Effect of fission products on tensile strength of UO₂ Σ3 (111)/[11¯0] and UO₂ Σ11 (11¯3)/[110] grain boundaries from first-principles study

The segregation of solute atoms at grain boundaries (GBs) can significantly alter the cohesive properties of materials, potentially leading to embrittlement and cracking. As uranium dioxide (UO₂) is the primary component of spent nuclear fuel, understanding the segregation behavior of fission products in UO₂ is crucial for ensuring the safe and efficient disposal of nuclear waste. This work investigates the segregation behavior of representative fission products (La, Nd, Zr, and Mo) and their effects on the mechanical strength of UO₂ Σ3 (111)/[11¯0] and Σ11 (11¯3)/[110] GBs using first-principles calculations. All these four fission products exhibit a strong preference for segregating to UO₂ GB regions, where La and Nd weaken both UO₂ Σ3 and Σ11 GBs, but Zr and Mo strengthen these boundaries. Despite the strengthening effects produced from Zr and Mo segregations, GB fracture remains a likely failure mode for spent nuclear fuel. These results come from the fact that the segregation of La/Nd elongates the U-O bonds at UO₂ GB planes and reduces the charge density distributions within UO₂ GBs, while the segregation of Zr/Mo does the opposite. These findings provide valuable insights for future theoretical and experimental investigations, and for GB engineering strategies to improve the mechanical reliability and safe disposal of oxide fuels.