Simulations of self- and Xe diffusivity in uranium mononitride including chemistry and irradiation effects

MWD Cooper and J Rizk and C Matthews and V Kocevski and GT Craven and T Gibson and DA Andersson, JOURNAL OF NUCLEAR MATERIALS, 587, 154685 (2023).

DOI: 10.1016/j.jnucmat.2023.154685

A combination of density functional theory and empirical potential atomic scale simulations have been used to determine a model for defect stability and mobility in uranium mononitride (UN), as a function of temperature (T) and N-2 partial pressure (P-N2). Using the model, predictions of hypo-stoichiometry under U-rich conditions compare favorably to CALPHAD calculations using the TAF-ID database. Furthermore, our predictions of U and N self-diffusivity are in good agreement with experiments carried out as a function of T at specific partial pressures under thermal equilibrium. The validated atomic scale data have then been implemented within a cluster dynamics method to simulate irradiation-enhanced defect concentrations. All defects and clusters studied have significantly enhanced concentrations, with respect to thermal equilibrium, as T is lowered. The irradiation- enhanced Xe diffusivity is compared to post-irradiation annealing and in-pile experiments. The contributions of various defects and clusters to non-stoichiometry, self-diffusivity, and Xe diffusivity are discussed.

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