Ruthenium Thermodynamics in Nuclear Waste Glasses

Abstract

ABSTRACTIn high level radioactive glasses, the low solubility platinoids (Pd, Ru, Rh) precipitate to form (Pd-Rh-Te, Ru-Rh, Ru) metallic particles and (RuO2, Rh2O3) oxides during the vitrification process. The composition and microstructures of these phases can significantly modify the physico-chemical properties and the electrical or thermal conductivities during melting.Several studies are undertaken at CEA in order to point out the reactions and the chemical interactions in the liquid and viscous states between the glass matrix and the platinoids present in the calcinated waste. Among these studies, a thermodynamic fission products database is being developed on the metallic (Pd-Rh-Ru-Te) and oxide (O-Pd-Rh-Ru-Te) systems. In this work, based on the Calphad method, the Gibbs free energies of each phase are modelled to provide an overall thermodynamic description of the platinoid phases in nuclear waste glasses. The objective of the database is to facilitate calculations of phase diagrams and thermodynamic properties. This flexible tool also enables calculations of the relative stability between metallic and oxide phases in function of the oxygen potential (RedOx equilibrium).For example, some solidification routes are calculated for typical Pd-Rh-Ru-Te compositions of LWR spent fuels. The calculated Pd-Rh-Ru-Te solidification paths are compared with the phases analysed in simplified laboratory scale glass samples. Using these results, the compositions of the Pd-Rh-Ru-Te inclusions are predicted. Furthermore, possible consideration of the RedOx equilibria for some ruthenium based phases makes it possible to explain the speciation between oxide and metallic phases partly due to the Pd-Te interaction.