ABSTRACT Technetium‐99 (Tc) is a long‐lived, volatile fission product in nuclear waste that poses challenges for immobilization due to its tendency to volatilize at high temperatures during vitrification. Understanding and minimizing Tc volatilization is essential to efficient waste processing and the reduction of secondary waste. In this study, we investigated the effect of silica particle size and primary foaming intensity on the retention of rhenium (Re), a nonradioactive surrogate for Tc, in borosilicate glasses produced from high‐level waste melter feeds. Contrary to expectations, we observed that finer silica particles—which increased primary foaming—led to higher Re retention in the final glass. X‐ray diffraction analysis showed that smaller silica particles with high surface area dissolved more rapidly, resulting in a higher fraction of transient glass‐forming melt in early stages. However, leaching tests revealed that the rate of Re incorporation into the glass‐forming melt was fast and largely independent of the used silica particle size. Instead, differences in final Re retention were caused by faster Re volatilization from glass‐forming melts with lower viscosity, which formed during the melting of feeds containing coarser silica due to their slower dissolution. These findings indicate that the particle size of silica—and potentially other glass‐forming or modifying additives—can indirectly affect Re retention by their influence on the transient glass‐forming melt viscosity. Moreover, our results suggest that the intensity of primary foaming has minimal effect on Re retention, despite often being associated with enhanced volatile losses due to increased surface exposure.
Eret et al. (Fri,) studied this question.