Effects of iron oxalate on rhenium incorporation into low-activity waste glass

Abstract Volatilization of technetium-99 (99Tc) is of concern during vitrification of Hanford low-activity waste (LAW) to produce a borosilicate glass waste form at the Waste Treatment and Immobilization Plant. Addition of reducing agents to LAW glass feed has been known to increase the retention of 99Tc in the final glass product. We investigated rhenium incorporation into glass by performing crucible melting tests on simulated LAW glass feeds, prepared without and with iron oxalate as a reducing agent, to understand the mechanism how the reducing agent influences Re retention in glass. Deionized water leaches were conducted on the dried feed and heat-treated samples to analyze the compositions of soluble and insoluble phases separately and to profile the partitioning of components into different phases during melting. The results in this study do not support the previous hypothesis that the low volatility of reduced Re4+ and Tc4+ is responsible for higher Re and Tc retention in glass when a reducing agent is added. We propose that early reaction of nitrate/nitrite and faster incorporation of sulfate into glass caused by iron oxalate are the two contributing factors that can explain the experimental findings in this study regarding the positive effect of iron oxalate on Re incorporation and retention.