PNNL

Abstract

Volatilization of Technetium-99 (99Tc) is of concern in the vitrification process of Hanford low-activity waste (LAW) to produce a borosilicate glass waste form at the Waste Treatment and Immobilization Plant (WTP). Addition of reducing agents to the LAW glass feed (a mix of liquid LAW and mineral/chemical additives) have been known to increase the retention of 99Tc in the final glass product. We investigated rhenium (a nonradioactive surrogate for 99Tc) incorporation into glass by performing crucible melting tests of two representative simulated LAW glass feeds, each prepared without and with a reducing agent, iron oxalate. The iron oxalate replaced hematite that is used as one of the mineral additives in the baseline feeds to supply Fe2O3. The slurry feeds were dried at 105°C and heated to 400–1100°C. Deionized water leach at room temperature and at 80°C was conducted on the dried feed and heat-treated samples to analyze the chemical compositions of soluble and insoluble phases separately and to profile the partitioning of various components into different phases during the melting process. Partitioning behavior of rhenium and sulfate is discussed in connection with major feed-to-glass conversion reactions. The mechanism on how iron oxalate affects rhenium incorporation during feed-to-glass conversion reactions, and so final retention of rhenium in glass, is proposed.