Grain-Boundary Corrosion in UO2+δ from a Defect Chemical Perspective: A Case Study of the Σ5(310)001 Grain Boundary

MJ Wolf and AL Usler and RA De Souza, ACS APPLIED MATERIALS & INTERFACES, 17, 7906-7915 (2025).

DOI: 10.1021/acsami.4c20688

We combine atomistic and continuum simulation methods to study the defect chemistry of a model grain boundary in UO2+delta. Using atomistic methods, we calculate the formation energies of oxygen interstitials, uranium vacancies, and hole polarons (U5+ ions) across the Sigma 5(310)001 symmetric tilt grain boundary. This information is then used as input in a continuum model of point-defect concentrations at the grain boundary and in its vicinity, taking into account electrostatic (space-charge) effects. Two scenarios are modeled: one in which oxygen interstitials are the majority ionic defect and one in which uranium vacancies are the majority ionic defect, with bulk charge neutrality being maintained by hole polarons in both cases. Our results indicate that, irrespective of the majority ionic defect, the Sigma 5(310)001 grain boundary in UO2+delta is negatively charged, with positively charged adjacent space-charge zones in which the hole-polaron concentration is enhanced. We propose that the enhanced U5+ concentration at the grain boundary and within the space-charge zones renders grain-boundary regions more susceptible to oxidative corrosion, an effect that could be counteracted by acceptor doping.

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