Submitted by Nuclear Energy Institute (NEI)

Comment: As the NUREG-1829 report may be considered to be the “most recent applicable data” upon finalization, it is important that the final report provide an alternative to continue using operational experience data for the determination of small break LOCA frequencies. Most PRAs currently reference NUREG-5750, which used such a basis (at the time there were 1,250 reactor years of operating experience) to estimate small break LOCA frequencies. Since issuance of NUREG-5750, over one thousand additional reactor years of operational experience have confirmed the conclusions of NUREG — 5750 relative to small break LOCAs. Draft NUREG-1829 notes that, when steam generator tube rupture data are excluded, there is general correlation on small break frequencies with NUREG-5750. However, our review of the report indicated that draft NUREG-1829 estimates these frequencies over one order of magnitude higher than the estimate of NUREG-5750. Using the NUREG-1829 small break LOCA frequency estimation, the US reactor fleet should be experiencing one small break LOCA on average every 4 years. However, no such LOCAs have occurred in the operating history of the US plants. Obviously, the incorporation of this frequency estimate into existing PRAs would lead to unwarranted impacts that are out of context with reality.

Response: Comparing results from NUREG-1829 and NUREG/CR-5750 must be done with some care due to differences in how the LOCA frequencies were calculated. The SB LOCA frequencies typically reported in NUREG-1829 for PWRs include contributions from SGTRs while the SB LOCA frequencies reported in NUREG/CR-5750 exclude SGTRs. Furthermore, the LOCA frequencies in NUREG-1829 are based on threshold leak rates (Category 1 LOCAs include all leaks greater than 100 gpm [380 lpm], Category 2 LOCAs include all leaks greater than 1,500 gpm [5,700 lpm], etc.). Conversely, the SB LOCA results in NUREG/CR-5750 include only those events with leak rates between 100 and 1,500 gpm (380 and 5,700 lpm).

These distinctions were considered in making the comparisons provided in Section 7.9 that are summarized in Table 7.20. The reported SB mean LOCA frequency estimates for NUREG/CR-5750 are 4.0E-04 per calendar year and 7.4E-03 per calendar year for BWR and PWR plants, respectively. Note that these PWR SB LOCA estimates include the steam generator rupture frequencies calculated in NUREG/CR-5750. The revised NUREG-1829 SB LOCA estimates are 5.2E-04 and 6.6E-03 for BWR and PWR plants, respectively (geometric mean with overconfidence adjustment). The ratio of the NUREG/CR-5750 to the NUREG-1829 results is 0.76 for BWRs and 1.12 for PWRs. These SB LOCA estimates are therefore similar.

It is also interesting to compare the PWR SB LOCA frequency estimates after excluding SGTR frequencies from the Category 1 LOCA estimates (Table 7.19). As reported in Section 7.8 of NUREG 1829, the SGTR rupture frequencies predicted by the elicitation, NUREG/CR-5750, and operational experience are consistent. The cumulative SB LOCA Category 1 and 2 estimates without SGTR contributions are 1.9E-03 and 4.2E-04 per calendar year (Table 7.19). Therefore, the interval value that corresponds to the historical SB LOCA definition (i. e., breaks between 100 and 1,500 gpm [380 and 5,700 lpm]) is 1.48E-03 per calendar year. This value is approximately 3.7 times higher than the mean SB LOCA frequency from NUREG/CR-5750 of 4E-04 per calendar year.

The increase in the elicitation result reflects the panelists’ opinion that current PWR SB LOCA frequencies for components other than steam generator tubes are higher than historical averages. This increase primarily stems from current PWSCC concerns (Section 6.3.2). The practical implications from these differences however are not striking. The NUREG/CR-5750 estimate translates to an expected PWR SB LOCA every 36 years for the current fleet of 69 operating PWRs. The elicitation frequency corresponds to an expected PWR SB LOCA every 10 years. Additional comparisons between the NUREG-1829 and NUREG/CR-5750 estimates are contained in Section 7.9.

Because of this and similar comments, Section 7.10 has been added to NUREG-1829 to compare the elicitation LOCA frequency estimates with estimates derived from operating experience. This comparison shows that the differences between the elicitation and operating experience-based estimates of the non-SGTR, PWR Category 1 LOCA frequencies are not statistically significant. However, even though these differences are not statistically significant, this does not imply that the two approaches are estimating the same frequency. Because an operating experience-based estimate is an historical average based on many years of operation, a difference will exist if the panelists believe that the current failure frequency differs from the historical average. In fact, the increased elicitation estimate is supported by the panelists’ qualitative and quantitative responses. As noted above, the panelists indicated that medium and, to a lesser extent, small LOCAs in PWRs are most dramatically impacted by PWSCC in relatively small diameter passive system component (e. g., CRDMs, instrument nozzles, etc.) (Section 6.3.2).

Additional details on this comparison are found in Section 7.10. Related information is also found in the responses to GC4, GC5, GC6, GC7, and 7-8.