The distribution and atomic structure of grain boundaries has been investigated in UO(2). Our scanning electron microscopic/electron backscatter diffraction experiments on a depleted UO(2) sample showed real nuclear fuels contain a combination of special coincident site lattice (CSL) and general boundaries. The experimental data indicated that similar to 16% of the boundaries were CSL boundaries and the CSL distribution was dominated by low Sigma boundaries; namely Sigma 9, Sigma 3, and Sigma 5 Based on our experimental observations, the structures of select low Sigma (Sigma 5 tilt, Sigma 5 twist, Sigma 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 gamma-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.