PNNL

Cementitious waste forms present a low-temperature, low-cost option for the immobilization of nuclear wastes. At the U.S. Department of Energy Hanford Site, the low activity waste (LAW) portion of the nuclear wastes stored in underground tanks at the site contain Tc-99, a long lived radionuclide capable of high environmental mobility. A cementitious waste form known as Cast Stone is under investigation as a possible candidate technology for the immobilization of LAW. Ensuring slow rates of release of Tc from a LAW waste form, be it glass or cementitious, is essential for safe disposal of the waste. This work focuses on the addition of getter materials to Cast Stone that can selectively immobilize Tc from the LAW, and in turn, lower Tc release from the Cast Stone. Two getters which produce different products upon sequestering Tc from LAW were tested in Cast Stone: Sn(II)-treated apatite (Sn-A) that removes Tc as a Tc(IV)-oxide and potassium metal sulfide (KMS-2) that removes Tc as a Tc(IV)-sulfide species, allowing for a comparison of stability of the form of Tc upon entering the waste form. The Cast Stone treated with KMS-2 generating a Tc-sulfide had the best performance. The observed diffusion (Dobs) of Tc decreased from 4.6 ± 0.2 × 10-12 cm2/s for Cast Stone that did not contain a getter to 5.4 ± 0.4 × 10-13 cm2/s for KMS-2 containing Cast Stone, where KMS-2 addition was equivalent to