Exploring the effects of solute segregation on strength of UO₂ and α-U₃O₈ Σ5 (210)/[001] symmetrical tilt grain boundaries by first-principles calculations

The segregation of fission products in nuclear materials significantly affects the cohesive properties of the grain boundaries (GBs), potentially leading to intergranular embrittlement and cracking of the nuclear fuels. Therefore, studying the segregation behavior of fission products in uranium oxides is crucial for ensuring the eventual and efficient disposal of spent nuclear fuel. This study investigates the segregation behavior of typical fission products (La, Nd, Zr and Mo) and their effects on the strength of both UO₂ and α -U₃O₈ Σ5 (210)/[001] GBs using first-principles calculations. It can be concluded that all these four fission products can easily segregate to UO₂ Σ5 grain boundary, while they exhibit a relatively weaker tendency to segregate to α -U₃O₈ Σ5 grain boundary. Additionally, both the Rice-Wang model and the first-principles tensile test calculations were employed to assess the strengthening/embrittlement potency. The findings reveal that La and Nd exert a weakening effect on both UO₂ and α -U₃O₈ GBs, whereas the presence of Zr and Mo strengthens these boundaries. But spent nuclear fuel tends to fracture along GBs in any case. Furthermore, the underlying mechanism of differences in GB strength due to the fission product segregation was profoundly discussed through bond lengths and charge density analyses. Finally, the results from this research are expected to be valuable for future theoretical and experimental investigations on UO₂, U₃O₈ as well as more other complex uranium oxides.