JPH06138295A - Method for reducing and chemically decontaminating radioactive metal waste - Google Patents

Abstract

PURPOSE:To provide a chemical decontaminating method for radioactive metal wastes, which enables the DC power supply required for electroytic reduction to be miniaturized and eliminates the need for adding source. CONSTITUTION:When radioactive metal wastes are decontaminated by chemically dissolving the surfaces of the radioactive metal wastes using a strong acid decontaminant after electrolytic reduction for a predetermined time, at the early stage of immersion of the radioactive metal wastes in the strong acid decontaminant, the above electrolytic reduction is carried out by bringing part of the metal wastes 1 into contact with the strong acid decontaminant 2, applying voltage and current to that part, and holding the metal wastes 1 for a predetermined time so that the immersion potential of the metal wastes 1 enters an active dissolution range. After the electrolytic reduction is stopped, the metal wastes 1 are entirely immersed in the decontaminant 2 for chemical dissolution and decontamination.

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 in particular, not only the clad (contamination oxide film) where the contamination of the metal waste adheres firmly to the surface of the waste but also the metal. The present invention relates to a reduction chemical decontamination method for radioactive metal waste made of stainless steel that has permeated and diffused into the 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 shredded into dedicated containers such as drums. It is put in and stored in the waste storage. However, the number is accumulating year by year, and securing the storage place is a big 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 heat melting has been proposed. However, in these methods, the contaminated portion of the surface contaminated with radioactive material and most of the non-contaminated portion are collectively treated, so that it cannot be said that the volume reduction effect is remarkable.

On the other hand, decontamination / volume reduction treatment, which enables to remove only the contaminated part on the surface and to dispose most of the uncontaminated part in the same manner as general waste, is a promising technique for reducing the volume of radioactive metal waste. Is being watched. As this decontamination / volume reduction technology, electro-decontamination method, blast decontamination method, etc. have been developed.In these methods, it is necessary to insert an electrode or nozzle into the decontamination target, and the valve, pump It is said that it is difficult to decontaminate a complicated shape such as. The most general decontamination method applicable to such complicated shapes is a chemical decontamination method in which a metal waste is immersed in a decontamination solution to dissolve the contaminated surface. As the decontamination solution, 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 achieve decontamination quickly and reliably. For the purpose of decontamination and volume reduction, a strong acid having a large ability to dissolve 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 comparable to that of general waste, both the clad and the metal base material should be efficiently treated. A chemical decontamination method that can dissolve and remove well is required. In general, a metal waste made of stainless steel has a dense passivation film, and therefore acid dissolution hardly progresses only by immersing it 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 oxidatively dissolved using a solution in which an oxidizing agent such as tetravalent cerium is added to nitric acid or sulfuric acid, and the clad is peeled off (for example, JP-A-61-204597). However, in this method, a large amount of expensive tetravalent cerium is consumed, so that it is necessary to regenerate cerium by an electrolytic operation, and there is a problem that the apparatus and the operation are complicated.

Therefore, in order to destroy and dissolve the clad and the dense passivation film on the surface of the radioactive metal waste made of stainless steel, in order to dissolve it, an electrochemical reduction reduction is carried out using an external power source, and then an oxidizing agent is added. A method has been proposed in which the metal base material is dissolved in an acid in a sulfuric acid solution to reduce the amount of an oxidizer such as tetravalent cerium used (JP-A-2-2259).
7 publication). However, in this method, for the decontamination object having a large area because the electrolytic reduction is performed as the first step, the voltage and current required for the electrolytic reduction become large, and a large-capacity DC power supply is required. Along with this, there is a problem that the device becomes complicated, such as a thick power cable.

[0006]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art, can efficiently decontaminate radioactive metal waste made of stainless steel having a large area, and downsizes the DC power supply required for electrolytic reduction. It is an object of the present invention to provide a chemical decontamination method that can be performed and does not require addition of an oxidizing agent.

[0007]

The present invention takes advantage of the dissolution characteristics of stainless steel in a strong acid solution, that is, when a portion of stainless steel is temporarily held in the active dissolution zone by electrolytic reduction, the cladding and impurities Even after the passive film is destroyed, the dissolution reaction starts, and the electrolytic reduction operation is stopped, the dissolution reaction is propagated to the entire stainless steel one after another by the action of the local battery, and the cladding and the passive film are chemically reacted in the strong acid solution. Utilizing the fact that it is dissolved and removed, a part of the metal waste is brought into contact with a strong acid decontamination solution for electrolytic reduction, and then the whole is brought into contact with a decontamination solution for chemical decontamination in the decontamination solution. By
The above-mentioned problems have been achieved.

That is, the method for reducing chemical decontamination of radioactive metal waste according to the present invention is such that the radioactive metal waste is electrolytically reduced for a predetermined time at the initial stage of immersion in the strong acid decontamination solution, and then the surface of the metal waste is decontaminated with strong acid. In a method for decontaminating radioactive metal waste by chemically dissolving it in a dyeing solution to remove radioactive substances, in the electrolytic reduction operation, a part of the metal waste is brought into contact with a strong acid decontaminating solution, and a voltage is applied to the part. By applying an electric current and holding the immersion potential of the metal waste for a predetermined time so as to be in the active dissolution region, then the electrolytic reduction operation is stopped, and then the entire metal waste is immersed in the decontamination solution, It is characterized by being chemically dissolved and decontaminated.

In the method of the present invention, as a method of contacting only a part of the radioactive metal waste with the decontamination solution, the radioactive metal waste is hung by a conductive suspending tool, and only a part of the lower part is used as the decontamination solution. It can be done by dipping or by exposing a part of the surface of the metal waste and covering the rest with a removable non-conductive shield, the whole radioactive metal waste is immersed in the decontamination solution. it can. In this way
After a part of the surface of the metal waste is brought into contact with the decontamination solution to carry out the electrolytic reduction operation, the electrolytic reduction operation is stopped, and in the case of the latter method, the non-conductive shield is removed, and then the metal waste is removed. Chemical dissolution is promoted by immersing the whole in the decontamination solution.

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

As a result, the exposed and activated metal base material starts melting as shown in the formula (2), and at this time, it emits electrons. M → M ⁺ + e (2) These electrons cause the remaining cladding and passive film to be reduced and dissolved as shown in the following formula (3) without electrolytic reduction. 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 / current to the entire metal waste. Therefore, after the electrolytic reduction operation is stopped, the entire metal waste is immersed in the decontamination liquid to allow the above reaction to proceed.

[0012]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is an explanatory view showing an embodiment of the present invention. In FIG. 1, only a part of the lower part of the radioactive metal waste 1 is immersed in the decontamination solution 3 in the decontamination tank 2 and connected to the cathode side of the DC power supply 4 and between the electrode 5 connected to the anode. To electrolyze. The potential at this time is measured by the 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 while (specifically, about 5 minutes) by the above-mentioned electrolytic reduction device, the clad and the passive oxide film on the surface are dissolved and removed, and the metal base material is removed. Dissolution reaction begins. After that, by immersing the whole radioactive metal waste 1 in the decontamination liquid 3 in the decontamination tank 2, the whole dissolution reaction proceeds by the reactions of the above formulas (1) to (3), and decontamination is performed. To be 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 removable and covered with a non-conductive shield 8. And 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, the entire radioactive metal waste 1 is brought into contact with the decontamination solution 3, and the dissolution reaction proceeds, Achieve decontamination. Examples of the material of the shield 8 used in this method include a non-conductive material such as plastic.

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

[0015]

[Table 1]

[0016]

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

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram 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]

 1 Radioactive metal waste 2 Decontamination tank 3 Decontamination liquid 4 DC power supply 5 Electrode 6 Potential measuring device 7 Reference electrode 8 Shield

Claims (3)

[Claims] 1. Radioactivity is obtained by electrolytically reducing a radioactive metal waste in a strong acid decontamination solution for a predetermined period of time, and then chemically dissolving the surface of the metal waste in the strong acid decontamination solution to remove radioactive substances. In the method for decontaminating a metal waste, in the electrolytic reduction operation, a part of the metal waste is brought into contact with a strong acid decontamination solution, and a voltage / current is applied to the part to adjust the immersion potential of the metal waste to an active dissolution zone. The radioactive metal waste is characterized in that it is held for a predetermined time so that the electrolytic reduction operation is stopped, and then the entire metal waste is immersed in the decontamination solution and chemically dissolved for decontamination. Reduction chemical decontamination method for materials. 2. The radioactive metal waste is suspended by a conductive suspender, and a part of the lower part is immersed in the decontamination solution to bring a part of the metal waste into contact with the decontamination solution.
Method for reducing chemical decontamination of radioactive metal waste as described. 3. A part of the metal waste is removed by immersing the whole radioactive metal waste in which a part of the surface is exposed and the remaining part is covered with a removable non-conductive shield in a decontamination solution. 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 liquid after the electrolytic reduction operation is carried out by bringing it into contact with a dyeing solution and the electrolytic reduction operation is stopped. Reductive chemical decontamination method.