Comment Number: 7-6

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station Comment: The various LOCA frequency tables provided results for the past 25-year operating time period and for the 40-year average plant life. The more suitable value to select for use in PRA models may well be the expected LOCA frequencies applicable to the next 15 years. That particular selection allows for frequency penalties at aging plants as well as credits at plants with new steam generators and/or improved methods of inspections & leak detections. In order to extract the next 15-year LOCA frequencies from the existing tables, the user has to perform simple arithmetic calculations which may lead to human errors and inconsistent applications across the industry. Please provide frequency estimates for the next 15-year time period.

Response: As stated in the Executive Summary, the frequency estimates are not expected to change dramatically over the next fifteen years for any size LOCA, or even the next thirty-five years for LOCA Category 4 and smaller. Because of the predicted stability in these estimates over the near-term, it is recommended that the 25 year (i. e., current-day) results be used to estimate the average LOCA frequencies over the next 15 years of fleet operation. This last point was incorporated in both the Executive Summary and Section 7.4 of the revised NUREG.

Comment Number: 7-7

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station

Comment: There is insufficient description of small LOCA frequency’ “comparison results” relating to those in NUREG 5750 (which is most typically used in current PRA models). If one excludes the contribution of steam generator tube rupture frequency, the draft NUREG-1829 small LOCA value is ONE order of magnitude higher. Please add justifications for that large difference.

Response: This comment is very similar to Comment GC3. See the response to Comment GC3 for the comparison of NUREG-1829 with both NUREG/CR-5750 and operating experience estimates. More detailed information on these comparisons is available in Sections 7.9 and 7.10.

Comment Number: 7-8

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station

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

Response: A new section has been added to the NUREG (Section 7.10) to compare SB LOCA frequency estimates from the elicitation with operating experience. As discussed in this section, the elicitation estimates for BWR and PWR small break LOCA frequencies are generally consistent with operational experience estimates. The BWR and total PWR elicitation mean frequency estimates are only 20 percent and 100 percent higher, respectively, than operational experience estimates. Further the PWR SGTR frequencies are virtually identical to operating experience predictions. The biggest difference between the elicitation results and operating experience occurs for SB LOCA estimates that are determined without SGTR contributions. The elicitation mean frequency estimate is approximately 5 times higher than the operating experience estimate which accounts for nearly all of the 100% increase in the total PWR SB LOCA frequencies indicated above. Although the five-fold increase in the elicitation non-SGTR SB LOCA frequencies is not inconsistent with operating experience (Section 7.10), this difference is physically supported by the panelists’ qualitative and quantitative responses. The panelists indicated that medium and, to a lesser extent, small LOCAs in PWRs are most dramatically impacted by current PWSCC concerns (Section 6.3.2). This increase reflects this concern.