PHASES IN THE SYSTEM LN2O3 – TIO2 AS MATRICES FOR ACTINIDE WASTE IMMOBILIZATION

Toxic transuranic actinides (An) generated in a closed nuclear fuel cycle must be isolated from the biosphere for a very long period of time. This term is equal to ten of half-life periods and can vary from a few thousands (Am-241) to tens millions of years (Np-237). Optimal way of management with the radionuclides is their incorporation into durable ceramic compounds (actinides host phases) and disposal in a deep geological repository. New extraction technologies are developed for separation Ln-An fraction from the high level waste (HLW) produced at spent nuclear fuel reprocessing. Here symbol Ln denotes cerium group lanthanides (La, Ce, Nd, Sm, Pr), and An – minor actinides, americium (Am) and curium (Cm), value of mass ratio Ln : An in the waste is close to 30. All these elements predominantly exist as trivalent cations with close radii. So they have similar crystal-chemical, mineralogical, and geochemical properties and can be jointly incorporated into the same host phases. In this work we have analysed properties of the phases produced in the system Ln₂O₃-TiO₂ by sintering or melting-crystallization as possible host for immobilization of the Ln-An fraction. The main attention is paid to the compounds with nominal formulae: Ln₂TiO₅ (orthorhombic symmetry, space group Pnma), Ln₂Ti₂O₇ (monoclinic, P21), and Ln₄Ti₉O₂₄ (orthorhombic, Fddd). The results of study of limits of U incorporation into the phases, their response to irradiation and corrosion stability in a hot water are discussed. Taking the data obtained into considiration optimal compounds for immobilization of lanthanide - actinide waste are suggested. This study is supported by the Russian Foundation for Basic Research (project #18-29-12032 mk).