Abstract: The fracture failure of the pressure bearing nut on the 500 kV busbar of a nuclear power plant occurred. The fracture cause of the nut was analyzed by chemical composition analysis, metallographic examination, hardness test, fracture morphology observation and energy spectrum analysis. The results showed that the cracked nut was made of austenitic stainless steel. Due to the high carbon content, a large amount of carbide precipitated along the grain boundary, resulting in chromium depletion at the grain boundary, which increased the stress corrosion sensitivity. In addition, there was work hardening in the production process of nuts, which further increased the stress corrosion sensitivity of nuts. At the same time, there were many non-metallic inclusions of type B and D in the raw materials used for nuts, which promoted the initiation and development of cracks. Under the combined action of ocean atmosphere and preload, the nuts eventually failed due to intergranular stress corrosion cracking.