Vanadium and chromium redox behavior in borosilicate nuclear waste glasses

X-ray absorption spectroscopy (XAS) was used to characterize vanadium (V) and chromium (Cr) environments in low activity nuclear waste (LAW) glasses synthesized under a variety of redox conditions. V₂O₅ was added to the melt to improve sulfur incorporation from the waste; however, at sufficiently high concentrations, V increased melt foaming, which lowered melt processing rates. Foaming may be reduced by varying the redox conditions of the melt, while small amounts of Cr are added to reduce melter refractory corrosion. Three parent glasses were studied, where CO-CO₂ mixtures were bubbled through the corresponding melt for increasing time intervals so that a series of redox-adjusted-glasses was synthesized from each parent glass. XAS data indicated that V and Cr behaviors are significantly different in these glasses with respect to the cumulative gas bubbling times: V⁴⁺/V_total ranges from 8 to 35%, while Cr³⁺/Cr_total can range from 15 to 100% and even to population distributions including Cr²⁺. As Na-content decreased, V, and especially, Cr became more reduced, when comparing equivalent glasses within a series. The Na-poor glass series show possible redox coupling between V and Cr, where V⁴⁺ populations increase after initial bubbling, but as bubbling time increases, V⁴⁺ populations drop to near the level of the parent glass, while Cr becomes more reduced to the point of having increasing Cr²⁺ populations.