JPS60140199A - Method of electrolytically decontaminating radioactive metallic waste - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for electrolytically decontaminating radioactive metal waste having an oxide film layer containing a radioactive substance and a bare metal portion containing a radioactive substance.

Radioactive metal waste is mainly waste materials such as pipes and valves, and conventionally, this type of waste has been cut into small pieces, unpacked into drums, and stored on-site.

However, the number of drums in storage is increasing year by year, and securing storage space is considered to be a major problem. In this type of waste, most of the radioactive cladding is present in the oxide film layer on the surface, but radioactive materials are also diffused, permeated, and activated in the bare metal.
Removing the oxide film layer deposited on the surface is not a complete decontamination; therefore, it is necessary to quickly dissolve and remove the base metal as well.

Therefore, the electrostatic dyeing method, which can quickly remove contamination from the surface and metal parts and dispose of it as general waste, is attracting attention. As an electrolytic dedying method, a method using a strongly acidic electrolytic solution containing phosphoric acid as a main component and a method using a neutral salt-based electrolytic solution are known. The former method uses a concentrated strong acid aqueous solution, so once the radioactivity concentration in the electrolyte reaches a certain value, it is neutralized and then solidified.
There is a drawback that the electrolyte needs to be renewed each time, and the amount of secondary waste increases. In addition, in the latter method, the metal ions dissolved during electrolytic de-dying become metal hydroxides in the electrolytic solution, so the hydroxide flocs can be separated into solid and liquid and the electrolytic solution can be reused. These electrolytes include
Sodium nitrate, sodium chloride, etc. are used as electrolytes.

These neutral salt electrolytes have the advantage of being easy to reuse, but on the other hand, sodium nitrate has the disadvantage of poor removal performance for oxide film layers due to anodic oxidation of nitrate groups. has.

In addition, sodium chloride has excellent ability to remove oxide films due to the reducing power of chlorine ions, and excellent ability to melt base metal due to its corrosive nature, but due to its corrosive nature, pitting corrosion is likely to occur. There is a problem in that the metal surface becomes rough and efficient decontamination cannot be performed, and furthermore, the molten metal base metal adheres and remains on the surface to be decontaminated as hydroxide or oxide sludge. Another drawback is that great consideration must be given to the cleaning solution in view of stress corrosion cracking of stainless steel caused by chlorine ions.

The object of the present invention is to solve the above-mentioned drawbacks of the prior art, to reduce the amount of secondary waste, to quickly and completely remove the oxide film layer and base metal containing radioactive materials, and to solve the problem in handling the electrolyte. The purpose is to provide a decontamination method for radioactive metal waste that is free from decontamination.

This objective is achieved according to the invention by sequential alternating and anodic electrolysis with appropriate selection of temperature and current density.

That is, the method of the present invention consists of a first electrolytic step in which the temperature of the electrolytic solution and the current density applied to the metal are maintained in an active melting region and electrolysis is carried out in an alternating manner; It is characterized by comprising a second electrolytic step in which anodic electrolysis is carried out while maintaining the method in a hyperpassive region.

In the present invention, when electrolytic de-dying is performed using a neutral salt electrolyte, the dissolution reaction in the active dissolution region (current efficiency 100%) and the oxygen gas generation and dissolution reaction in the hyperpassive region (current 9 J > rate approx. 5%)
This method utilizes the following methods to perform quick and effective de-dying.

Although neutral salt electrolytes are used in the present invention, sodium sulfate solutions are preferred.

Hereinafter, in order to simplify the explanation, an example in which a neutral salt (sodium sulfate) is used will be described with reference to the drawings.

The polarization state of carbon steel, which is a material for piping, etc., was investigated in a 20% by weight aqueous sodium sulfate solution, and the polarization curve is shown in FIG. The polarization curve represents the anodic dissolution characteristics of metals in an electrolyte, including the active dissolution region of iron (current efficiency 100%), the region of passive film formation of iron, and the generation of oxygen due to electrolysis of water and a small amount of iron. Divide into a hyperpassive region (current efficiency 5%) where melting occurs. The present invention utilizes the fact that the dissolution rate of iron is high in the active f4 decomposition region, and that oxygen gas generation and iron melting occur in the hyperpassive region, which has a great effect on removing surface deposits, and removes radioactive metal waste. This is an attempt to efficiently decontaminate contamination.

As mentioned above, most of the contamination of waste to be decontaminated is captured in the oxide film layer on the surface, and the present invention applies alternating electrolysis to remove the oxide film layer. Figure 2 shows the oxide film removal curve, and curve a shows a current density of 0.3 A/aR1 at a cathode temperature of 60°C for 30 seconds;
Curve C shows the case where alternating electrolysis was carried out under alternating conditions for 30 seconds at the anode.Curve C shows the case where alternating electrolysis was carried out in the same manner as in a except that the electrolysis temperature was 25°C.Curve C shows the case where anodic electrolysis was carried out at a temperature of 60°C. This shows the case where As can be seen from Fig. 2, the effect of removing the oxide film by alternating electrolysis is much greater than that by anodic electrolysis.
The oxide film with composition Pe404 could be removed in 15 to 20 minutes.

However, as shown in Figure 3, the radioactive decontamination effect remains even after washing with spray water, under normal conditions without radioactive contamination (bank The ground) had not been decontaminated from radioactivity. In the case of alternating electrolysis at 60° C. in a, when the surface after film removal was observed, 11 particles were deposited, which hindered the radioactive decontamination effect. On the other hand, b
In the case of alternating electrolysis at 25°C, the film was completely removed and no deposits were observed, which is thought to be due to the radioactivity still remaining in the bare metal. Therefore, a and bo) the samples after the oxide film removal are further subjected to anodic electrolysis.

As a result of this anodic electrolysis, as shown in FIG.
In the case of anodic electrolysis (indicated by a-25 in the figure), 1
Hack grounds could be decontaminated in minutes, and even the presence of Kimono was not recognized. In addition, when a was subjected to anodic electrolysis at 60°C (indicated by a-60 in the figure), the deposits were not removed, and even after 5 minutes, 6b, which had hardly been decontaminated with radioactivity, was heated to 60°C. In the case of anodic electrolysis with C (in the figure, b-6
(indicated by 0), new deposits were formed and the radioactivity decontamination effect was low. In addition, when b is anodic electrolyzed at 25°C (
Although no deposits were observed (indicated by b-25 in the figure), the decontamination effect was low.

It has been found that such a phenomenon does not simply occur under the above conditions, and that effective decontamination can be achieved by appropriately setting the electrolysis temperature and current density.

That is, at a constant current density, as shown in FIG. 5, the amount of iron dissolution increases rapidly with a certain electrolytic temperature as a threshold value. This range in which the amount of iron/8 dissolved is increased, which is the active melting region described in FIG. 1, and is characterized by a high dissolution rate. However, since the amount of iron dissolved is large, a large amount of iron hydroxide exists near the surface to be decontaminated, and there is no disturbing effect due to the generation of oxygen gas, even in anodic dissolution, the anodic deposition phenomenon causes occurs. This deposit cannot be removed by ordinary stirring. On the other hand, when the temperature is lowered, oxygen gas generation and a slight iron dissolution reaction occur due to the reaction in the hyperpassive region, so that no deposits are formed and the effect of removing deposits is great.

The results are shown in FIG. 6 when the current density and the electrolytic temperature were variously changed, and the active solubility region and the overly immobile suspension region were determined. Based on Figure 6, most of the decontamination can be carried out by performing alternating electrolysis in the active dissolution zone, removing most of the oxide film, and quickly dissolving the bare metal. When anodic electrolysis is performed in the passive region, deposits generated during alternating electrolysis can be easily removed and radioactivity can be completely decontaminated.

Figure gA7 is a system diagram of an apparatus suitable for carrying out the method of the invention. This device mainly consists of a DC power supply 1, an alternating control device 2, a first electrolytic cell 3, a counter electrode 4, a second electrostatic cell 5, a cleaning bath 6, a centrifugal dehydrator 7, and a cement solidifying device 8. The radioactive metal waste 9 is immersed in a first electrolytic tank 3 filled with a sodium sulfate electrolyte 12 maintained at a predetermined temperature by a heater 11, and subjected to alternating electrolysis together with a counter electrode 4. If the temperature of the electrolytic solution is initially heated to a predetermined temperature, it will be maintained by the heat generated during electrolysis during subsequent constant-rich operation, so there is no need to continue heating. The electrolysis time depends on the thickness of the oxidized film, and is preferably 15 to 20 minutes for a film of about 40 μm.

The contaminants removed by the alternating electrolysis are precipitated as hydroxides or oxides, and sent to the centrifugal dehydrator 7, and then sent as sludge via piping 10 to the cement solidification device 8, where it is deconsolidated in a drum. On the other hand, the dehydrated solution after dehydration is reused as an electrolyte.

Although most of the radioactive metal waste 9 has been decontaminated by alternating electrolysis, it still contains some debris, and is then subjected to anodic electrolysis in the second electrolytic cell 5.

The waste 13 that has been decontaminated by anodic electrolysis is decontaminated by being washed with water by a spray washer 14 in the cleaning tank 6, and becomes a radioactive intensity GJ hack ground area.

With this configuration, according to the present invention, it is possible to decontaminate extremely effectively, to completely remove deposits generated during alternating electrolysis, and to use neutral salts as the electrolyte. By using lithium sulfate, there is no need to worry about corrosion to component and peripheral equipment, and it can be used as the main component of waste liquid normally generated at BWR power plants, such as recycled waste liquid of ion exchange resin. Since they are the same, this recycled waste liquid can be used as an electrolyte. In addition, the cleaning liquid can be treated with a normal waste liquid treatment system by reducing its radioactivity level.

Next, the present invention will be explained in detail with reference to Examples, but the present invention is not limited thereto.

Example 1 Steel piping contaminated with radioactivity at a nuclear power plant was electrolyzed at a temperature of 70°C and a current density of 0 using a 20% by weight aqueous sodium sulfate solution as an electrolyte and SUS as a counter electrode using the apparatus shown in Figure 7. .4A/c+d, anodic electrolysis 30 seconds, cathode electrolysis 30 seconds
Alternating electrolysis was performed for about 10 minutes. As a result, the oxide film layer could be completely removed and a decontamination coefficient of 102 could be achieved. After the alternating electrolysis, the piping is then electrolyzed at a temperature of 2 in the second electrolytic tank.
When anodic electrolysis was performed at 5°C for 1 minute at a current density of 0.3 A/ctK and further cleaning was performed, a decontamination coefficient (ratio of radioactivity level or concentration before and after decontamination) of 103 was obtained, and it was reduced to bank ground. Decontamination was possible.

Example 2 Using the equipment shown in Figure 7 for pipes manufactured by wI, which were contaminated with radioactivity at a nuclear power plant, electrolysis temperature was set at 50°C with a 20% by weight aqueous sodium sulfate solution as the electrolyte and SUS as the counter electrode.
Current density 0.2A/c, anodic electrolysis 30 seconds, cathodic electrolysis 30 seconds
Alternating electrolysis was performed for about 10 minutes. As a result, the oxide film layer could be completely removed and a decontamination coefficient of 5×102 could be achieved. After the alternating electrolysis, the piping is then subjected to an electric current of 1η in the second electrolytic tank.
When anodic electrolysis was carried out for 1 minute at a temperature of 25° C. and a current density of 0.2 A/CJA, and further cleaning was performed, a decontamination coefficient of 104 was obtained, and decontamination to the background level was achieved.

In addition, in the above example, the sulfuric acid sulfuric acid solution was
Although the explanation was given as weight %, it may be any concentration as long as it is below the saturation solubility, and considering the amount of electricity used,
As long as the electrolytic conditions in the active dissolution region and the overpassive region are maintained, it is desirable to have the concentration as high as possible1 and to lower the liquid resistance.

[Brief explanation of drawings]

Figure 1 is a polarization curve diagram of 20 wt% sodium sulfate-carbon steel system, Figure 2 is a diagram of changes in oxide film removal rate over time, Figure 3 is a diagram of changes in radioactivity intensity due to electrolysis, and Figure 4 is a diagram of anode FIG. 5 is a diagram of changes in radioactivity intensity due to electrolysis, FIG. 5 is a diagram of changes in melting type with respect to electrolysis temperature, FIG. 6 is a state diagram of reactive species, and FIG. 7 is a system diagram of an apparatus for carrying out the method of the present invention. 2... Alternating control device, 3... First electrolytic cell, 4...
Counter electrode, 5...Second electrolytic cell, 9...Radioactive metal waste. Patent Applicant: Hitachi Plant Construction Co., Ltd. Figure 1: Electrolysis time (VvsSCE) Figure 2: Electrolysis time (min) Figure 3: Electrolysis time (min) Figure 4: View of electric field (min) Figure 5 @M Wen Ying (0C) Figure 6 Electric l! l! Temperature (0C) 71st

Claims (2)

[Claims] (1) In a method for electrolytically dedying radioactive metal waste that has an oxide film layer containing a radioactive substance and a bare metal part containing a radioactive substance, the temperature of the electrolytic solution and the current density applied to the metal are set to be in the active dissolution region. It is characterized by consisting of a first electrolytic step in which the temperature of the electrolytic solution and the degree of current applied to the metal are maintained in a hyperpassive region and conducts anodic electrolysis. A method for electrolytic decontamination of radioactive metal waste. (2) The metal waste is carbon steel and the electrolyte is 10 to 20%
wt% sodium sulfate solution, and the first electrolytic step was carried out at 40% by weight.
The second electrolysis step was carried out at ~80°C with a current density of 0.1-0.4 A/cml, and the second electrolysis step was performed at 20-60°C with a current density of 0.2-0.
5 A/cnl.