JPH0420159B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a method for disposing of radioactive waste, and more specifically, to a method for producing a radioactive waste storage body in which a solidified body made of glass or ceramics containing radioactive waste is surrounded by a metal coating. Regarding how to. [Technical background of the invention] When disposing of radioactive waste generated from nuclear power plants and spent nuclear fuel reprocessing plants, it is necessary to solidify the waste into a form that minimizes the diffusion of radioactive materials to the surrounding area. It is necessary that the stored storage medium be chemically and mechanically stable and not cause environmental pollution even during long-term storage. From this point of view, vitrification is the mainstream solidification method conventionally used. In this method, radioactive waste is melted together with a glass-forming material such as borosilicate glass or phosphate glass, and solidified into a glass ingot of a certain shape. The glass ingot thus solidified is usually stored in a metal container. Furthermore, many relatively small glass ingots are buried in the metal. A so-called metal composite solidification method has also been proposed. However, the above-mentioned vitrification method or metal composite solidification method has the following problems. (b) Assuming that the metal sheath that houses or covers the vitrified material is damaged due to corrosion, etc., the solidified material inside will come into direct contact with the external environment, so for long-term stable storage, It is required to minimize the rate of radioactive material leaching into the external environment (for example, water). (b) Vitrified materials have the disadvantage that cracks are likely to occur during the production of the solidified material, since there are restrictions on the composition of glass, which is the basic material. In particular, since the metal cladding and the vitrified body have different coefficients of thermal expansion, when molten glass solidifies in the metal cladding, cooling is accelerated at the interface between the glass and metal, creating thermal stress in this area. As a result, cracks are likely to occur on the surface or inside the solidified material. Cracks that occur in the solidified body will inhibit the dissipation of radioactive decay heat generated inside the solidified body, and the resulting temperature rise may damage the mechanical and chemical stability inside the solidified body. . Furthermore, the cracks increase the surface area of the vitrified body, thereby increasing the leaching area when the vitrified body is in direct contact with the external environment. [Object of the Invention] The present invention has been made in view of the above-mentioned problems, and provides a method for producing a solidified storage body that has even better resistance to leaching of radioactive substances and can be safely stored for a long period of time without causing cracks. The purpose is to provide [Summary of the Invention] In order to achieve the above object, the radioactive waste solidification treatment method of the present invention provides a solidification storage body in which a solidified body made of glass or ceramics containing radioactive waste is further surrounded by a metal coating. When manufacturing, an intermediate layer made of glass or ceramics that does not contain radioactive substances is provided between the solidified body and the metal coating, and the temperature of this intermediate layer is within the temperature range from room temperature to the softening temperature of the solidified body. The average coefficient of thermal expansion is a value between the average coefficient of thermal expansion of the solidified body in the temperature range and the average coefficient of thermal expansion of the metal cladding in the temperature range. [Specific Description of the Invention] Production of Solidified Materials Radioactive waste to be treated in the present invention includes, for example, after processing spent nuclear fuel,
In addition to the radioactive waste remaining after recovering U and Pu, it includes various types of waste liquid such as recycled waste liquid, floor drain, and various solid wastes such as filter sludge and sedimentation sludge. Furthermore, examples of the solidified body containing radioactive waste used in the present invention include solidified bodies of radioactive waste made of glass, ceramics, or the like. For example, containing radioactive waste,
Vitrified materials such as borosilicate glass and phosphoric acid glass made through heat treatment processes such as melting or sintering, crystallized vitrified materials such as Diopside type, Al ₂ O ₃ −SiO ₂ type, TiO ₂ type, _MnO2 − _SiO2
Solidified ceramics such as ZrO ₂ -based and ZrO 2 -based materials are preferably used. The content of radioactive waste in the solidified body is suitably in the range of 10 to 40%. Formation of Intermediate Layer When surrounding the solidified body with a metal covering, an intermediate layer is formed between the solidified body and the metal covering. This intermediate layer is made of glass or ceramics that do not contain radioactive waste, such as crystallized glass such as borosilicate glass, phosphate glass, diopside glass, Al ₂ O ₃ , ZrO ₂ , TiO ₂ , Al ₂ O 2 _{3 −SiO2}
Ceramics such as SiC, _Si3N4 , _etc. may be used. Further, in order to prevent cracks from occurring in the solidified product, it is preferable to appropriately select the thermal expansion coefficient of the intermediate layer. In other words, in order to reduce the thermal stress that occurs when a molten solidified body cools and solidifies in a metal coating (or when an already formed solidified body is surrounded by molten metal), and to prevent the occurrence of cracks. The average coefficient of thermal expansion of the intermediate layer in the temperature range from room temperature to the softening temperature of the solidified body is the average coefficient of thermal expansion of the solidified body in the temperature range, and the average coefficient of thermal expansion of the metal coating in the same temperature range. It is preferable that the value is between , and more preferably a value approximately in between. If the thickness of the intermediate layer is too thin, its function as a barrier to prevent radioactive substances in the solidified material from leaching to the outside world will be reduced, while if it is too thick, the weight of waste relative to the total amount of the composite solidified storage material will be reduced. Since the ratio (ie, content) decreases, a range of 0.02 mm to 100 mm is appropriate. As a method for forming the intermediate layer, (a) the intermediate layer is formed in advance on the inner surface of a metal covering (for example, a metal container) surrounding the solidified glass or ceramic body by a melt coating method, a flame spray method, etc.; A glass or ceramic melt containing radioactive waste is injected,
Solidification method: (b) The already formed solidified body is charged into a metal container, and then a molten glass or ceramic material, which will be the material for the intermediate layer, is injected into the gap between the metal container and the solidified body and solidified. (c) Flame spray method, plasma spray method, CVD method,
The intermediate layer is coated on the surface of the solidified material that has already been formed using the PVD method, vacuum evaporation method, or direct immersion into the melt of the intermediate layer material, and then this is coated with the metal by a conventional method. A method such as surrounding it with a covering can be used. [Embodiments of the Invention] Example 1 The drawing shows a longitudinal cross-sectional view of a composite solidified storage body obtained in this example. First, a melt containing simulated radioactive waste with the composition shown in Table 1 below and borosilicate glass with the composition shown in Table 2 below in a weight ratio of 3:7 was placed in a carbon mold with an inner diameter of 25 mm and a height of 80 mm. The mixture was injected into the solution and slowly cooled to solidify.

【table】

[Table] On the other hand, a cylindrical SUS304 storage container 1 (canister) with an inner diameter of 30 mm is prepared, and the glass melt for the intermediate layer having the composition shown in Table 3 below is first poured into the bottom of the canister and slowly cooled and solidified. By doing so, an intermediate layer with a thickness of 3 mm was formed on the bottom of the canister.

[Table] Next, the vitrified body was taken out of the carbon mold and placed in a canister 1. moreover,
The glass melt for the intermediate layer is injected into the gap between the inner surface of the canister 1 and the vitrified body 2 and the upper surface opening in the same manner as described above, and is slowly cooled and solidified to form an intermediate layer between the canister 1 and the vitrified body 3. Form layer 2, cover with SUS top cover 4 and seal by welding.
A composite solidified body was obtained. In addition, the vitrified body (diameter 25 mm, height 80 mm) containing the simulated radioactive waste created in this example
and this solidified body was further coated with an intermediate layer having the composition shown in Table 3 above (the thickness of the intermediate layer was 2.5 mm),
The leaching rate of radioactive substances in the solidified material with respect to water was measured. After each sample was immersed in 1000 ml of pure water at 100°C for 24 hours, Mo ions in the solution were quantified. For those without covering the intermediate layer, the leaching rate is
The leaching rate was 2.5×10 ^-5 g/cm ² ·day, and the leaching rate of the intermediate layer coated one was below the detection limit of 1×10 ^-6 g/cm ² ·day. Example 2 The simulated radioactive waste having the composition shown in Table 1 above and the ceramic forming material having the composition shown in Table 4 below were adjusted at a weight ratio of 3:7, and this was sintered to form a ceramic material with a diameter of Solidified ceramic particles of 7 mm were obtained.

〔Effect of the invention〕

The composite solidified storage body obtained by the present invention has an intermediate layer that does not contain radioactive materials between the solidified body containing radioactive waste and the metal coating surrounding it, so that it has high leakage resistance. Further improvement can be achieved. Furthermore, since it is possible to effectively prevent cracks from forming in the solidified body, it is possible to prevent a decrease in the dissipation of radioactive decay heat generated inside the solidified body and an increase in the leaching area due to the cracks.

[Brief explanation of drawings]

The drawing is a longitudinal sectional view of a composite solidified storage body according to an embodiment of the present invention. 1... Canister, 2... Intermediate layer, 3... Solidified body containing radioactive waste, 4... Lid.

Claims (1)

[Claims] 1. When producing a solidified storage body in which a solidified body made of glass or ceramics containing radioactive waste is further surrounded by a metal coating, glass or ceramics that do not contain radioactive materials are placed between the solidified body and the metal coating. An intermediate layer made of ceramics is provided, and the average coefficient of thermal expansion of this intermediate layer in the temperature range from room temperature to the softening temperature of the solidified body is equal to the average coefficient of thermal expansion of the solidified body in the temperature range and the metal in the temperature range. characterized by a value between the average coefficient of thermal expansion of the coating,
Solidification treatment method for radioactive waste.