A thermodynamic approach to predict the metallic and oxide phases precipitations in nuclear waste glass melts

Among the large number of matrixes explored as hosts for high-level nuclear wastes, conditioning of fission products and minor actinides into a homogeneous borosilicate glass is the most promising technique already implemented at the industrial scale. The advantage of this vitrification process is the volume reduction of the high level waste coming from the spent fuel reprocessing and its stability for the long-term storage. Nevertheless, some fission products are poorly soluble in molten glasses: - Platinoids (Pd, Ru, Rh) which precipitate as (Pd-Te, Ru-Rh) metallic particles and (Rh,Ru)O₂ oxide phases with acicular or polyhedral shapes during the vitrification process. - Molybdenum oxide (MoO₃) which can form complex molybdates. In order to point out the chemical interactions between the glass and these precipitated phases issuing from the calcinated waste, a thermodynamic approach is developed using the Calphad method. The objective of this work is to calculate thermodynamic properties for complex fission product systems in order to predict the precipitation of platinoids or molybdate phases. This thermodynamic database is being developed on the Mo-Pd-Rh-Ru-Se-Te-O complex system. This flexible tool enables to predict phase diagrams, composition and relative stability of the metallic or oxide precipitated phases as a function of both temperature and oxygen potential in the glass melt. (authors)