SORPTION OF URANIUM(VI) TO GRAPHITE UNDER POTENTIAL NUCLEAR REPOSITORY CONDITIONS

Currently, the United States’ Next Generation Nuclear Plant program has focused on the development of reactors with novel fuel forms that incorporate significant quantities of graphite into the nuclear fuel element. Accordingly, any model for the repository performance of this new fuel should account for the effect of the surrounding graphite matrix on release of the radioactive inventory. However, there is little in the literature on the potential impact of graphite on the transport (and eventual release) of uranium or other isotopes under environmental conditions. This work was undertaken to examine the sorption behavior of uranium for a variety of environmental conditions that have been identified as impacting uranium transport to assist in the development of uranium transport models for the widest possible range of repository-relevant environmental conditions.

The sorption and desorption behavior of uranium(VI) was examined through a series of batch experiments to examine the equilibrium sorption and reaction kinetics across a range of potential repository conditions. Uranium(VI) showed pH dependent sorption behavior with pH varied from 2-10 with maximum values being recorded in the near-neutral and slightly acidic pH region (K_d ranging from 0 – 125 mL/g). Sorption behavior was also found to behave in a non-linear fashion with changing U(VI) concentration in solution, best fit by a Freundlich isotherm. Ionic strength was not observed to have a significant effect on the sorption up to 4 molal NaCl. Sorption was also strongly dependent on the solution concentration of dissolved carbonate with elevated dissolved carbonate being able to completely suppress sorption in previously identified high sorbing regions and reduced dissolved carbonate concentrations enhancing sorption in areas where sorption had been previously minimized. Sorption and desorption kinetics were examined revealing significant differences in the sorption and desorption rates, as well as incomplete desorption of the uranium from the graphite over the experiment duration.