(528c) Technetium Leaching Cementitious 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, Tc persist in the environment as an anion in the +7 oxidation state, i.e., pertechnetate (TcO₄^-). In soil partitioning studies, the oxidized species displays limited retention generally associated with amphoteric soil oxides. However, Tc(VII) is subject to chemical reduction to the +4 oxidation state, which is less soluble and mobile in the environment. One disposition strategy for immobilizing ⁹⁹Tc containing waste is the production of cementitious materials that contain reducing agents, such as blast furnace slag (BFS), to enhance the retention of redox sensitive contaminants. At the Saltstone Disposal Facility (SDF) on the Department of Energyâ??s Savannah River Site (SRS), low-level radioactive saltwaste is mixed with BFS, fly ash and portland cement to produce a solidified cementitious material known as Saltstone, which is deposited in a series of vaults for long-term disposal.

In the current study, contaminant mass transfer rates for ⁹⁹Tc and other contaminants from saltstone simulants spiked with ⁹⁹Tc and rhenium (Re), often used as surrogate for ⁹⁹Tc, and actual SDF saltstone samples were assessed using EPA Method 1315, a method for evaluating contaminant leaching from intact monolithic materials. Since redox state plays a key role in controlling ⁹⁹Tc solubility, the monoliths were exposed to one of three different test environments during leaching: aerobic (ambient air), anoxic (N₂), and reducing (H₂/N₂). Rhenium leaching rates (and other poorly sorbing contaminants like NO₃^- and iodine) were much higher than ⁹⁹Tc, which was attributed to poor initial reduction or more rapid oxidation, indicating that Re is not a suitable chemical analog for evaluating Tc partitioning under reducing conditions.