Grain boundaries in uranium dioxide: Scanning electron microscopy experiments and atomistic simulations

The distribution and atomic structure of grain boundaries has been investigated in UO₂. Our scanning electron microscopic/electron backscatter diffraction experiments on a depleted UO₂ sample showed real nuclear fuels contain a combination of special coincident site lattice (CSL) and general boundaries. The experimental data indicated that ∼16% of the boundaries were CSL boundaries and the CSL distribution was dominated by low ∑ boundaries; namely ∑9, ∑3, and ∑5 Based on our experimental observations, the structures of select low ∑ (∑5 tilt, ∑5 twist, ∑3 tilt) and a random boundary were analyzed in greater detail using empirical potential atomic-scale calculations. Our calculations indicate that the boundaries have very different structures and each CSL boundary had multiple minima on the γ-surface. The presence of a significant fraction of CSL boundaries and the differences in their structures are expected to have important consequences on fuel properties.