Atomistic simulation study of the structure, segregation and stability of grain boundaries in the U-Zr metallic fuel

Uranium-zirconium (U-Zr) metallic fuel is considered as fuel candidate for sodium-cooled fast reactors due to its compatibility with sodium, intrinsic passive safety characteristics and higher burnup capability compared to the oxide systems. In the present study, we performed molecular dynamics (MD) and static simulations to investigate the stability, cohesive and structural properties of U-Zr grain boundaries, with 10 and 20 at. % of Zr content. The effect of the Zr composition, temperature and atomic configurations were investigated, and the fracture mechanism highlighted. The cohesive energies of special low energy GBs indicate that the intergranular fracture mechanism plays a significant role in the brittle regime. Moreover, we investigated the role of the atomic configurations in determining the GB properties of the alloys. Indeed, for a given misorientation angle, we found that different atomic configurations do not only yield different energies, but also different strain distribution, atomic coordination and different net expansions (free volume at the boundary). In general, we observed that Zr atoms tend to segregate in low energy configurations. We also observed a systematic increase in GB and surface energies with the Zr concentration, in which the surface energies are systematically higher than the corresponding GB energies. The reported findings highlight important effects of the atomic configurations on properties of investigated GB structures. Therefore, it clearly indicates that the atomic configurations of GBs must be correctly determined for the accurate modeling of the alloys properties.