Patent Number: 052563383
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solidifying material and a waste container suitable for the final disposal of radioactive wastes generated from a nuclear power plant and the like, a structure for disposal, a back-filling material, and a process for solidifying radioactive wastes. 2. Description of the Related Art Hitherto, hydraulic inorganic solidifying materials such as cement and water-glass (sodium silicate) have been used for waste containers, solidifying materials, back-filling materials and structures for disposal site of so-called a low level radioactive waste such as a concentrated liquid waste, a spent ion exchange resin and various solids generated from nuclear power plants and nuclear fuel reprocessing facilities. The above-mentioned hydraulic inorganic solidifying materials have the merits of (1) an easy operation, (2) an inexpensiveness and (3) an excellent radiation resistance and are suitable for disposal of low level radioactive wastes. Furthermore, for a disposal of low level radioactive wastes, it is necessary that the solidifying materials can maintain safety even under such a condition that solidifying materials or disposal facilities sink under the water and besides can greatly retard leakage of internal radioactive nuclides out of the waste forms or disposition facilities. A conventional method for assuring long-term endurance of waste forms is to add glass fibers to hydraulic solidifying materials as described in Japanese Patent Kokai (Laid-Open) No. 60-202398. Since fibrous materials have tensile strength several times that of the base hydraulic solidifying materials, they have a reinforcing effect to remarkably improve tensile strength and flexural strength of the whole waste form. Therefore, even if the waste form undergoes a change in volume of the filler or is applied with an external force, cracks or breakage of the waste form do not occur and even in the case of the waste form being disposed into the land, it is considered that the waste form can never deteriorate during from several ten years to several hundred years in which radioactivity decays to sufficiently low level. Furthermore, for retardation of leakage of radioactive nuclides, Japanese Patent Kokai (Laid-Open) No. 58-40000 has proposed to provide a protective layer for a waste container for radioactive wastes and to embed a filler having ion exchangeability and adsorbability in this protective layer. Among the above-mentioned conventional methods, the method described in Japanese Patent Kokai (Laid-Open) No. 60-202398 does not consider reduction of leaching rate of radioactivity from wastes and has the problem that a countermeasure to reduce leaching rate of radioactivity is required when wastes higher in level of radioactivity than the present level are disposed of or when wastes containing carbon-14 or technetium-99 longer in half-life are disposed of. On the other hand, since the invention disclosed in Japanese Patent Kokai No. 58-40000 has not the property to improve the strength of the waste container, the invention has a problem that cracks occur in the waste container due to the cycles of dry/wet and hot/cold of the disposal site, and hence the maintenance of soundness thereof is impossible. SUMMARY OF THE INVENTION The object of the present invention is to provide solidifying materials, for disposal of radioactive waste (solidifying materials for making waste form, back-filling materials used in disposal site and so on) and structures for radioactive waste disposal (waste containers, structures of disposal site and so on) which can simultaneously attain improvement of long-term endurance of waste form and the like and reduction of leaching rate of radioactivity from waste form and the like. The above object has been attained by adding fibrous materials having property to adsorb onto the surface the radioactive nuclides in the form of ion or molecule to cement type hydraulic solidifying materials which are used for solidifying materials for disposal of radioactive wastes (solidifying materials for making waste forms and back-filling materials used in disposal site), waste containers and structures of disposal site. The fibrous materials which have the property to adsorb on the surface thereof the radioactive nuclides in the form of ion or molecule and are added to cement type hydraulic solidifying materials as a base material have the following actions. Since the fibrous materials have a tensile strength several times the strength of said base material, they have an action to enhance tensile strength of final hardened bodies depending on the addition amount thereof. Besides, from the point of fracture mechanics, the above fibrous materials have an action to stop the development of cracks which have occurred in the hardened bodies and hence can markedly increase brittle fracture value of set bodies which are inherently brittle materials and can inhibit deterioration of the waste forms. Especially, when solidifying materials, waste containers, structures of disposal site and back-filling materials which concern with disposal of radioactive wastes undergo wet-and-dry cycle or temperature cycle, cracks may occur in hardened bodies. Leakage of radioactivity from waste forms of wastes when they sink under the water is accelerated owing to these fine cracks. When the above-mentioned fibrous materials are added, not only the final tensile strength of the waste forms is improved, but also generation of fine cracks and development thereof can be prevented not so as to affect mechanical strength. Therefore, increase of leaching rate of radioactivity from waste forms can be prevented even under such severe environmental conditions as dry-and-wet cycle or temperature cycle. Furthermore, since the fibrous materials have the property to adsorb radioactive nuclides in the form of ion and molecule, distribution coefficient of the solidifying materials as base materials for radioactive nuclides can be improved by the addition of the fibrous materials and leaching rate of radioactivity from waste form can be diminished.