Molecular dynamics modeling of thermodiffusion in solids with charged defects using uranium dioxide as the case study

G Bareigts and S Maillard and JM Simon, CHEMICAL ENGINEERING SCIENCE, 281, 119141 (2023).

DOI: 10.1016/j.ces.2023.119141

Although thermodiffusion is generally much less efficient than diffusion for chemical species transport, this phenomenon can sometimes have a significant impact on natural or technological systems. More specifically, oxygen thermodiffusion is reported as a key mechanism for nuclear fuel evolution during high power transients in nuclear plant operation. To contribute to our understanding and characterization of this phenomenon, oxygen thermodiffusion in UO2 is simulated by molecular dynamics (MD) in order to evaluate the oxygen heat of transport. Specific formulae for the heat flux are derived to take into account the non-pairwise embedded atom model (EAM) term of the empirical potential used. No electronic defects are considered and a specific strategy is built to address this flaw, including the way to ensure electroneutrality with a background charge compensating for the deviation from stoichiometry. A specific thermodynamic model is designed, including the effect of the background charge and each point defect contribution.Five MD techniques are tested and compared, among which the Green-Kubo formalism and the imposed electric field give similar results. The results are interpreted in terms of the defect heats of transport whose combination yields the complete oxygen heat of transport, which is a function of the oxygen potential. The calculated ranges of the heats of transport for the oxygen vacancy and interstitial are 0.20,0.86 eV and -3.75,-2.60 eV, respectively.

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