Lawrence Berkeley National Laboratory

Technetium (Tc), found in nuclear waste, is of particular concern with regard to long-term waste storage because of its long half-life and high mobility in the environment. One method of stabilization of such waste is through vitrification to produce a durable borosilicate glass matrix. The fate of Tc under hydrothermal conditions in the Vapor Hydration Test (VHT) was studied to assess and possibly predict the long-term rate of release of Tc from borosilicate waste glass. For comparison, the fate of rhenium (Re), the preferred non-radioactive surrogate for Tc, was similarly studied. X-ray absorption spectroscopy (XAS) and scanning electron microscopy (SEM) measurements were made on each original borosilicate glass and the corresponding sample after the VHT. Tc K-edge XAS indicates that, despite starting with different Tc(IV) and Tc(VII) distributions in each glass, both corresponding VHT samples contain 100 percent Tc(IV). The Tc reduction within the VHT samples may be driven by simultaneous oxygen depletion from corrosion of the surrounding stainless steel vessel. From SEM analyses, both of the Tc-containing VHT samples show complete alteration of the original glass, significant Tc enrichment near the sample surface, and nearly complete depletion of Tc toward the sample center. XAS indicates Tc(IV)O6 octahedra, possibly within gel-like amorphous silicates in both VHT samples, where Tc-Tc correlations are observed in the higher Tc-content VHT sample. Re LII-edge XAS and SEM indicate quite different behavior for Re under VHT conditions. Re oxidation state appears to be invariant with respect to the VHT treatment, where perrhenate (Re(VII)) species are dominant in all Re-containing samples investigated; Re2O7 concentrations are low NEAR the sample surface and increase to approach the concentration of the un-reacted glass toward the sample center.