Role of di-interstitial clusters in oxygen transport in UO2+x from first principles

Using density functional theory, we examine a recently discovered structure for di-interstitial oxygen clusters in UO2+x in which three oxygen ions share one lattice site. This di-interstitial cluster exhibits a fast diffusion pathway; the migration barrier for these clusters is approximately half of that for mono-interstitials. Using kinetic Monte Carlo, we calculate the diffusivity of oxygen with and without the di-interstitial mechanism as a function of x and find that oxygen transport is significantly increased for higher values of x when the di-interstitial mechanism is included, agreeing much more closely with experimental data. These results emphasize the importance of clustering phenomena in UO2+x and have implications for the evolution of UO2+x.