Re volatilization at elevated temperatures: Interest of Re-doped SnO2 for nuclear waste vitrification

Abstract Rhenium (Re), a non-radioactive surrogate for technetium (Tc), was incorporated into the lattice of SnO2 by reductive co-precipitation process at room temperature, and Re volatility from Re(SnO2) under different atmospheres (in air and N2 and with glass frit) at elevated temperatures was carefully investigated. Re began to evaporate from Re(SnO2) both in air and N2 at 400 °C, while Re exhibited a faster volatilization in air than in N2, as all Re was gone at 1200 °C in air, but ∼40% of Re still remained at 1200 °C in N2. Additionally, Re did not volatilize from Re(SnO2)-containing glass frit until 800 °C and only ∼15% of Re volatilized at 1200 °C, suggesting the loss of Re decreased by ∼30 percentage points through incorporation of Re into SnO2 before vitrification The analysis of Re valence transition revealed that the reduced Re4+ was easily oxidized to volatile Re7+ in air, but in N2, Re4+ disproportionated to form Re7+ and the lower valence states of Re at temperatures from 600 °C to 1000 °C. In Re(SnO2)-containing glass frit, the formation of NaReO4 during vitrification inhibited Re7+ volatilization before 800 °C, while the presence of Re4+ in the glass melt enhanced Re retention at temperature above 800 °C.