Rhenium (Re) As a Surrogate for Technetium (Tc) Partitioning in Radioactive Waste Materials.

Technetium (Tc), a nuclear fission product with no stable isotopes, represents much of the long-term radioactivity associated with nuclear waste. Under oxidizing conditions, Re and Tc persist in the environment as anions in the +7 oxidation state, i.e., pertechnetate (TcO₄^-) and perrhenate (ReO₄^-). In soil partitioning studies, the two oxidized species display similar sorption properties, with limited partitioning generally associated with amphoteric soil oxides. Both Tc and Re are subject to chemical reduction to the +4 oxidation state, which is far less soluble and mobile in the environment. Therefore, one disposition strategy for immobilizing ⁹⁹Tc containing waste is the production of cementitious materials that contain reducing agents, such as blast furnace slag, to enhance the retention of redox sensitive contaminants. In the current study, reducing grout materials spiked with either Re or ⁹⁹Tc were allowed to cure for various durations ranging from three months to a year and then subjected to EPA Method 1315, a recently adopted method for evaluating the leaching potential of contaminants found in cementitious materials. Under similar conditions, Re leaching was higher than ⁹⁹Tc from the reducing grout, which was attributed to the more rapid oxidation of Re. These results indicate that Re is not a suitable chemical analog for evaluating Tc partitioning under reducing conditions.