Thermodynamic Modelling of the Pd-Te-Ru System for Nuclear Waste Glasses Application
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Thermodynamic Modelling of the Pd-Te-Ru System for Nuclear Waste Glasses Application S. Gossé1, S. Schuller2, C. Guéneau1 1
CEA/DEN/DANS/DPC/SCP/LM2T Commissariat à l’Energie Atomique – Centre de Saclay, F-91191 Cedex, France 2 CEA/DEN/DTCD/SECM/LDMC Commissariat à l’Energie Atomique – Centre de Marcoule, F-30207 Cedex, France ABSTRACT The waste management process of the French nuclear spent fuels is managed by a new vitrification conditioning step. High level radioactive borosilicate glasses are melted by induction in a cold crucible to accommodate a wide range of minor actinides and fission products. Among the fission products, platinoids precipitate in the form of (Pd-Te, Ru-Rh, Ru) metallic particles in the glass. The microstructure of these phases can affect the physicochemical properties of the melt during the vitrification process. To predict the thermodynamic properties of these alloys in the glass, a database is being developed on the Pd-Rh-Ru-Te system using the Calphad method. The binary (Pd-Te, Pd-Ru, Ru-Te) and ternary (Pd-Te-Ru) systems have been modelled. The Pd-Te-Ru ternary system built by extrapolation from the binaries enables to calculate ternary isothermal sections and thermodynamic properties of the platinoid phases between 873 K and 1573 K. Solidification paths are also calculated for Pd-Te alloys representative for those observed in the glasses. The phase composition at equilibrium and the Ru solubility limit in Pd-Te alloys are also calculated. INTRODUCTION In France, fission products and actinide solutions from nuclear spent fuels are vitrified with a new direct induction-heated melter technology in sodium borosilicate glass matrices. The platinum-group metals exhibit a low solubility in the glass and partly precipitate as metal or oxide phases in the molten glass. For example, Pd mainly reacts with Te to form Pd-Te alloys. The precipitation of these platinoid phases in the glass is of major importance due to its impact on the thermophysical properties of the molten glass (electrical and thermal conductivities) [1-3]. The microstructure of the metallic particles depends on the temperature, the composition and the oxygen potential of the melt. To control the nature and quantity of these precipitates, a description of the thermodynamic properties of the platinoid systems is required. In the present work, a thermodynamic database on the Pd-Te-Ru system has been developed using the Calphad method. THERMODYNAMIC CALCULATIONS: The Calphad Method The Calphad (Calculation of Phase Diagrams) method consists in describing the thermodynamic data on a system by coupling both phase diagrams and thermodynamic properties [4]. For given temperature, pressure and number of component moles, the thermodynamic equilibrium is calculated by minimization of the total free Gibbs Energy of the system. The database contains a mathematical description of the Gibbs energy of all the phases (solid, liquid, gas) as a function of temperature and composition. The Gibbs energy functions of the phases are adj
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