Failure analysis on premature fracture of anchor bolts in seawater booster pump of nuclear power plant

Three of the four TP321 stainless steel anchor bolts used in an indoor seawater booster pump of a nuclear power plant were encountered with premature fracture during service. A comprehensive analysis was performed to investigate the operational conditions, base materials and macro/ micro morphologies of the failed bolts to identify the failure causes. The results showed that a collection of banded inclusions was distributed among the grains by metallographic examination, and typical pitting holes were observed and high levels of chlorides were detected on the surfaces of the failed bolts' fractures by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). In addition, branched cracks were found on the axial sections of the fractured bolts. Given all that, the premature fracture of the anchor bolts was determined to be mainly caused by stress corrosion cracking (SCC) from chlorides, which was then proved to be induced by splashing of the seawater during routine maintenance of the pump. Besides, the sequence of fracture for these three anchor bolts was ascertained by fractography from three aspects, including shapes of fractures, morphologies of corrosion products and chemical compositions of corrosion products. Finally, countermeasures were put forward as well.