Plutonium Desorption from Nuclear Melt Glass-Derived Colloids and Implications for Migration at the Nevada National Security Site, USA.

Abstract

The migration of low levels of plutonium has been observed at the Nevada National Security Site (NNSS) and attributed to colloids. To better understand the mechanism(s) of colloid-facilitated transport at this site, we performed flow cell desorption experiments with mineral colloid suspensions produced by hydrothermal alteration of NNSS nuclear melt glass, residual material left behind from nuclear testing. Three different colloid suspensions were used: (1) colloidal material from hydrothermal alteration of nuclear melt glass at 140 °C; (2) at 200 °C; and (3) plutonium sorbed to SWy-1 montmorillonite at room temperature. The 140 °C sample contained only montmorillonite, while zeolite and other phases were present in the 200 °C sample. Overall, more plutonium was desorbed from the 140 °C colloids (ca. 9-16%) than from the 200 °C colloids (ca. 4-8%). Furthermore, at the end of the 4.5 day flow cell experiments, the desorption rates for the 140 °C colloids and the Pu-montmorillonite colloids were similar while the desorption rates from the 200 °C colloids were up to an order of magnitude lower. We posit that the formation of zeolites and clays hydrothermally altered at 200 °C may lead to a more stable association of plutonium with colloids, resulting in lower desorption rates. This may give rise to more extensive colloid-facilitated transport and help explain why trace levels of plutonium are found downgradient from their original source decades after a nuclear detonation. Interestingly, in the case of cesium (a co-contaminant of plutonium), no difference was observed between the 140 and 200 °C colloids. This reflects intrinsic differences between cesium and plutonium sorption/desorption behavior (charge, cation size) and suggests that the Cs sorption mechanism (cation exchange) is not similarly affected by colloid formation temperature.