JPH0727075B2 - Method for solidifying radioactive waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to a method for treating intermediate-level to low-level radioactive waste generated in a nuclear facility such as a nuclear fuel reprocessing facility, and particularly to packing. The present invention relates to a method of solidifying radioactive waste, which can easily produce a solidified body having a high rate and excellent stability and durability over a long period of time.

(Prior art) As a method of treating radioactive waste, such as radioactive concentrated waste liquid and sludge, that has been conventionally generated in nuclear fuel reprocessing facilities,
The concentrated waste liquid is subjected to asphalt solidification treatment, and the sludge is stored as it is.

In this solidification method, the radioactive waste liquid is concentrated and dried into a powder mainly composed of sodium nitrate, and then this radioactive waste is solidified by a solidifying material composed of asphalt. However, even if a solidified body of radioactive waste is obtained by solidification by such a method, most of them are still in the state where the final disposal method has not been established at present. On the other hand, for radioactive waste generated from the BWR power plant, a method of temporarily storing it as an intermediate storage has been proposed in recent years.

This method is a method in which radioactive waste is dried and greatly reduced in volume, and then pelletized to produce a stable intermediate storage, which is temporarily stored in a storage tank in a nuclear facility. According to this method, the powder radioactive waste after the drying treatment is compressed and pelletized, so that a high volume reduction rate can be obtained.

However, with ¹³⁷ Cs and Sr generated in nuclear fuel reprocessing facilities, the half-life of radioactivity is about 30 years, and it is virtually impossible to attenuate radioactivity by such a method, and even if it is possible, it will be constant. After being stored for a period of time and the radioactivity is attenuated, it is necessary to solidify again as a stable solidified package.

In addition to such wastes, it is known that miscellaneous solid wastes such as metal fragments, filters, and packings are generated from reprocessing facilities. These are so diverse in type and indefinite in shape that at present, they are appropriately cut if necessary and put in a storage container. Such miscellaneous solids also need to be solidified together as a stable solidified package.

As a method for packaging such a radioactive waste into a solidified body package, treatment with the aforementioned solidified material which has been conventionally used can be considered.

(Problems to be Solved by the Invention) However, the above-mentioned asphalt solidification method has a problem in that it is stable for several hundred years or more because the solidifying material is an organic substance. A method of using cement as a solidifying material is also conceivable, but in this case, a large amount of water is required, so that water is solidified during solidification treatment, particularly when solidifying pelletized radioactive waste, water absorption of pellets, pellets due to swelling and Degradation of pellets and solidification material can be prevented by using cement with a minimum amount of water required for hardening, which may cause deterioration of solidification material. Is large, which makes it difficult to densely fill the pellets.

In addition, it is necessary that the solidified solid of the formed radioactive waste does not release the radioactive substance in the solidified body into the environment even at the time of final disposal. If present in the body, this excess water may evaporate due to drying or the like during formation of the solidified body, and the solidified body may become microscopically porous. As a result, the stability and durability of the solidified product will be significantly reduced.

For the above reason, in order to manufacture a stable solidified package, the solidified material needs to have as little added water as possible and low viscosity. However, it has been extremely difficult to produce a solidifying material that simultaneously satisfies two contradictory conditions of low water content and low viscosity.

The present invention is intended to eliminate the drawbacks of the conventional solidification treatment method of radioactive waste, radioactive waste, especially pellet-shaped radioactive waste intermediate storage excellent in volume reduction,
An object of the present invention is to provide a method of integrally solidifying as a solidified package that has excellent heat resistance and is chemically and mechanically stable upon storage for a long period of time.

[Structure of the Invention] (Means for Solving Problems) A method for solidifying radioactive waste according to the present invention is a method of converting radioactive waste generated in a nuclear facility into (a) Portland cement and (b) aggregate. (C) An inorganic fluidizing material and (d) a dispersant composed of a condensed phosphate are mixed to be integrally solidified by a hydraulic solidifying material.

The radioactive waste to be treated in the present invention may be any of powder, pellets, miscellaneous solids, sludge, or a mixture thereof.

The (a) component Portland cement used in the present invention is a hydraulic cement mainly composed of calcium silicate and calcium aluminate.

Also, as the inorganic fluidizing material of (C) component, the particle size is 10 μm
The following inorganic oxides can be used, and examples thereof include fine alumina powder, fine silica powder, and a mixture thereof.

As the dispersant of the component (d), a condensed phosphate having a large dispersion effect and being inorganic and stable is used, and examples of the condensed phosphate include sodium pyrophosphate, sodium tripolyphosphate, and the like.
Examples thereof include sodium tetrapolyphosphate, sodium hexametaphosphate, and sodium ultrapolyphosphate.

(Function) In the present invention, by adding a dispersant and an inorganic fluidizing agent to the solidifying material, each particle of the inorganic fluidizing agent whose particle size is adjusted is dispersed, and the fine particles of 10 μm or less in the binder are also dispersed. It is dispersed and penetrates between the binder and the aggregate to improve the slip between these particles. As a result, the particles forming the solidifying material are uniformly dispersed in the added water, and are in a state of easy movement. Therefore, when the solidifying material is filled, the solidifying material easily flows on the surface of the waste to be treated, it is possible to easily fill the container of the solidified material package without a gap, and it is possible to form a dense solidified material package.

And in the conventional solidifying material, in addition to water necessary for curing, a large amount of water had to be added in order to uniformly mix the binder and the aggregate at the time of kneading and to give an appropriate fluidity. In the solidifying material of the present invention, since the fluidity is provided by the inorganic fluidizing material and the dispersant, the amount of added water is the minimum amount necessary for hardening the binder. Therefore, since water other than the water of crystallization taken into the crystal hardly exists in the solidified body, it is possible to obtain a dense and stable solidified body.

Furthermore, by using an inorganic condensed phosphate as a dispersant without using an organic system, it is possible to obtain a stable solidified body which is not deteriorated for a long period of time.

(Example) Hereinafter, the Example of this invention is described.

Examples 1 to 5 With the composition shown in Table 1, Portland cement was uniformly mixed with an aggregate and an inorganic fluidizing agent, added water and a dispersant were added, and these were kneaded to obtain 5 kinds of slurry solidifying materials. Got The values in Table 1 are given in parts by weight. Kneading 20-25
1 ~ 5 after mixing the solidified product and the kneading liquid in an atmosphere of ℃
Mechanical kneading was performed for a minute.

In addition, as a comparative example, four kinds of solidifying materials were similarly obtained with the composition shown in Table 1 without adding the inorganic fluidizing material or the dispersant.

The fluidity of each of the above-mentioned solidified materials was evaluated. The results are shown in Table 1.

From this result, it can be seen that the solidifying material using the inorganic fluidizing material and the dispersant has significantly improved fluidity as compared with the solidifying material not using these. Moreover, it has a significantly lower viscosity than the conventional solidifying material and has a small water content.

Next, in order to investigate the filling property of the solidifying material and the degree of elution of the pellet components, the solidifying materials of Examples 1 and 2 and Comparative Examples 3 and 4 were used to simulate pellets of radioactive waste liquid generated from the nuclear fuel reprocessing facility. When the solidified material was filled and solidified,
It was as shown in the table.

From this result, it is understood that the solidifying material using the inorganic fluidizing material and the dispersant has a low viscosity and a large fluidity, so that the voids between the pellets can be sufficiently filled. Moreover, since the water content in the solidified body is the minimum amount necessary for curing, elution and swelling due to moisture absorption of the pellet did not occur. Further, since the solidified body had a remarkably small amount of free water, no elution from the pellet component or elution in water was observed after evaporation of free water by drying.

Next, in order to investigate the filling property at the time of filling miscellaneous solid waste, the solidifying material of Example 1 and Comparative Example 3 was used to fill and solidify a substance simulating the miscellaneous solid waste generated from the nuclear fuel reprocessing facility. The results are shown in Table 3. However, in the table, ◯ means “good filling state”, Δ means “not good filling state”, and x means “poor filling state”.

As can be seen from these results, it is not possible to fill small voids with solidified materials using ordinary cement, even with relatively simple shapes such as rubber packing and refractory bricks, and with complex materials such as metal cutting pieces. Although it could not be filled, the solidifying material of the embodiment can fill and solidify any shape of coarse solid without gaps.

The solidifying material used in the conventional cement solidification method contains crystal water that is taken into the crystal in the solidified body that is formed because it contains a large amount of water, and free water, especially pelletized radioactive waste is solidified. In the case of liquefaction, there is a problem in that the pellets are eroded by the free water, and the pellets are relatively poor in water resistance, so that the pellets are dissolved or swelled and partially destroyed. However, as shown in the examples, by adding the inorganic fluidizing agent and the dispersant to the solidified body, the amount of added water added to the solidified body can be suppressed to the necessary minimum, so that the solidified body formed is free to be added. There is almost no water. Therefore, when solidifying pelletized radioactive waste, swelling and dissolution due to water absorption of the pellets, deterioration of the solidifying material does not occur, and the solidified body does not become porous due to evaporation of water during solidified body formation. A stable solidified body can be obtained. Also,
By adding a dispersant and an inorganic fluidizing material to the solidifying material,
A solidified material having high fluidity can be obtained, whereby a dense solidified body having no gap can be formed.

As a result, the radioactive substance can be completely contained in the solidified body, and a solidified body package that is chemically and mechanically stable for a long period of time can be easily formed.

[Effects of the Invention] As described above, in the method for solidifying radioactive waste according to the present invention, the radioactive waste is solidified by solidifying the radioactive waste with a low-viscosity hydraulic solidifying material having a low water content. With a high filling rate, a solidified package that is chemically and mechanically stable over a long period of time can be easily formed.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Masakazu Ota 1-1-7 Uchisaiwaicho, Chiyoda-ku, Tokyo Inside Nihon Nuclear Power Co., Ltd. (72) 1-1-1 Shibaura, Minato-ku, Tokyo Toshiba Headquarters Office (72) Inventor Takanao Sugino No. 1 Nanto, Ogakie-cho, Kariya City, Aichi Toshiba Cera Mix Co., Ltd. Kariya Factory (72) Inventor, Kazuaki Matsuo No. 1 Nanto, Ogakie-cho, Kariya City, Aichi Prefecture Toshiba Ceramics Co., Ltd., Kariya Plant (56) Reference Japanese Patent Laid-Open No. 61-215999 (JP, A) Okada Kiyoshi, Rokusha Hen Edition Revised New Edition "Concrete Engineering Handbook" (Head 2-7-1) Asakura Shoten P. 48-55, 84-85, 139

Claims (3)

[Claims] 1. A radioactive waste generated at a nuclear facility, (a) Portland cement, (b) aggregate, (c) inorganic fluidizing material, and (d) a dispersant composed of condensed phosphate. A method for solidifying radioactive waste, which comprises solidifying integrally with a hydraulic solidifying material mixed with. 2. The solidification of radioactive waste according to claim 1, wherein the radioactive waste comprises one or more selected from powders, pellets, miscellaneous solids and sludge. Processing method. 3. An inorganic fluidizing material comprising one or more selected from inorganic oxides such as alumina fine powder and silica fine powder having a particle size of 10 μm or less. The method for solidifying radioactive waste according to item 1 or 2.