JPS6042439B2 - Vitrification method of radioactive waste solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating radioactive waste solutions, and more particularly to an improvement in the so-called vitrification method.

As the amount of radioactive waste solution generated from the reprocessing process of spent nuclear fuel has increased in recent years, there has been an increasing demand for the development of methods for processing and disposing of it.

One of these methods is the vitrification method, in which powdered silicic anhydride, boric anhydride, aluminum oxide, lithium oxide (lithium hydroxide), and sodium oxide (sodium hydroxide, sodium carbonate) are usually used as solidifying agents in the radioactive waste solution.
A predetermined amount of magnesium oxide, potassium oxide (potassium hydroxide), zinc oxide, calcium oxide (calcium hydroxide), etc. are added to make a slurry, and after the water is evaporated and distilled off, the remaining metal compound components containing radioactive substances and glass constituent components are mixed. It is heated to 1,200 to 1,300°C to vitrify it to form an insoluble solid.

In addition, if the radioactive waste solution contains nitrate ions, a nitric acid decomposer such as formic acid is added, and the nitric acid is decomposed by heating and concentrating. There is also a method of concentrating the metal compound components, adding the above-mentioned solidifying agent, and vitrifying by heating. However, the slurry prepared in the process of vitrifying the radioactive waste solution is water-based. Since the additive contains a hardening agent that is hardly soluble, the added components easily precipitate, and it is necessary to redisperse the precipitate before or during the heat treatment to prevent the precipitate from settling.

In addition, during the heating process after adding a solidifying agent to the radioactive waste solution, when the water content decreases, the solidifying agent splashes and
They tend to adhere to the walls of the equipment, making it difficult to obtain a uniform glass composition and completely capture radioactive substances in the vitrified body.

The present inventors have found that by using ultrafine silica as the added silicic anhydride, it is possible to obtain a slurry in which the added solidifying component does not cause sedimentation, and that the slurry does not splash (splatter) during heating. discovered this and completed the present invention. According to the present invention, in the so-called vitrification method in which a vitrification solidification agent is added to a radioactive waste solution and heated to fix the radioactive substance as a glassy solid, a hydrolyzable volatile silicic acid component of the solidification agent is used. There is provided a method characterized in that ultrafine silica obtained by burning a silicon compound in an oxyhydrogen flame is used.

According to the present invention, the slurry prepared by adding the vitrification component using the ultrafine silica is thick but has thixotropy (thixotropy), and when stirred, the viscosity weakens and becomes fluid. While it can be easily transported by pump, it becomes gel-like when left to stand, does not cause sedimentation of difficult-to-tolerate components, and maintains an agar-like state with a uniform composition almost semi-permanently.

In particular, waste with a high radioactivity level is subject to various restrictions on the shape of the equipment in order to ensure the safety of workers during operation, but the slurry prepared according to the present invention can be A viscous slurry can be obtained regardless of the concentration, and there is no need for stirring operations to prevent sedimentation or redispersion, and it can be handled simply by transporting and storing fluids, so it can be processed by remote control inside the shield. This is particularly advantageous for the disposal of radioactive waste that must be disposed of.

In addition, when ultrafine silica is used, it is difficult for the concentrated slurry of added components or metal components containing radioactive isotopes to splash during the final stage of water distillation due to heating of the radioactive waste solution. This is presumed to be because the ultrafine silica particles gradually gel during the process of evaporation to dryness, and during drying, metal components are trapped in the network structure of the silica.

The silicic anhydride used in the present invention is
359, Japanese Patent Publication No. 47-46274, etc., if necessary, a mixed gas to which a flammable gas has been added and a hydrolyzable active silicon compound are vigorously mixed to make them homogeneous, and then they are uniformly mixed together. Obtained by burning in a single flame.

Incidentally, in the above-mentioned Japanese Patent Publication Publication No. 36-33, 1, the combustion gas is homogeneously mixed with the volatile compound to be hydrolyzed while forming water, and the oxygen in the raw material mixture is converted into hydrogen or hydrogen with respect to the formation of water. subdivided oxidation, characterized in that it is maintained in at least stoichiometric proportions with respect to the gases emitted and that an intense homogeneous mixing of all components is obtained, followed by reaction all at once in one flame. The method of manufacturing the product is disclosed, and the ultrafine silica of the present invention is obtained by burning a hydrolyzable volatile silicon compound in an oxyhydrogen flame.

In addition, the composition of the additive components for solidification, denitrification, water distillation,
The temperature profile of vitrification does not need to be changed at all by using ultrafine silica for silicic anhydride, and can be appropriately determined by a person skilled in the art taking into consideration the desired glass properties. The composition of the components is based on the weight of the metal compound containing the radioactive material in the radioactive waste aqueous solution.
Ultrafine silica 150-220 Anhydrous boric acid 30-65 Aluminum oxide 10-25 Lithium oxide 3-14 Sodium oxide 30-60 Magnesium oxide O-10 Potassium oxide O-10 Zinc oxide 0-10 Calcium oxide O- 10 is appropriate, and when the above components are contained in the solution, the amount added is adjusted so that the composition of the solidified product falls within the above range.

For example, since the radioactive waste solution contains sodium, the amount of sodium oxide added to form the vitrified material is often preferably about 1 to 1 part by weight. The invention will now be illustrated with reference to an example using a simulated radioactive waste solution.

Example 1 Cobalt ion 200g/'sodium ion 78.0
A nitric acid acidic sodium nitrate aqueous solution containing the ratio of g/e was used as a simulated solution of radioactive waste solution, and 200 r
To T1', 66.6 g of the ultrafine silica described above, 1 boric acid (H
3BO3) 38.0g1 Aluminum oxide (Ae2O3
) 5.18g Lithium hydroxide hydrate (LfOH-H
2O) 12.9, sodium hydroxide (NaOH) 1.9
1g, potassium carbonate (K2CO3) 4.57gzinc acid (ZnO), 3.12g1 calcium hydroxide (Ca(
0H)2) 4.12g was added and mixed well, and the pH of this liquid was 4.5.

This mixture was colloidal and gelled without producing any supernatant when left to stand, and remained the same even after being left for one month (no supernatant was produced), but it easily flowed when stirred. and could be transferred using a funnel.

This mixture was placed in a crucible and left to stand still, and then heated gradually in an electric furnace without scattering.The mixture was kept at 650°C for 3 minutes, then heated further and melted at 1200°C for 1 hour, then slowly cooled. When taken out, 185 g of vitrified material was obtained.

Example 2 A nitric acid acidic sodium nitrate aqueous solution containing uranyl ions at a rate of 20 g/' (uranium equivalent) and 6.8 g/e of sodium ions was used as a simulated solution of radioactive waste, and this solution 1
Add formic acid to 1 to decompose nitric acid, then 200n1e
After heating and condensing, 40.3 g of ultrafine silica, 7.26 g of boric anhydride, 3.78 g of aluminum oxide, and 1 lithium oxide were obtained.

48 g, 0.222 g of sodium oxide, and 1.70 g of magnesium oxide were added and mixed with thorough stirring.

This mixture also exhibited similar properties as described in Example 1,
Its pH was 9.5.

Put this in a crucible and gradually heat it in an electric furnace to 130℃.
After melting at 0℃ for 1 hour, it was slowly cooled and taken out.
92 g of vitrified material was obtained. Although the above embodiments have been exemplified using nitrate solutions for the reprocessing of nitric acid-based radioactive waste, which is a product of the currently practiced Piurex method, the principles of the present invention can be applied regardless of the liquid nature of the solution. It will be understood.

Claims (1)

[Claims] 1 In the so-called vitrification method, in which a vitrification solidification agent is added to a radioactive waste solution and heated to fix the radioactive substance as a glassy solid, a hydrolyzable volatile silicon compound is used as the silicic acid component of the solidification agent. A method characterized by using ultrafine silica obtained by combustion in a hydrogen flame.