JP3010943B2 - Reduction chemical decontamination method for radioactive metal waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for chemical decontamination of radioactive metal waste, and more particularly to a method of chemically contaminating radioactive metal waste in addition to cladding (contaminated oxide film) in which metal waste is strongly adhered to the surface of waste. The present invention relates to a method for reducing chemical decontamination of stainless steel radioactive metal waste which has also penetrated and diffused into a base material.

[0002]

2. Description of the Related Art Radioactive metal waste generated in nuclear power plants is mainly waste materials such as pipes and valves. Conventionally, this kind of waste is cut into dedicated containers such as drums by shredding. And stored in waste storage. However, the number is accumulated year by year, and securing a storage place is a major problem. Therefore, it is required to reduce the volume of this type of metal waste, and a method of reducing the volume by compression or heating and melting has been proposed. However, in these methods, since the contaminated portion of the surface contaminated with the radioactive substance and most of the non-contaminated portion are treated collectively, the volume reduction effect cannot be said to be remarkable.

On the other hand, a decontamination / volume reduction process that removes only the contaminated portion of the surface and allows most non-polluted portions to be disposed of in the same manner as general waste is promising as a technology for reducing the volume of radioactive metal waste. Have been watched. Electrode-release and blast decontamination methods have been developed as decontamination and volume reduction technologies.However, these methods require the insertion of electrodes or nozzles into the object to be decontaminated, and require valves and pumps. It is said that it is difficult to decontaminate complicated shapes such as. The most common decontamination method applicable to such complicated shapes is a chemical decontamination method in which metal waste is immersed in a decontamination solution to dissolve the contaminated surface. As the decontamination liquid, those mainly composed of organic acids such as oxalic acid and citric acid, and strong acids such as sulfuric acid, nitric acid, hydrochloric acid and hydrofluoric acid are used, but it is necessary to quickly and surely achieve decontamination. For the purpose of certain decontamination volume reduction, a strong acid having a large dissolving capacity for metals is used.

By the way, in order to decontaminate radioactive metal waste not only in the clad but also in the metal base material to a radioactivity level equivalent to that of general waste, both the clad and the metal base material need to be efficiently treated. A chemical decontamination method that can be dissolved and removed well is required. In general, since a dense passive film is present in stainless steel metal waste, acid dissolution hardly progresses simply by dipping in a strong acid solution such as sulfuric acid. Therefore, a decontamination method has been proposed in which a metal base material made of stainless steel is oxidized and dissolved using a solution in which an oxidizing agent such as tetravalent cerium or the like is added to nitric acid or sulfuric acid, and the clad is peeled off (for example, JP-A-61-204597). However, this method consumes a large amount of expensive tetravalent cerium, so that it is necessary to regenerate cerium by an electrolytic operation, and there has been a problem that the apparatus and operation are complicated.

[0005] Therefore, in order to break and dissolve the cladding and the dense passive film on the surface of the radioactive metal waste made of stainless steel, it is electrochemically reduced and dissolved using an external power supply, and then an oxidizing agent is added. A method has been proposed in which the amount of an oxidizing agent such as tetravalent cerium is reduced by dissolving a metal base material in acid in a sulfuric acid solution (JP-A-2-2259).
No. 7). However, in this method, for the decontamination target having a large area to perform electrolytic reduction as the first step, the voltage and current required for electrolytic reduction become large, and a large-capacity DC power supply is required. Along with this, there has been a problem that the apparatus becomes complicated, for example, the power cable becomes thicker.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art and can efficiently decontaminate radioactive metal waste made of stainless steel having a large area, and reduces the size of the DC power supply required for electrolytic reduction. It is an object of the present invention to provide a chemical decontamination method which can be carried out without adding an oxidizing agent.

[0007]

The present invention utilizes the dissolution characteristics of stainless steel in a strong acid solution. That is, when a portion of stainless steel is temporarily held in an active dissolution zone by electrolytic reduction, cladding or non-cladding occurs. The active film is destroyed, the dissolution reaction starts, and even after the electrolytic reduction operation is stopped, the dissolution reaction propagates to the entire stainless steel one after another by the action of the local battery, and the cladding and passive film are chemically formed in a strong acid solution. Performing electrolytic reduction by contacting a part of metal waste with strong acid decontamination solution using the fact that it is dissolved and removed, and then performing chemical decontamination in the decontamination solution by contacting the whole with the decontamination solution By
The above object has been achieved.

That is, according to the method for reducing chemical decontamination of radioactive metal waste according to the present invention, the radioactive metal waste is electrolytically reduced for a predetermined time at the initial stage of immersion in a strong acid decontamination solution, and then the surface of the metal waste is subjected to strong acid decontamination. In a method of decontaminating radioactive metal waste by removing a radioactive substance by chemically dissolving in a dyeing liquor, the electrolytic reduction operation is performed by contacting a part of the metal waste with a strong acid decontamination liquid and applying a voltage to the part. An electric current is applied, and the immersion potential of the metal waste is maintained for a predetermined time so as to be in the active dissolving region.After that, the electrolytic reduction operation is stopped, and then the entire metal waste is immersed in the decontamination solution, It is characterized by chemical dissolution and decontamination.

In the method of the present invention, as a method of bringing only a part of the radioactive metal waste into contact with the decontamination liquid, the radioactive metal waste is hung by a conductive hanging tool, and only a part of the lower part is converted into the decontamination liquid. This can be done by immersing or immersing the entire radioactive metal waste in a decontamination liquid, exposing a part of the surface of the metal waste and covering the remaining part with a removable non-conductive shield. it can. In this way,
Performing an electrolytic reduction operation by bringing only a part of the surface of the metal waste into contact with the decontamination liquid, stopping the electrolytic reduction operation, and removing the non-conductive shield in the case of the latter method, and then removing the metal waste. By immersing the whole in the decontamination solution, chemical dissolution proceeds.

In the method of the present invention, a strong acid such as sulfuric acid or nitric acid is used as a decontamination solution. Hereinafter, the case where sulfuric acid is used will be described for simplicity. Sulfuric acid decontamination solution for stainless steel base material (H ₂ SO ₄ concentration 1.8 mol /
FIG. 3 shows a polarization (potential-current) curve showing the dissolution characteristics in (I, temperature 80 ° C.). In FIG. 3, the stainless steel radioactive metal waste in the passive region is immersed in a sulfuric acid decontamination solution, and the electric potential is operated in a negative direction. The potential is manipulated for a period of time to enter the chemically active active region. The current required to achieve this potential increases in proportion to the area of decontamination of the metal waste. Specifically, a DC power supply of about 100 A was required for a 1 m ² contaminated surface. By the way, the chemical reaction in the present invention proceeds as shown in equation (1), and the cladding and the passive film on the surface of the metal waste are destroyed by reduction and dissolution, exposing the metal base material. MO + 2H ⁺ + e → M ⁺ + H ₂ O (1)

The exposed and activated metal base material starts melting as shown in the equation (2), and emits electrons at this time. M → M ⁺ + e (2) The remaining cladding and passive film are reduced and dissolved by the electrons without electrolytic reduction as shown in the following equation (3). MO + 2H ⁺ + e → M ⁺ + H ₂ O (3) That is, if the reaction of the formula (1) occurs in a part of the metal waste, the reactions of the formulas (2) and (3) proceed one after another. Therefore, it is not necessary to apply voltage and current to the entire metal waste. Therefore, after stopping the electrolytic reduction operation, the entire metal waste is immersed in a decontamination solution to allow the above reaction to proceed.

[0012]

Next, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is an explanatory diagram showing one embodiment of the present invention. In FIG. 1, only a part of the lower part of the radioactive metal waste 1 is immersed in a decontamination solution 3 in a decontamination tank 2, connected to the cathode side of the DC power supply 4, and between the electrode 5 connected to the anode. To perform electrolysis. The potential at this time is measured by a reference electrode 7 such as a platinum electrode and the potential measuring device 6. By holding a part of the radioactive metal waste 1 in the active dissolution zone for a short period of time (specifically, about 5 minutes) by the above-described electrolytic reduction apparatus, the cladding and passive oxide film on the surface are dissolved and removed, and the metal base material is removed. The dissolution reaction starts. Thereafter, the entire radioactive metal waste 1 is immersed in the decontamination solution 3 in the decontamination tank 2, whereby the entire dissolution reaction proceeds by the reactions of the above formulas (1) to (3), and decontamination is performed. Achieved.

FIG. 2 is an explanatory view showing another embodiment of the present invention. In the method shown in FIG. 2, only a part of the radioactive metal waste 1 is exposed, and the remaining part is covered with a detachable and non-conductive shield 8, so that only the exposed part is decontaminated liquid 3 , The electrolytic reduction operation is performed in the same manner as described above, and after the electrolytic reduction operation is stopped, the shield 8 is removed, and the entire radioactive metal waste 1 is brought into contact with the decontamination solution 3 to allow the dissolution reaction to proceed. Achieve decontamination. As a material of the shield 8 used in this method, a non-conductive material such as plastic is used.

Next, the effects of the present invention will be concretely described by numerical values through actual decontamination experiments. In the conventional method, a metal waste having an area of 1 m ² requires a current of 100 A, and when a current to a current-carrying jig or the like is further applied, a current of 12 A is obtained.
Power supply equipment having a power supply capacity of 0 A was required. On the other hand, according to the present invention, it is sufficient to energize about 1/5 of the area,
Since there is no need for extra current to the current-carrying jig, 20 A
Decontamination could be achieved. Furthermore, the immersion dissolution time required for decontamination hardly changed. Table 1 shows the experimental results.

[0015]

[Table 1]

[0016]

According to the method of the present invention, it is possible to reduce the DC power supply capacity required for causing a reaction in the active dissolution zone in the electrolytic reduction operation, and it is possible to reduce the capacity of stainless steel having a large area. Even in the case of decontaminating radioactive metal waste, it is possible to reduce the size and simplification of decontamination equipment and to reduce the required amount of electricity. Further, chemical dissolution can be performed without adding an expensive oxidizing agent.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing one embodiment of the present invention.

FIG. 2 is an explanatory view showing another embodiment of the present invention.

FIG. 3 is a polarization curve diagram of stainless steel in a sulfuric acid decontamination solution.

[Explanation of symbols]

 Reference Signs List 1 radioactive metal waste 2 decontamination tank 3 decontamination liquid 4 DC power supply 5 electrode 6 potential measurement device 7 reference electrode 8 shield

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G21F 9/28

Claims (3)

(57) [Claims] At the initial stage of immersing a radioactive metal waste in a strong acid decontamination liquid, electrolytic reduction is performed for a predetermined time, and then the surface of the metal waste is chemically dissolved in the strong acid decontamination liquid to remove radioactive substances. In the method for decontaminating metal waste, the electrolytic reduction operation is performed by bringing a part of the metal waste into contact with a strong acid decontamination solution, applying a voltage / current to the part, and changing the immersion potential of the metal waste to an active dissolution zone. Radioactive metal waste, characterized in that the electrolytic reduction operation is stopped, and then the entire metal waste is immersed in the decontamination solution and chemically dissolved for decontamination. Method of reducing chemical decontamination of materials. 2. The method according to claim 1, wherein the radioactive metal waste is suspended by a conductive suspending tool, and only a part of the lower part is immersed in the decontamination liquid to bring a part of the metal waste into contact with the decontamination liquid.
The method for reduction chemical decontamination of radioactive metal waste according to the above. 3. A part of the metal waste is removed by immersing the whole of the radioactive metal waste in a decontamination solution, exposing a part of the surface and covering the remaining part with a removable non-conductive shield. The radioactive metal waste according to claim 1, wherein the non-conductive shield is removed and the entire metal waste is immersed in the decontamination solution after performing the electrolytic reduction operation by contacting with the dyeing solution and stopping the electrolytic reduction operation. Reduction chemical decontamination method.