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Impact of Environmental Curium on Plutonium Migration and Isotopic Signatures

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journal contribution
posted on 02.12.2014, 00:00 by Hiromu Kurosaki, Daniel I. Kaplan, Sue B. Clark
Plutonium (Pu), americium (Am), and curium (Cm) activities were measured in sediments from a former radioactive waste disposal basin located on the Savannah River Site, South Carolina, and in subsurface aquifer sediments collected downgradient from the basin. In situ Kd values (Pu concentration ratio of sediment/groundwater) derived from this field data and previously reported groundwater concentration data compared well to laboratory Kd values reported in the literature. Pu isotopic signatures confirmed multiple sources of Pu contamination. The ratio of 240Pu/239Pu was appreciably lower for sediment samples compared to the associated groundwater. This isotopic ratio difference may be explained by the following: (1) 240Pu produced by decay of 244Cm may exist predominantly in high oxidation states (PuVO2+ and PuVIO22+) compared to Pu derived from the disposed waste effluents, and (2) oxidized forms of Pu sorb less to sediments than reduced forms of Pu. Isotope-specific Kd values calculated from measured Pu activities in the sediments and groundwater indicated that 240Pu, which is derived primarily from the decay of 244Cm, had a value of 10 ± 2 mL g–1, whereas 239Pu originating from the waste effluents discharged at the site had a value of 101 ± 8 mL g–1. One possible explanation for the isotope-specific sorption behavior is that 240Pu likely existed in the weaker sorbing oxidation states, +5 or +6, than 239Pu, which likely existed in the +3 or +4 oxidation states. Consequently, remediation strategies for radioactively contaminated systems must consider not only the discharged contaminants but also their decay products. In this case, mitigation of Cm as well as Pu will be required to completely address Pu migration from the source term.