JPS60162800A - Method for decontaminating surface of metallic member - Google Patents

Abstract

PURPOSE:To remove efficiently a contaminant by immersing a pair of metallic members having surfaces contaminated by radioactivity in an electrolytic soln. at an interval and by supplying AC using the metallic members as electrodes. CONSTITUTION:An electrolytic cell 1 is filled with an electrolytic soln. 2 consisting of about 70wt% phosphoric acid having 1.84 specific gravity, about 20wt% sulfuric acid having 1.84 specific gravity and about 10wt% water. A pair of metallic members 31, 32 having surfaces contaminated by radioactivity are immersed in the soln. 2 so that they confront each other. The members 31, 32 are connected to a DC power source 4, and they are electropolished. Conditions during the electrodeposition include about 60 deg.C temp., 60Hz AC cycle, about 70A/dm<2> current density and about 15min electrolysis time.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing surface contamination of metal members, and particularly to a method useful for decontaminating metal members contaminated with radioactivity.

At nuclear power plants, waste metal parts contaminated with radioactivity are generated during periodic inspection work, etc., but at present, these contaminated wastes are cut into appropriate sizes and stored in sealed drums etc. It is stored in containers. However,
In the future, as the number of nuclear power plants increases and the number of years of operation increases, the amount of radioactively contaminated waste will inevitably increase, and difficult problems are expected in terms of securing and managing storage space.Contaminated waste that should be stored It is desired to develop a method that can reduce the radioactivity of objects.

By the way, conventional methods are used to remove radioactive contaminants adhering to the surface of metal members.

The first method is a physical decontamination method, in which solid particles such as alumina, silica sand, ice particles, glass bulbs, l1llf, and dry ice are used as they are, or water, detergent, and sometimes acids or alkalis are added to them. This is an abrasive that is sprayed onto the surface to be decontaminated or physically rubbed to remove contaminants.

The m2 method is a chemical decontamination method that uses non-acid such as sulfuric acid, hydrochloric acid, and nitric acid, strong alkali such as caustic soda and caustic potash, and am
By using an aqueous solution containing acid such as citric acid or citric acid, and sometimes EDTA (ethylenediaminetetraacetic acid) as a treatment liquid, or by blowing the treatment liquid,
It removes contaminants through its chemical dissolving action.

However, among the conventional methods, the third method described below, that is, the electrochemical method, has the highest decontamination efficiency. This method uses a single solution such as phosphoric acid, sulfuric acid, or a mixture thereof as an electrolyte, and conducts an electric current using the metal member to be decontaminated as an anode (anode) and lead, platinum, graphite, stainless steel, etc. as a cathode (cathode). This method performs electrolytic polishing and is generally called the electrolytic dyeing method. When decontaminating contaminated stainless steel parts using this electrolytic dedying method, a method using a phosphoric acid/sulfuric acid mixture as the electrolyte is considered to be the most efficient and promising method. However, depending on the degree of contamination, there is an advantage that a neutral salt aqueous solution such as sodium sulfate or sodium chloride can be used as the electrolyte.

However, in the case of the conventional electrolytic decontamination method described above, the object to be decontaminated is used as an anode and the other metal electrode is used as a cathode to pass a direct current electrolytic current, so that decontamination is performed only on the anode side. If the object to be decontaminated is used for both electrodes, and it becomes possible to decontaminate not only the anode side but also the cathode side, the decontamination efficiency can be dramatically improved.

The present invention has been made in view of the above circumstances, and provides a method for removing surface contamination of metal members, which improves the conventional electrolytic decontamination method and enables decontamination of radioactively contaminated metal members on both the anode side and the cathode side. This is what we provide.

That is, the first method of removing surface contamination from a metal member according to the present invention
The method is characterized in that a metal member whose surface is contaminated with radioactivity is held apart in an electrolytic solution, and an alternating current is passed through the metal member as an electrode.

In addition, the second method for removing surface contamination according to the present invention is to use the same metal member as an electrode after carrying out the first method described above.
Furthermore, it is characterized by flowing a direct current.

In the present invention, since alternating current is used as the electrolytic current, the anode and cathode are alternately reversed, and the object to be decontaminated can be electrolytically polished using both poles. Therefore, the processing capacity is approximately twice that of the conventional electrode dyeing method using direct current, and is significantly improved.

In addition, in the conventional electrode dyeing method using a DC power supply, polarization (a type of electrical resistance) occurs on the anode side due to metal elution and oxygen gas generation, and on the cathode side, metal elution does not occur, but hydrogen gas is generated. Because polarization occurs due to
In order to counteract this, it is usually necessary to load a high voltage, which causes an increase in power consumption. In contrast, in the decontamination method of the present invention that employs AC electrolysis, metal elution is observed from both electrodes, but the polarization is destroyed because oxygen gas and hydrogen gas are generated alternately at the same electrode (this phenomenon is generally called negativity). Therefore, it is possible to perform electrolysis at a lower voltage than in the conventional decontamination method using DC electrolysis, and power consumption can be reduced. Furthermore, since polarization does not occur, even if the object to be decontaminated has a complicated shape, the current will flow to every corner of the object, thus further improving the electrolytic decontamination effect.

On the other hand, electrolytic polishing using AC electrolysis has the problem that the surface condition inevitably becomes rough, but this problem is addressed in the second part of the present application.
This problem can be solved by performing AC electrolysis and then performing DC electrolysis as in the invention.

Hereinafter, the present invention will be explained in more detail based on Examples.

FIG. 1 shows an example of a specific apparatus for implementing the present invention. In the figure, 1 is an electrolytic cell, and the electrolytic cell 11 is filled with an electrolytic solution 2. A pair of metal members 31 and 32 to be decontaminated are immersed in the electrolytic solution 2 and hung facing each other as shown. On the other hand, as an electrolytic power source, the DC power source 4 is used as an electrolytic power source. The electrolysis source 4 is connected to the metal members 3r and 32 to be decontaminated via a DC/AC converter 5 (inverter). Further, an ammeter 6 is provided to measure the electrolytic current, and a voltmeter 7 is provided to measure the voltage between the electrodes.

Next, an electrolytic polishing experiment conducted using the apparatus shown in FIG. 1 will be described as an example.

Example 1 5us304 with dimensions of 50aX 100aX1 rum
The electrode metal member 31°32 in Fig. 1 is made of stainless steel.
This was used for electrolytic polishing under the following conditions.

In addition, as a comparative example, the conventional electrolytic fA polishing method using DC electrolysis, that is, the 5 us 304 stainless steel electrode 31.32
Similar electrolytic polishing was performed by connecting directly to the DC power source 4.

[Electrolysis conditions] Composition of electrolytic solution; Phosphoric acid (specific gravity 1.84) 70% by weight Sulfur III (specific gravity 1.84) 20 Water 10! l Liquid temperature; 60°C AC cycle; 60H2 Current density; 2OA/dm Electrolysis time: 15 minutes For the above Example 1 and Comparative Example, the amount of reduction in the layer due to elution of the electrode metal member (only on the anode side in the Comparative Example) was In addition, the difference in power consumption was determined by measuring the voltage during electrolysis at the same current density. The results are shown in Table 1 below.・Table 1 As is clear from the results, Example 1 using AC electrolysis
In this case, the elution of the electrode material is slightly inferior to that in the comparative example of DC electrolysis, but in the case of Example 1, the electrode metal members 3 of both electrodes are removed in one electrolysis.
1.32 can be electrolyzed, so the processing amount is 2 in the comparative example.
It turned out to be twice as powerful. Further, the surface of the electrode metal member after electrolysis was not as good in Example 1 as in the comparative example using DC electrolysis, but no problem was observed in the decontamination work.

On the other hand, the voltage during electrolysis was also 10 to 30% lower in Example 1 than in the comparative example, confirming that power consumption could be reduced.

Next, change the frequency of the AC electrolysis II source to 3H2,101-1z
, 30H7, respectively, and performed the same electrolytic polishing as in Example 1, and examined the influence of frequency. It was found that electrolytic polishing is possible at both poles at any frequency, and electrolytic dedying can be performed using alternating current. was confirmed.

Example 2 Using the same electrode metal members 31 and 32 as used in Example 1, this was electrolytically polished using AC under the same conditions as Example 1. After that, the operation of the inverter 5 was stopped and the conventional DC current was used. Electrolytic polishing was performed for 3 minutes using a solution. As a result, the surface roughness of the electrode metal member on the cathode side is 0.8 to 1.2μ.
The results were the same as in Example 1, but the surface condition of the electrode metal member on the anode side was improved to 0.5 to 0.8 μm, the gloss increased, and a mirror finish was achieved.

The fact that the electrolytically polished surface of the metal member is mirror-finished in this way has the following meaning in decontaminating the radioactively contaminated member. In other words, when a decontaminated metal member used as an electrode is pulled up from the electrolyte after the completion of electrolytic de-dying, if the surface is rough, the electrolyte will adhere to it and be easily carried away. In this case, the electrolyte is less likely to adhere to the surface of the metal member to be decontaminated, and less likely to be carried away to the outside.

In the case of electrolytic de-dying of radioactively contaminated parts, contaminants are eluted into the electrolytic solution, which means that external diffusion of radioactivity can be prevented, which is extremely advantageous in terms of safety management.

Example 3 A 5us304 steel chevron 8M having the cross-sectional shape shown in Fig. 2
(a-60am, b-60ms+, C=5ao+
) were used as electrode metal members 3s and 32, and electrolytic polishing by AC electrolysis was performed under the same conditions as in Example 1 (however, the electrolysis time was 60 minutes). At that time, the angle irons 31 and 32 were placed in the electrolytic solution facing each other as shown in the figure.

After 60 minutes of electrolytic polishing, the thickness of the angle iron used for the electrode metal member was measured at five locations (10M intervals) shown in Figure 3.
It was measured with.

Further, as a comparative example, the same angle steel as above was electrolytically polished by conventional DC electrolysis, and the thickness of the angle steel used for the anode was measured in the same manner as in Example 3 after completion of electrolysis.

FIG. 3 shows the results of Example 3 (curve X) and the results of Comparative Example (track 4IIY). From this result, it was confirmed that AC snow melting produced the same level of electrolysis deep into the corners of the angle steel, and that it was superior in uniform electrolysis performance compared to the DC electrolysis of the comparative example.

As described in detail above, the method for removing surface contamination of metal members according to the present invention, which is an improvement over the conventional electrolytic decontamination method, makes it possible to decontaminate radioactively contaminated metal members on both the anode side and the cathode side. Uniform decontamination can be achieved even when the metal member has a complicated shape.In an example conducted to confirm the uniform decontamination effect, the electrode metal member Fig. 3 is a cross-sectional view showing the shape of the angle iron used in the above, Fig. 3 is a side view showing the position of thickness measurement, and Fig. 4 shows the results of an example conducted using the angle iron shown in Fig. 2. It is a line diagram.

DESCRIPTION OF SYMBOLS 1... Electrolytic tank, 2... Electrolyte, 31, 32... Electrode metal member, 4... DC power supply, 5... Inverter,
6... Ammeter, 7... Voltmeter.

Claims (2)

[Claims] (1) A method for removing surface contamination from a metal member, which is characterized by holding a metal member whose surface has been contaminated by radioactivity in an electrolytic solution at a distance, and passing an alternating current through the metal member as an electrode. (2) A metal member whose surface is contaminated with radioactivity is held separately in an electrolytic solution, an alternating current is passed through the metal member as an electrode, and then a direct current is passed through the same metal member as an electrode. A method for removing surface contamination from metal parts.