Calculation of Self-Diffusion Coefficients in UO₂ Using Atomic Scale Modelling

Uranium dioxide is the most used nuclear fuel in pressurized water nuclear reactors. Thermally or radiation induced atomic transport properties impact many important engineering aspects of nuclear fuel behaviour, during fabrication and in reactor. For example, uranium diffusion leads to significant modifications of the microstructure and controls creep in certain regimes, while the oxygen diffusion governs the local oxide over metal ratio. Therefore, the precise knowledge of diffusion mechanisms and coefficients, starting from the atomic scale, is crucial to model and predict the microstructure evolution of nuclear fuels.

Cationic self-diffusion in UO₂, however, is still not fully quantified and understood. There is a large gap among the experimental results and between modelling and experimental data. Common kinetic modelling methods such as molecular dynamics struggle to compute the cationic self-diffusion coefficients because of the high cationic migration barriers compared to the anionic ones, making cationic migration sampling difficult.

In this work, we propose a multiscale approach combining the activation relaxation technique (ART-nouveau) [1], the Kinetic Cluster Expansion (KineCluE) [2] and atomic scale calculations of migration energy barriers, for instance the Nudged Elastic Band Method [3], to compute the self-diffusion coefficients in oxide fuels at thermal equilibrium. We employ ART-nouveau to explore the energy landscape and uncover non-trivial migration mechanisms, and atomic scale calculations to compute their corresponding energy barriers. We then feed these mechanisms and energy barriers in KineCluE to determine transport coefficients. This approach allows for a wide exploration of the possible migration trajectories and therefore for an accurate evaluation of self-diffusion coefficients. We apply this approach to the calculation of the self-diffusion coefficients of Uranium in UO₂.

References
[1] N. Mousseau, L. K. Béland, P. Brommer et al., J. Atom. Mol. Opt. Phy. 2012, 1-14 (2012)
[2] T. Schuler, L. Messina, M. Nastar, Comp. Mater. Sci. 172, 109191 (2020)
[3] H. Jonsson, G. Mills, K.W. Jacobsen, Classical and Quantum Dynamics in Condensed Phase Simulations p. 385-404, 1998