*2.2. Model Building*

The structure of GBs is essential for predicting the behavior of Xe bubbles. Up to now, most theoretical studies on the properties of GB have been based on molecular statics (MS) or DFT [35,36] without considering the influence of temperature. However, experimental observations have shown that GB microstructures and corresponding properties reflect the effect of temperature on GBs to a certain extent [37–39] because temperature changes may cause the relative sliding of grains at GBs [17,19]. Therefore, heat treatment of GBs is a necessary process to expand the understanding of GB. Hong et al. [40] reported that high temperature can cause complex ion transitions in some GBs. In this study, the symmetrically

tilted GB, Σ5(310)/[100], in UO2 was used as a GB model because the Σ5 symmetric tilt GB has been extensively studied both experimentally and theoretically [17,41–43]; thus, we could compare our results with the available data.

In this study, the symmetrically tilted GB Σ5(310)/[100] in UO2 were formed by rotating perfect UO2 crystals. Symmetrically tilted GB means that the crystals on both sides of the GB are tilted towards each other and the interface of the GB is symmetrical to the two grains. The specific process of constructing the GB is as follows. First, half of the perfect UO2 crystal simulation box is rotated along the axis by half of the orientation difference angle. Then, the second crystal is constructed through mirror inversion, taking the first crystal as a reference. However, this method causes an overlap of the atoms at the crystal interface. Thus, we removed the overlapped interface atoms to maintain a neutral charge.

The annealed structures of the Σ5(310) GB based on MD are shown in Figure 1 and a common neighbor analysis was performed to identify the GB region and bulk region. The atomic configuration was viewed using OVITO software [44]. At 1000 K, the structure of the interface is conserved with the repetition of a triangular-like pattern (depicted by lines in Figure 1); however, at 2000 K, a structural change occurs with the boundary showing reduced diamond shapes arranged end-on-end. Previous simulation results show that, with an increase in temperature, the disorder degree of GB increases [45]; e.g., at 2000 K, the Σ5(310) GB has a more distorted triangular pattern [19].

**Figure 1.** Structures of grain boundaries (GBs) simulated at (**a**) 1000 K and (**b**) 2000 K. Oxygen atoms are removed for clarity. The atoms in the GB and bulk regions are colored white and green, respectively.
