The migration of lanthanide fission products to cladding materials is recognized as one of the key causes of fuelcladding chemical interaction (FCCI) in metallic fuels during operation.We have performed first-principles density functional theory calculations to investigate the segregation behavior of lanthanide fission products (La,Ce,Pr,and Nd) and their effects on the intergranular embrittlement at ∑3(111) tilt symmetric grain boundary (GB) in α-Fe.It is found that La and Ce atoms tend to reside at the first layer near the GB with segregation energies of-2.55 eV and-1.60 eV,respectively,while Pr and Nd atoms prefer to the core mirror plane of the GB with respective segregation energies of-1.41 eV and -1.50 eV.Our calculations also show that La,Ce,Pr,and Nd atoms all act as strong embrittlers with positive strengthening energies of 2.05 eV,1.52 eV,1.50 eV,and 1.64 eV,respectively,when located at their most stable sites.The embrittlement capability of four lanthanide elements can be determined by the atomic size and their magnetism characters.The present calculations are helpful for understanding the behavior of fission products La,Ce,Pr,and Nd in α-Fe.