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Geocheminar - spring-2023

Field shift effects in the transuranic elements Pu and Np

June 1, 2023
3:30 p.m. - 4:30 p.m.
Slichter conference room

Presented By:

  • Alexander Sedlak -
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The nuclear field shift is an equilibrium phenomenon driven by the electron density at the nucleus of an atom as well as the nuclear charge radius, and has the potential to drive large isotopic shifts for many systems. Significant nuclear field shift effects have been modeled and observed in the uranium isotopic system driven primarily by changes in redox state, but to date no one has examined the same phenomenon in the transuranic elements. Due to their geochemical similarities to uranium, including the presence of multiple redox states at environmentally relevant conditions, neptunium and plutonium are good candidates for study. Like uranium, plutonium and neptunium are radioactive (though for several isotopes their half-lives may still be significant in human timescales,) but in contrast to uranium nearly all of the material found in the modern environment is anthropogenic in origin. Both elements are of interest due to their presence in the environment as a consequence of nuclear fallout from weapons tests, nuclear disasters, and potential leaks due to storage of nuclear waste. In this study we examine the field shift effect for Pu and Np using the DFT-PAW method of Schauble (2013). We then calculate the expected isotopic fractionation across the expected redox states and species that are potentially significant in the natural environment. We determine that the Np system likely has very subtle 237Np/235Np fractionations of ~0.3 ‰ for the environmentally relevant redox pair Np(V)/Np(IV). In contrast, Pu can be present as Pu(III), Pu(IV), Pu(V), and Pu(VI) and large isotopic fractionations may result for any system containing Pu(III) and/or Pu(VI), up to 5 -6 ‰ in the 242Pu/239Pu system, and generally over 1 – 2 ‰.