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

Rhenium (Re), a non-radioactive surrogate for technetium (Tc), was incorporated into the lattice of SnO 2 by reductive co-precipitation process at room temperature, and Re volatility from Re(SnO 2 ) under different conditions (in air and N 2 and with glass frit) at elevated temperatures was carefully investigated. Re began to evaporate from Re(SnO 2 ) both in air and N 2 at 400 °C, while Re exhibited a faster volatilization in air than in N 2 , as all Re was gone at 1200 °C in air, but ∼40% of Re still remained at 1200 °C in N 2 . Additionally, Re did not volatilize from Re(SnO 2 )-containing glass frit until 800 °C and only ∼15% of Re volatilized at 1200 °C, suggesting the loss of Re decreased by ∼30% points through incorporation of Re into SnO 2 before vitrification. The analysis of Re valence transition revealed that the reduced Re 4+ was easily oxidized to volatile Re 7+ in air, but in N 2 , Re 4+ disproportionated to form Re 7+ and the lower valence states of Re at temperatures from 600 °C to 1000 °C. In Re(SnO 2 )-containing glass frit, the formation of NaReO 4 during vitrification inhibited Re 7+ volatilization before 800 °C, while the presence of Re 4+ in the glass melt enhanced Re retention at temperature above 800 °C.