First-principles total energy calculations were used to investigate the behaviour of

helium, and its diffusion properties in uranium dioxide. The investigations were

based upon density functional theory within the generalized gradient

approximation. The trapping behaviour of He in UO2 was modelled using a super

cell which contained 96 atoms as well as uranium and oxygen vacancy trapping

sites. The calculated incorporation energies showed that, for He, an uranium

vacancy was more stable than an oxygen vacancy or an octahedral interstitial site.

Interstitial site hopping was found to be the rate-determining mechanism for He

diffusion, and the corresponding migration energy was estimated to be 2.79eV at

0K (with spin-orbit coupling included) and 2.09eV by using the thermally

expanded lattice parameter of UO2 at 1200K. This was relatively close to the

experimental value of 2.0eV. The lattice expansion coefficient of the He-induced

swelling of UO2 was calculated to be 9 x 10−2. In the case of two He atoms, it was

found that they formed a dumb-bell configuration if they were close enough to

each other, and the lattice expansion induced by such a dumb-bell was larger than

produced by two distant interstitial He atoms. The clustering tendency of He was

studied for clusters of up to six He atoms. It was found that He tended strongly to

cluster in the vicinity of an octahedral interstitial site, and that the collective action

of the He atoms was sufficient to create additional point defects spontaneously

around a He cluster in the UO2 lattice.

Theory of He Trapping, Diffusion, and Clustering in UO2. Y.Yun, O.Eriksson,

P.M.Oppeneer: Journal of Nuclear Materials, 2009, 385[3], 510-6