B12412, Forwards Summary of Results & Insights Gained from Fire PRA & Planned Actions to Reduce the Probabilistic Risk Due to Fires

Forwards Summary of Results & Insights Gained from Fire PRA & Planned Actions to Reduce the Probabilistic Risk Due to Fires

ML20214T858
Person / Time
Site:	Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date:	06/01/1987
From:	Mroczka E CONNECTICUT YANKEE ATOMIC POWER CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
B12412, NUDOCS 8706100450
Download: ML20214T858 (9)

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CONNECTICUT YANKEE ATOMIC POWER COMPANY B E R L I N. CONNECTICUT P o. Box 270 e HARTFORD, CONNECTICUT 06141-0270 203 465-5000 June 1,1987 Docket No. 50-213 B12412 Re: Integrated Safety Assessment Program U.S. Nuclear Regulatory Commission s

Attn: Document Control Desk Washington, D.C. 20555 Gentlemen:

Haddam Neck Plant Probabilistic _ Safety Study - Fire Analysis In a letter dated March 31, 1986,(1) Connecticut Yankee Atomic Power Company (CYAPCO) docketed to the NRC Staff a summary report of a plant-specific Probabilistic Safety Study (PSS) for the Haddam Neck Plant. At that time, we noted that the study only considered internally-initiated events and that we were planning to complete and document an analysis of fire-initiated events.

Northeast Utilities Service Company, on behalf of CYAPCO, has completed a probabilistic risk analysis of fire-initiated events at the Haddam Neck Plant.

Attachment 1 provides a ::ummary of results and insights gained from the fire PRA analysis. Attachment 2 provides a discussion of planned actions to reduce the probabilistic risk due to fires. The complete fire analysis, included as Attachment 3 to this letter, was performed in order to identify and evaluate the probability and consequences of postulated fires on the operation of the Haddam Neck Plant. Specifically, a set of event trees was developed which modeled the pathways via which a fire would lead to plant events similar to those previously analyzed as originating from internally-initiated events. Fires in critical areas of the plant can act as transient initiating events as well as lead to degradation or loss of vital safety equipment, thus resulting in a decrease in the likelihood of successful transient mitigation.

The fire analysis calculated a mean core melt frequency of 6.8 x 10-4 per reactor-year from fire-initiated events, which, when added to the internally-Initiated events core melt frequency, results in a total core melt fregaency of 1.23 x 10-3 per reactor-year. However, modifications planned for the 1937 refueling outage will reduce this overall core melt frequency by about 20%, i.e.

to about 1.0 x 10-3 per reactor-year. Modifications planned for the 1989 refueling outage will further reduce this overall core melt frequency by approximately 35%; i.e. to about 5.8 x 10-4 per reactor-year. CYAPCO considers this to be a significant reduction. CYAPCO has established a (1) 3. F. Opeka letter to C. I. Grimes, "Probabilistic Safety Study - Summary Report and Results," dated March 31,1986.

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U.S.'Nucitar Regulatory Commission B12412/Page 2 June 1,1987 continuous fire watch in the switchgear room which will reinain in place until the switchgear room Halon system is upgraded during the 1937 refueling outage.

CYAPCO has also established h roving fire patrol to continuously monitor those plant areas identified in the schedular exemption until the new switchgear building is completed during the 1989 refueling outage. Both of these fire watches have been credited in the attached analysis. Further, CYAPCO is firmly committed to reducing this core melt frequency due to fires as much as practicable.

Per Staff request, we are sending 20 copies of the Haddam Neck Plant probabilistic risk-based fire analysis to the !?AP Project Directorate for distribution within the NRC (including NRR, Region I, ACRS, etc.). We have documented the fire analysis as Chapter 7 of the Haddam Neck PSS. As such, the attached analysis and revised index should be inserted in Volume 5 of the Haddam Neck Plant PSS.

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

Very truly yours, CONNECTICUT YANKEE ATOMIC POWER COMPANY 2d/

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Senid hce President cc: W. T. Russell, Redi on 1 Administrator F. M. Akstulewicz, NRC Project Manager, Haddam Neck Plant P. D. Swetland, Resident Inspector, Haddam Neck Plant

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Docket No. 50-213 B12t4 12 ATTACHMENT 1 Haddam Neck Plant Summary of Results and Insights from the Probabilistic Risk Assessment Of Fire-Initiated Events s

June, 1987

U. S. Nuclear Regulatory Commission B12412/ Attachment 1 Page 1 Summary of Results The core melt frequency due to fire-initiated events is 6.84 x 10-4/ reactor year, and the dominant contributors are summarized below in descending order.

1. Switchgear Room - 3.09 x 10-4/r-y (45.2% of total)

Any of the following sequences leads to early core melt.

a. Loss of all overhead cables due to fire initiating in one of the following:

o 4160V Buses 1-1B and 1-2 o 4160V Buses 1-1 A and 1-3 o 4160V/480V Oil-Filled Transformers o 480V Switchgear o MCC-5 o MCC-6 o Vital AC Inverters, or

b. Fire-induced loss of vital AC and MCC-5 due to fire in either 4160V Buses 1-1 A and 1-3 or 480V switchgear, or

c. Fire-induced loss of vital AC and ECCS systems due to fire in 4160V Buses 1-1 A and 1-3.

2. Primary Auxiliary Building - 1.12 x 10-4/r-y (16.4% of total)

Any of the following sequences also leads to early core melt.

a. Propagation of fire from MCC-8, liquid waste panel, or charging, HPSI, LPSI or CCW pumps to all overhead cables, or

b. Fire-induced loss of the running charging pump and subsequent random failures of the other charging pumps, or

c. Potential loss of overhead cabling in the PAB if any flammable liquids are present in the immediate area.

3. Switchgear Room - 1.03 x 10-4/r-y (15.1% of total)

The dominant sequence, which leads to late core melt, involves a total loss of control air, charging system, safety injection valves, PORV block valves, and main feedwater resulting from a postulated fire.

4. Cable Spreading Area - 3.87 x 10-5/r-y (5.7% of total)

Any of the following sequences also leads to early core melt.

a. Propagation of fire to all cables in cable spreading area, or

b. Fire spreads throughout one area causing loss of all ECCS systems, or l.

U. S. Nuclear Regulatory Commission B12412/ Attachment i Page 2

c. Fire spreads throughout another area which causes loss of service water system and ECCS equipment.

5. Switchgear Room - 3.65 x 10-5/r-y (5.3% of total)

A fire initiating in the battery charger propagates to both DC buses, and either steam generator cooling fails or the ability to maintain primary ,

integrity fails. This sequence results primarily in early core melt, and '

secondarily in late core melt.

6. Switchgear Room - 3.07 x 10-5/r-y (4.5% of total)

The dominant sequence, which results in early core melt, is the total loss of vital AC due to a fire in 480V switchgear or MCC-6, and subsequently, failure of steam generator cooling or failure to maintain primary integrity.

7. Containment Cable Vault - 2.56 x 10-5/r-y (3.7% of total)

Propagation of fires throughout cables in the cable vault results in total loss of instrumentation, and early core melt.

8. Turbine Building - 7.35 x 10-6/r-y (1.1% of total)

Fire-induced service building collapse results in early core melt.

9. Containment Cable Vault - 6.97 x 10-6/r-y (1.0% of total)

Fire in a particular area can result in total loss of instrumentation. If steam generator cooling fails or if the ability to maintain primary integrity fails, the result is early core melt.

The above mentioned fire damage contributors account for about 9896 of the overall core melt frequency attributable to fire-initiated events.

i Docket No. 50-213 B12412 ATTACHMENT 2 Haddam Neck Plant Planned Actions To Reduce The Core Melt Frequency Due To Fires June,1937

U. S. Nucinr Rsgulatory Commission B12412/ Attachment 2 Page1 Planned Actions to Reduce The Core Melt Frequency Due to Fires As a result of the recently completed probabilistic fire analysis, CYAPCO has initiated studies and corrective actions that would lower the core melt frequency due to fire-initiated events at the Haddam Neck Plant. One of the dominant contributors to the risk of a switchgear room fire is from the presence of oil-filled transformers in the switchgear room. Since these were identified as a significant fire hazard, they have been scheduled for replacement during the 1987 refueling outage with dry-type transformers which would eliminate the potential for oil rapidly spreading a fire from the transformers to other safety-related equipment and cabling in the switchgear room, and reduce the CMF due to fires by approximately 32 percent. Another potential risk contributor involved the use of transient flammables. Current procedural controls are being used to resolve these concerns.

The major modifications associated with the completion of the new switchgear room in 1989 and the completion of all scheduled Appendix R modifications will further reduce the core melt frequency due to fires at Haddam Neck. The new switchgear room will house a new safety-related 480-V load center, one set of station batteries with associated charger and DC bus, two static inverter vital power supplies, and a remote instrumentation panel. These changes clearly reduce the core melt contribution from fires in the old switchgear room. In addition, as part of the accompanied Appendix R modifications, cables for various equipment will be routed outside the Primary Auxiliary Building. The rerouting of cabling will reduce the core melt frequency contribution from fires in the Primary Auxiliary Building and also the cable spreading area. Additionally, a fire protection modification already implemented has been determined to provide a reduction in the risk associated with turbine building fires. The modification involved the upgrade of the turbine building sprinkler system to provide water spray coverage on required structural steel. This modification reduced the risk due to fire-induced collapse of the turbine building-service building common wall.

Preliminary studies have or are currently being performed for other possible modifications to reduce the core melt frequency due to fires. One of these, a redundant and diverse means of fire detection in the containment cable vault area would augment the current smoke detection and automatic suppression system and provide additional early warning of cable fires. With early detection and suppression, minimum damage would occur and the risk associated with containment cable vault fires could potentially be reduced. Additionally, fire-rated cabling is being considered for this area to prevent loss of instrumentation due to fires in this area.

Specific projects already completed or scheduled for completion in the 1987 refueling outage (i.e., removal of oil-filled transformers, turbine building fire protection modifications, etc.), along with continued administrative control of combustibles, will result in approximately a 34 percent decrease in CMF (i.e., to approximately 4.5 x 10-4/ reactor-year) due to fires. Other Appendix R modifications in the PAB to be accomplished during the 1989 refueling outage, including the new switchgear room, will further reduce the CMF due to fires by approximately 10 percent. This will result in a revised CMF due to fires of approximately 3.8 x 10-4/ reactor year.

U. S.~ Nuclear Regulatory Commission

. B12412/ Attachment 2 Page 2.

Analyses of potential modifications which could further reduce the core melt

. frequency due to fires at Haddam Neck is ongoing. CYAPCO is firmly committed to reducing this risk as much as practical and will continue to use the ISAP process to arrive at the proper resource allocation for risk reduction. In this context and in recognition of the near-term planned improvements, CYAPCO concludes that appropriate action is being taken .to reduce potential fire vulnerabilities at the Haddam Neck Plant.

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Docket No. 50-213 B12I12 4

ATTACHMENT 3 Haddam Neck Plant Probabilistic Safety Study - Fire Analysis i

l June,1987