Molecular dynamics investigation of xenon, uranium, and oxygen diffusion in UO2 nuclear fuel

SM Zamzamian and A Zolfaghari and Z Kowsar, COMPUTATIONAL MATERIALS SCIENCE, 211, 111553 (2022).

DOI: 10.1016/j.commatsci.2022.111553

In this paper, atomistic simulations based on molecular dynamics (MD) are used to calculate the diffusion coefficients of xenon, uranium and oxygen in uranium dioxide containing different percentages of xenon atoms replaced by uranium from 2%Xe to 10%Xe at ten temperatures between 300 and 2500 K (covering three temperature regimes of athermal, intermediate and intrinsic). Since we did not take the irradiation conditions into account in the simulations, the calculated diffusion coefficients are "out-of-pile". Initially, the validity of the interatomic potential was performed by calculating the formation energies of interstitial, vacancy, Frankel and Schottky defects by using molecular statics (MS) simulations. Then, the migration energies of Xe/U/O based on considering various mechanisms were obtained by employing the Nudge Elastic Band (NEB) method. The results were generally in good agreement with those reported in the literature and the potential was validated. The simulations of UO2 containing different percentages of xenon (percentage of replacement of uranium atoms with xenon atoms) from 2 to 10 showed that with increasing the xenon percentage, the number of xenon clusters, cluster size, and radius of gyration considerably increases, and bubble nucleation was considerable in the case of 10%Xe. Then, by determining the slope of the mean square displacements, diffusion coefficients of xenon, uranium and oxygen were obtained. The results showed that they are both temperature-dependent and also dependent on the percentage of xenon (which was previously assumed to be independent of concentration). In the end, three temperature and xenon percentage dependent equations were proposed for the diffusion coefficients of xenon, uranium and oxygen. Finally, by applying a reduction factor (constant) to the diffusion coefficients, they were compared with experimental data available in the literature and significant satisfaction was obtained.

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