PNNL

Abstract

The electronic properties, structure and phase transformation of UO2 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method. The generalized gradient approximation (GGA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized U 5f electrons. GGA+U gives an antiferromagnetic insulating ground state for the effective Hubbard parameter Ueff =2.0 eV and this ordering is consistent with experimental measurement. Our results also reveal that by choosing an appropriate Ueff =3.0 eV it is possible to consistently describe structural properties of UO2 and model phase transition processes. The distribution of the local electrostatic potential indicated that the phase transition pressure for UO2 under operant conditions is about 20 GPa. In addition, the formation energies of intrinsic defects, which play a critical role in UO2 fuel under operant conditions, are found to be depended on whether the environment is under U-rich condition or O-rich condition.