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Nuclear Event – Unanalyzed Condition that significantly degrades plant safety, Equipment Failure: Watts Bar Nuclear Plant, Tennessee


North America – USA | State of Tennessee, Rhea County, Spring City, Watts Bar Nuclear Plant
Location: 35°36’10.0″N 84°47’22.0″W
Site Operational Age: ~31 years
Event: UNANALYZED CONDITION – Hazard Mitigation

Nuclear Event in USA on Wednesday, 08 June, 2016 at 15:26 [EDT]


“At 1526 Eastern Daylight Time on 6/8/2016, a determination was made involving the potential impact of a tornado on the Emergency Diesel Generators (EDGs). The EDGs are required to be operable to provide power to ensure that acceptable fuel design limits, reactor coolant system pressure boundary limits, and containment integrity are not exceeded during abnormal transients. Further, the EDGs are designed with a crankcase pressure trip (setpoint = 1 inch water), which is bypassed during an emergency start. Engineering has determined that a tornado could potentially cause actuation of the crankcase pressure trip due to a low barometric condition. If an emergency start signal has NOT previously occurred, then during a tornado, actuation of the crankcase pressure trip would energize the shutdown relay causing an EDG lockout condition. The EDG lockout condition prevents subsequent EDG starts (normal or emergency) until operators manually reset the lockout condition locally at the EDG. This condition could potentially affect all four EDGs simultaneously.

“The EDGs are operable but degraded. All EDGs have successfully passed their required surveillances within the appropriate frequency. No severe weather warnings or watches are forecast in the local areas, which could challenge the crankcase pressure trip.

“This condition places both units in an unanalyzed condition that potentially significantly degrades plant safety, 10 CFR 50.72 (b)(3)(ii)(B).

“A compensatory measure has been established, that upon notification of a Tornado Warning, the EDGs would be ’emergency started’ and run during the time the Tornado Warning was in effect. This action bypasses the crankcase pressure trip function and allows the EDGs to perform their required safety function.

“The NRC Senior Resident Inspector has been notified.”

Source: NRC  Event Number: 51995


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