某核电厂循环水泵盘根压盖螺栓发生断裂，导致轴封水泄漏，通过宏微观分析、化学成分分析、金相检验、力学性能测试及断口分析等方法，对螺栓的断裂原因进行了分析。结果表明：螺栓断裂的主要原因是螺纹切削加工时产生刀痕，在加工刀痕处形成了应力集中并萌生了的疲劳裂纹源，同时高强度和高的硫化物夹杂含量也促进了疲劳裂纹的萌生，在交变载荷作用下裂纹不断扩展，最终导致螺栓发生疲劳断裂。

The bolt of packing gland of circulating water pump in a nuclear power plant fractured, which resulted in the leakage of shaft seal water. The causes of fracture on bolt were analyzed by means of macro and micro analysis, chemical composition analysis, metallographic examination, mechanical property test and fracture analysis. The results show that the main reason of fracture on bolt was that the cutting of thread produced the cutting mark, and the stress concentration and fatigue crack source were generated at the machining tool mark. At the same time, the high strength and high sulfide inclusion content promoted the initiation of fatigue crack. The crack propagated continuously under the action of alternating load, and finally led to the fatigue fracture of bolt.