USNRC Regulatory Perspective on Unanticipated Thermal Fatigue in LWR Piping

The regulatory response of the United States Nuclear Regulatory Commission (USNRC) to unanticipated thermal fatigue in light water reactor (LWR) primary piping systems is based on a long history of such events in both US and foreign commercial nuclear plants. This history includes operational conditions leading to surge line stratification, and cracks in feedwater nozzles, high pressure safety injection lines, and residual heat removal lines. US nuclear power plant experience with thermal fatigue has prompted two studies by the USNRC Office for Analysis and Evaluation of Operational Data (AEOD). In addition, the USNRC has provided generic communications to its licensees addressing problems arising from thermal stresses and stratification. Thermal fatigue in general has also been examined by the NRC as a generic safety issue. All these evaluations have alerted the staff to concerns over the broader implications of unanticipated thermal fatigue in piping, including potential problems with plant conditions not being consistent with licensing basis design and inspection commitments. Of particular concern are those problems arising from unanticipated thermal fatigue in un-isolable piping connected to the reactor coolant system. Current USNRC acceptance criteria of licensee actions are based on the USNRC's mission to preserve the public health and safety. (authors)