JPS6112237B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for solidifying radioactive waste, and more specifically, it involves forming vitrified particles containing radioactive waste, coating the surfaces of these particles with metal, and then compressing and sintering the particles. This invention relates to a method for obtaining a radioactive waste storage body that is chemically and mechanically stable and suitable for semi-permanent storage. With the spread of nuclear power generation, highly concentrated radioactive waste fluid generated from spent nuclear fuel reprocessing plants is
It tends to increase year by year. Since there are safety issues in storing these radioactive waste liquids in their liquid state in tanks, there is a strong desire to establish a technology to convert them into solid storage bodies that can be stored more safely. Generally, when disposing of radioactive waste, the waste is solidified in a form that minimizes the diffusion of radioactive materials into the surrounding area, and the resulting solid storage medium is chemically and mechanically stable and has a long-term lifespan. It is necessary that storage does not cause environmental pollution. From this point of view, the conventional solidification methods are:
Vitrification technology (for example, Special Publication No. 46-3240,
50-4840, etc.) are the mainstream, in which highly concentrated radioactive waste is melted together with glass materials such as phosphoric acid or borosilicate glass, and solidified into glass ingots of a certain shape. The solidified product is then sealed in a storage container and stored, for example, in underground storage. According to the above method, by considering the waste content, the composition of the glass, etc., it is possible to obtain a vitrified material with relatively high mechanical strength, but it has the following problems. (b) If the storage container is damaged, the solidified material will be released into the external atmosphere, such as groundwater in the case of underground storage.
Therefore, for long-term stable storage, it is necessary to minimize the rate of leaching of radioactive materials by water. In this respect, since the glass, which is the basic material of the vitrified body, is subject to restrictions due to the composition, it is not necessarily easy to achieve both the mechanical strength or integrity of the solidified body and a reduction in the leaching rate using a glass material. (b) Since the thermal conductivity of glass is inherently lower than that of metals, the temperature inside the vitrified body rises due to the heat generated by the radioactive decay of the radioactive substances it contains, reaching 500 to 700°C in the center. It may even extend to. Such temperature increases weaken the structure of the vitrified body, impair its mechanical and chemical stability, and make safe long-term storage of radioactive waste difficult. In order to eliminate the above-mentioned disadvantages of vitrification treatment, for example, small particles of vitrification are made, and a molten substance of a low melting point metal such as Sn or Pb is injected into the aggregate and solidified to form glass in the metal matrix. Methods have also been developed to obtain more stable waste storage bodies in the form of embedded solidified bodies. However, although this method provides a storage medium with significantly improved thermal conductivity and chemical stability, low-melting point metals generally have low mechanical strength, and thermal conductivity and chemical stability are low, such as Cu and Ni. The stability of the resulting storage medium is still far from satisfactory, as it is significantly inferior to that of metals with higher melting points. Furthermore, since the process involves injecting molten metal, the vitrified particles are not uniformly dispersed in the metal matrix, and contact areas between the vitrified particles remain, which becomes an obstacle to heat dissipation. Furthermore, in order to avoid contact between the vitrified particles, it is necessary to lower the mixing ratio of the vitrified particles to the metal matrix, and as a result, the composite solidified material required to store a certain amount of radioactive waste The disadvantage is that the volume and weight of the product increase significantly. The object of the present invention is to eliminate the drawbacks of the conventional radioactive waste treatment method by vitrification as described above, to contain radioactive waste uniformly dispersed with a high degree of filling, and to achieve chemical stability, mechanical strength, and thermal stability. The object of the present invention is to provide a method for producing a composite solidified storage body that has excellent conductivity and can be safely stored for a long period of time. According to the research of the present inventors, in order to achieve the above object, the surface of the glass solid particles is preferably coated with a metal having excellent chemical stability, mechanical strength and thermal conductivity, It has been found that it is extremely effective to integrate these particles by utilizing the bonding force of the metal under heating and compression. That is, the method for solidifying radioactive waste of the present invention includes the steps of coating the outer surface of solidified particles containing radioactive waste with metal, and converting the resulting aggregate of metal-coated vitrified particles into glass. It is characterized by comprising a step of compressing and sintering at a temperature above the softening point and below the melting point of the metal. The present invention will be explained in more detail below. In the following description, "parts" and "%" are based on weight unless otherwise specified. Radioactive waste to be treated by the present invention includes, for example, U, Pu,
In addition to the remaining radioactive waste collected, various waste liquids containing radioactive materials, such as concentrated liquid waste recycled from mixed-bed desalination equipment, concentrated waste liquid from floor drains or equipment drains generated from buildings, and even atomic Various types of solid waste such as used ion exchange resins and filter sludge generated from the filtrate purification system, fuel pool system, condensate system, and drain system, and precipitated sludge generated from coagulation and sedimentation treatment of waste liquid, etc. This includes low- and medium- and low-level radioactive waste. Similar to the conventional method, these radioactive waste solutions or powders are dispersed or co-melted in a melt of borosilicate glass, phosphate glass, etc. at a ratio of 70 parts to 30 parts by weight of the dry oxide. The particles are then reduced in diameter by conventional methods in the glass industry, such as crushing and dropping, to form vitrified particles. There are no particular restrictions on the size of the vitrified particles, but if they are too small, problems will arise when controlling the coating film thickness in the metal coating process and particle aggregation when the plating method is used for coating, and if they are too large. In this case, since the temperature rise due to radioactive decay in the center becomes large, a diameter in the range of 2 mm to 50 mm is appropriate. The shape of the vitrified particles can be spherical, ellipsoidal, cylindrical, polygonal, convex, etc.; Also, from the viewpoint of uniformity of filling, particles that are spherical and have a uniform diameter are desirable. According to the invention, the outer surface of the vitrified particles containing such radioactive waste is coated with metal. The covering metal has enough strength to hold the glass inside at a temperature above the glass's softening point during the subsequent compression and sintering process, and has enough deformability to deform to fill the space. , it is necessary to further bond by mutual diffusion, and Cu,
Ni, Fe, Ag and alloys thereof are preferably used. In addition, as coating methods, since vitrified particles are generally insulators, a conductive film is formed on the surface by electroless plating, sputtering, vapor deposition, paste baking, etc., and then various methods including electrolytic plating are used. A metal coating as described above is formed by the coating method. Further, after the metal coating is applied, a thin coating of a low melting point metal such as Sn, Pb, Zn, or an alloy thereof may be formed on the surface to promote diffusion between the metal coatings. The thickness of the metal coating varies depending on the particle size of the glass solidification particles, but if it is too thin, the coating will break during the subsequent compression sintering process and the glass will seep out. Since the filling rate of glass in the composite solidified body decreases, a range of 0.02 to 10 mm is preferably used. Next, the vitrified particles 2 coated with the metal covering 1 thus obtained are filled into a mold 3 made of a heat-resistant material such as carbon and non-reactive with metals, as shown in FIG. Then, by applying a load 5 through a lid 4 made of the same material while heating, the vitrified particles and the metal coating are deformed as shown in FIG. 1a), and through the interface 6 between the metal coatings, heating diffusion between the coating metals,
In other words, sintering occurs. The heating temperature is set to be above the softening point of the glass and below the melting point of the metal so as to obtain the desired filling effect, and the coated vitrified particles are placed in a non-oxidizing atmosphere so as not to interfere with the interdiffusion of the coated metal. (inert or reducing atmosphere). The metal-glass composite solidified body obtained through the above steps may be stored as is, but if necessary, for safer storage, e.g.
It may be placed in a container made of a corrosion-resistant alloy such as SUS304, sealed, and then stored. Furthermore, in the compression and sintering process described in FIGS. 1 and 2, a container made of a corrosion-resistant alloy may be used as the mold 3, and after compression and sintering, the container may be closed and stored with a lid. In the radioactive waste solidified material obtained by the method of the present invention described above, each particle of the vitrified material containing radioactive waste is individually wrapped in a homogeneous metal coating, and The coated particles are deformed and bonded by mutual diffusion of the coated metals, completely filling the space, so they have excellent chemical stability and mechanical strength, and they also resist heat from the vitrified material due to radioactive decay. is also sufficiently dissipated. In addition, since the contact between the vitrified particles is completely blocked by the coating metal, the filling rate of the vitrified material can be increased and the volume ratio of the entire vitrified material can be increased.
It is also possible to increase it to more than 50%. Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples. Example 1 Spherical borosilicate glass particles (diameter 5 mm) containing 30% of simulated radioactive waste having the composition shown in the table below were prepared.

【table】

[Table] After performing Cu electroless plating on the above particles, Cu electroplating was performed at room temperature in a copper sulfate-sulfuric acid bath to form a Cu coating with a thickness of 0.63 mm. Next, the obtained Cu-coated particles were
Fill a carbon container with a diameter of 50 mm and a height of 150 mm, cover the top with a carbon disc-shaped lid with an outer diameter slightly smaller than the inner diameter of the container, apply a load of 10 kg/ ^cm2 , and heat at 750°C in a hydrogen stream for 30 minutes. Heat treatment was performed for several hours. The obtained solidified body has a filling rate of 96% (volume basis)
It was a nearly dense solidified body with no glass seeping through the coating. Separately, 20 coated particles were heat-treated for 3 hours at 750°C in a hydrogen stream in a carbon container without contacting each other, and then immersed in 100ml of 100°C pure water for 1 hour. When we measured Mo ions and calculated the leaching rate, we found that the detection limit was 1×10 ^-6 g/cm ² day
It was less than Example 2 As in Example 1, tin plating with a thickness of approximately 5 μm was applied to the vitrified particles on which a Cu coating with a thickness of 0.63 mm was formed, and then Cu plating was applied to the inner surface with an inner diameter of 50 mm and a height of 150 mm. The mixture was filled into a SUS304 container, and the top was covered with a SUS304 disk whose bottom surface was Cu-plated and had an outer diameter slightly smaller than the inner diameter of the container. Next, a load of 50 kg/cm ² was applied through this lid, and sintering was performed at 700° C. for 3 hours in a hydrogen stream. After sintering, a SUS304 disk was welded to the container at that position and sealed. The filling rate of the obtained solidified material is approximately 95% (based on volume)
It was hot. Comparative Example In Example 1, 10 vitrified particles before metal coating were immersed in 100 ml of pure water at 100°C for 1 hour, Mo ions in the solution were measured, and the leaching rate was determined to be 2.5. A value of ×10 ⁻⁵ g/cm ² , day was obtained. As is clear from the above Examples and Comparative Examples, in the storage body according to the present invention, even if the outermost container is damaged and the solid inside is exposed to the outside world, the vitrified particles are still contained in the coated metal matrix. Because it is uniformly dispersed and coated, leaching of radioactive elements is prevented, and it also has excellent heat dissipation and mechanical properties, allowing a high content of vitrified material, as well as safety and long-term storage. Excellent in sex.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a mold filled with vitrified particles according to one embodiment of the method of the present invention before compression and sintering, and FIG. 2 is a longitudinal sectional view of the same after compression and sintering. 1... Metal coating, 2... Vitrified particles, 3...
Mold, 4... Upper lid, 5... Load, 6... Interface between metal coatings, and the subscript a indicates the state after compression and sintering.

Claims (1)

[Claims] 1. The step of coating the outer surface of vitrified particles containing radioactive waste with metal, and compressing the resulting aggregate of metal-coated vitrified particles at a temperature above the softening point of glass and below the melting point of metal. A method for solidifying radioactive waste, comprising a step of sintering.