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Comments Related to Section 7 of NUREG-1829

Comment Number: 7-1

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station 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. I strongly recommend that Category 1 LOCA frequency estimates 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. They are statistically significant to be used to estimate 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: Operating experience becomes more relevant as the LOCA size decreases, especially for events such as SGTR, because the database is populated with actual events. However, there is still scant data on SB LOCAs due to limited passive piping or non-piping component failures. The panelists were made aware of this operating experience data, and SGTR was specifically used as a base case estimate, along with frequencies for other piping and non-piping precursor events (e. g., cracking). However, one of the cautions with using operating experience solely to calculate Category 1 LOCA frequency estimates is that past experience is not necessarily indicative of current or future performance. Common cause material degradation can result in systematic increases in generic frequencies as a function of time compared to frequencies based on operating experience. Conversely, wholesale mitigation programs (as with IGSCC) can result in systematic decreases in LOCA frequencies over time.

These common cause considerations are one reason why elicitation is preferable to simple operating experience estimates for even small break LOCAs. In fact, the elicitation results are most justified for this LOCA category because of operating experience data. The operating experience data was fully considered by the panelists to estimate past LOCA frequencies, and they then used their best judgment to modify these estimates, as appropriate, based on their current knowledge. It is preferable to use current knowledge in this way rather than wait for several years for operating experience to adequately reflect the new conditions. It is worth noting, however, that the elicitation results and operating experience data are generally consistent for LOCA Category 1. The only differences, which are not statistically different, are for the frequencies associated with non-SGTR, Category 1 PWR failures. The panelists justified the higher NUREG-1829 estimates as a result of the effects of PWSCC on small diameter piping failures.

Key elements of this response have been incorporated into Section 7.10 of the revised NUREG. This comment is also very similar to GC5. Please see the response to this comment for additional information.

Comment Number: 7-2

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station

Comment: The draft NUREG combined a variety of LOCA sources into each LOCA category. Piping LOCAs and several non-piping LOCAs were pooled together to form each of the LOCA categories. It would be useful for each of the 6 LOCA categories to add a table of LOCA sources and frequency contributions. This breakdown is particularly important for the small and medium LOCA categories.

Some contributors to the small and medium LOCAs are modeled separately in most PRA models (SGTR, RCP seals, inter-system LOCAs and others). If the end user does not subtract the separately-modeled LOCA contributors, then the contribution to CDF (core damage frequency) from those contributors would be conservatively and redundantly modeled.

Response: First, it is important to recall that this study was only concerned with estimating passive system failure frequencies of structural system components (SSCs) within the primary system (Section 2). Therefore, LOCAs associated with RCP seals, interfacing system LOCAs, and active system LOCAs are not included in the NUREG-1829 estimates. There are previous estimates for these types of events (e. g., NUREG/CR-5750). The revised NUREG (Section 7.8) includes a table of SGTR frequencies as requested by this comment because all the panelists provided sufficient information to determine individual LOCA frequency estimates for this system. Section 7.8 of the revised NUREG also contains estimates for all non-SGTR passive-system SSC failures.

In general, however, it was not possible to develop information for individual system failures from the elicitation responses. For each LOCA category there was a potential contribution from either 12 or 13 different piping systems (12 for the PWRs and 13 for the BWRs) plus a contribution from either 3 or 5 non-piping components (3 for the BWRs and 5 for the PWRs). In addition, for each of the non-piping components, there were up to 8 (for the RPVs) subcomponents (e. g., vessel body, CDRMs, nozzle, head bolts, etc.) that were potential contributors. In order to make the responses tractable, the panelists were requested to only provide information for systems that they expected to make up at least 80 percent of the failure frequency contribution. Thus, not every panelist provided responses for every piping system or non-piping component or subcomponent. Furthermore, it was not possible to impute this data accurately because the relative frequency contributions to each panelist’s estimates are small. Consequently, it was not possible to calculate group estimates of individual system failure frequencies because this would have required each panelist to provide estimates for every piping system and non-piping component..

Key elements of this response have been incorporated into Section 2 of the revised NUREG. Additionally, Section 7.8 has been added to the revised NUREG to provide the SGTR and non-SGTR estimates for PWR plants.

Comment Number: 7-3

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station

Comment: The steam generator tube rupture frequency (merged with LOCA category 1) was reported as mean value = 3.5E-03 based on number of industry events averaged over years of reactor operations. This mean value frequency should be separated from the main small LOCA category and estimated as a “range” consisting of upper and lower bounds. Plants with aging steam generators are encouraged to use the upper bound of the range. And, plants with new steam generators may use the lower bound of the range. Of course, numerous plants would use the overall mean value of 3.5E-03.

Response: Separate SGTR frequencies have been calculated from the elicitation responses and are provided in the revised NUREG (Section 7.8). The current-day median STGR frequency is 2.6E-3 and the mean frequency is 3.7E-3 (Table 7.18). These values compare favorably with the operating-experience Category 1 frequency of 3.5E-3 for SGTRs as reported in Section 4.4.1. The 5th and 95th percentiles of the group estimates are 5.0E-4 and 1.0E-2, respectively. Additional discussion concerning the SGTR estimates is also contained in Section 7.10 of the revised NUREG.

Comment Number: 7-4

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station

Comment: The various LOCA frequencies are reported in the several tables as cumulative values. In order to isolate the frequency of each LOCA category, one has to subtract the frequency of the next higher ranking category. This reporting format may lead to human errors. Some users may not become aware of the cumulative table format since that description is briefly stated at the later sections of a very large report. Please add a footnote under each LOCA frequency table explain how to obtain the frequency of each LOCA category.

Response: The six LOCA categories were defined in terms of cumulative thresholds because the panelists felt more comfortable with providing their responses as cumulative thresholds rather than to intervals defined by consecutive thresholds. (Appendix B) Because the differences between the results for consecutive LOCA categories are typically much smaller than their uncertainties (see Table 1, Figure 1, Figure 7.36 and Figure 7.37), there is no statistical difference between using the cumulative frequencies or interval frequencies determined by subtraction. If interval-defined LOCA frequencies are required, simply use the NUREG cumulative threshold estimates in Section 7.7. However, Section 7.9 does use interval-defined frequencies for consistency in comparing the NUREG-1829 estimates with other prior study estimates. This point has been clarified in the revised NUREG in the Executive Summary, Section 3.4.1, and Section 7.9.

Comment Number: 7-5

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station

Comment: The equivalent break diameters used in various PRA models that form boundaries between various LOCA categories are not necessarily matching those used in the draft NUREG. For example, a small LOCA range may extend up to 0.03 square feet equivalent break area (derived through existing capability of high pressure safety injection). The draft NUREG should give a clear guideline on interpolations between the various LOCA frequency values, including advice on arithmetic or geometric preference for interpolation.