JP3144141B2 - Electropolishing decontamination method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic polishing decontamination method.

[0002]

2. Description of the Related Art Generally, a nuclear power plant whose service life has passed has been planned to be dismantled and separated. In this dismantling work, the surfaces of various equipment and pipes constituting the facility were polished and ⁶⁰ Co, ¹³⁷ C
A so-called decontamination treatment for removing scales containing various radioisotopes such as s, ²³⁹ Pu is performed in advance. This reduces the exposure of workers performing dismantling work, and
The disposal of the waste generated as a result of dismantling can be facilitated.

[0003]

However, most nuclear power plants are made of metallic materials such as stainless steel. Therefore, as one of the methods for decontaminating such metal materials, an electrolytic polishing method utilizing an electrolytic separation action has been proposed. The electrolytic polishing method is, for example, a method in which a metal material to be polished is used as an anode and immersed in an electrolytic solution of an electrolytic cell to perform electrolysis, and the scale is separated and removed in a short time to smooth the surface of the metal material to be polished. Is what you can do. Conventionally, an aqueous solution of phosphoric acid or sulfuric acid has been generally used as an electrolytic solution in the electrolytic polishing method.

[0004] However, when such an electrolytic solution is used, the separated scale is retained in a state of being dissolved in the electrolytic solution after one electrolytic polishing operation.
Each time, it is formed in a large amount as a secondary radioactive contaminant, and there is a problem that its treatment method needs to be considered separately.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and provides an electrolytic solution capable of facilitating treatment of an electrolytic solution after an electrolytic polishing operation and improving reusability of the electrolytic solution. It is intended to provide a polishing decontamination method.

[0006]

In order to achieve the above object, the present invention provides an electrolytic polishing step using nitric acid as an electrolytic solution, and heating the used electrolytic solution produced in the electrolytic polishing step to nitric acid. Comprises a calcining step of separating radioactive substances as solid waste by separating water, a washing step of washing the separated nitric acid, and a concentration step of removing the water content of the nitric acid washed in the washing step. And an electrolytic polishing decontamination method in which nitric acid concentrated in the concentration step is reused in the electrolytic polishing step.

[0007]

According to the electrolytic polishing decontamination method of the present invention, the radioactive substance adhered to the member to be polished in the electrolytic polishing step is separated and dissolved in nitric acid as an electrolytic solution. Then, by separating the nitric acid in which the radioactive substance is dissolved in the calcining step, the radioactive substance is deposited as solid waste. On the other hand, the nitric acid separated from the radioactive substance is concentrated in the concentration step after being washed in the washing step, so that it can be reused in the electrolytic separation step.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the electrolytic polishing decontamination method according to the present invention will be described below with reference to FIGS. Electrolytic polishing decontamination method of this embodiment, as shown in FIG. 1, an electrolytic polishing process S ₁ for peeling the scale M (radioactive material) containing a radioactive isotope from the surface of the metal material (non-abrasive metal material), calcining the step S _2, the electrolytic polishing nitric acid vapor V and nitrogen oxides NO _X produced in the temporary baking process step S ₁ of heating the spent electrolyte L ₁ used in electropolishing polishing step S ₁ It has and a recycling step S ₃ for reusing at.

[0009] The electrolytic polishing process S ₁ is a metal material such as stainless steel are those which electrolysis is immersed in the electrolytic solution L ₀ as an anode, in the present embodiment, as the electrolytic solution L _0, using nitric acid ing. Nitric acid causes an electrolytic action similar to that of a conventionally used sulfuric acid or phosphoric acid-based electrolytic solution, and separates scale M adhering to the surface of a metal material as an anode from the surface of the metal material. It is known that a nitric acid compound can be used and is decomposed in a relatively low temperature state (about 500 ° C.) to become nitrogen oxide NO _x . Here, in the electropolishing polishing step S _1, using nitric acid concentrated to 80% or more concentration. Then, as a result of the electrolytic action, the scale M of the radioactive substance peeled off from the surface of the metal material is converted into the electrolytic solution L
With nitric acid is ₀ as spent electrolyte L _1, is adapted to discharge the calcining step S ₂ to be described later.

[0010] The calcination step S _2, for example, it is assumed that the use of rotary Khalsa Inner 1 as shown in FIG. The rotary calciner 1 includes a straight tube 2 arranged in an inclined state, and a cylindrical heater 3 arranged around the tube 2. The tube 2 is supported so as to be rotatable around its axis, and is rotated around the axis by a drive motor (not shown). Further, the heater 3 is arranged in a state of being divided into a plurality in the axial direction of the tube 2 so that the temperature of the tube 2 can be changed in the axial direction.

[0011] wherein the upper end opening 2a of the tube 2, the electrolytic polishing process inlet tube 4 the spent electrolyte L ₁ discharged from S ₁ is introduced into the tube 2 and, nitric acid, which is then generated in the tube 2 the discharge pipe 5 for discharging the steam V and nitrogen oxides NO _X in the outer tube 2 is provided. on the other hand,
The lower end opening 2b of the tube 2, the waste discharge pipe 7 for discharging waste 6 from the spent electrolyte L ₁ removed nitrate component (solid waste) is provided.

In the recycling step S ₃ , the discharge pipe 5
A cleaning step S ₄ of washed and nitric acid vapor V and nitrogen oxides NO _X emissions from the concentration step S ₅ that is concentrated by evaporating the water in an aqueous nitric acid solution produced in the washing step S _4, the the high produced by concentration step S ₅ the concentration of nitric acid has and a feedback line 8 for feeding back to said electrolytic polishing step S _1.

[0013] Examples of cleaning step S _4, for example, assumes a dedusting apparatus such as a scrubber (not shown). The said scrubber, the are discharge pipe 5 disposed in the rotary Khalsa Inert 1 is connected, by washing a mixture of nitric acid vapor V and nitrogen oxides NO _X is is inserted inside of the scrubber, Removing dust from the mixture,
In addition, the nitric acid aqueous solution is regenerated.

In order to decontaminate the metal material to which the scale M containing a radioisotope is attached by the electrolytic polishing decontamination method configured as described above, first, the electrolytic polishing step S ₁
The metal material is introduced into the electrolytic solution L ₀ to generate a potential difference to cause an electrolytic action. Thus, the scale M adhering to the surface of the metallic material will be dissolved in nitric acid stripped an electrolytic solution L ₀ as nitric compounds.

Next, the used electrolytic solution L ₁ generated by the electropolishing operation is introduced into the rotary calciner ₁ in the calcination step S _{2 through} the introduction pipe 4. At this time, in the rotary calciner 1, the tube 2 is rotated around its axis by operating the drive motor, and the heater 3 is operated to maintain a constant temperature along the axial direction. A gradient is formed. The temperature gradient is caused by the upper end opening 2 of the tube 2.
a side, about 100 ° C., lower end opening 2b of tube 2
The side is formed to have a temperature of about 500 ° C.

As a result, the used electrolyte L ₁ charged into the tube 2 is first heated to about 100 ° C., whereby a part thereof is evaporated to become nitric acid vapor V. Then, the nitric acid vapor V flows into the upper end opening 2 of the tube 2.
is discharged from the discharge pipe 5 provided on a on the outer tube 2 will be put into the washing step S ₄ in the subsequent stage. Further, the remaining used electrolyte L _{1 that} has not been evaporated is heated while being lowered along the axial direction of the tube 2, and is heated to the nitrogen oxide NO _X and the oxide of the scale M as the waste 6. Will be decomposed into And nitrogen oxide NO
_X, while being put into the washing step S ₄ through the discharge pipe 5 together with the nitrogen oxides NO _X, waste 6 is discharged from the waste discharge pipe 7, the waste treatment facility which is disposed downstream (not shown ).

In this case, since the waste 6 discharged from the calcining step S ₂ is formed into a powdery solid by the rotary calciner 1, the waste 6 is greatly compared with the used electrolyte L _1. There is an advantage that the volume can be reduced and the handling property can be improved because it is solid.

Moreover, nitrogen oxides are turned from the rotary Khalsa Inert 1 in the washing step S ₄ NO _X and nitric acid vapors V
, By being washed in the washing step S _4, is taken out as a nitric acid solution removed dust, the concentration of a degree that can be used in the electrolytic polishing process S ₁ by being subsequently remove moisture in the concentration step S ₅ It becomes nitric acid which has. Then, by re-introducing the nitric acid through feedback line 8 to the electrolytic polishing process S _1, reuse nitric acid, there is an effect that it is possible to carry out the decontamination of a large amount of metallic material.

[0019]

As described in detail above, the electrolytic polishing decontamination method according to the present invention comprises an electrolytic polishing step using nitric acid as an electrolytic solution, and heating the used electrolytic solution to separate nitric acid and remove radioactive substances. A calcining step of precipitating as solid waste, a washing step of washing the separated nitric acid, and a concentration step of removing the water content of the washed nitric acid, and by reusing the concentrated nitric acid, It has the effect of. Since the electrolytic solution used in the electropolishing step is nitric acid which can be decomposed at a relatively low temperature, the radioactive substance can be taken out as solid waste by separating in the calcination step. As a result, the volume of waste discharged by the decontamination work can be significantly reduced, the processing space can be reduced, and the handling property can be improved because the waste is deposited as a solid. In the calcining step, nitric acid can be easily separated from radioactive waste, and the separated nitric acid can be reused by washing and concentrating it. As a result, a large amount of decontamination work can be performed with a small amount of the electrolytic solution.

[Brief description of the drawings]

FIG. 1 is a schematic process diagram showing one embodiment of an electrolytic polishing decontamination method according to the present invention.

FIG. 2 is a schematic view showing a calcining step of the electrolytic polishing decontamination method of FIG.

[Explanation of symbols]

L ₀ electrolyte L ₁ used electrolyte M scale (radioactive substance) NO _X nitrogen oxide S ₁ electropolishing step S ₂ calcination step S ₃ recycling step S ₄ cleaning step S ₅ concentration step V nitric acid vapor 1 rotary calciner 2 Tube 2a Upper end opening 2b Lower end opening 3 Heater 4 Introductory pipe 5 Discharge pipe 6 Waste (solid waste) 7 Waste discharge pipe 8 Feedback line

Claims (1)

(57) [Claims] 1. A method for removing a radioactive substance from a member to be polished to which the radioactive substance is attached, comprising: an electrolytic polishing step using nitric acid as an electrolytic solution; A calcining step of heating the liquid to separate out nitric acid to precipitate radioactive substances as solid waste, a washing step of washing the separated nitric acid, and a concentration step of removing the water content of the nitric acid washed in the washing step Wherein the nitric acid concentrated in the concentration step is reused in the electrolytic polishing step.