Comment Number: 7-9

Submitted by Zouhair Elawar — Palo Verde Nuclear Generating Station

Comment: Provide a section on probabilistic validation of the small LOCA frequency. Using a Poisson distribution with failure rate of 2.9E-03 (NUREG-1829 category 1 LOCA frequency excluding steam generator tube rupture events). And, approximately 1500 reactor-years, the probability of ZERO small LOCA events (actual industry performance) is ONLY 0.013 (i. e.1.3% chance). This result shows an excessive conservatism in the category 1 LOCA frequency. Are we to expect an average of ONE small LOCA event (with 74% chance) per 345 reactor-years (equivalent to ~4 calendar years)? I think not.

Response: This comment is similar to both GC7 and 7-8. See the responses to these comments for comparisons between the NUREG-1829 Category 1 LOCA estimates and operating experience. Section 7.10 has also been added to provide an in-depth comparison with operating experience.

Comment Number: 7-10

Submitted by Westinghouse Owners Group

Comment: It would be useful to the PRA community, and help facilitate plant to plant consistency if, for the smaller break sizes, more information or guidance were provided to help separate out the frequencies of SGTR from small break LOCA, and CRDM nozzle breaks from medium LOCAs, as well as any other contributors other than primary system piping. Although there is no current intention to use the results of the expert elicitation to update the various LOCA frequencies assumed in individual plant PRAs, as plants go forward with peer reviews of PRAs, it is likely that LOCA frequencies for small, medium and large LOCAs will be compared with NUREG-1829 results. The NUREG-1829 frequencies for the smaller LOCA sizes are several times higher than the values presented in NUREG-5750. The NUREG-1829 values listed for the small break frequency (> 100 gpm) are so high that one would expect to have seen an event in the US every three or four years, whereas to date we have seen none. It would be helpful if the conservatism in the estimates for the smaller beak sizes was discussed and some caveat provided so that plant PRAs don’t end up with excessive conservatism in their small LOCA risk estimates.

Response: This comment is similar to Comment 7-2. See this response to this comment for a discussion about separating failure of individual components like steam generator tubes and CRDMs from the total

LOCA frequency estimates. Section 7.8 in the revised NUEREG provides separate SGTR estimates.

Also, the responses to Comments GC3, GC4, GC5, GC7, 7-1, 7-3, 7-8, and 7-11 summarize comparisons between NUREG-1829 and either NUREG/CR-5750 or operating experience estimates. See also Sections 7.9 and 7.10 of the revised NUREG for more detailed comparisons.

Comment Number: 7-11 Submitted by Nuclear Energy Institute

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: This comment is similar to Comment 7-2. See the response to this comment for a discussion about separating failure of individual components like steam generator tubes and CRDMs from the total LOCA frequency estimates. Section 7.8 of the revised NUREG provides separate SGTR estimates.

Comment Number: 7-12 Submitted by Nuclear Energy Institute

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: This comment is identical to Comment 7-4. See the response to this comment for guidance on obtaining interval values from the cumulative results reported in NUREG-1829. This point has been clarified in the revised NUREG in the Executive Summary, Section 3.4.1, and Section 7.9.