Technetium (Tc) Partitioning in Cementitious 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, Tc persists in the environment as a readily mobile anion in the +7 oxidation state, i.e., pertechnetate (TcO₄^-).  One strategy for immobilizing ⁹⁹Tc at the Department of Energy’s Savannah River Site (SRS) is the production of cementitious materials that contain blast furnace slag (BFS) as a reducing agent to enhance the retention of redox sensitive contaminants.  Low-level liquid radioactive wastes containing ¹³⁷Cs, ¹²⁹I and other radionuclides in addition to ⁹⁹Tc (high pH, and high levels of Na⁺, NO₃^-, and NO₂^-) is mixed with cement, BFS and fly ash to form a cementitious material known as Saltstone, which is poured into a series of large concrete vaults for curing. EPA Method 1315, a method for evaluating the leaching potential of contaminants found in monolithic materials, was used to evaluate the leachability of contaminants from actual Saltstone (SDFS) collected by drilling core sections from a vault at the Saltstone Disposal Facility (SDF), and simulant laboratory Saltstone (LS) made from the same dry feed materials, spiked with a similar level of ⁹⁹Tc and cured for 3 to 6 months under high humidity before testing. EPA leaching tests indicated that ⁹⁹Tc was retained in both the SDFS and LS when compared to other more readily mobile contaminants, such as Na and NO₃, indicating that the LS material was an effective surrogate for predicting ⁹⁹Tc behavior.