Dissimilar behavior of technetium and rhenium in borosilicate waste glass as determined by X-ray absorption spectroscopy

Technetium-99 is an abundant, long-lived (tau(1/2) = 213 000 years) fission product that creates challenges for the safe, long-term disposal of nuclear waste. Although Tc-99 receives attention largely because of its high environmental mobility, it also causes problems during its incorporation into nuclear waste glass because of the volatility of Tc(VII) compounds. This volatility decreases the amount of Tc-99 stabilized in the waste glass and causes contamination of the waste glass melter and off-gas system. The approach to decreasing the volatility of Tc-99 that has received the most attention is the 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 Tc-99 to avoid the radioactive contamination problems caused by volatile Tc-99 compounds. However, Re(VII) is more stable toward 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, we prepared a series of glasses 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.