Thermodynamics of fission products in dispersion fuel designs - First-principles modeling of defect behavior in bulk and at interfaces

Density functional theory (DFT) calculations of fission product (Xe, Sr, and Cs) incorporation and segregation in alkaline earth metal oxides, HfO₂ and UO₂ oxides, and the MgO/(U, Hf, Ce)O₂ interfaces have been carried out. These calculations demonstrate that the fission product incorporation energies in MgO are higher than in HfO₂. However, this trend is reversed or reduced for alkaline earth oxides with larger cation sizes. In the case of UO₂, the calculations were performed using spin polarization and with a Hubbard U term characterizing the on-site Coulomb repulsion between the localized 5f electrons. The fission product solution energies in bulk UO_2 ± x have been calculated as a function of non-stoichiometry x, and were compared to that in MgO. The solution energies of fission products in MgO are substantially higher than in UO_2 ± x, except for the case of Sr in hypostoichiometric UO₂. Due to size effects, the thermodynamic driving force of segregation for Xe and Cs from bulk MgO to the MgO/fluorite interface is strong. However, this driving force is comparatively weak for Sr.