Redox behavior of ruthenium in nuclear glass melt: ruthenium dioxide reduction reaction

Among platinoid fission products, ruthenium is a key element during the vitrification of high-level waste because it is poorly soluble in the glass matrix. It occurs mainly as RuO2 particles and sometimes as Ru0 particles in the glass melt and final cooled glass. Because the physical properties of ruthenium are very different from those of glass, the RuO2 particles change the glass melt properties such as the rheology, thermal conductivity and electrical conductivity. In addition, because the Ru0 properties are even more distinct from the glass properties, glass melts can be more sensitive to the presence of Ru0 particles. Thus, ruthenium speciation, specifically the RuO2 reduction into Ru0, was investigated in air and glass environments, and the experimental results were compared to thermodynamic calculations (Calphad method). Under air conditions, the thermodynamic modeling and experimental results are in good agreement. They show a reduction of RuO2 at 1403 °C and 1405 ± 7 °C, respectively. The experimental results indicate that the Ru0 particle formation may result from an autocatalysis mechanism, which involves the gaseous species of ruthenium. In the investigated glass, the first Ru0 particles appear at approximately 1280 °C. At 1280 °C < T < 1400 °C, only few particles are observed attributed to minor reduction phenomena: local redox reaction with iron or disproportionation reaction. At higher temperatures (T > 1400 °C), a much more significant reduction takes place. This one is attributed to the decomposition under temperature and oxygen pressure conditions, predicted by thermodynamic calculations. Based on the results obtained under air conditions, it is assumed that the reduction mechanism involves dissolved ruthenium species.