm. If it is less than 1000 ppm, the foamability is poor, and
This is because no improvement in foaming property is observed even if it exceeds 000 ppm. By preheating the metal waste to 50 to 100 ° C, the dissolution reaction is promoted and the dissolution time can be shortened.

Further, according to the decontamination apparatus of the present invention, the decontamination solution containing acid can be foamed in the foaming machine and the dissolution tank of the metal waste can be filled with the foamed decontamination solution. The decontamination method of the present invention described above can be easily carried out simply by immersing the metal waste. Hereinafter, these inventions will be described in more detail with reference to Examples.

[0010]

[Example] [Example 1] Nitric acid / hydrochloric acid / hydrofluoric acid in weight ratio
To 99 parts of the decontamination solution mixed in a ratio of 20: 2: 3, a surfactant commercially available under the trade name of Trax K-40 from NOF CORPORATION was mixed in a ratio of 1 part, and air was added thereto. It was blown in to create a foamy decontamination solution. On the other hand, 214 x 151 x 4 mm
The surface of 2 sheets of SUS 304 (surface area 1350 cm) polished with sandpaper was prepared as a simulated sample of radioactive metal-contaminated metal waste, and this was decontaminated in the form of foam at room temperature (20 ° C). It was immersed in the liquid. In this case, the decontamination liquid volume is 75m
There were 5 types of L, 100 mL, 130 mL, 200 mL, and 300 mL, and each was immersed for 60 minutes, then the sample was taken out, and the amount of dissolution was calculated from the weight change of the sample before and after immersion.

When the temperature change of the decontamination liquid (100 mL of liquid volume) was measured after the start of the immersion, as shown in the graph of FIG. 1, even though no external heat was applied, the decontamination was foamy. It can be seen that the temperature near the sample surface rises due to the adiabatic effect of the liquid, and the dissolution of the metal surface is progressing. Also, the solution thickness and the eluted metal ion concentration change depending on the liquid ratio (sample surface area / decontamination liquid amount), but as shown in Fig. 2, the metal ion concentration is about 70 g / L when the liquid ratio exceeds 0.7 m ² / L. L
And the solubility limit of metal ion concentration by the conventional method is about 55.
Considering that it was g / L, it can be seen that the metal surface can be dissolved more efficiently than the conventional method.

[Example 2] In Example 1 described above, the sample at room temperature was immersed in the foam-like decontamination solution, but in Example 2, the sample was preheated to 50 ° C in the same manner as in Example 1. It was immersed in a foamy decontamination solution at room temperature. The decontamination solution amount was 100 mL, and the other conditions were the same as in Example 1. The results are shown in FIGS. 3 and 4.

As shown in FIG. 3, when the sample was preheated to 50 ° C., the dissolution reaction reached a peak about 20 minutes after immersion,
It can be seen that decontamination can be performed faster than in the case of Example 1. Fig. 4 shows changes in the concentration of eluted metal ions. When the sample was preheated to 50 ° C, it was about 20 after immersion.
It can be seen that the metal ion concentration reaches saturation in minutes, and the dissolution proceeds sufficiently in a short time. In addition, in order to heat the actual radioactive metal waste, it is preferable to adopt a remote heating method using high frequency.

[Embodiment 3] In the above Embodiments 1 and 2, data was collected by using the experimental apparatus, and FIG. 5 shows a decontamination apparatus of the embodiment that is actually used. In FIG. 5, 1 is a dissolution tank, 2 is a decontamination liquid supply tank, 3
Is a circulation tank for the decontamination liquid, and 4 is a pump for feeding the decontamination liquid from the circulation tank 3 for the decontamination liquid to the dissolution tank 1. In the present invention, a foam generator 5 is attached to the dissolution tank 1, and compressed air is blown into the decontamination solution sent to the dissolution tank 1 by the pump 4 to foam the decontamination solution. It has a structure to send to 1.

A decontamination waste liquid tank 6 stores the decontamination waste liquid discharged from the dissolution tank 1 and returns it to the decontamination liquid circulation tank 3 by a pump 7. A defoaming device 8 passes the gas discharged from the dissolution tank 1 through the defoaming device 8, and further includes a scrubber 9, a HEPA filter 10, and a blower.
It is released into the atmosphere via 11. By using the apparatus of Example 3, the foaming decontamination liquid can be constantly filled in the dissolution tank 1, and the method for decontaminating metal waste according to the present invention can be carried out on a large scale. .

[0016]

As described above, according to the decontamination method of the present invention, the heat of dissolution of metal waste is insulated by the foam-like decontamination solution without heating the decontamination solution from the outside. By raising the temperature of the metal waste to increase its dissolution rate, it is possible to perform decontamination efficiently and superior to conventional decontamination. Further, since the melting tank is not heated, its life can be extended, and energy for heating is not required, so that there is an advantage that running cost can be reduced. Furthermore, the decontamination apparatus of the present invention has an advantage that the above decontamination method can be carried out on a large scale. Therefore, the present invention greatly contributes to the industry as a method and apparatus for decontaminating a metal waste that solves the conventional problems.

[Brief description of drawings]

FIG. 1 is a graph showing a temperature change in Example 1.

FIG. 2 is a graph showing the relationship between the liquid ratio, the metal ion concentration, and the dissolution depth in Example 1.

FIG. 3 is a graph showing temperature changes in Example 2.

FIG. 4 is a graph showing changes in dissolved metal ion concentration in Example 2.

FIG. 5 is a block diagram showing a decontamination apparatus of Example 3.

[Explanation of symbols]

 1 melting tank, 5 foam generator

Claims (5)

[Claims] 1. A decontamination liquid in the form of a foam containing an acid is adhered to the surface of the metal waste, and the decontamination liquid adhered to the surface of the metal waste is removed by holding for a predetermined time. Decontamination method for metal waste. 2. Removal of metal waste according to claim 1, wherein the foam formed by adding a surfactant to the acid has a diameter of 0.1 to 2 mm and a liquid / foam ratio of 1 to 10% by volume. Dyeing method. 3. The method for decontaminating a metal waste according to claim 1, wherein the acid used is nitric acid 10 to 30% by weight, hydrochloric acid 0 to 10% by weight, and hydrofluoric acid 0 to 10% by weight. 4. The method for decontaminating a metal waste according to claim 1, wherein the metal waste is preheated to 50 to 100 ° C. for the purpose of shortening the treatment time. 5. A decontamination device for metal waste, wherein a foam generator is attached to a dissolution tank for metal waste in which a decontamination solution containing an acid is circulated.