Estimation of Redox Condition of Simulated High-level Waste Glass Based on the Chemical Composition of Crystalline Phases of Platinum-group Elements

The redox condition of simulated high-level waste (HLW) glass was investigated by thermodynamic calculations for the phase equilibrium of crystalline phases composed of platinum group elements (PGE, Pd, Rh, and Ru) based on the thermodynamic data for Pd-Rh-Ru and (Ru,Rh)O2 solid solutions. Phase equilibrium calculation indicates that the stability field of the Pd-rich metal phase with an FCC structure and the (Ru,Rh)O2 phase is nearly independent of the relative abundances of Pd, Rh, and Ru. Thermodynamic calculation for rhodium partitioning between Pd-Rh-Ru and (Ru,Rh)O2 can provide the temperature and oxygen fugacity conditions under which these phases are stable. The redox condition of HLW glasses retrieved from a full-scale mock-up melter (KMOC) in JAEA, Tokai, Japan, was evaluated by comparing the results of chemical composition analysis for PGE crystalline phases in the glasses with those of phase equilibrium and/or thermodynamic calculations. The obtained results suggest that the oxygen fugacity is higher than the atmospheric condition (fO2>0.21) for the glass retrieved in a normal melter operation, while it is lower at 10－1.5-10－2.0 for the PGE-rich residual glass retrieved from the melter after glass draining. The calculated fO2 values are consistent with the redox condition estimated from Ce3+/Ce4+ ratios of KMOC glasses reported previously.