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 Co -60, Sr-90, Cs-137, U-233, Pu-239, and Am-241 in a basalt sample and a comp osite of sedimentary interbed from the Snake River Plain at the Idaho Natio nal Engineering and Environmental Laboratory. The radionuclides were spiked into a synthetic groundwater (pH 8, ionic strength = 0.004 M) and introduc ed into the columns (D = 2.6 cm, L = 15.2 cm) as finite steps with a width of I pore volume followed by unspiked synthetic groundwater. The effluent c oncentrations were measured continuously for up to 200 pore volumes. Hydrog en-3 was used as a nonreactive tracer in all of the experiments to monitor for channeling. In the basalt sample, the behavior of Sr-90, Cs-137, and U- 233 was quire different from that of Co-60, Pu-239, and Am-241. The column effluent curves for the former were characterized by single peaks containin g, within the limits of experimental uncertainty, all of the activity in th e spike. The mobilities were ordered as follows: U-233 ((R) over bar = 5.6) > Sr-90 ((R) over bar = 29) > Cs-137 ((R) over bar = 79). The curves for t he other radionuclides were characterized by two or three fractions, each h aving a distinctly different mobility. Cobalt-60 had high- ((R) over bar < 3), intermediate- ((R) over bar = 34), and low- (R > 200) mobility fraction s. Although a majority of the Pu-239 and Am-241 had low mobility (R > 200), there were high-mobility (R < 3) fractions of each (17 to 29% for Pu-239 a nd 7 to 12% for Am-241). 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 facto r of 30. However, high-mobility fractions were observed for Co-60 (I to 4%) and Pu-239 (1.1 to 2.4%). These results could have important implications with respect to transport modeling. If the multiple-mobility fractions obse rved here are also present in the field, transport predictions based on cla ssical modeling approaches that incorporate mobilities from batch sorption experiments are likely to be in error.