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Lindsay Shuller-Nickles1

1, Clemson University, Anderson, South Carolina, United States

The fate of a single atom seems inconsequential in the evaluation of risk for long term disposal of radioactive waste. However, consider the incorporation of a single atom (e.g., Cs) in a ceramic (e.g., hollandite) waste form. The release of Cs from the solid waste form may occur prior to matrix dissolution via diffusion along preferential pathways, such as tunnels in the hollandite structure, to grain boundaries. Quantification of the energetic stability of Cs in the crystalline waste form, based on the coordination chemistry of the Cs and the composition-dependent structure of the matrix, is critical to evaluate the risk-mitigation for a specific waste form. Further, atomic scale calculations can identify energetically favorable incorporation mechanisms. For example, divalent cation incorporation into UO2 (as a reference for spent nuclear fuel) reveals oxidation of the uranium as a dominant incorporation mechanism, which may enhance dissolution of the UO2 matrix. The position of the incorporated cation at or near a surface of UO2 will affect the surface energy, and ultimately change the equilibrium morphology and potential for alteration. Non-equilibrium morphologies can be probed using quantum-mechanical calculations of higher energy configurational states, such that compositional controls on radionuclide release may be predicted. This presentation will explore radionuclide incorporation into nuclear waste forms, specifically hollandite and UO2, and discuss the significance of calculated equilibrium morphologies on the prediction of phase formation and alteration.

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