OXYGEN AND URANIUM DIFFUSION IN URANIUM DIOXIDE (A REVIEW)

Oxygen and uranium self-diffusion coefficients determined experimentally by a variety of techniques are reviewed and compared. In the case of oxygen selfdiffusion for essentially stoichiometric UO2, the expression, D = 1.15 exp(-56700 ± 1000/RT)cm2/sec, fits the data for three separate sets of observations covering a range of 10 orders of magnitude in the diffusion coefficient. Variations in the diffusion coefficient and activation energies with composition for UO2 + x are not as definitive but are in qualitative agreement with the Thorn-Winslow statistical-mechanical model of a crystal having a perfect cation lattice and anionic vacancies and interstitials. In the case of uranium self-diffusion, on the other hand, agreement among the various sets of data for nominally stoichiometric UO2 is poor. Experimentally determined activation energies range from 70 to about 105 kcal/mole. Two investigations of the effect of composition on uranium self-diffusion coefficients in UO2 + x indicate the diffusion coefficient, D, to be proportional to x2. This behavior is consistent with the theoretical model developed by Lidiard which relates defects in the anion and cation sub-lattices by way of Frenkel and Schottky products. © 1969.