Effects of sulfate on rhenium incorporation into low-activity waste glass

Abstract Technetium-99 ( 99 Tc) is a major radionuclide of concern in the Hanford low-activity waste (LAW), which will be vitrified into borosilicate glass at the Waste Treatment and Immobilization Plant (WTP). Sulfate in LAW has been known to be a critical factor affecting the volatile loss of 99 Tc. We investigated rhenium (a nonradioactive surrogate for 99 Tc) incorporation by crucible melting tests of two representative simulated LAW glass feeds, each prepared with varied sulfur concentrations in glass. The slurry feeds were dried and heated to 400–1100 °C. Soluble and insoluble phases of heated feeds were analyzed to profile the partitioning of various components during the melting process. The mechanism of how sulfate affects rhenium incorporation during feed-to-glass conversion reactions, and so final retention of rhenium in glass, is proposed based on the two hypotheses that segregated sulfate-rich phase forms during feed conversion reactions and Re preferentially partitions to this sulfate-rich phase.