Influence of rhenium concentration and alumina source on rhenium retention in nuclear waste glasses

Technetium-99 ( 99 Tc) volatility during low-activity waste (LAW) vitrification limits waste loading, as recycling the captured 99 Tc introduces other volatile elements into the melt that increase waste glass volume and processing cost. In this study, rhenium (Re) was used as a non-radioactive surrogate for 99 Tc to examine the effects of Re concentration and alumina source on the Re retention in LAW glass formulations. At low Re concentrations, particularly 5 ppm, both LAW feeds tested exhibited increased Re retention, potentially due to faster kinetics of Re incorporation into the glass-forming melt or due to a reduced volatility associated with dilute perrhenate phases. At higher Re concentrations (10–300 ppm), the Re retention in the produced glasses remained nearly constant. With respect to alumina sources, high-temperature visual observations revealed that gibbsite and boehmite increased the primary foaming of LAW feeds during conversion to glass, while gas evolution behavior was not significantly affected. Interestingly, the substitution of gibbsite or boehmite for kyanite did not produce consistent trends in retention. This is contrary to current literature, which suggests that the formation of reactive nanocrystalline alumina during melting of gibbsite- or boehmite-containing feeds leads to increased Re retention. These results suggest that feed-specific factors and experimental conditions may play an important role.