Tc and Re behavior in borosilicate waste glass vapor hydration tests II

Technetium (99Tc) is a significant environmental risk factor to consider for nuclear waste disposal repositories. Rhenium (Re), in the same column of the periodic table as Tc, is often used as a non-radioactive surrogate for Tc. Six waste glasses containing both Tc and Re were synthesized under a variety of redox conditions to produce different distributions of Tc and Re oxidation states. These glasses were exposed to vapor hydration tests (VHT) at 200 °C for 23 to 30 days; and the Tc and Re oxidation state, coordination environment, and spatial distribution within the altered coupons were determined. Compared with the original glasses, the corresponding VHT samples showed substantial reduction of Tc species, except where the original glass contained only reduced Tc (Tc4+). Similar to earlier findings, Tc is more sensitive to redox conditions than Re with respect to both glass synthesis conditions and VHT alteration processes. Glasses that originally contained more oxidized Tc (near 100% Tc7+) showed the most Tc enrichment in the altered VHT sample layers, where Tc was largely reduced to Tc4+. Re is generally more oxidized than Tc in the samples measured and has similar spatial distributions as Tc in some VHT samples, while having very different spatial distributions compared with Tc in others. Glasses that originally had a distribution of Tc oxidation states (approximately 1:1 Tc4+ to Tc7+), had Tc concentrations in the VHT altered layers that were approximately equal to or less than those found in the unaltered glass. However, in the same samples, Re concentrations were highest in the altered layers. Overall, with regard to spatial distributions within the altered VHT layers, the behavior of Re was not a good predictor of Tc behavior. Therefore, at least under VHT conditions, using Re as a non-radioactive surrogate for Tc in borosilicate waste glasses can provide misleading results.

► Tc, an environmental risk for nuclear waste storage, is commonly modeled using Re. ► Tc and Re valences can diverge in original and hydrothermally altered glasses. ► Tc and Re spatial distributions can diverge in hydrothermally altered glasses. ► Re can be an unreliable surrogate for Tc in situations where redox is important. ► Using Re as a surrogate for Tc in waste glasses can provide misleading results.