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.

Technetium-99 is an abundant, long-lived (t1/2 = 213,000 yr) fission product that creates challenges for the safe, long-term disposal of nuclear waste. While 99Tc receives attention largely due to its high environmental mobility, it also causes problems during its incorporation into nuclear waste glass due to the volatility of Tc(VII) compounds. This volatility decreases the amount of 99Tc stabilized in the waste glass and causes contamination of the waste glass melter and off-gas system. The approach to decrease the volatility of 99Tc that has received the most attention is reduction of the volatile Tc(VII) species to less volatile Tc(IV) species in the glass melt. On engineering scale experiments, rhenium is often used as a non-radioactive surrogate for 99Tc to avoid the radioactive contamination problems caused by volatile 99Tc compounds. However, Re(VII) is more stable towards reduction than Tc(VII), so more reducing conditions would be required in the glass melt to produce Re(IV). To better understand the redox behavior of Tc and Re in nuclear waste glass, a series of glasses were prepared under different redox conditions. The speciation of Tc and Re in the resulting glasses was determined by X-ray absorption fine structure spectroscopy. Surprisingly, Re and Tc do not behave similarly in the glass melt. Although Tc(0), Tc(IV), and Tc(VII) were observed in these samples, only Re(0) and Re(VII) were found. In no case was Re(IV) (or Re(VI)) observed.