Ruthenium Thermodynamics in Nuclear Waste Glasses
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Ruthenium Thermodynamics in Nuclear Waste Glasses S. Gossé1, S. Schuller2, C. Guéneau1, H. Boucetta2 1
CEA, DEN, DPC, SCCME, LM2T Commissariat à l’Energie Atomique et aux Energies Alternatives – Centre de Saclay 91191 Gif-sur-Yvette Cedex, France 2
CEA, DEN, DTCD, SECM, LDMC Commissariat à l’Energie Atomique et aux Energies Alternatives – Centre de Marcoule 30207 Bagnols sur Cèze Cedex, France ABSTRACT In 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. INTRODUCTION Regardless of the vitrification process or the type of phase based matrix of high level radioactive glasses (phosphate or borosilicate), the platinoid fission products Pd-Rh-Ru from nuclear spent fuel segregate in the glass [1,2,3]. These very low solubility elements precipitate both under (Pd-Te, Ru-Rh, Ru) metallic particles [4] and/or ((Rh,Ru)O2, Rh2O3) oxide phases during vitrification [3,5-6]. Composition and microstructures of these phases can significantly affect the physico-chemical properties and the electrical or thermal conductivities during melting. Several studies are undertaken at CEA [1-2] in order to point out the reactions and the chemical interactions in the liquid and viscous states between the glass matrix and the platinoids issuing from the calcinated waste. Among these studies, a thermodynamic FissionProducts database is being developed on the metallic (Pd-Rh-Ru-Te) and oxide (O-Pd-Rh-RuTe)
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