Tilt grain boundary stability in uranium dioxide and effect on xenon segregation

L Yang and BD Wirth, JOURNAL OF NUCLEAR MATERIALS, 577, 154302 (2023).

DOI: 10.1016/j.jnucmat.2023.154302

Grain boundaries (GBs) play a key role in the accumulation and release of xenon (Xe) in uranium diox-ide (UO2). The stability and structure of tilt GBs with low sigma values in UO2 has been investigated by molecular statics/dynamics (MS/MD) simulations with three different empirical potentials. The atom-istic modeling results confirm that E3 and E11 are more stable than both E5 and E9, consistent with previous work J. Nucl. Mater. 517 (2019) 286. In general, the higher distortion of atoms at a GB corre-sponds to a higher GB formation energy. Density functional theory (DFT + U ) calculations indicate that a defective triangle-like pattern in the E5(310 is preferred rather than an oxygen-ring pattern. The en-ergetics of Xe with or without vacancies at the most stable GBs have been studied by MS simulations. The incorporation energy profiles indicate that the Xe interstitial sites at the GBs are always energetically favorable in comparison to the bulk, while Xe incorporation in some Schottky defects (SDs) or uranium vacancy (VU) sites at the E5(210, E5(310 and E9(221 boundaries are energetically unfavorable rela-tive to the bulk. However, in general, interstitial Xe, Xe-SD, and Xe-VU complexes tend to segregate to the GBs. Notably, the symmetry degree of the distribution of excess atomic volume at a GB is correlated to the charge character of the GB. An asymmetric distribution of excess atomic volume results in the for-mation of electrostatic dipoles in GB systems, which significantly influences the segregation behavior of charged Xe-vacancy complexes at the GB. The segregation energy of Xe with or without vacancies is to some extent related to the GB excess volume.(c) 2023 Elsevier B.V. All rights reserved.

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