MINERALOGY AND GEOCHEMISTRY OF MATRICES FOR THE IMMOBILIZATION OF HIGH-LEVEL RADIOACTIVE-WASTES

According to the IAEA requirements, the liquid high-level radioactive wastes (HLW) of weapon plutonium production and spent nuclear fuel rep rocessing are to be transformed to a solid form. For this purpose, alu mophosphate glass is used in Russia, and borosilicate glass is used in all other countries. The highly efficient technology of HLW incorpora tion into glasses is available due to the low melting temperature of t hese glasses, minimizing radionuclide volatilization and power consump tion. These glasses are rather highly soluble in aqueous solutions and tend to recrystallize. The immobilization of actinide-bearing wastes requires safer matrices providing isolation of the waste for many thou sands of years. The crystalline matrices where radionuclides are bound in crystal lattices are the best candidates in this respect. This pap er contains a review of published data on borosilicate glasses and cov ers the results of the study of alumophosphate glasses performed at IG EM RAS. The Synroc ceramics synthesized from the same starting materia l by the hot pressing technique in Australia and by inductive melting in a cold crucible in SIA Radon were compared. The latter ceramic mate rials are composed of mineral grains with larger sizes, include pores (up to 5 vol %), are free of metal alloys, and do contain molybdates o f alkali or alkali earth elements. The formation of molybdates is caus ed by higher oxic conditions of synthesis. When interacting with aqueo us solutions, the melted ceramics confine actinides securely and Cs an d Sr poorly due to the high solubility of molybdates. Zirconolite cera mics with high concentrations of elements simulating radionuclides wer e also produced and investigated. The distribution of different elemen ts among the phases of ceramics was studied in detail. It was conclude d that the melted zirconolite materials are suitable for conservation of the actinide portion of HLW after its fractionation. The fractionat ion technology is being promoted now at the radiochemical plant of the Mayak Production Association (PA Mayak) (Glagolenko et al., 1996; Dze kun et al., 1996).