Cause of Fracture on Packing Gland Bolt of Circulating Water Pump in Nuclear Power Plant

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.