ML20238C469

Revised Response to NRC Observation 5.7 from Insp Repts 50-327/86-55 & 50-328/86-55.Corrective Actions:Util Evaluating three-diesel Generator Loading Sequences & Implementing Necessary Mods to Meet Sequence Before Restart

ML20238C469
Person / Time
Site:	Sequoyah
Issue date:	09/01/1987
From:	Gridley R TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
NUDOCS 8709100130
Download: ML20238C469 (4)

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TENNESSEE VALLEY AUTHORITY CHATTANOOGA, TENNESSEE 37401 SN 1578 Lookout Place SEP 011987 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Gentlemen:

In the Matter of ) Docket Nos. 50-327 Tennessee Valley Authority ) 50-328 SEQUOYAH NUCLEAR PLANT - REVISED RESPONSE TO NRC OBSERVATION NO. 5.7 FROM INSPECTION REPORT NOS. 50-327/86-55 AND 50-328/86-55 During the recent NRC inspection of the Design Baseline and Verification Program (DBVP), TVA notified the inspection team that the original response to v NRC Observation No. 5.7 from Inspection Report Nos. 50-327/86-55 and 50-328/86-55 should be revised. TVA was requested formally to transmit the revised response to NRC.

NRC's original concern was stated as follows in the subject inspettion report:

Observation No. 5.7 - Diesel Breaker Trip The team's review of ECN L-5363 indicated that the design process failed to evaluate failure of the load breaker to trip or the effects of delayed tripping. TVA issued the ECN to revise the closing circuit of the diesel breakers to prohibit closing of this breaker unless load shedding has occurred. A tripping delay could be due to the single or combined effects of drift, aging or continued operation on an overloaded condition. During certain plant conditions such as the appearance of a blackout signal during a routine test of the diesel generator before it is synchronized with the bus or the appearance of a blackout signal following a LOCA, a race between tripping loads and closing diesel breaker will occur, and a failure of the load breakers to trip or a delay in tripping will result in closing of the diesel breaker on a partially loaded bus, contrary to the ECN's design objectives.

TVA's response was transmitted to NRC as an enclosure to a letter from me dated April 22, 1987. In this response, TVA stated that the Electrical Engineering Branch (EEB) concurred with the NRC team's findings and that Significant Condition Report (SCR) SQNEEB86206 had been issued to disposition the concern.

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8709100130 870901 PDR ADOCK 05000327 G PDR An Equal Opportunity Employer

< U.S. Nuclear Regulatory Commission SEP 011987 In the subsequent reviews and evaluations, EEB determined that SCR does not present a condition adverse to quality (CAQ). SCR states that the interlocks on the emergency feeder breaker to the 6.9-kV shutdown boards will allow the diesel generator to connect to its respective board without assurance that the loss of voltage initiated load-stripping relays operated for the condition where the diesel was running before the loss of voltage (i.e., with the diesel i

generator in the test mode).

The failure of a load breaker to operate is a single failure. Under the single failure criteria for SQN electrical power systems, Institute of Electrical and Electronics Engineers, Standard 279-1971, the failure of any one component or the entire power train will not prevent the redundant system from performing the required safety function. In addition, no two diesel generators run in the test mode simultaneously. This restriction ensures that a common-mode failure will not occur.

As documented in a TVA letter to NRC dated March 12, 1987, TVA is evaluating three-diesel generator loading sequences and implementing necessary modifications required to meet these sequences before restart. However, the sequence of safety injection with a delayed loss of offsite power is not among the cases being analyzed because it does not significantly contribute to the probability of core melt. SQN F$nal Safety Analysis Report is scheduled to be revised in the 1988 annual update to eliminate this sequence as a design basis event.

To ensure that the load breakers are maintained in an operable state, SQN Surveillance Instructions series 26 provide for operational tests of the load shedding features of the breakers every 18 months. SQN Maintenance Instruction MI-10.4 specifies that a complete disassembly and inspection occur every five years.

TVA's position is that the concern regarding diesel generator breaker trip for SQN has been adequately addressed, and for the reasons cited above, does not present a CAQ nor require further action by TVA.

Very truly yours, TENNESSEE VALLEY AUTHORITY

, Gridley, Director Nuclear Safet and Licensing cc: See page 3

U.S. Nuclear Regulatory Commission SEP 01B87 cc: Mr. G. G. Zech, Assistant Director-for Inspection Programs Office Of Special Projects U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, NH, Suite 2900 Atlanta, Georgia 30323 Mr. J. A. Zwolinski, Assistant Director for Projects Office of Special Projects U.S. Nuclear Regulatory Commission 4350 East West Highway EHH 322 Bethesda, Maryland 20814 Sequoyah Resident Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy, Tennessee 37379 l

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