Defect energetics in uranium dioxide is investigated using electronic structure and atomistic level simulations. The stability range of intrinsic point defects in uranium dioxide is determined as a function of temperature, oxygen partial pressure, and non-stoichiometry. In particular, the density functional theory (DFT) calculations are performed at the level of the spin polarized, generalized gradient approximation and includes the Hubbard U term as a result they predict the correct anti-ferromagnetic insulating ground state of uranium oxide. The predicted equilibrium properties and defect formation energies for neutral defect complexes match trends in the experimental literature quite well.Next, the stabilities of selected fission products---Xe, Cs, and Sr---are investigated as a function of non-stoichiometry x in UO2 using DFT and empirical potentials. In general, higher charge defects are more soluble in the fuel matrix and the solubility of fission products increases as the hyperstoichiometry increases. The solubility of fission product oxides is also explored. These observations mirror experimentally observed phenomena.Finally, the segregation of Xe to different grain boundaries is studied using empirical potentials. The segregation behavior is observed to be similar for the different boundaries qualitatively but the ease with which segregation occurs is found to depend on the type of boundary.
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