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

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