Submitted by Nuclear Energy Institute (NEI)

Comment: The report should provide a discussion on statistical validation of small LOCA frequency. By using the method of Jeffrey’s non-informative prior (over the past 2,500 reactor years with zero events excluding steam generator tube ruptures), the expected small LOCA frequency is at or below the 1E-04 level. This frequency is over one order of magnitude lower than the frequency reported in the draft NUREG. Plant operational experience of over 2,500 reactor-years should be considered as a valid predictor of small LOCAs. That consideration is further strengthened by improved methods and increased requirements for in-service-inspections and leak detections.

Response: The use of Jeffrey’s non-informative prior to estimate a frequency if no events have been observed leads to a significant underestimate of the mean frequency. Using this method, NEI states that the expected frequency is no more than 1E-04, based on a denominator of 2,500 reactor years. With zero events, a reasonable estimate of the mean is based on using a 50% confidence bound or a value of 0.7 in the numerator. This yields 0.7/2,500 = 2.8E-04 which is a factor of 3 larger than the estimate using Jeffrey’s non-informative prior. Furthermore, the use of 2,500 reactor years as the denominator clearly combines both PWR and BWR operating experience. More rigorous evaluation of operational experience data (See Section 7.10) has demonstrated that the operational experience-based and elicitation-based SB LOCA estimates are not inconsistent. Furthermore, differences are supported by the panelists’ responses and rationale. Section 7.10, which was added to address this and similar comments, provides additional details on the comparison between the elicitation estimates and operational experience data. Also, see the responses to GC3, GC4, GC5, GC7, and 7-8 for related information.