Effect of alumina source on the retention of rhenium during low-activity waste feed conversion to glass

Volatility of technetium poses a challenge during the vitrification of low-activity waste (LAW) feed to glass. Although this is typically resolved through recycle loops, this can lead to other problems, such as sulfate phase formation. Thus, ensuring high single-pass retention of Tc (or Re, its non-radioactive surrogate) is required for enabling high waste-loading formulations. In our previous study, we observed significantly higher Re retention in final waste glass produced from a LAW melter feed containing gibbsite as the alumina source compared to a compositionally similar feed containing kyanite. To investigate this effect, we prepared representative LAW melter feeds with kyanite and gibbsite as alumina sources and measured the Re retention in the produced glasses. We found that the Re retention is consistently higher in glasses produced with gibbsite, by up to 20 %. We attribute this result to the formation of nanocrystalline alumina in melter feeds containing gibbsite. Possessing a high surface area, nanocrystalline alumina can adsorb the perrhenate-containing molten salt, increasing the rate at which Re is incorporated into the alkali-alumino-boro-silicate glass-forming melt. In addition, we demonstrate that replacing kyanite with gibbsite in LAW melter feeds has no adverse effects on their processing during vitrification.