Near-field behavior of 99Tc during the oxidative alteration of spent nuclear fuel

99Tc is a long-lived radioactive fission product with a half-life of 2.13x105 yr and a fission yield of 6.13% in nuclear reactors. 99Tc is a prominent contributor to dose in safety assessments of nuclear waste repositories. Under Eh-pH conditions corresponding to the oxidative corrosion of spent nuclear fuel, which is constrained by the stability of uranyl phases relative to that of UO2+x phases, TcO4- is the predominant species of technetium with log [TcO4-]/[TcO(OH)20]>2.15 in the range of pH=4-10. Because of the low solubility of TcO2·H2O and high adsorption of Tc(4+) by geological materials and clays, the concentration of Tc(4+) in groundwater is expected to be less than 10-8 M, and the incorporation of Tc4+ into alteration uranyl phases is not considered to be an important retardation mechanism. In contrast, TcO4- is highly soluble and weakly adsorbed in the near-field. The incorporation of Tc7+ into the structure of uranyl phases that are expected to occur as alteration products of spent nuclear fuel will result in underbonding at the U6+ site and will destabilize the structure, suggesting that significant substitution of (TcO4-) will not occur in uranyl phases