某核电机组应急柴油发电机固定螺钉发生断裂，采用宏观观察、硬度测试、拉伸试验、金相检验、扫描电镜分析及能谱分析等方法对螺钉的断裂原因进行了分析。结果表明：螺钉拧入通孔的深度不够，增大了应力集中部位的应力水平，且螺钉的螺纹牙表面存在较多的折叠缺陷，裂纹在交变应力作用下于应力集中部位的折叠缺陷处萌生并扩展，最终导致螺钉断裂。

The fixed screws of a diesel generator of a nuclear power unit fractured. The fracture causes of the fixed screws were analyzed by macro observation, hardness test, tensile test, metallographic examination, scanning electron microscopy analysis, energy spectrum analysis and so on. The results show that the depth of the screws into the through-hole was not enough, which increased the stress level of the stress concentration point. And there were more folding defects on the surface of the screw thread. The cracks initiated and propagated at the folding defect in the stress concentration area under the action of alternating stress, which eventually led to the fracture of the screws.