ML20245A322

Forwards Response to PRA Audit on 881101-02

ML20245A322
Person / Time
Site:	Browns Ferry
Issue date:	06/15/1989
From:	Shell R TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
TAC-63178, NUDOCS 8906210223
Download: ML20245A322 (5)

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' ?, y TENNESSEE . VALLEY AUTHORITY

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CH ATTANOOGA. TENNESSEE 37401 SN 157B Lookout Place JUN 151989 U.S. Nuclear Regulatory Commission ATTN: -Document Control Desk Washington, D.C. 20555' Gentlemen:

In the Matter of ) Docket Nos. 50-259 Tennessee Valley Authority ) 50-260

) 50-296 BROWNS FERRY NUCLEAR PLANT-(BFN) - NUCLEAR REGULATORY COMMISSION (NRC) AUDIT-0F TVA*S PROBABILISTIC RISK ASSESSMENT (PRA)

Reference:

S. A. Black letter to 0. D. Kingsley dated March 29, 1989~

The NRC conducted an audit of the BFN PRA in Knoxville, TN, on November 1-2,,

1988. The referenced letter identifies an audit finding involving potential single failure vulnerability. TVA's response to this. issue is contained-in enclosure 1.

Enclosure 2 provides a summary list of commitments made by TVA in this submittal.

Ifthereareanyquestions,pleasetelephoneR.J.'McMahonat(615)632-6568.

Very truly yours, TENNESSEE VALLEY AUTHORITY

. ku anager, Nuclear Licensing and Regulatory Affairs cc: See page 2 003 e

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, An Equal opportunity Employer

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JUN 15.1989 U.S. Nuclear Regulatory Commission  ;

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Enclosures 'l

.cc (Enclosures): .!

Ms. S. C. Black, Assistant Director for Projects ':

TVA Projects. Division U.S. Nuclear Regulatory Commission ,

One White Flint, North- 1 11555 Rockville Pike i' Rockville, Maryland 20852

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Mr. B. A.' Wilson, Director j

for Inspection Programs j TVA Projects Division -l U.S. Nuclear Regulatory Commission -

j Region II- 'J 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 Browns Ferry Resident Inspector Browns Ferry Nuclear Plant Route 12, Box 637 Athens, Alabama 35609-2000 i

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ENCLOSURE 1 BR0HNS FERRY NUCLEAR PLANT (BFN) l PROBABILISTIC RISK ASSESSMENT (PRA)

NRC AUDIT FINDING ~

l The-following is an audit finding on dominant sequence 5 of the revised Browns Ferry PRA (September 1987 version):

Sequence 5: 1 A loss of offsite power event'followed by failure of the high pressure l coolant makeup systems, and failure of the low pressure. injection system i and the core spray system to provide coolant inventory makeup to the i reactor.and required pool cooling.

l Finding: The frequency estimate seems somewhat conservative. However, during the audit review, the licensee indicated that failure.of a single shutdown board 1A following a loss of offsite power event will result in failure of two RHR pumps which are required for successful pool cooling.

There is a possibility that this situation may result in a violation of General Design Criteria (GDC) 17 and 34 of 10 CFR Part.50, Appendix A.

GDC 17 and 34 generally require that the licensed nuclear facility be provided a decay heat removal design that is single failure proof.

Therefore, we recommend that the adequacy of the existing decay heat removal design and the significance of the loss of two RHR pumps resulting from a single failure of shutdown board 1A be carefully considered. This issue must be resolved prior to Unit-2 restart.

Remarks: The modeling of the recovery of the 480 V shutdown board 1A during a period of five to six hours could reduce the sequence frequency.

The Browns Ferry plant-specific procedures should be. reviewed carefully for the feasibility estimate is also based on a success criterion that one

, RHR pump is needed for vessel injection and two RHR pumps are I simultaneously needed for pool cooling (referred to as the RC event in the

PRA). This criterion may be overly stringent, assuming that RHR pumps could be re-started manually; and considering an availability of a i recovery time of at least two to three hours for the RHR system prior to a substantial pool heat-up. The future revisions to the PRA should explore i a realistic success criterion for a combination of the vessel injection  ;

and pool cooling functions. The failure of a single shutdown board 1A following a transient such as loss of offsite power should result in a potential loss of two RHR pumps needed for pool cooling function.

TVA RESPONSE The NRC audit finding was based on BFN PRA sequence 5 which assumed failure of one loop of Low Pressure Coolant Injection (LPCI) and one loop of core spray from loss of pump cooling due to a single electrical board failure. Based on the discussions with TVA during the audit, it appeared this failure would result in the inability to meet the minimum pump flowrate requirements for a recirculation' discharge line break as. listed-in Table 6.5-P of the BFN Final Safety Analysis Report (FSAR). Subsequent review by TVA revealed the BFN PRA sequence had applied the short-term requirements of BFN FSAR Table 6.5-2, to the long-term LPCI function of RHR and incorrectly applied the design basis.for the RHR motor cooler.

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Page 2 ENCLOSURE 1 (Continued) 1 BROWNS FERRY NUCLEAR PLANT (BFN) J PROBABILISTIC RISK ASSESSMENT (PRA)

The design calculation on which BFN FSAR Table 6.5-2 is based states both short-term and long-term requirements for core cooling. 'Only the short-term (less than 10 minutes) requirements are listed in FSAR. Table

6.5-2. The'long-term requirements were not identified in BFN FSAR Table j 6.5-2. The long-term (beyond 10 minutes) requirements'are that one loop j of core spray is necessary for core cooling'du' ring the design basis Loss-of Coolant Accident (LOCA) and two RHR pumps are necessary for. containment cooling.

BFN PRA sequence 5 assumed that a single failure of'an electrical board .

would disable one loop of LPCI'(RHR) and one loop of CS due to the loss of power to the RHR pump motor cooler and the room cooler for the core spray pumps. If a recirculation line break then occured on the remaining RHR I loop, only one loop of CS would be available to provide ~ core injection.

The assumption that the LPCI loop would be unavailable due to loss of the motor cooler is inconsistent with the design basis. The design basis for the short term does not require the RHR motor coolers. Therefore, one '

loop of CS and one RHR pump are available which meets the short term ~

cooling requirements. .

The BFN PRA only evaluated the short term sequence. The pump requiremer.ts for the long term are met by one loop of CS providing core injection and the loop of LPCI (RHR) made unavailable by the recirculation line break:

being aligned to containment cooling by the operator.

The discussion above confirms that in this situation, BFN is in compliance with GDC 17 and 34 of 10 CFR 50, Appendix A which requires a licensed-nuclear facility be provided a decay heat removal design that is' single j failure proof.

TVA will re-confirm the adequacy of the RHR pump with the RHR motor cooler unavailable to meet the short-term pump combination requirements. This action will be complete by December 15, 1989. TVA will also revise the BFN PRA to reflect the correct long-and short-term pump combination requirement for core spray and RHR in the next update. 'In addition,. FSAR Table 6.5-2 will be revised in the next annual update to reflect the correct short-and-long term pump combination requirements for core spray  ;

and RHR (LPCI). This action will be complete by July 31, 1990.  !

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ENCLOSURE 2 BROWNS' FERRY NUCLEAR PLANT (BFN)

PROBABILISTIC RISK ASSESSMENT (PRA)

SUMMARY

LIST OF COMMITMENTS _

l. TVA Will~ reconfirm.the adequacy'oflthe RHR pump with the_RHR motor cooler ,

unavailable to meet the short-term pump combination' requirement. This, action will be completed by December 15, 1989.

2. TVA will revise the BFN PRA to reflect the correct long and short-term. 1 pump combination requirements for core. spray and RHR (LPCI) in the next 1 update. 'j i

3. FSAR Table 6.5-2 will be revised in the~next annual update to' reflect thel l correct.long and short-term pump combination requirements for core spray and RHR (LPCI). This action will be complete by July 31, 1990.

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