05000425/LER-2017-002

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LER-2017-002, Valid Undervottage Condition results in Automatic Actuation of a Safety System
Vogtle Electric Generating Plant
Event date:
Report date:
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
LER 17-002-00 for Vogtle, Unit 2, Regarding Valid Undervoltage Condition Results in Automatic Actuation of a Safety System
ML17321B052
Person / Time
Site: Vogtle Southern Nuclear icon.png
Issue date: 11/17/2017
From: Myers D J
Southern Nuclear Operating Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
NL-17-1750
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.

Because this event resulted in the automatic actuation of a system listed in 10 CFR 50.73(a)(2)(iv)(B), this event is reportable under 10 CFR 50.73(a)(2)(iv)(A)

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.

E. Previous Similar Events:

None