JPH06249999A - Method and apparatus for removing radioactive contamination - Google Patents

Abstract

PURPOSE:To provide a method and apparatus for removing radioactive contamination efficiently from not only the surface layer but also the metallic body. CONSTITUTION:A hollow cylindrical electrode 1 has a rotary shaft 11 penetrating through the electrode. The shaft 11 is also hollow except the lower end part thereof and an umbrella valve 12 is formed integrally thereabout. A hole 13 is made through the side part of the electrode 1 at a position shifted slightly from the tip of the valve 12. The cylindrical part is covered with an unwoven abrasive cloth 2 applied with abrasive grains. Plus and minus voltages are then applied, respectively, to an object S to be polished and the electrode 1 which is rotated by means of a motor while feeding an electrolyte to the abrasive cloth 2 thus pressing the object S against the abrasive cloth 2 and polishing the object S. The composite electrolytic polishing unit is moved by means of a truck and guided by means of guide rollers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing metal contaminated by radiation, including reactor wells, and an apparatus used for the same.

[0002]

BACKGROUND OF THE INVENTION Contamination of metal with radioactive substances reaches a depth not only on the surface but also up to a depth of 20 μm, so residual mechanical activity can be reduced by conventional mechanical brushing. It was very difficult to reduce to the background level.

FIG. 3 is a schematic diagram showing a contamination model of a primary cooling system stainless steel pipe. At a depth of 5 μm from the surface, soft clad 31 (Co-60, Mn-54, Z
The contamination is due to r-95-Nb-95 = αFe ₂ O ₃ ) and is mainly hard clad 32 (C) from 5 μm to 7 μm.
o-60, Mn-54, = αFe 2 O 3 · Fe 3 O 4)
Pollution. Further, the lower oxide film 33 and the stainless steel base material 34 are contaminated with Co-60, and the contamination reaches about 20 μm from the surface of the stainless steel base material 34.

FIG. 4 is a graph showing the relationship between the amount of polishing and the residual radioactivity. When the surface is removed by, for example, 10 μm by the conventional method, the radioactivity level is slightly reduced, but to reduce it to the level in the background (BG) region (10 ^-4 μCi / cm ^-2 ), further polishing of the metal base material is possible. However, there is a problem that the work time is long. Further, when mechanically polishing the surface, the decontamination performance for an object to be polished having a complicated surface shape was extremely poor.

Therefore, instead of mechanical polishing, a method of immersing an object to be polished in an electrolytic solution and performing electrolytic polishing utilizing the electrolytic action caused by passing an electric current can be considered. When electrolytic polishing is performed, the current does not directly act on the soft clad 31 on the surface layer, but easily passes through it and acts on the hard clad 32 on the lower layer to separate the hard clad. Along with this, the soft clad 31 is also removed. Furthermore, the electric current reaches the oxide film 33 and the stainless steel base metal 34,
Decontamination in these is possible. However, the immersion type electropolishing requires a container and an electrolytic solution for immersing the object to be polished, and when the object to be polished is large, a large container and a large amount of the electrolytic solution are required.

The present invention has been made in view of such circumstances, and by applying the composite electropolishing method, utilizing the electrolytic elution action and the abrasive grain abrasion action, not only the surface but also the oxide film and the coating It is an object of the present invention to provide a method for removing a radioactive contaminant capable of efficiently decontaminating even a base material for an abrasive and an apparatus used for the implementation.

[0007]

A method for removing a radiation contaminant according to the present invention is a method for removing a metal contaminated by radiation, which comprises mechanical polishing by rubbing an abrasive, and an electrolytic solution. It is characterized in that the metal is removed by using a composite electropolishing method which is a combination of electropolishing utilizing an electrolytic action obtained by passing an electric current as a medium.

The composite electropolishing apparatus according to the present invention scrapes a polishing material, applies a-voltage to the electrodes while supplying an electrolytic solution, and applies a + voltage to the object to be polished, thereby performing mechanical polishing and electrolysis. A composite electropolishing apparatus for performing polishing, comprising a substantially cylindrical electrode having a rotation axis at its center, a polishing material mounted so as to cover the cylindrical portion of the electrode, and electrolytically applied to a plurality of locations of the polishing material. And means for supplying a liquid.

[0009]

According to the present invention, by applying the complex electropolishing method to remove the radiation contaminant, the removal can reach the base material of the object to be polished, and the effect of reducing the exposure can be obtained. That is, the composite electropolishing method removes only the film formed on the convex portion of the passivation film formed on the anode surface (the object to be polished) due to the electrolysis phenomenon by abrasive grain abrasion and selectively concentrates the electrolytic elution action on this part. The metal mirror surface is efficiently obtained. FIG. 5 is a schematic view showing a processed state of composite electrolytic polishing. On one surface side of the electrode 21, a polishing material 22 made of a non-woven fabric having abrasive grains adhered is attached, and an electrolytic solution is supplied to the polishing material 22. Then, a voltage of + is applied to the object to be polished 23, a voltage of-is applied to the electrode 21, and the abrasive 22 is pressed against the object to be polished 23 while rotating the abrasive 22 (electrode 21) to perform polishing. . The method of the present invention can be applied to objects to be polished of any shape and size, and the applicable range can be easily expanded only by changing the shape of the abrasive 22 (electrode 21).

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings showing the embodiments. FIG. 1 is a schematic view showing a composite electropolishing apparatus used when decontaminating a sidewall of a reactor well by the method for removing a radioactive contaminant according to the present invention.
1 (a) and 1 (b) show a front view and a sectional view taken along the line AA, respectively. The electrolysis roller 4 is supported inside the housing 3 lacking one side surface with its shaft portion in the vertical direction in a state of slightly protruding. At the four corners of the housing 3, guide rollers 5, 5, 5, 5 having a smaller diameter than the electrolysis roller 4 are provided.
The shaft portions are supported in the vertical direction with the same protrusion. A motor 6 for rotating the electrolytic roller 4 and the guide rollers 5, 5, 5, 5 is attached to the upper portion of the housing 3. Further, a hose 7 for supplying an electrolytic solution via a shaft portion is attached to the electrolytic roller 4 by a pressure pump from an electrolytic solution tank (not shown), and power supply for supplying electricity to the electrode of the electrolytic roller 4 is provided. A cable 8 is attached. The main body of the composite electropolishing apparatus having the above-described structure is suspended by the wire 9 from the traveling carriage 10 provided on the upper portion thereof.
The position of the composite electrolytic polishing apparatus main body can be adjusted by controlling the movement of the composite electrolytic polishing apparatus.

FIG. 2 is a schematic sectional view of the electrolytic roller.
In the figure, reference numeral 1 is an electrode having a hollow, substantially cylindrical shape, and is provided with a shaft portion 11 which is a rotating shaft penetrating the center. The inside of this shaft portion 1a is hollow except for the lower end portion, and umbrella-shaped valves 12, 12 ... Are integrally formed around the periphery. Further, on the side surface of the electrode 1, holes 13, 13 are formed at positions slightly offset from the tips of the valves 12, 12.
… Has been opened. The columnar portion is covered with the abrasive 2 made of a non-woven fabric to which abrasive grains are attached.

A positive voltage is applied to the object S to be polished, a negative voltage is applied to the electrode 1, and the electrode 1 (abrasive material 2) is rotated by the motor 6 while supplying the electrolytic solution to the abrasive material 2. Then, the abrasive 2 is pressed against the object S to be polished. The main body of the complex electropolishing apparatus is moved by the traveling carriage 10, and the guide rollers 5, 5, 5, 5 are rotated by the motor 6 to guide it. A 20% sodium nitrate aqueous solution (neutral salt) was used as the electrolytic solution.

Removal of radiation contaminants by using the method of the present invention has a great effect of reducing exposure due to residual radioactivity, and simplifies the work such that the next decontamination can be performed only with jet water. You can

When stainless steel is polished, a polished surface having a maximum surface roughness of 0.05 to 0.1 μm is obtained. Table 1 shows a comparison of characteristics between the composite electrolytic polishing method used in the method of the present invention and the immersion electrolytic polishing method.

[0015]

[Table 1]

Most of the processing discharge discharged after decontamination becomes ions and dissolves in the liquid. This processing discharge is in a dissolved state as an ion in the acidic electrolytic solution used in electropolishing, but becomes an oxide in the neutral electrolytic solution used in the method of the present invention and precipitates in the solution. . Therefore, if the processed discharge is removed by filtering using filter paper, etc., the solution can be circulated and used many times, and it is not necessary to monitor the amount of contamination of the electrolytic solution as in electrolytic polishing. Liquid volume is relatively small. Further, since the surface layer of the object to be polished is also polished by mechanical polishing, polishing is performed at a higher speed than electrolytic polishing, and the working time can be shortened.

In the method of the present invention, the electrolytic solution is of the jet type, so that the processing posture does not matter. That is, by simply exchanging the shape of the electrode and the shape of the abrasive material, pipes and valves that are the decontamination target of nuclear facilities, equipment of large and small objects, and all shapes and sizes from flat to uneven surfaces. Can respond.

[0018]

As described above, in the method for removing radiation pollutants according to the present invention, by applying the composite electropolishing method, not only the surface but also the oxide film is utilized by utilizing the electrolytic elution action and the abrasive grain abrasion action. Also, the present invention has excellent effects such as effective decontamination of the base material of the object to be polished to reduce exposure.

[Brief description of drawings]

FIG. 1 is a schematic view showing a composite electropolishing apparatus according to the present invention.

FIG. 2 is a schematic cross-sectional view showing an electrolytic roller in the device of the present invention.

FIG. 3 is a schematic diagram showing a contamination model.

FIG. 4 is a graph showing the relationship between the amount of polishing and the residual radioactivity.

FIG. 5 is a schematic view showing a processing state of composite electrolytic polishing.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electrode 2 Abrasive material 3 Housing 4 Electrolytic roller 5 Guide roller 6 Motor 7 Hose 8 Power supply cable 9 Wire 10 Traveling trolley 11 Shaft 12 Valve 13 Hole S Object to be polished

Claims (2)

[Claims] 1. A method of removing a metal contaminated by radiation, which comprises mechanical polishing by rubbing an abrasive, and electrolytic polishing utilizing electrolytic action obtained by passing an electric current using an electrolytic solution as a medium. A method for removing radiation contaminants, characterized in that the metal is removed using a composite electropolishing method in which 2. A composite electropolishing apparatus that performs mechanical polishing and electropolishing by rubbing an abrasive, applying a − voltage to an electrode while supplying an electrolytic solution, and applying a + voltage to an object to be polished. And a substantially cylindrical electrode having a rotation axis at the center thereof, an abrasive material mounted so as to cover the cylindrical portion of the electrode, and means for supplying an electrolytic solution to a plurality of locations of the abrasive material. A composite electropolishing device characterized by the above.