**4. Conclusions**

In this paper, molecular dynamics simulations were used to study the stable occupancy of Xe/Kr clusters in defect-free configurations and configurations with five different defects. The results show that the system was energetically favourable when Xe/Kr clusters were trapped by vacancies, especially U vacancies, because they can provide a larger space, and the complex of Xe/Kr and vacancies were more stable. The diffusion of Xe/Kr atoms and small clusters at octahedral interstitial sites in bulk UO2 was also studied. The diffusion of Xe/Kr atoms in bulk UO2 was relatively complex. The activation energy of Xe/Kr small clusters is ~2 eV, and the Xe/Kr clusters are difficult to diffuse. The nucleation of Xe/Kr in systems containing different defect concentrations was also investigated. At 2500 K, the Xe/Kr atoms dispersed in the system gather into clusters after a 20 ns relaxation. A comparison of the different systems revealed that the vacancy-containing system is more likely to form large clusters than a system containing interstitial atoms.

**Author Contributions:** Conceptualization, W.H. and H.D.; methodology, L.W. (Li Wang), Z.W., Y.X., Y.C., Z.L., Q.W. and W.H.; validation, Y.X., Z.W., L.W. (Li Wang), Y.C., Z.L., Q.W. and L.W. (Lu Wu); formal analysis, L.W. (Li Wang), Z.W., Y.X., Y.C., Z.L. and Q.W.; investigation, L.W. (Li Wang); resources, Z.W., Z.L. and Q.W.; data curation, Y.C. and W.H.; writing—original draft, L.W. (Li Wang); writing—review & editing, L.W. (Li Wang), L.W. (Lu Wu) 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 not applicable.

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

#### **References**

