Invited Paper AC+AS+SA-TuM5
Physical and Chemical Characterization of Solid Pu and Np Sources after Multi-year Exposure to Environmental Conditions
Tuesday, October 31, 2017, 9:20 am, Room 22
A field scale radionuclide vadose zone transport experiment at the United States Department of Energy Savannah River Site is being operated by Savannah River National Laboratory and Clemson University scientists. In this experiment, plutonium and neptunium solid sources are buried in 61 cm long x 10 cm diameter lysimeters which are open to precipitation. Such experiments provide the opportunity to observe changes in the microcrystalline structure of plutonium and neptunium solid phases under various environmental conditions. In the current experiments, sources of neptunium and plutonium in multiple initial oxidation states and chemical forms have been deployed for 2-5 years of field exposure. These sources include Pu(V)NH4CO3(s), Pu(IV)(C2O4)2(s), Pu(III)2(C2O4)3(s), Pu(IV)O2(s), Np(IV)O2(s) and NpO2NO3(s). X-ray absorption spectroscopy (XAS) analysis of initially Pu(V)NH4(CO3) sources archived in an inert atmosphere and sources exposed to lysimeters indicate some reduction to Pu(IV) within the sources leading to the formation of Pu(IV)O2(s). Thus, there appears to be an auto-reduction of NH4Pu(V)CO3(s) to Pu(IV)O2(s) even under inert conditions. However, solvent extraction on archived and field-deployed sources show the archived source still contains around 40% Pu(V) whereas the same source from a field lysimeter deployed for 2.5 years contained less than 10% of Pu(V). XAS and electron microscopy studies have demonstrated differences between Pu(IV)O2(s) formed via reduction of Pu(V)O2NH4CO3 and initially Pu(IV)O2 formed from precipitation of a Pu(IV) solution. The behavior of initially Np(IV)O2 sources was quite different showing oxidation to Np(V) and subsequent downward transport of more soluble Np(V)O2+. The oxidation of Np(IV)O2(s) leads to formation of a much more disordered solid phase with a significantly altered morphology than the initial Np(IV)O2(s). The results of these experiments will be discussed in terms of evaluating the history of the sample through analysis of the microcrystalline structure and the influences of aging under various environments.