ML12020A243

Supplement to Relief Request RR-04-11, Implementation of a Risk-Informed Inservice Inspection Program Based on ASME Code Case N-716

ML12020A243
Person / Time
Site:	Millstone
Issue date:	01/12/2012
From:	Price J Dominion Nuclear Connecticut
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RR-04-11, 11-109A
Download: ML12020A243 (5)

Text

Dominion Nuclear Connecticut, Inc.

5000 Dominion Boulevard, Glen Allen, Virginia 23060 ,ikominioniif Web Address: www.dom.com January 12, 2012 U.S. Nuclear Regulatory Commission Serial No. 11-109A Attention: Document Control Desk NLOSNVDC RO Washington, DC 20555 Docket No. 50-336 License No. DPR-65 DOMINION NUCLEAR CONNECTICUT. INC.

MILLSTONE POWER STATION UNIT 2 SUPPLEMENT TO RELIEF REQUEST RR-04-1 1, IMPLEMENTATION OF A RISK-INFORMED INSERVICE INSPECTION PROGRAM BASED ON ASME CODE CASE N-716 By letter dated March 30, 2011, Dominion Nuclear Connecticut, Inc. (DNC) requested authorization to implement a Millstone Power Station Unit 2 (MPS2) risk-informed inservice inspection (RI-ISI) program based on the American Society of Mechanical Engineers Code Case N-716. Following this submittal, additional fire protection piping was identified that had previously screened out of the MPS2 probabilistic risk assessment (PRA) model. This piping, which is part of the fire suppression system in the MPS2 auxiliary building cable vault, may contribute sufficient risk to be considered high safety significant and will be conservatively included in the MPS2 RI-ISI program until a flow rate analysis and PRA model update are performed.

The information provided in this letter does not affect the conclusions of the significant hazards consideration presented in the March 30, 2011 DNC letter.

If you have any questions regarding this supplement, please contact Wanda Craft at (804) 273-4687.

Sincerely, J. .P e Viq esident - Nuclear Engineering

Attachment:

Supplemental Information for Proposed Relief Request RR-04-1 1, Implementation of a Risk-Informed Inservice Inspection Program Based on ASME Code Case N-716

Serial No. 11-109A Docket Nos. 50-336 Relief Request RR-04-11 Page 2 of 2 Commitments made in this letter:

1. None cc: U.S. Nuclear Regulatory Commission Region I 475 Allendale Road King of Prussia, PA 19406-1415 C. J. Sanders Project Manager - Millstone Power Station U.S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Stop 08-B3 Rockville, MD 20852-2738 NRC Senior Resident Inspector Millstone Power Station

Serial No. 11-109A ATTACHMENT SUPPLEMENTAL INFORMATION FOR PROPOSED RELIEF REQUEST RR-04-11, IMPLEMENTATION OF A RISK-INFORMED INSERVICE INSPECTION PROGRAM BASED ON ASME CODE CASE N-716 DOMINION NUCLEAR CONNECTICUT, INC.

MILLSTONE POWER STATION UNIT 2

Serial No. 11-109A Attachment, Page 1 of 2 SUPPLEMENTAL INFORMATION FOR PROPOSED RELIEF REQUEST RR-04-1t, IMPLEMENTATION OF A RISK-INFORMED INSERVICE INSPECTION PROGRAM BASED ON ASME CODE CASE N-716 By letter dated March 30, 2011, Dominion Nuclear Connecticut, Inc. (DNC) requested authorization to implement a Millstone Power Station Unit 2 (MPS2) risk-informed inservice inspection (RI-ISI) program based on the American Society of Mechanical Engineers (ASME) Code Case N-716. Following this submittal, additional fire protection piping was identified that had previously screened out of the MPS2 probabilistic risk assessment (PRA) model. This piping, which is part of the fire suppression system in the MPS2 auxiliary building cable vault, may contribute sufficient risk to be considered high safety significant (HSS) and will be conservatively included in the MPS2 RI-ISI program until a flow rate analysis and PRA model update are performed.

The cable vault fire suppression system consists of two headers, one 4" header and one 6" header that feed a series of 3", 2.5", 2" and 1"lines which terminate at sprinkler nozzle heads designed to suppress a cable tray fire. The cable vault has ventilation ducts in the floor which lead to the battery rooms below. In the MPS2 PRA model, if a flood occurred in the cable vault and water flowed into the ventilation ducts, it could eventually fail both direct current (DC) buses and result in core damage. The cable vault has flooding mitigation features designed to preclude these consequences.

These features include a floor drain system, berms around the ventilation ducts, and a gap under a door through which water can flow out of the room. However, if a pipe break of sufficient size occurred, these features may not allow sufficient time to mitigate the flood.

Postulated failure of the 6" and 4" headers, or the 3" fire protection piping require detailed analysis to assess the pipe failure frequencies and the capacity of the cable vault flooding mitigation systems. As a conservative measure, until the flowrate analysis is completed and the PRA model is updated, the identified fire protection piping will be treated as HSS. Additional inspections determined by the guidelines of Code Case N-716 will be added to the RI-ISI schedule and will be performed within the ten year interval. Following completion of the flow rate analysis and PRA model update, the cable vault fire protection piping confirmed as HSS will be incorporated into the RI-ISI program as part of the normal update process. The periodic update process for the RI-ISI program is documented in-house and available for review by the NRC.

To ensure that the MPS2 internal flooding PRA model has captured the flood scenarios which could contribute significantly to core damage frequency (CDF) and large early release frequency (LERF) and impact the scope of the RI-ISI program, an independent review was performed of the plant areas that, if flooded, would result in a HSS flood scenario. A walkdown of these areas was performed to identify the flooding sources, propagation paths, and flood mitigation features. The internal flooding model was then reviewed to verify these flooding scenarios were included.

Serial No. 11-109A Attachment, Page 2 of 2 In addition to the fire protection piping in the cable vault, one additional pipe segment was identified that was not included in the internal flooding model used in development of the proposed RI-ISI Program. This fire protection piping segment is located in the turbine building cable vault. A flooding event resulting from a failure of this piping segment would not contribute significantly-to CDF or LERF because there would be adequate time for operators to mitigate the flood from this source.