Dominion Virginia Power has determined that a five-by-four inch electronic circuit card was the culprit of a recent automatic shutdown of North Anna Power Station’s Unit 2.
The card failed during the early morning hours of Oct. 24 and closed the four governor valves that control the flow of steam to the generator’s turbine system, knocking the unit offline, according to Dominion spokesman Richard Zuercher.
Zuercher said that the incident occurred on the secondary or non-nuclear side of the plant and that all systems functioned properly during the shutdown. He added that Unit 1 was not affected and remains at full power.
Dominion replaced the faulty circuit card and restarted Unit 2 the following day.
Zuercher said that operators continue to monitor temperatures and pressures in the turbine system. He said the reactor, which is operating at 99 percent power, should reach full power soon.
The trip came about a week after operators manually shut down Unit 2 to replace a failing reactor coolant pump seal. During the manual shutdown, technicians replaced a second seal as preventative maintenance. Unit 2 was powered down for 10 days for the repairs.
Unit 2 has had eight automatic shutdowns since 2007, several of which were caused by lighting strikes, according to Zuercher.
He said the unit’s grounding grid could not properly handle the strikes, but added the problem has been resolved.
Zuercher said that prior to the incident with the faulty electronic card and excluding the manual shutdown, Unit 2 had operated for 427 straight days without incident since the August 2011 earthquake, which tripped both of North Anna’s reactors offline.
North Anna’s Unit 2 was refueled in 2011 and is scheduled for a refueling in 2013.