Submitted by Nuclear Energy Institute (NEI)

Comment: The Nuclear Energy Institute offers the following comments on the subject Federal Register notice, which solicited public comments on draft NUREG-1829. This NUREG was intended to provide technical support for the proposed rulemaking to 10 CFR 50.46 which would establish the option to revise the design basis LOCA break size. Thus, the emphasis of the expert elicitation was on estimating frequencies for large break LOCAs. Our comments are limited to the report’s estimation of small break LOCA frequencies, which, unlike large break LOCAs, are important contributors to PRA risk profiles.

PRA standards have been developed by consensus bodies and endorsed by NRC, with the expectation that plants will be expected to conform to these standards to support regulatory applications. ASME PRA standard RA-S-2002 (endorsed through NRG Regulatory Guide 1.200) contains the following requirement relative to initiating event frequency estimation:

IE-C1: Calculate the initiating event frequency from plant-specific data, if sufficient data are available. Otherwise, use generic data. Use the most recent applicable data to quantify the initiating event frequencies

Response: Contrary to the commenter’s assertion, the objective of the expert elicitation (Section 2) was to calculate LOCA frequencies for all break sizes, not just large break LOCAs. Also, in general, the objectives and results of the expert elicitation as summarized in NUREG-1829 are consistent with the above statement in ASME PRA standard RA-S-2002. The NUREG-1829 small break LOCA estimates represent generic values. Calculations based on plant-specific data, specifically for PRA, are acceptable for use provided that a sufficient technical basis supporting these alternative estimates has been established. There is also an implication by this comment (and subsequent Comment GC5) that the NUREG-1829 estimates do not compare favorably with operator experience. As documented in the newly-added Section 7.10, the differences between the NUREG-1829 estimates and operating experience are not statistically significant. Those differences that do exist are also supported by the panelists’ qualitative and quantitative responses. More details on the comparison between operating experience and the NUREG-1829 LOCA frequency estimates are provided in Section 7.10 which was added to address this and similar comments. See also the responses to GC3, GC5, GC6, GC7, and 7-8 for similar discussions.

Comment Number: GC5

Submitted by Nuclear Energy Institute (NEI)

Comment: Draft NUREG-1829 used plant experiences to estimate the steam generator tube rupture (SGTR) frequency which amounts to greater-than 50% of the total small LOCA frequency. Estimate of the remaining 50% of Category 1 LOCA was entirely based on expert elicitation. The resulting Category 1 frequency estimates from the panel showed a significant divergence of opinions. It is recommend that Category 1 LOCA frequency estimate should continue to be related to the large number of years of plant experiences similar to the method used in NUREG-5750. The current lengths of those experiences amount to thousands of reactor-years, and are statistically significant for use in estimating the annual frequency of events at the 1E-2, 1E-3, and 1E-4 levels. Similar estimates are used in PRA models for numerous other important PRA parameters (such as SGTR).

Response: The commenter incorrectly states that plant experiences were used to estimate the SGTR frequency while the remaining contributions were based on expert elicitation. All contributions to the Category 1 LOCA frequency estimates were determined by the expert elicitation process. For the reasons documented in Section 1 of NUREG-1829, the authors believe that the expert elicitation results are more accurate than results calculated simply from operational experience.

However, the SGTR contributions provided by the panelists are nearly identical to estimates determined simply from operational experience. This consistency, however, simply reflects the panelists’ combined opinion that the operational experience data is relevant for calculating the current day SGTR contributions. The contributions of non-SGTR failures to the PWR LOCA Category 1 frequencies and the BWR LOCA Category 1 estimates are also not inconsistent with estimates based solely on operational experience (See Section 7.10). However, the panelists do expect some increase in the PWR LOCA Category 1 frequencies compared to operational experience-based estimates as a result of current PWSCC issues (Sections 6.3.2 and 7). See Section 7.10, which was added to address this and similar comments, for additional details on the comparison between the elicitation estimates and operational experience data. Additional relevant information is also provided in the responses to GC3, GC4, GC6, GC7, and 7-8.