Increasing Tc retention during nuclear waste vitrification: Effect of alumina source on the rate of rhenium incorporation into glass

Abstract

Technetium (Tc) volatilization during nuclear waste vitrification poses a significant challenge, necessitating recycle loops that may lead to reduced waste loading in the glass, or secondary Tc treatment in alternative waste forms. Thus, reducing the volatilization of Tc (or Re, its non-radioactive surrogate) is crucial for improving vitrification process efficiency and minimizing the long-term costs associated with nuclear waste management. In our previous study, we demonstrated that using gibbsite, instead of kyanite, as the alumina source in low-activity nuclear waste melter feeds increased the Re retention by up to 20 %, a result attributed to the formation of nanocrystalline alumina in the gibbsite-containing melter feeds. In this work, we show that using boehmite, another Al-source that forms nanocrystalline alumina polymorphs upon heating, results in an even higher retention of Re, by up to 25 %. Using X-ray diffraction and deionized water leaching tests on heat-treated feed samples, we analyzed the compositions of soluble oxyanionic salt phase and the insoluble glass-forming phase as a function of temperature, and evaluated the rate of Re incorporation into the developing transient alkali-alumino-borosilicate melt. We found that the formation of nanocrystalline alumina in the boehmite-containing feed occurred at temperatures approximately 100 °C lower than in the gibbsite feed, accelerating reactions between alumina, oxyanionic salts, and the borosilicate melt, which likely accounts for the enhanced Re retention in the boehmite-containing feeds.