Atomistic simulations of oxides of nuclear interest

In this presentation we will be interested by uranium dioxide UO2 which is the standard nuclear fuel in nuclear reactors and by pyrochlores Gd2Ti2O7 and Gd2Zr2O7 which are considered as potential immobilization matrices for the actinides produced in nuclear power plants. These compounds have been extensively studied both experimentally and theoretically over the last decades. More particularly, atomistic computer simulation techniques, using interatomic potentials of first-principles methods, were used in order to improve knowledge of the properties of point-defects and their role on the structural stability under irradiation. Since few years we develop the so-called SMTB-Q model (Second-Moment Tight-Binding - QEq model) aimed at describing bulk, surface and defect properties of oxides. This model allows the ionic charges to vary with respect to the structural environment of the ions thanks to the charge-equilibration (QEq) procedure. Moreover the iono-covalent metal-oxygen bond is correctly described in the framework of the tight-binding approach. These points establish strong improvements with regard to purely ionic models usually used in previous works and allow providing results which convincingly compare with ab initio calculations with considerable gain in CPU resources. After a brief presentation of the SMTB-Q model, applications to bulk structural and elastic properties, atomic relaxations at the (111), (110) and (100) surfaces, Frenkel and Schottky defects properties in UO2 will be presented. The charge transfer at surfaces and around defects was calculated. Then, new results explaining the origin of the difference in the radiation tolerance of Gd2Ti2O7 (high susceptibility to amorphization) and Gd2Zr2O7 (no amorphization) will be provided. All the results will be compared with ab initio calculations and experimental data when available. (author)