**4. Conclusions**

In this study, MD and static simulations were employed to investigate the energy and diffusion of a single Xe atom and small Xe clusters and the nucleation and growth of Xe bubbles at the UO2 Σ5(310) GB. The results show that the formation energies of single Xe atoms and small Xe clusters at the GB are much lower than those in the bulk, indicating that impure Xe atoms are more stable at the GB and can precipitate into clusters. The diffusion activation energy of a single interstitial Xe atom at the GB is approximately 1 eV lower than that in the bulk. The energy barrier for interstitial Xe atoms to diffuse through the interstitial mechanism is high, which is mainly due to lattice deformation caused by the movement of Xe between two adjacent interstitial sites. The growth of Xe bubbles at the GB was simulated by sequentially inserting Xe atoms into a pre-existing Xe bubble. Then, the bubble volume and pressure were estimated for different numbers of Xe atoms. The pressure dropped with an increase in temperature, whereas the volume increased. At low

Xe concentrations, the Xe bubble volume increased approximately linearly with an increase in the number of Xe atoms. We also compared the configurations of Xe atoms at the GB and in the bulk at the same temperature and found that Xe atoms are more regular in the bulk, whereas, at the GB, multiple Xe atoms form a planar structure.

**Author Contributions:** Conceptualization, H.D.; Investigation, Y.X.; Methodology, Y.X., Z.W., L.W. (Li Wang), Y.C., Z.L., Q.W. and L.W. (Lu Wu); Validation, Y.X., Z.W., L.W. (Li Wang), Y.C., Z.L., Q.W. and L.W. (Lu Wu); Formal analysis, Y.X., L.W. (Li Wang), Y.C. and Z.L.; Resources, Z.W., Z.L., Q.W., L.W. (Lu Wu) and H.D.; Writing—original draft, Y.X.; Writing—review & editing, Y.X. and H.D.; Supervision, H.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
