The cation diffusion occurs in actinide dioxides via cation vacancies in the vicinity of oxygen vacancy according to Jackson eta/.234 As a consequence, cation diffusion may occur in hyperstoichiometric dioxides MO2 + x (with x > 0). The diffusion coefficient increases drastically with departure from stoichiom­etry as x2 in MO2 + x as follows:

D(x, T)=A, x2exp^—[15]

Recently, Gao et a/.235 obtained the prefactor D0 = 2.341 x 10-2m2s-1 and migration energy E0 = 2.5 eV for UO2 + x.

Knorr et a/.236 mentioned that the effect of the intergranular grain boundary is to change the pre­factor to a linear dependence upon stoichiometry. The diffusion coefficient is expressed as follows:

D = Dox ex^kE^^ [16]

They found for UO2 + x, D0 = 7.5 x 10-^m2s—1 and E0 = 2.47 eV.236

This apparent activation energy is very close to the one obtained by Gao et a/.235 A systematic study of the difference between monocrystals and polycrystals done by Sabioni et a/.220 led the authors to the conclusion that most of the activation ener­gies reported by previous authors probably refer to polycrystals.