Chancellor Water Colloids: Characterization and Radionuclide Associated Transport

Abstract

Column transport experiments were conducted in which water from the Chancellor nuclear test cavity was tranported through crushed volcanic tuff from Pahute Mesa. In one experiment, the cavity water was spiked with solute 137 Cs, and in another it was spiked with 239/240 Pu(IV) nanocolloids. A third column experiment was conducted with no radionuclide spike at all, although the 137 Cs concentrations in the water were still high enough to quantify in the column effluent. The radionuclides strongly partitioned to natural colloids present in the water, which were characterized for size distribution, mass concentration, zeta potential/surface charge, critical coagulation concentration, and qualitative mineralogy. In the spiked water experiments, the unanalyzed portion of the high-concentration column effluent samples were combined and re-injected into the respective columns as a second pulse. This procedure was repeated again for a third injection. Measurable filtration of the colloids was observed after each initial injection of the Chancellor water into the columns, but the subsequent injections (spiked water experiments only) exhibited no apparent filtration, suggesting that the colloids that remained mobile after relatively short transport distances were more resistant to filtration than the initial population of colloids. It was also observed that while significant desorption of 137 Cs from the colloids occurred after the first injection in both the spiked and unspiked waters, subsequent injections of the spiked water exhibited much less 137 Cs desorption (much greater 137 Cs colloid-associated transport). This result suggests that the 137 Cs that remained associated with colloids during the first injection represented a fraction that was more strongly adsorbed to the mobile colloids than the initial 137 Cs associated with the colloids. A greater amount of the 239/240 Pu desorbed from the colloids during the second column injection compared to the first injection, but then desorption decreased significantly in the third injection. This result suggests that the Pu(IV) nanocolloids probably at least partially dissolved during and after the first injection, resulting in enhanced desorption from the colloids during the second injection, but by the third injection the Pu started following the same trend that was observed for 137 Cs. The experiments suggest a transport scale dependence in which mobile colloids and colloid-associated radionuclides observed at downstream points along a flow path have a greater tendency to remain mobile along the flow path than colloids and radionuclides observed at upstream points. This type of scale dependence may help explain observations of colloid-facilitated Pu transport over distances of up to 2 km at Pahute Mesa.

Colloid-Facilitated Radionuclide Transport: Current State of Knowledge from a Nuclear Waste Repository Risk Assessment Perspective

“…6. The first example, involving 137 Cs transport in the NNSS system, is fully documented in Reimus and Boukhalfa (2014).…”

“…Groundwater collected from the Chancellor nuclear test cavity, which had an unusually high concentration of colloids (~120 mg/L) and measurable concentrations of 137 Cs (1.3 x 10 -12 mol/L), was used in an experiment involving 3 sequential injections into the same column, with a portion of the effluent from earlier column injections being re-injected into the column. The colloids were determined by qualitative XRD analysis to consist predominantly of smectite and illite clay (Reimus and Boukhalfa, 2014). The water was spiked with additional Cs to a concentration of 2.24 x 10 -7 mol/L, although the majority of the spiked Cs was nonradioactive, so the resulting 137 Cs concentration was only about 18% higher than in the unspiked water.…”

“…4-1 through 4-3 along with model fits to the data, and the best fitting model parameters are listed in Table 4-1. Details of the experiments and the interpretive procedure are provided in Reimus and Boukhalfa (2014). Table 4-1 for bestfitting model parameters.…”

International Collaboration Activities in Geologic Disposal Research: FY2021 Progress

“…The ability of bentonite to adsorb radionuclides onto its large surface area is part of what makes it attractive as a barrier material, preventing the release of dissolved radionuclides into the environment. However, that same property poses a risk of enhanced transport of those radionuclides should bentonite with sorbed radionuclides erode (Reimus and Boukhalfa, 2014). The behavior and interactions of various radionuclides when adsorbed onto bentonite colloids are the subject of numerous past and ongoing studies (Birgersson et al, 2009;Missana et al, 2003Missana et al, , 2008Missana et al, , 2018.…”