Symmetric tilt grain boundaries in uranium nitride and uranium carbide.

The development of UN as a nuclear fuel requires good understanding of its microstructure. Here is presented a study of a set of 14 symmetric tilt grain boundaries (GBs) in UN and UC using density functional theory (DFT). From this set, both for UN and UC, the Σ03 (111)[[Formula: see text]10] is the most stable GB. Correlations between properties of the GBs such as tilt angle, excess volume and energies were obtained. For both materials the GB energies have a global minimum at the tilt angle of 70°. The plot of the tilt angles vs. GB energies converges naturally to the previously published stacking fault energy for UN at 0°. Using this method, a stacking fault energy for UC was obtained. The correlation between GB excess volume and GB energy is strong for UN, it is single-valued and of the simple parabolic type and shows that the excess volume is a good predictor of the GB energy for this material. For UC, the corresponding correlation is moderately predictive. Similarly to what is known for MgO with rock salt structure, this set of GBs is expected to be a good description of coincidence site lattice GBs in UN and UC that occur in microstructures close to equilibrium.