99Tc is a long-lived radioactive fission product with a half-life of 2.13×105 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.

Citation: Chen, F., Burns, P.C., and Ewing, R.C. "Near-Field Behavior of 99Tc During the Oxidative Alteration of Spent Nuclear Fuel." Journal of Nuclear Materials. 278(2-3) April 2000: 225-232.

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