| Classical atomic-scale and first principles, quantum-based computational methods are used to examine impurity behavior in UO2 and surface phenomena on SrTiO3 and TiN.;Firstly, the accommodation of fission products in UO2 fuels is investigated. In particular, the stability and clustering of Ru is examined using density functional theory in combination with classical thermodynamics. As observed in experiments, Ru is predicted to be insoluble and metallic and its aggregates are identified as the probable nucleus of metallic precipitates. For further investigation of fission product behavior especially near a Sigma5 grain boundary, segregation energies of various fission products to the boundary are calculated using empirical potentials and their dependency on site, charge, and ionic radius is determined. These results provide insights into the way in which the microstructure of the nuclear fuel influences fission product retention, which is important for fuel design to length lifetime. Additionally, the microstructure of UO2 affects the accommodation of Cr, a common grain enlarger of UO2. The bonding within the grain boundary is investigated using density functional theory and the results indicate that Cr prefers to reside in U substitutional sites. They further predict that Cr will segregate to grain boundaries and form bonds with neighboring O atoms that weaken the ionic nature of adjacent U-O bonds.;Secondly, the surface diffusion mechanism of adatoms and vacancies on SrTiO3 (001) is explored using temperature accelerated dynamics. Ad-species are predicted to be mobile with relatively low diffusion barriers on the SrO-terminated surface, whereas they are predicted to be largely immobile on the TiO2-terminated surface. An additional important finding is that, of the two lowest binding sites on the SrO-terminated surface, one is typically very close in energy to the saddle point.;Finally, the surface oxidation of the TiN (001) surface with a monatomic step is examined using density functional theory. The energy released during the adsorption and dissociation of O2 on the stepped surface is predicted to be much larger than on the flat surface with the same orientation. Furthermore, a TiO2 formation reaction associated with O2 dissociation is predicted to be especially favorable at the step edge. |
