Uranium dioxide (UO2) is the typical fuel that is used in the current nuclear power plant; fission gas atoms are produced during and after the nuclear reactor operation; the fission gas atoms have a significant effect on the performance of UO2 fuel in the nuclear reactor. In this paper, we investigated the diffusion of the fission gas atoms in the UO2 fuel by using the first-principles calculation method based on the density functional theory (DFT). The results indicate that the volume of the UO2 cell increased when there is a fission gas atom enters in the UO2 supercell; the elastic properties of UO2 are in good agreement with other simulation results and experimental data and the fission gas atoms make the ductility of UO2 decreased; the fission gas atoms prefer to occupy the octahedral interstitial site (OIS) over the uranium vacancy site and the oxygen vacancy site, and the oxygen vacancy site is the most difficult occupied site due to the formation of an oxygen vacancy is more difficult than that of the uranium vacancy; the diffusion barrier of a He atom in the UO2 supercell is higher than that of an oxygen atom, that means that the diffusion of the He atom in UO2 fuel is weaker than that of the oxygen atom. Our works may shed some light on the formation mechanism of the bubbles caused by the fission gas atoms in the UO2 fuel.

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