First-principles study of defects and phase transition in UO₂

Defect properties and phase transition in UO₂ have been studied from first principles by the all-electron projector-augmented-wave (PAW) method. The generalized gradient approximation with empirical self-interaction correction, (GGA)+U, formalism has been used to account for the strong on-site Coulomb repulsion among the localized U 5f electrons. The Hubbard parameter U_eff, magnetic ordering, chemical potential and heat of formation have been systematically examined. By choosing an appropriate U_eff = 3.0eV it is possible to consistently describe structural properties of UO ₂ and model the phase transition processes. The phase transition pressure for UO₂ is about 20GPa, which is less than the experimental value of 42GPa but better than the LDA+U value of 7.8GPa. Meanwhile our results for the formation energies of intrinsic defects partly confirm earlier calculations for the intrinsic charge neutral defects but reveal large variations depending on the determination of the chemical potential and whether the environment is O-rich or U-rich. Moreover, the results for extrinsic defects of Xe, which are representative of mobile insoluble fission product in UO ₂, are consistent with experimental data in which Xe prefers to be trapped by Schottky defects.