Trends in the energetics and defect clustering tendencies for UO 2 compounds substituted with trivalent rare earth cations (M 3+ ) are investigated computationally using methods based on density functional theory. Higher energetic stability of U 1−x M x O 2−0.5x solid solutions relative to constituent oxides and increased preference for higher oxygen coordination around the trivalent cation are found with increasing size of the M 3+ species. The implications of the computational results for the effect of trivalent fission products on oxygen ion mobility in spent fuel are discussed.