B12396, Forwards Probabilistic Risk Analysis of Events Caused by Internal Flooding,Per 860731 Commitment.Analysis Identifies & Evaluates Consequences of Postulated Accidents Due to Internal Flooding.Insights Gained Listed

Forwards Probabilistic Risk Analysis of Events Caused by Internal Flooding,Per 860731 Commitment.Analysis Identifies & Evaluates Consequences of Postulated Accidents Due to Internal Flooding.Insights Gained Listed

ML20211B017
Person / Time
Site:	Millstone
Issue date:	02/13/1987
From:	Mroczka E NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
B12396, NUDOCS 8702190288
Download: ML20211B017 (103)

Text

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General Offices e Selden Street, Berlin, Connecticut 9 5

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(20 4 665-5000 February 13,1987 Docket No. 50-245 B12396 Re: Integrated Safety Assessment Program U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555 Gentlemen:

Millstone Nuclear Power Station, Unit No.1 Probabilistic Safety Study -Internal Flooding Analysis In a letter dated July 10, 1985,(1) Northeast Nuclear Energy Company (NNECO) submitted a summary report of the results of a plant-specific Probabilistic Safety Study (PSS) for Millstone Unit No.1. The study considered internally-initiated events. Subsequently, in a letter dated March 26, 1986,(2) NNECO submitted a summary report of the results of a plant-specific fire risk assessment. Further, by letter dated July 31, 1986,(3) NNECO committed to complete and submit to the Staff an analysis of postulated accidents caused by internal flooding.

Northeast Utilities Service Company, on behalf of NNECO, has recently completed a probabilistic risk arealysis of events caused by internal flooding at Millstone Unit No. 1. This analysis, included as an attachment to this letter, was performed in order to identify and evaluate the probability and consequences of postulated accidents due to internal flooding. Specifically, a set of event trees was developed which modeled the pathways via which internal flooding would lead to plant events similar to. those previously analyzed as originating from other internally-initiated events. Internal flooding in certain areas of the plant can act as initiating events -

that could lead to loss of vital safety equipment.

The analysis calculated a mean core melt frequency due to internal flooding initiated events of 2.51 x 10-7 per reactor-year. This is negligible when compared (1) 3. F. Opeka letter to J. A. Zwolinski, " Millstone Unit No.1 Probabilistic Safety Study - Results and Summary Report," dated July 10,1935.

(2) 3. F. Opeka letter to C. I. Grimes, " Millstone Unit No.1 Probabilistic Safety Study - Fire Analysis," dated March 26,1986.

(3) J. F. Opeka letter to C. I. Grimes, " Millstone Unit No.1 Integrated Safety Assessment Program - Final Report," dated July 31,1936.

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to the core melt frequency from all other internally-initiated and fire-initiated events, which is currently 5.7 x 10-4 per reactor-year. Hence, this represents an increase in core melt frequency of 0.044%

The following insights have been gained as a result of the internal flooding analysis effort:

o Internal flooding which results in additional reactor transients is the dominant damage sequence.

o Internal flooding that results in a reactor transient and loss of safe shutdown equipment accounts for only 2296 of the internal flooding risk.

o The reactor building LPCl/CS corner rooms were determined to be the critical impact location for Millstone Unit No.1, with several critical flood sources within the reactor building. However, due to the effective use of curbing, plant drainage and equipment pedestals, the frequency of flooding an LPCI/CS corner room is low.

! o In the turbine building, the areas most susceptible to flooding do not contain critical flooding sources or propagation pathways from other areas containing flood sources. Therefore, the turbiae building was determined not to be a significant contributor to internal flooding risk.

Per NRC request, we are sending twenty (20) copies of the Millstone Unit No. I probabilistic risk-based internal flooding analysis to the ISAP Project Directorate for distribution within the NRC (including NRR, Region I, ACRS, etc.). We have documented the internal flooding analysis as Chapter 7 of the Millstone Unit No.1 PSS. As such, the attached analysis and revised index should be inserted in Volume 4 of the Millstone Unit No.1 PSS.

If you have any questions on this material, please feel free to contact my staff.

Very truly yours, NORTliEAST NUCLEAR ENERGY COMPANY es/

E.J.yoc'zka ()

Senior Vice President cc: Dr. T. E. Murley - Region 1 Administrator

3. Shea - Millstone Unit No.1 Project Manager T. Rebelowski- Millstone Unit No. I Resident Inspector G. Kelly -ISAP Project Directorate

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