Severed Tubes in Operating Nuclear Steam Generators: Case Analysis — II

During an eddy current inspection in April 2002, it was found that three tubes downstream of an out-of-service tube in a once-through nuclear steam generator (OTSG) had significant wear indications. Wear indications of these tubes has been recorded as early as 1995 and apparently has been progressing with time. A shell-side video inspection revealed that this tube was completely severed at the bottom tubesheet. This severed tube was discovered immediately after a tube in another OTSG of similar design was found to have severed at the top tubesheet. In the root-cause analysis of the other severed tube, the possibility of a similar tube failure due to flow-induced vibration at the bottom tubesheet was explicitly ruled out and corrective action was implemented based on this conclusion. This discovery cast doubt on the earlier analysis and raised concern on both steam generators. The purpose of this study is to carry out a root cause analysis of the failure of the second severed tube and show that even though the two failures appeared to be similar, the causes and failure mechanisms were different. The study also showed that what appeared to be an obvious fix turned out to be invalid upon more in-depth flow-induced vibration analysis.