Causes of fracture of Ni-Al bronze bolt used in the condenser seal structure of a nuclear power station

The Ni-Al bronze bolt used for the sealing structure of a certain nuclear power plant condenser fractured. The causes of bolt fracture were analyzed by macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, mechanical property testing and metallographic examination. The results show that the bolts experienced stress corrosion cracking in a humid NH₃ environment. During service, fractured bolts were subjected to uneven stress loads, resulted in an increased tendency for surface stress corrosion. Moreover, the bolts were exposed to their own stress corrosion sensitive medium for a long time. Under the coupling effect of the two, stress corrosion cracks initiated on the bolts. The crack propagated inward along the phase boundary area with high residual stress, ultimately led to bolt fracture.