First-principles total energy calculations were performed in order to investigate the

defect energetics and diffusion behaviour of Xe in UO2 and ThO2 matrices. All of

the calculations were carried out using density functional theory within the

generalized gradient approximation and by applying the projector augmented-wave

method. The results showed that the formation and migration energies of vacancy

defects were more than twice as high in ThO2 as compared with UO2. Another

marked difference between the two oxides was the role played by an oxygen

vacancy in the movement of a cation vacancy. The vacancy enhanced the

movement of a uranium vacancy by lowering its migration energy by about 1eV.

However, no similar effect was observed in ThO2. The differing behaviours of

cation vacancies in the two oxides strongly affected the mobility of fission gases

and led to differences in their respective diffusion behaviours. It was suggested that

the strong resistance to oxidation of ThO2 prevented the creation and migration of

defects, and resulted in a lower mobility of fission gases in ThO2 as compared with

UO2.

Defect Energetics and Xe Diffusion in UO2 and ThO2. Y.Yun, P.M.Oppeneer,

H.Kim, K.Park: Acta Materialia, 2009, 57[5], 1655-9