Migration mechanisms of oxygen interstitial clusters in UO2

Understanding the migration kinetics of radiation-induced point defects and defect clusters is key to predicting the microstructural evolution and mass transport in nuclear fuels. Although the diffusion kinetics of point defects in UO2 is well explored both experimentally and theoretically, the kinetics of defect clusters is not well understood. In this work the migration mechanisms of oxygen interstitial clusters of size one to five atoms. (1O(i)-5O(i)) in UO2 are investigated by temperature-accelerated dynamics simulations without any a priori assumptions of migration mechanisms. It is found that the migration paths of oxygen interstitial clusters are complex and non-intuitive, and that multiple migration paths and barriers exist for some clusters. It is also found that the cluster migration barrier does not increase with increasing cluster size and its magnitude has the following order: 2O(i) < 3O(i) < 1O(i) < 5O(i) < 4O(i). Possible finite-size effects are checked with three systems which are of different sizes. The results show good agreement with other available experimental and theoretical data. The cluster migration sequence might explain the interesting relationship measured experimentally between the oxygen diffusivity and stoichiometry in UO2+x.