RBG-47401, Preliminary Accident Sequence Precursor Analysis Review
| ML13302B679 | |
| Person / Time | |
|---|---|
| Site: | River Bend  |
| Issue date: | 10/24/2013 |
| From: | Clark J Entergy Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RBG-47401 | |
| Download: ML13302B679 (5) | |
Text
SEntergy Entergy Operations, Inc.
River Bend Station 5485 U.S. Highway 61 N St. Francisville, LA 70775 Tel 225-381-4177 Joseph A. Clark Manager, Licensing October 24, 2013 RBG-47401 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555
Subject:
Preliminary Accident Sequence Precursor Analysis Review River Bend Station - Unit 1 Docket No. 50-458 License No. NPF-47
References:
- 1.
River Bend Station, Licensee Event Report 458/12-003 (RBG-47266)
- 2.
NRC Letter; "River Bend Station, Unit 1 - Preliminary Accident Sequence Precursor Analysis for Licensee Review" dated August 26, 2013 (RBC-51160)
Dear Sir or Madam:
In Reference 2 the NRC provided a preliminary result of an Accident Sequence Precursor (ASP) analysis of an operational event that occurred at River Bend Station (RBS), per Reference 1.
The attachment to this letter provides RBS comments to the preliminary ASP.
This letter does not contain any commitments.
Please contact me if you require further information.
Sincerely, JAC/bm b Ac-)b
RBG-47401 Page 2 of 2 RBF1-13-0132 Attachment - RBS Comments on Preliminary Precursor Analysis cc: U.S. Nuclear Regulatory Commission Attn: Mr. Alan Wang Mail Stop MS 0-8 B1 Washington, DC 20555-0001
Attachment to RBG-47401 RBS Comments on Preliminary Precursor Analysis
RBG-47401 Attachment Page 1 of 2 Entergy has the following comments regarding the Preliminary Accident Sequence Precursor Analysis in the NRC letter (TAC NO. MF2599) dated August 26, 2013, for the May 24, 2012, scram at River Bend Station:
Cutsets 1 through 8 and 13 and 14 of LONSW Sequence 10 include test and maintenance events for Standby Service Water Pumps, Residual Heat Removal pumps, and Containment Unit Coolers. This equipment was available at the time of the scram, thus these are not valid cutsets.
Cutsets 3 and 4 of LONSW Sequence 34 also include test and maintenance unavailability events for Standby Service Water Pumps, thus are also not valid cutsets for evaluating the CCDP for the event.
The cutsets in sequence LONSW 61 appear to be non-minimal to similar cutsets in sequence LONSW 34. The LONSW 34 cutsets involve a loss of Normal Service Water (NSW) with a failure of all Standby Service Water. The LONSW 61 cutsets involve the same events but with RCIC failure, thus would be already accounted for under LONSW 34.
Several cutsets include an event SSW-MOV-OO-F096A as an event which fails Train A of Standby Service Water (SWP-P2A and SWP-P2C). As documented in the River Bend Service Water Probability Risk Analysis (PRA) system notebook, flow diversion from the Standby Cooing Tower to the Normal Service Water system through SWP-MOV96A(B) does not result in failure of the Standby Service Water System. Since the Normal Service Water system is a closed loop system, any flow diversion would only occur for the short period of time required to fill the Service Water System surge tank, which has a volume of approximately 20,000 gallons.
Thus, if Service Water flow were diverted to the tank, it would take less than three minutes to fill the tank. Such a momentary flow diversion would not fail a Train of Standby Service Water.
The ASP analysis is incorrectly assuming that a common cause failure of the Service Water return valves, SWP-MOV55A(B), will by itself lead to core damage. (e.g.,
second cutset of sequence LONSW 34). This is incorrect. For this scenario, RCIC would be available to respond and would prevent initial core damage. Fire Protection Water would also be available for injection. Valve SWP-AOV599 would open to provide a bypass path around SWP-MOV55A, providing a return path for Service Water to the Standby Cooling Tower. This allows time (roughly four hours) to manually open SWP-MOV55A. Restoration of this return path would allow for service water flow to the RHR A heat exchanger, providing suppression pool cooling and thus preventing core damage which would otherwise result from long-term containment overpressurization.
Although not consequential to the analysis of this event, note that the Service Water Cooling (SWC) pumps do not have the same power supply as the Normal Service Water (NSW) Pumps. The description of event IEFT-LONSW-NONVITAL mistakenly implies NSW and SWC are powered from the same busses. NSW pumps SWP-P7A(B)(C) are powered from NNS-SWG2A(B), which in turn are powered from NPS-SWG1A(B). The SWC pumps are powered from different switchgear,
RBG-47401 Attachment Page 2 of 2 NNS-SWG6A(B), which are powered from NPS-SWG3A(B), which in turn are connected to NPS-SWG1A(B). Thus, it would have been possible to restore "A" train power to SWC pumps after the event if it had been needed.
In the Precursor Analysis Event Description, bus NNS-SWG2B is incorrectly described as a 13.8 kV bus. NNS-SWG2B is a 4.16 kV bus, which in turn is supplied from 13.8 kV bus NPS-SWG1B. Similarly, the second bullet under "Key Modeling Assumptions" incorrectly states that NNS-SWG2A(B) are 13.8 kV busses vice 4.16 kV busses.
Accounting for these considerations in the NRC's analysis would reduce the calculated Conditional Core Damage Probability (CCDP) from 2.55E-04, per Attachment A, to a value of approximately 1.2E-04.
Entergy's PRA of this event calculated a CCDP of 1.24E-04 based on the plant conditions at the time of the scram, using the average maintenance unavailability model and with the same E-1 2 truncation limit as used for the RBS base model. When the average maintenance unavailability terms are removed to correspond to the actual plant configuration, the calculated CCDP is 7.5E-05. This is of a similar magnitude but about 40%
smaller than the value calculated by the NRC.