At the present time the primary problem in a closed nuclear fuel cycle is the management of high level liquid waste (HLLW) generated by the recovery of uranium and plutonium from spent nuclear fuel. Long-term storage of the HLLW, even in special storage facilities, poses a real threat of ecological accidents. This problem can be solved by incorporating the radioactive waste into solid fixed forms that minimize the potential for biosphere pollution by long-lived radionuclides and ensure ecologically acceptable safe storage, transportation, and disposal. In the present report, the advantages of a two-stage HLLW solidification process using a “cold” crucible induction melter (CCIM) are considered in comparison with a one-stage vitrification process in a ceramic melter.This paper describes the features of a process and equipment for a two-stage HLLW solidification technology using a “cold” crucible induction melter (CCIM) and identifies the advantages compared to a one-stage ceramic melter. A two-stage pilot facility and the technical characteristics of the equipment are described using a once-through evaporator and cold-crucible induction melter currently operational at the IA.Mayak. facility in Ozersk, Russia. The results of pilot-plant tests with simulated HLLW to produce a phosphate glass are described. Features of the new mineral-like waste form matrices synthesized by the CCIM method are also described. Subject to further development, the CCIM technology is planned to be used to solidify all accumulated HLLW at Mayak – first to produce borosilicate glass waste forms and then mineral-like waste forms.
According to the multibarrier concept adopted in this country for the burial of high-level wastes, the main barriers of the burial system which isolate the system from man are the matrix which contains the radionuclides and the geological formation in which the burial site is located. The packing of the wastes, the driving complex of the burial site, and so on are viewed as additional engineering barriers. The properties of each component of the system must meet certain requirements and, on the whole, they must be coordinated with one another so that the radionuclides are reliably isolated from the biosphere until they decay to a safe level.The first step in assessing the reliability of the burial site is to characterize the properties of the solidified highlevel wastes which are included in different matrices. Phosphate, borosilicate, and aluminosilicate glasslike and minerallike materials were considered as matrices. The composition of these materials is given in Table 1.To choose the optimal composition, the properties of the wastes determining the conditions of temporary storage and burial are investigated: the chemical stability of the materials in de-ionized water and water of the corresponding geological formations, the thermal stability, the mechanical strength, the radiation resistance, and the thermophysical characteristics (linear expansion coefficient, thermal conductivity, and heat capacity).The assessment of the properties of the solidified wastes starts with the hydrolytic stability, which determines the main possible escape of radionuclides from the matrix during storage. According to GOST-211-91, the chemical stability of the materials is determined by the rate of leaching of the radionuclides (Table 2).To assess the behavior of the solidified wastes during storage in geological formations, the escape of nuclides from solidified wastes in simulators of formation waters of different formations in the temperature range 20-150~ was studied. It was shown that the true behavior of the solidified wastes during geological storage will depend not only on the temperature but also on the rate of the diffusion of water, the ratio of the flow volume of the soil water to the contact surface area with the solidified material, the composition of the ground waters (salt content and pH), and other factors that must be taken into account for the specific conditions of a geological storage site. For example, at high temperatures (100~ and higher) separate components of the formation water (for example, calcium and iron) can interact with the products of leaching of separate components of the solidified wastes with formation of insoluble compounds at the surface of the solidified waste. The "secondary" layer of insoluble compounds that forms screens the surface of the materials and slows down to some degree the escape of leached nuclides in the contact waters. The thickness and composition of the secondary layer depend on the composition of the solidified material and the contact water (Table 3) and the interaction ...
A technique for solidification of high-level liquid wastes (HLLW) to obtain mineral-like forms through the use of water-cooled melters with direct induction heating of a melt (CCIM) has been developed at the State Scientific Center of the Russian Federation VNIINM (SSC RF VNIINM).Mineral-like materials of different classes such as pyroxenes, pyrosilicates, garnets of the andradite group, titanosilicates and Synroc D were synthesized by the CCIM method at temperatures of 1250 – 1550°C.The preliminary X-ray diffraction studies of these materials indicate that the structures synthesized by the CCIM method have compositions that are identical to those of the corresponding natural analogs. The chemical durabilities of the synthesized materials upon their exposure to distilled water at 20°C have been determined.In addition to the investigation of solidification of liquid HLW, the CCIM method is examined and applied to the synthesis of mineral-like compositions involving simulated waste components (nonsoluble residues, ashes, and pulps) produced by the chemical and metallurgical manufacturing of fissile materials.
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