Characterization of the Mobilities of Selected Actinides and Fission/Activation Products in Laboratory Columns Containing Subsurface Material from the Snake River Plain

Laboratory column tests were performed to characterize the mobilities of ⁶⁰Co, ⁹⁰Sr, ¹³⁷Cs, ²³³U, ²³⁹Pu, and ²⁴¹Am in a basalt sample and a composite of sedimentary interbed from the Snake River Plain at the Idaho National Engineering and Environmental Laboratory. The radionuclides were spiked into a synthetic groundwater (pH 8, ionic strength = 0.004 M) and introduced into the columns (D = 2.6 cm, L = 15.2 cm) as finite steps with a width of 1 pore volume followed by unspiked synthetic groundwater. The effluent concentrations were measured continuously for up to 200 pore volumes. Hydrogen-3 was used as a nonreactive tracer in all of the experiments to monitor for channeling. In the basalt sample, the behavior of ⁹⁰Sr, ¹³⁷Cs, and ²³³U was quite different from that of ⁶⁰Co, ²³⁹Pu, and ²⁴¹Am. The column effluent curves for the former were characterized by single peaks containing, within the limits of experimental uncertainty, all of the activity in the spike. The mobilities were ordered as follows: ²³³U ([overbar]R = 5.6) > ⁹⁰Sr ([overbar]R = 29) > ¹³⁷Cs ([overbar]R = 79). The curves for the other radionuclides were characterized by two or three fractions, each having a distinctly different mobility. Cobalt-60 had high- ([overbar]R = < 3), intermediate- ([overbar]R = 34), and low- (R > 200) mobility fractions. Although a majority of the ²³⁹Pu and ²⁴¹Am had low mobility (R > 200), there were high-mobility (R < 3) fractions of each (17 to 29% for ²³⁹Pu and 7 to 12% for ²⁴¹Am). In sedimentary interbed, mobilities were generally much lower than in basalt. Uranium-233 was the only radionuclide with 100% recovery within 200 displaced pore volumes, and it had a retardation factor of 30. However, high-mobility fractions were observed for ⁶⁰Co (1 to 4%) and ²³⁹Pu (1.1 to 2.4%). These results could have important implications with respect to transport modeling. If the multiple-mobility fractions observed here are also present in the field, transport predictions based on classical modeling approaches that incorporate mobilities from batch sorption experiments are likely to be in error.