|Vogtle Electric Generating Plant|
|Reporting criterion:||10 CFR 50.73(a)(2)(iv)(B), System Actuation|
10 CFR 50.73(a)(2)(iv)(A), System Actuation
|4252017002R00 - NRC Website|
|Person / Time|
|From:||Myers D J|
Southern Nuclear Operating Co
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17321B052 (4)|
A Event Description On September 26, 2017, during the performance of Engineered Safety Features Actuation System (ESFAS) testing, operators simulated an under voltage (UV) condition and Safety Injection (SI) signal in accordance with their procedure. The Emergency Diesel Generator (EDG) started and was carrying the safety related electrical bus as expected. As operations continued in the procedure, the UV condition was cleared once the diesel began carrying the electrical bus and the SI signal was manually reset per procedure from the control room. By design, the diesel generator will continue to run in emergency mode even after the UV or SI signals are cleared until the diesel is reset locally from the Loss of Site Power (LOSP)/SI signal.
When the operator reset the diesel locally, the engine control system sent a shutdown signal to the engine which caused the output breaker to open and deenergized the bus. Because the EDG was the only source carrying the emergency bus, this caused an undervoltage condition on the bus and the EDG received another emergency start signal.
The EDG, which had not coasted to a stop, but did indicate that the speed dropped below 440rpm, subsequently restarted, the output breaker closed, and the loads were sequenced back on to the bus.
The unit operated in Mode 6 at 0 percent power during this event.
B. Cause of Event
The cause of the diesel trip and subsequent loss of the safety related bus was an air leak on the shutdown logic board within the pneumatic control system. This leak was large enough to degrade the air pressure such that when the emergency signal that was blocking non-emergency trips was removed, the diesel controls processed an engine shutdown signal and opened the output breaker.
C. Safety Assessment When the EDG started and loaded the bus. it was already performing it's specified safety function. As such, the safety significance of this event is very low.
D. Corrective Actions
The issue was corrected by replacing the leaking part on the logic board.
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