Pankaj Nerikar

Thermodynamics of fission products in UO2 ± x

The stabilities of selected fission products-Xe, Cs, and Sr-are investigated as a function of non-stoichiometry x in UO(2 ± x). In particular, density functional theory (DFT) is used to calculate the incorporation and solution energies of these fission products at the anion and cation vacancy sites, at the divacancy, and at the bound Schottky defect. In order to reproduce the correct insulating state of UO(2), the DFT calculations are performed using spin polarization and with the Hubbard U term. In general, higher charge defects are more soluble in the fuel matrix and the solubility of fission products increases as the hyperstoichiometry increases. The solubility of fission product oxides is also explored. Cs(2)O is observed as a second stable phase and SrO is found to be soluble in the UO(2) matrix for all stoichiometries. These observations mirror experimentally observed phenomena.

The role of grain boundary structure in stress-induced phase transformation in UO2

Recently, we reported on a phase transformation in UO2 aided by the presence of high-energy amorphous grain boundaries (Desai and Uberuaga 2009 Scr. Mater. 60 878). Here, we examine the role that the structure of the grain boundary plays in aiding this phase transformation. By examining three bicrystalline systems with different types of grain boundaries (amorphous, Σ5 tilt and Σ5 twist), we study the influence on the phase transformation behavior by the critical applied stress to induce the phase transformation, the effects of grain size and the grain boundary type. We find that there is a critical applied stress below which the phase transformation does not occur, and that the phase transformation occurs more readily in systems with smaller grain sizes. Finally, we observe a weak dependence on the applied stress to cause the phase transformation with the grain boundary energy.

Simulation of xenon, uranium vacancy and interstitial diffusion and grain boundary segregation in UO2

In light water reactor fuel, gaseous fission products segregate to grain boundaries, resulting in the nucleation and growth of large intergranular fission gas bubbles. Based on the mechanisms established from density functional theory (DFT) and empirical potential calculations 1, continuum models for diffusion of xenon (Xe), uranium (U) vacancies and U interstitials in UO2 have been derived for both intrinsic conditions and under irradiation. Segregation of Xe to grain boundaries is described by combining the bulk diffusion model with a model for the interaction between Xe atoms and three different grain boundaries in UO2 ( Σ5 tilt, Σ5 twist and a high angle random boundary),as derived from atomistic calculations. All models are implemented in the MARMOT phase field code, which is used to calculate effective Xe and U diffusivities as well as redistribution for a few simple microstructures.