JPH02201199A - Treatment of high-radioactive waste - Google Patents

Abstract

PURPOSE:To simplify a treatment stage by subjecting high-radioactive waste to a heating treatment or heating reduction treatment at a high temp. to evaporate, separating a part of the elements contained therein and cooling and capturing the elements. CONSTITUTION:The high-radioactive waste 24 to be treated is put into a heating container 14 and is heated. The high-radioactive waste 24 is the calcined body obtd. by heating the nitric acid soln. generated from a retreatment stage for, prescribed used nuclear fuel to evaporate the moisture and nitric acid. This calcined body is heated from 500 deg.C to about 3,000 deg.C, more preferably to about 1,000 to 2,500 deg.C. The elements contained in the calcined body evaporate at the sublimation points or boiling points corresponding to the respective chemical forms and are sent through a gas passage 20 to cooling and capturing devices 12a,...12n. The evaporated elements are discretely cooled and captured by the respective cooling and capturing devices 12a...12n which are controlled to the temps. corresponding to the sublimation points or boiling pots. in their chemical forms.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention treats highly radioactive waste generated in spent fuel reprocessing processes at high temperatures, and in the heating process, the chemical forms of each element are This invention relates to a highly radioactive waste treatment method that uses sublimation or boiling phenomena to separate each element and recover the residue as an assimilate.

[Conventional technology] Highly radioactive waste generated during the reprocessing of spent nuclear fuel contains transuranium elements with long half-lives and Tc (technetium), which accounts for most of the heat generation and is therefore attracting attention in processing, storage and disposal. It contains platinum group elements such as elements Cs (cesium) and Sr (strontium) and useful metals Rh (rhodium) and Pd (palladium).

Therefore, it is important to separate and recover these materials before processing them as a solidified material and use them as radiation sources, heating elements, and precious metals.
This is extremely important in making effective use of resources.

The following three methods are known as conventional techniques for recovering these elements from highly radioactive waste. These are: (j) Pre-extraction method in which target nuclides are separated from the highly radioactive waste generated in one culm of reprocessing using a special solvent: Highly radioactive waste generated in the lower stage of the reprocessing process. waste,
Ion-exchange method to separate target nuclides through Q treatment
This is a lead extraction method for platinum group elements in the vitrification process in which A is separated by 1.1% ('5).

[Problem to be Solved by the Invention 8] However, each of the prior art techniques as described in Section 1 has the following drawbacks.

■In the extraction separation method, a new solvent is introduced into the reprocessing process, which complicates the solvent treatment process and reduces the efficiency of the main extraction process.

(2) In the case of the ion exchange method, there is a problem in terms of safety because combustible substances are formed when the ion exchange resin comes into contact with nitric acid.

■In the lead extraction method of platinum group elements in the vitrification process,
Although it is possible to separate platinum group elements, secondary treatment is required to extract platinum and other parent elements alone.

A further problem with these methods is that no matter which method is adopted, highly radioactive waste cannot be treated to reduce the volume of highly radioactive waste.

The purpose of the present invention is to eliminate the drawbacks of the prior art as described above, to be able to separate and recover useful elements using an extremely simple method of heat treatment, and to do so without generating secondary waste.
Another object of the present invention is to provide a method for treating highly radioactive waste that can yield a solidified material with a high volume reduction.

[Means for Solving the Problems] The present invention, which can solve the technical problem described in 1. , a method of processing highly radioactive waste that involves cooling and collection.

Highly radioactive waste is usually a nitric acid solution obtained as an extraction residue in the reprocessing process of spent nuclear fuel F4, and contains almost all the fission products and actinides in the spent fuel. In the present invention, this nitric acid solution is heat-treated to generate moisture and nitric acid to obtain a calcined body, and the calcined body is heated to 500 to 3000°C, more preferably 1000 to 25°C.
For example, as a first step, elements that sublimate or boil in their oxide chemical form are heat treated under normal pressure or reduced pressure to reduce the sublimation point or boiling point of each element's chemical form. Each element is separated and collected using a cooling collection device set at a temperature corresponding to the temperature. Next, in the second step, highly radioactive substances are reduced by heat treatment while introducing a single processing agent such as hydrogen, and elements that boil or sublimate in the chemical form of metal are separated, and as in the first step, each It is separated and collected using a cooling collection device with a temperature corresponding to the element. Of course, the elements metallized during the temperature raising process in the first stage can be separated by sublimation boiling treatment without going through the reduction reaction process in the second stage.

Poroxide duration is a technology for removing radioactive materials from spent fuel, but this method uses krypton, iodine,
It only targets non-metallic elements such as tritium. The present invention targets metal elements, and not only removes high-boiling point radioactive materials by simply heating high-level waste to high temperatures, but also combines reduction reactions with high-calorific elements that pose problems in disposal. C, s and Sr
Both can be removed.

The residue after recovery is a metal or a mixture of metal and oxide,
It can be recovered as a highly radioactive assimilate with reduced volume.

``Action'' Most elements have boiling points or sublimation points that are different from other elements. Some elements contained in highly radioactive waste have relatively low sublimation or boiling points in the form of oxides or metals. For example, metal cesium is 690℃,
Technetium oxide is 311℃, metal cadmium is 765℃
゛C, metallic strontium has a boiling point of 1384℃ etc.
By taking advantage of this fact, highly radioactive waste can be treated at high temperatures to form oxides, or reduced with hydrogen etc. to form metals, sublimation or boiling υ, and cooling them at predetermined temperatures to produce f. ↑It is possible to separate and recover the elements for each element.

Removal of cS, & Sr reduces the effectiveness of highly radioactive solids￥
1054, and the burial density in the disposal space is greatly improved. Incidentally, if only Cs is removed, the effective injection will be only 50%, and no great effect can be expected.The boiling point of Sr oxide is at least 2430°C or higher,
As mentioned above, the boiling point of strontium is 1384°C, so strontium can be separated by combining reduction reactions as in the present invention.

Note that by carrying out the heat treatment or the thermal reduction treatment under reduced pressure, each element can be vaporized at an even lower temperature.

[Actual Example 1] FIG. 1 is a conceptual diagram showing an example of a processing apparatus used for carrying out the method of the present invention. This processing equipment has a processing bag: ηl
O, and a plurality of cooling collection devices 12a connected thereto,
..., 12n is input. The heat treatment equipment IO is
Consisting of a heating container 14, a heating element 16, etc., a processing gas supply port 18 is provided at the top, and a cooling collection device! A gas passage 20 is connected between the gas passage 2a and the gas passage 2a. A heat-retaining heating element 22 is attached around the gas passage 20.

The heating container 14 is made of, for example, a heat-resistant metal material such as tungsten, or a ceramic material 4 such as alumina or high-chromium refractory brick, depending on the heat treatment temperature. As a heating method, in addition to external heating by energizing the heating element 16 as shown in the figure, methods such as high frequency heating, microwave heating, and direct energization heating of highly radioactive waste can also be used. It is also important to effectively utilize the heating generated by the decay heat of highly radioactive waste.

Highly radioactive waste 24 to be treated is placed inside the heating container 14 and heated.This highly radioactive waste 24 is heated, for example, by heating a nitric acid solution generated from the reprocessing process of spent nuclear fuel to evaporate moisture and nitric acid. This is the calcined body obtained by Of course, from the state of the nitric acid solution, i, I k? It is also possible to perform processing manually. This calcined body is heated to about 500°C to 3000°C, more preferably about 100°C to 2500°C.

By heating, the elements contained in the calcined body are vaporized at the sublimation point or boiling point depending on their chemical form,
The cooling collection devices 12a, . . . , 12 pass through the gas passage 20.
Sent to n.

The two vaporized elements are stored in each cooling collection device 1 whose temperature is controlled to correspond to the sublimation point or boiling point of their chemical forms.
2a5..., 12n are individually cooled and collected.

The heating step may be performed under normal pressure, but when performed under reduced pressure, the sublimation or boiling temperature is lowered, so the process can be performed at a lower temperature, which is also desirable in terms of energy efficiency.

In fact, in the first step, elements that sublimate or boil in the chemical form of oxides are heat-treated under normal pressure or reduced pressure.
Next, in the second step, heating is performed while introducing a reducing treatment agent such as hydrogen gas, carbon, or carbon monoxide to reduce highly radioactive substances and separate elements that boil or sublimate in the form of metals.

Finally, the residue inside the heating container 14 is recovered. The method for discharging the residual melt 25 from the heating container 14 may be a bottom flow method as shown in FIG.
An overflow type as shown in the figure may be used. In any case, a highly reduced volume assimilate can be obtained by discharging the residual melt 25 into the solidified material container 26 in this manner and cooling it as it is.

(Experimental Example 1) A calcined body was created by evaporating a simulated nitric acid solution of highly radioactive waste in which radionuclides were simulated with stable elements. The calcined body was then heat-treated at a high temperature of 1.000°C for 4 hours in a mixed gas flow of I(2He (1:4). During this time, it was cooled?
TeCd, Se, Cs, and Na were precipitated and collected in the first collecting part.

The temperature applied to each element in the cooling collection section is 200 to 6 for Te.
00℃, Cd 200-300℃, Se about 600℃,
Cs is 900-1000℃, Na is 6 Q O ~ + 0
It was 00'c.

(Experimental Example 2) The calcined body after the high temperature heating reduction treatment in Experimental Example 1 was
Processed in vacuum at 50°C to 1050°C. It was found that Pd and Ru were precipitated in the cooling collection section.

1. Effects of the Invention] As described above, the present invention is a method of separating and collecting some of the contained elements by subjecting highly radioactive waste to heat treatment or heat reduction treatment at high temperatures. Compared to this, the treatment process is simple, and there is no need to add new reagents or ion exchangers during the reprocessing or solidification process. &i Use as i, f as a useful metal
For example, it can be used without complex secondary processing such as annihilation processing due to nuclide transmutation.

In addition, the solidified material obtained by the method of the present invention contains almost no additives other than fission products and actinides, and occupies an extremely small volume during storage and disposal compared to conventional solidified material, significantly reducing costs. can be reduced. Moreover, this solidified body is very suitable as an irradiation body for nuclide conversion by neutron irradiation, etc. due to its small volume and high irradiation efficiency.

[Brief explanation of the drawing]

Fig. 1 is a conceptual diagram showing an example of an apparatus suitable for carrying out the method of the present invention, Fig. 2 is an explanatory diagram showing a method for discharging residual melt using a bottom flow type, and Fig. 3 is an explanatory diagram showing a method for discharging residual melt using a single overflow type. FIG. 2 is an explanatory diagram showing an iJ output method. 10... Heat treatment device, 12a,..., 12n...
- Cooling collection device, 14... Heating container, 16... Heating element, 20... Gas passage, 22... Heating element for heat retention, 2
4...Highly radioactive waste.

Claims (1)

[Claims] 1. A method for processing highly radioactive waste, which comprises heating the highly radioactive waste at a high temperature, vaporizing and separating some of the contained elements, and cooling and collecting the waste. 2. A method for processing highly radioactive waste characterized by subjecting highly radioactive waste to heat reduction treatment at high temperatures, vaporizing and separating some of the contained elements, and cooling and collecting the waste. 3. heat treatment to vaporize and separate some of the contained elements, cool and collect them, then heat and reduce the residue at high temperatures, vaporize and separate some of the contained elements, and cool it. A method for processing highly radioactive waste, characterized by collecting the waste and recovering the residue as a solidified substance. 4. The treatment method according to claim 2 or 3, wherein the reduction treatment is performed using hydrogen, carbon, or carbon monoxide.