Effect of sulfate sequestration by Ba Sn composite material on Re retention in low-activity waste glass

Incorporating radioactive Tc-99 into glass during vitrification of low-activity waste (LAW) at the U.S. Department of Energy's Hanford Site is a scientific challenge. Previous studies have suggested that the presence of sulfate salts in the melt hinder the incorporation of Re, a non-radioactive surrogate for Tc-99, into nuclear waste glass. In this study, we assessed the capability of a BaSn composite material to sequester sulfate during vitrification of AN-102 LAW feed and the resulting effects on Re retention in the glass. We also investigated a feed in which the initial soluble sulfate source in the simulated LAW (Na2SO4) was replaced with insoluble BaSO4. The BaSn composite-treated feed and BaSO4-substitute feed were compared to a control, AN-102 baseline. The feeds were prepared and then heated to temperatures ranging from 400 to 1000 °C at a 5 °C/min rate and then air-quenched to room temperature. The heated samples and dried feeds were leached in deionized water. The solutions from the leach test were analyzed by inductively coupled plasma optical emission spectroscopy to determine the amount of soluble sulfate as a function of temperature, and the resulting solids were analyzed for Re retention. Although the BaSn composite material successfully sequestered sulfate in the as-prepared LAW feed, heating to 400 °C resulted in most of the sulfate partitioning to a soluble salt phase. By 700 °C, sulfate partitioning in the composite-treated feed was identical to that of the baseline feed. Throughout the entire temperature range, the BaSO4-substitute feed behaved similarly to the composite-treated feed. The results of the study show that sequestering sulfate in a BaSO4 phase is not effective for Re retention as the sulfate salt behavior was nearly identical in the three feeds at ≥700 °C. The fraction of Re retained in the glass at 1000 °C varied from 39.4 to 46.6% for the three feeds, which is shown to be a statistically insignificant difference.