Abstract: The connecting stud between the outer cylinder and the bottom plate of the safety spray pump of a nuclear power plant fractured after installation. The fracture causes of the stud were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the mass fraction of hydrogen in the fractured stud was as high as 0. 000 46%, and the yield strength and tensile strength at room temperature were 970 MPa and 1 200 MPa, respectively. The fracture was clean and relatively flat, without obvious plastic deformation. The fracture surface was crystalline granular, the crack initiation zone had intergranular characteristics, and there were claw-like lines on the crystal plane. The fracture property belonged to typical hydrogen-induced delayed fracture. Metallographic examination showed that there were segregation defects in the core of the fractured stud, which led to the expansion and aggregation of hydrogen elements at the defects and accelerated the fracture rate of the stud.