Thermodynamic assessment of platinoid and molybdate phases in nuclear waste glasses

Abstract : Fission products and actinides arising from the spent UOX fuel reprocessing are vitrified in borosilicate glass matrices. Among the fission products, platinum-group metals (Pd-Rh-Ru) exhibit very low solubility and partly precipitate as metal or oxide phases in the glass melt and molybdenum interacts with the glass frit to form molybdate phases which are known to precipitate as a complex phase called yellow phase. These platinoid and molybdate phases may induce modifications of the physico-chemistry of the glass melt and have an impact on the final glass confinement properties.To understand the relative stability of these phases depending on both temperature and oxygen potential of the melt, a thermodynamic database is being developed using the Calphad method. This database includes the metallic and oxide complex platinoid system and the interactions with tellurium Pd-Rh-Ru-Te-(O). To consider the formation of molybdates, the CaO-MoO$_3$ and of Na$_2$O-MoO$_3$ pseudo binary systems are taken into account. The modeling of the Na$_2$O-SiO$_2$, MoO$_3$-SiO$_2$ and of the ternary SiO$_2$-Na$_2$O-MoO$_3$ system was carried out based on the literature and on new experiments in order to consider a simplified glass melt.Using this tool, the thermodynamic state of the platinoid and molybdate phases is calculated as a function of temperature and composition. This study throws new light on the interactions between molybdenum and platinum-group metals with the glass melt during the vitrification process of high level nuclear wastes.