a b s t r a c tVolatile loss of radioactive technetium-99 ( 99 Tc) to off-gas is a major challenge when vitrifying low-activity waste (LAW) at the U.S. Department of Energy's Hanford Site in Washington State. We investigated the partitioning and incorporation of rhenium (Re) (a nonradioactive surrogate for 99 Tc) into the glass melt during crucible melting of two simulated LAW feeds that have exhibited a large difference in 99m Tc/Re retention in glass from small-scale melter tests. Each feed was prepared from a simulated liquid LAW and additives (boric acid, silica sand, etc.). The as-mixed slurry feeds were dried at 105°C and heated to 600-1100°C at 5 K/min. The dried feeds and heat-treated samples were leached with deionized water for 10 min at room temperature followed by 24-h leaching at 80°C. Chemical compositions of the resulting solutions and insoluble solids were analyzed. Volume expansion measurements and X-ray diffraction (XRD) analyses were performed on dried feeds and heat-treated samples to characterize the progress of feed-to-glass conversion reactions. We found that incorporation of Re into the glass melt was virtually completed during the major feed-to-glass conversion reactions that occurred at ≤700°C. The results of our study suggest that the different compositions of the salt phases formed during early stages of melting at ≤700°C are responsible for the large difference in Re incorporation into the glass melt in these two feeds.

Experiments were conducted to determine sulfur solubility in Hanford low‐activity waste (LAW) glass melts by a sulfur saturation method. Sulfur‐incorporated glass melts were prepared by salt saturation and bubbling methods. The salt saturation method was performed by mixing crushed premelted baseline glasses with an excess amount of Na2SO4 prior to melting the mixture at 1150°C for 1 hour. The bubbling method involved bubbling the glass melt at 1150°C in a Pt crucible with an SO2/O2/N2 gas mix to equilibrate the melt at a known pressure of SO3. Preliminary results suggested that performing 1 cycle of mixing and melting was not sufficient to saturate the glass. The bubbling method successfully incorporated sulfur into the glass but caused significant losses of sodium from the melt. In order to saturate the glass melt with sulfate without causing noticeable sodium loss, a modified crucible salt saturation method was developed by repeating the mixing and melting of the glass and salt mixture. For all 3 representative LAW glasses tested in this study, it was found that after 3 mixing and melting cycles, the sulfur concentration reaches a plateau, indicating reasonable sulfur saturation.

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Hanford low-activity waste (LAW) glasses with high sulfur concentrations are subject to salt segregation in the melter, which hinders melter operation by corroding components and shortening the melter life. To better predict the point at which salt accumulates on the melt surface, the development of sulfate solubility models is needed. Using a sulfur saturation method, crucible scale melts for 13 LAW glasses with varying sulfur solubilities were conducted. The resulting salt and glass compositions were reported and the change in component partitioning following the saturation process was examined to better understand potential changes in overall glass composition. It was shown that both Cr 2 O 3 and Cl experience significant losses, with ~28% of Cr 2 O 3 partitioning into the salt phase and Cl primarily volatilizing out of the melt (~23% partitioned to salt and ~40% lost as off gas). These patterns can be accounted for during model development. Measured sulfur solubilities were also compared to previously reported data. It was found that crucible sulfur solubility ranged from 0.95 to 2.14 wt% SO 3 with a high correlation ([R 2 fit = 0.94]) between crucible solubility and melter tolerance. These results suggest that crucible scale sulfate solubility data can be used to predict SO 3 tolerance in the melter feed. K E Y W O R D Scharacterization, glass forming melts-chemical analysis, glass forming systems-volatilization, modeling, properties-borosilicate, properties-glass forming melts