JPS62238B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for decontaminating metal surfaces, particularly for removing oxide films containing radioactivity attached to metal surfaces of equipment and equipment in nuclear power plants. Concerning a decontamination method suitable for

[Background of the invention]

Conventionally, methods for decontaminating metal surfaces, particularly metal surfaces of devices and equipment used in nuclear power plants, can be broadly classified into the following methods.

(1) High-pressure water jet method This method decontaminates the surface of the object to be decontaminated using a high-pressure water jet. Although this method is effective for decontaminating soft metal surfaces such as mild steel, it is not necessarily effective for decontaminating metal surfaces such as stainless steel. for example,
A high-pressure water jet of 600 atmospheres shaves 0.16 mm off mild steel, but not stainless steel. However, in reality, many objects to be decontaminated are made of stainless steel, such as fuel pools and casks. Therefore, in order to improve this method so that it is possible to decontaminate stainless steel surfaces, there are three ways to improve this method: (a) extend the time the water jet is applied, (b) increase the pressure of the water jet, and (c) add an abrasive material to the water jet. (d) Raise the water jet temperature. Or (e) Adding chemicals to the water jet could be considered. However, (a), (b), and (c) increase the amount of decontamination waste, and (d) is due to the equipment used. There are problems such as it is difficult to raise the temperature above the above level, (c) and (e) are concerned about damage to materials, and (e) generates a large amount of chemical waste. Therefore, it cannot be said that the high-pressure water jet method is necessarily suitable for decontamination.

(2) Brush method This method decontaminates the surface of the object to be decontaminated by scrubbing it with a brush. However, even with this method, it is difficult to remove the radioactive elements contained in the oxide film that has grown and adhered to the metal surface, and is hardly removed.
Furthermore, since radioactive substances are removed by adhering to the brushes, the brushes must be replaced as appropriate in order to improve decontamination efficiency, which is cumbersome.

(3) Electrochemical method This is a method that generally uses electrochemical methods to remove radioactive elements adsorbed onto metal surfaces or incorporated into surface oxide films. There are two types of this electrochemical method. One is an electrolytic polishing method that uses an anodic oxidation method, which makes it easier to remove surface contaminants by dissolving the surface of the base material itself into the metal. The other method uses cathodic reduction to easily dissolve surface oxides and decontaminate them. Since these methods dissolve the surface of the object to be decontaminated, the mechanical action force may be smaller than in mechanical decontamination as in (1) and (2) above. However, since the electrochemical method also requires an electrolyte solution, there is a problem in the amount of decontamination waste. Particularly when the object to be decontaminated is a tank, for example, which has a large ratio of internal volume to internal surface area, a large amount of decontamination waste is generated.

[Purpose of the invention]

Therefore, an object of the present invention is to provide a simple, low-temperature, and efficient metal surface decontamination method that uses electrochemical techniques and can greatly reduce the amount of decontamination waste.

[Summary of the invention]

In the metal surface decontamination method of the present invention, an electrolyte film is brought into contact with the metal surface to be decontaminated, an electrode as a counter electrode is brought into contact with the surface of the electrolyte film, and the metal surface is placed between the metal surface and the electrode. The idea is to allow current to flow through the surface with the surface as a positive or negative pole.

The electrolyte film can be made of a polymeric material mixed with a complexing agent or an electrolyte, a gel mixed with a complexing agent or an electrolyte, or a polymeric material that is itself an electrolyte.

To explain the basic concept of the present invention with reference to FIG. 1, first, a polymeric or gel-like film, that is, a film 2, is brought into contact with the surface of a metal object 1 to be decontaminated, and a counter electrode is placed on the surface of this film. The metal electrode 3 is brought into contact. A complexing agent or the like capable of capturing eluted metal ions is placed in the film 2. Therefore, the film becomes an electrolyte and becomes conductive. In this state, when electricity is applied from the power source 4 so that the object to be decontaminated 1 becomes the positive electrode and the counter electrode 3 becomes the negative electrode, the metal base material dissolves and the eluted metal ions are captured by the complexing agent in the film 2. be done. As a result of the base material surface being dissolved in this way, the surface oxide becomes loose and easily removed from the object to be decontaminated. If the film 2 is made sticky, when the film 2 is peeled off from the surface of the object 1 to be decontaminated after decontamination, loose surface oxides will also be removed.

When the direction of the current is reversed to the above, the metal base material does not dissolve, but the surface oxide dissolves, and the metal ions eluted at this time are captured by the complexing agent in the film 2 and fixed in the film. Similarly, when the film 2 is peeled off, it is removed together with the film 2. As a result, in the present invention, the electrolyte required for electrochemical decontamination is only the above film, so the amount of decontamination waste is significantly reduced.

[Embodiments of the invention]

First, we will explain that it is possible to decontaminate stainless steel surfaces using electrochemical methods.

Figure 2 shows the current-potential curve of stainless steel with an oxide containing a radioactive element attached to its surface. This curve shows a mixed aqueous solution (PH ₌
6) Obtained by measuring a stainless steel test piece immersed in the water. The curve in the figure is the current-potential curve of stainless steel with this oxide film attached. The curve is a current-potential curve for stainless steel whose oxide film has been mechanically removed for comparison.
Positive current corresponds to anode melting of the stainless steel and decomposition of H ₂ O to generate O ₂ . The negative current is used to dissolve the oxide film and
Corresponds to the generation of _H2 gas. Generation of H ₂ or O ₂ gas was visually observed when the current density exceeded ±several tens of mA/cm ² . However, a significant portion of the increase in current on the positive and negative sides is due to the dissolution of the oxide film or base material.

Figure 3 shows the negative current density and the
The relationship with the removal rate of Co ⁶⁰ is shown. This curve is the second
This was obtained by measuring in the same solution as in the measurement of the current-potential curve shown in the figure. The energization time was 1 hour. As is clear from this figure, decontamination efficiency improves as the current density increases up to approximately 0.1 mA/cm ² .
Furthermore, when this test piece was subjected to ultrasonic cleaning, the removal rate of Co ⁶⁰ was improved to over 90%. This is thought to be because the oxide film on the stainless steel surface was dissolved by electrochemical operation, making it easier to peel off, and was peeled off by applying ultrasonic waves. That is, it can be seen that the oxide film becomes easier to peel off mechanically when electrochemical manipulation is applied.

FIG. 4 shows the relationship between the positive current density and the Co ⁶⁰ removal rate. The solution used was 65% phosphoric acid,
It is a mixed solution of 20% H ₂ SO ₄ and 15% H ₂ O. Electrolysis time was 30 seconds, and after electrolysis, ultrasonic cleaning was performed. As is clear from this figure, it can be seen that decontamination is also possible with a positive current.

Figure 5 shows the electrolysis time when a positive current is applied.
The relationship with Co ⁶⁰ removal rate is shown. In this case, the current density is 50 mA/cm ² and the solution is the same as in FIG. As is clear from this figure, it can be seen that the decontamination effect improves within a short time.

Next, an example of an electrochemical decontamination method using a polymer film based on the present invention will be described.

First, a polyvinyl alcohol aqueous solution was prepared.
In this case, the molecular weight was approximately 1000 and the concentration was 10%. This was mixed with EDTA- _2NH4 salt (50%
salt). As shown in FIG. 6, this mixed solution was applied to a thickness of about 5 mm on the stainless steel 1 on which the oxide film containing the radioactive element had been deposited to form a film 2. An electrode 3 made of stainless steel was bonded onto this polymer film 2 as a counter electrode. This stainless steel electrode 3 and the oxide film are connected to the ammeter 5 between the stainless steel electrode 3 and the stainless steel 1.
Power was supplied from the power source 4 via the.

FIG. 7 shows the relationship between the Co ⁶⁰ removal rate and the electrolysis time when the oxide coated stainless steel 1 side is set as the positive electrode. The current density was 10 mA/cm ² . In addition, Co ⁶⁰
The removal rate was obtained by measuring after electrolyzing, drying the film 2 for about one day, and peeling it off.

FIG. 8 shows the relationship between electrolysis time and Co ⁶⁰ removal rate when stainless steel with an oxide film is used as the negative electrode. The current density was 0.1 Am/cm ² . In this case as well, after electrolysis, the coating 2 was dried for one day and peeled off, after which the Co ⁶⁰ removal rate was measured.

Figure 9 shows the relationship between electrolytic density and Co ⁶⁰ removal rate when a negative current is applied. The electrolysis time was 50 minutes. As described above, the removal rate of Co ⁶⁰ increases as the current density increases. However, since there is a slight tendency to saturation, even if the current density is extremely increased, the current will be consumed in H ₂ gas generation and there will be a large loss, so there is no point.

From the above results, it is clear that by using a polymer electrolyte film and an electrochemical method, it is possible to decontaminate while significantly reducing the amount of decontamination waste.

As an example of covering the surface of the object to be decontaminated with an electrolyte film, that is, coating 2, as described in the above example,
The following method is also effective. First, the polymer for making the polymer electrolyte film does not need to be limited to polyvinyl alcohol, and the complexing agent added thereto does not need to be limited to EDTA.

Furthermore, it is also possible to use a gel instead of a polymer to make the electrolyte film 2. In this case,
Instead of peeling off the polymer film, after decontamination,
The gel can be washed away with water, etc. There is no specific type of gel, but glycerophthalic acid gel (ester of glycerol and phthalic acid), glycerophosphonic acid gel, deposide gel, gelatin, agar, silica gel, etc. are effective.

The chemicals to be mixed into the polymer film or gel are not particularly limited, but include (1) inorganic acids such as HNO ₃ , H ₂ SO ₄ , phosphoric acid, (2) EDTA, oxalic acid, citric acid,
organic acids such as nitrilotricomplex acid (NTA), formic acid,
or these Na or NH ₄ salts, (3) ready-made commercially available decontamination liquids such as Citrox, Turcodecone, Clidecone (all trade names) are effective. These concentrations are most effective in the range of 0.01 to 10%.

If the adverse effect of the residual decontamination agent on the material to be decontaminated is a problem, a polymer in which the polymer itself becomes an electrolyte may be selected. In this case, if the film is peeled off after decontamination, the remaining chemicals will disappear. Effective polymers of this type include polyacrylamide, polyacrylic acid, polyamine, and the like.

Furthermore, as explained in FIG. 2, even if the current density is increased in order to increase the decontamination efficiency, O ₂ or H ₂ gas is generated and the decontamination efficiency does not increase as much as expected.
To improve this problem, it is effective to use an organic solvent that does not generate H ₂ and O ₂ by electrolysis without using an aqueous solution. For example, ethanol, methanol, dimethylsulfoxide (DMSO), dimethylsulfoformaldehyde (DMF), trimethylformaldehyde (TMF), etc. are effective. At this time, the polymer is not particularly specified, but synthetic rubber, nylon polystyrene, vinyl chloride, methacrylic acid ester, etc. are effective.

When decontaminating a metal wall surface used below the water surface, such as the side of a pool, as shown in Figure 10,
It is more effective to use an adhesive polymer electrolyte that has already been formed into a film. In this case, an adhesive polymer electrolyte film 2 integrated with a metal foil 3 is bonded onto the surface of a metal wall 1 to which radioactive elements have adhered, and electricity is supplied from a power source 4 between the wall surface 1 and the metal foil 3. do. After electrolysis is finished,
This film is peeled off and removed.

In the above electrolysis, the surface of the metal to be decontaminated can be on either the positive or negative side, but when setting it on the positive side, it is most effective to set the current density in the range of 1 to 500 mA/ ^cm2 , and also on the negative side. When the current density is on the side, it is most effective to set the current density in the range of 0.01 mA/cm ² to 10 mA/cm ² . Further, an alternating current of 1 to 500 mA/cm ² may be applied on top of these direct currents, and it is most effective if the frequency is between 0.001 Hz and 1000 Hz.

In each of the examples described above, the electrolyte film was attached to the entire surface of the metal to be decontaminated. However, an alternative embodiment is also possible, as shown in FIG. 11, in which the electrolyte and counter electrode are moved in the form of a roller. That is, a roller-shaped polymer electrolyte film 2 and a counter electrode 3 supporting it are brought into contact with and moved over the metal surface 1 to be decontaminated.
Electricity is applied between the electrode 3 and the counter electrode 3 using a power source 4. 6 is the handle of the roller. As the polymer electrolyte film 2, it is convenient to use an already formed film as described in the above description. The range of current density is the same as in the previously described embodiment.

In each of the embodiments described above, the counter electrode 3 was brought into contact with the entire surface of the electrolyte film 2. However, in this case, when an electrolyte film with a large area is used, the solvent in the electrolyte film becomes difficult to evaporate due to the presence of a counter electrode covering it.
As a result, there is a problem that the drying time of the electrolyte film is prolonged.

Therefore, in order to improve this point, as shown in FIG.
It is effective to open a large number of

In addition, as shown in FIG. 13, the same effect can be obtained by making the counter electrode 3 into a roller shape and moving it in contact with the electrolyte film 2 applied to the surface of the object to be decontaminated. This movable counter electrode 3
When using this method, it is convenient to use the target surface metal 1 as a positive electrode because the decontamination time can be shortened.

In addition, a method in which only the counter electrode made of metal foil is peeled off from the electrolyte film after electrolysis is completed, and then the polymer electrolyte film is peeled off and removed is also effective in shortening the drying time of the electrolyte film.

When decontaminating metal surfaces with high radiation dose rates, it is necessary to reduce the amount of radiation exposure during decontamination work. For this purpose, it is convenient if the electrolyte film is automatically adsorbed onto the metal surface.

FIG. 14 shows one embodiment for doing so, in which a large number of magnetic particles 8 are mixed inside the adhesive polymer electrolyte film 2, and these magnetic particles and the target metal surface are mixed. The film 2 is attracted to the surface 1 by applying a magnetic force between the film 2 and the surface 1. Note that instead of mixing the magnetic particles 8 into the film 2, a plate-shaped magnet may be attached to the upper surface of the metal foil 3, which is the counter electrode.

Further, as shown in FIG. 15, a counter electrode 3 having a spring function is placed on the surface of the polymer electrolyte film 2.
Before using it, roll it into a spiral shape, so to speak, like a Baumkuchen shape, and when you use it,
An embodiment in which decontamination is performed by electrolyzing the electrolyte film 2 by pressing the electrolyte film 2 against the surface 1 to be decontaminated by the force of a spring is also possible. In this case, as above, the current density is 1 to
500mA/ ^cm2 range, 0.01 for negative side
It is effective to set the current to ~10 mA/cm ² .

〔Effect of the invention〕

According to the present invention, since an electrochemical method is used, a large mechanical force is not required as in mechanical decontamination methods such as a high-pressure water jet method or a brush method. In addition, when the object to be decontaminated has a large internal volume compared to the area to be decontaminated, such as a container, or when it is a device that required immersion in a tank containing an electrolyte solution in conventional technology. Even in
According to the present invention, since a polymeric or gel-like electrolyte film is used, the amount of decontamination waste can be significantly reduced compared to the conventional method.

[Brief explanation of the drawing]

Figure 1 is a basic conceptual diagram of the present invention, Figure 2 is a current and potential curve in a stainless steel solution with an oxide film attached, and Figure 3 is a graph showing the flow of a negative current in a solution to a sample coated with ^Co60 . Figure 4 shows the relationship between Co ⁶⁰ removal rate and current density when ^{a positive current is passed in a solution to a sample with Co 60 attached} . The figure 5, which shows
A diagram showing the change over time in the removal rate of Co ⁶⁰ when a positive current is applied to a sample with Co ⁶⁰ attached in a solution. Figure 6 is a conceptual diagram of one embodiment of the present invention, and Figure 7 is the same diagram. Figure 8 shows the change over time in the Co ⁶⁰ removal rate when a positive current is passed through the decontamination target in the example. Figure 8 shows the Co ⁶⁰ removal rate when a negative current is passed through the decontamination target in the same example. Figure 9 shows the relationship between Co 60 removal rate and current density when a negative current was passed through the object to be decontaminated in the same example, and Figure 10 shows the relationship between the Co ⁶⁰ removal rate and the current density. A schematic diagram of an example in which the polymer electrolyte film is integrated. Figure 11 shows an example in which the electrolyte film and the counter electrode are shaped like a roller. Figure 12 shows a number of holes in the counter electrode that covers the electrolyte film. FIG. 13 is a diagram showing an embodiment in which a roller-like counter electrode is brought into contact with an electrolyte film, FIG. 14 is a diagram showing an embodiment in which a magnet is mixed in the electrolyte film, and FIG. 15 is a diagram showing an embodiment in which a magnet is mixed in the electrolyte film. FIG. 2 is a diagram showing an example in which a spring-like counter electrode is integrated with an electrolyte film. 1...Surface of decontamination target, 2...Electrolyte film, 3...Counter electrode, 4...Power source, 5...Ammeter, 6
...Handle, 7...hole, 8...magnet.

Claims (1)

[Claims] 1. An electrolyte film is brought into contact with the metal surface to be decontaminated, and an electrode as a counter electrode is brought into contact with the surface of the electrolyte film, and the metal surface is placed between the metal surface and the electrode. A metal surface decontamination method characterized by flowing current as a pole or a negative pole. 2. The electrolyte film is made of a polymer material mixed with a complexing agent or an electrolyte.
Metal surface decontamination method described in section. 3. The metal surface decontamination method according to claim 1, wherein the electrolyte film is made of a gel mixed with a complexing agent or an electrolyte. 4. The metal surface decontamination method according to claim 1, wherein the electrolyte film is made of a polymeric substance that is itself an electrolyte.