ML091890802
| ML091890802 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 07/06/2009 |
| From: | Bhalchandra Vaidya Plant Licensing Branch 1 |
| To: | Coddington C, Filchner D, Hoffman C, O'Rourke B Susquehanna |
| vaidya B, NRR/Dorl/lpl1-1, 415-3308 | |
| References | |
| TAC ME0969, TAC ME0970 | |
| Download: ML091890802 (3) | |
Text
From:
Vaidya, Bhalchandra Sent:
Monday, July 06, 2009 8:07 AM To:
'dlfilchner@pplweb.com'; 'O'Rourke, Brenda W';
'ckhoffman@pplweb.com'; 'Coddington, Cornelius T' Cc:
Salgado, Nancy
Subject:
FW: PRA Comments and Follow-up RAIs on PPL Response to PRA RAIs for T-20 TS submittal, ME0969 & Me0970 The PRA staff has the following comments on PPL Susquehanna Responses to PRA RAIs dated July 1, 2009, as described in the e-mail message below.
Please contact me ASAP, if you need a phone call discussion. If not, when can I expect the supplemental submission ?
Bhalchandra K. Vaidya Licensing Project Manager NRC/NRR/DORL/LPL1-1 (301)-415-3308 (O) bhalchandra.vaidya@nrc.gov From: Howe, Andrew Sent: Thursday, July 02, 2009 3:37 PM To: Vaidya, Bhalchandra Cc: Harrison, Donnie
Subject:
RE: PPL Response to PRA RAIs for T-20 TS submittal, ME0969 & Me0970 Bhalchandra -... There are still a couple of areas the RAI responses, which require further clarification for the safety evaluation to be completed. I have run these through Donnie Harrison and he concurs that the responses need additional clarifications.
RAI #1:
The intent of this RAI was to allow the staff to understand specifically how the effect of the unavailability of ST No. 20 was modeled in the PRA and how the plant-centered LOOP initiator frequency increase was calculated. The response to RAI #1a identifies transformer failure probabilities (9.9E-3/year, 8.21E-5/3-days), the qualitative basis of the plant-centered LOOP fault tree, and the total plant-centered LOOP frequencies (2.43E-3/year nominal, 3.98E-2/year during ST No. 20 outage). From this information, the staff can infer that the plant-centered LOOP frequency contribution due to failure of both transformers (not due to CCF) is 9.9E-3/year x 8.21E-5 x 2 = 1.63E-6/year, which is a negligible contributor to the 2.43E-3/year total frequency. Similarly, the 9.93E-3/year contribution of failure of one transformer is only 25% of the reported 3.98E-2/year LOOP frequency during the ST No. 20 outage. Therefore, it would seem from these numbers that independent transformer faults are not significant contributors, and that common mode failures of transformers and other faults leading to a plant-centered LOOP dominate the frequency calculation. Therefore, the staff still does not have an understanding of how the licensee determined that the single transformer plant-centered LOOP frequency increased from 2.4E-3/year to 3.98E-2/year. The licensee needs to provide the staff specific details of how the frequency increase in plant-centered LOOP was determined.
The licensee also identified that in the event of a plant-centered LOOP during the ST No. 20 outage, the PRA model still credits restoration of offsite power. If the cause of a LOOP is a failure of the remaining transformer, then offsite power is not recoverable. The contribution of failure of the remaining transformer needs to be excluded from offsite power recovery. Since the frequency of plant-centered LOOP increases by a factor of 16 (3.98E-2/2.43E-3) when ST No. 20 is unvailable, it would seem that the majority of LOOP events would be transformer faults of ST No. 10 (since no other failure modes should be impacted by unavailable transformers),
and therefore would not be recoverable. The licensee needs to further address this issue.
RAI #3:
It is still not clear to the staff how all three statements made in the initial submittal and the supplemental submittal are reconciled. Again, it is stated that "all fire progress to large fires",
"all fires not manually suppressed progress to large fires", and "fire risk considers both the probability of non-suppression and the probability of a fire progressing to a large fire". The first quoted statement would mean that all fires which start become large. The second quote would mean that all fires which start become large unless manually supressed (i.e., after crediting manual suppression, no further risk reduction is assumed for fire progression). The third quote would mean that a realistic assessment of both fire suppression and progression is being made. From the RAI #3a response, specifically based on the presentation of the fire CDF and LERF equations, it appears that the third quote is what was actually done. The licnesee needs to identify which of the three statements are valid for this application. Further, the staff is concerned that if PRA credit is being taken for both manual suppression and some fraction of all fires which progress to large fires, there may be "double counting" credit for manual supression in the fire progression data. The licensee needs to confirm the basis for its calculations to assure that this is not the case.
From: O'Rourke, Brenda W [1]
Sent: Wednesday, July 01, 2009 2:04 PM To: Vaidya, Bhalchandra
Subject:
PPL Response to PRA RAIs for T-20 TS submittal Importance: High Bhalchandra, Attached is the signed letter. If you have any problems viewing it, please let me know.
Thank You, Brenda O'Rourke Senior Engineer - Nuclear Regulatory Affairs PPL Susquehanna (570) 542-1791
<<PLA-6526 signed version.pdf>>
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