Systematic investigation on the segregation behavior of non-metallic atoms and their synergistic regulation mechanism of the hydrogen embrittlement at uranium grain boundaries

Abstract

As a core fuel material for nuclear reactors, the segregation of non-metallic impurities and the hydrogen embrittlement effect at the grain boundaries of metallic uranium pose a serious threat to the long-term service safety of fuel elements. In this study, density functional theory (DFT) was employed to systematically compare the segregation behaviors of six typical non-metallic atoms (C, N, O, Si, P, and S) at uranium grain boundaries and reveal their synergistic regulatory mechanisms on grain boundary strength and hydrogen embrittlement. The key findings are as follows: all non-metallic elements exhibit an intrinsic tendency to segregate spontaneously at grain boundaries; the closer the site is to the grain boundary core, the stronger the segregation tendency and the more stable the binding state. Non-metallic elements weaken grain boundaries, with Si, P, and S inducing a significantly more pronounced weakening effect than C, N, and O. This difference is primarily attributed to the size-mismatch strain caused by the disparity in atomic radius between non-metallic dopants and the uranium matrix. The dominant mechanism underlying the synergistic grain boundary weakening by hydrogen and non-metallic elements is the chemical contribution: the weak bonds formed between hydrogen and non-metallic atoms replace the original strong non-metal-U bonds, resulting in a significant reduction in electron cloud density at grain boundaries. This study clarifies the non-metallic segregation at uranium grain boundaries and its influence on hydrogen behavior at the atomic scale, providing a key theoretical basis for the design of uranium-based fuels with enhanced resistance to hydrogen embrittlement.