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Unscheduled Shutdown at Limerick Nuclear Plant

NRC: Electrical fault in transformer was impetus for manual shutdown; plant operator says 'no threat' to public safety

Last updated 9:48 p.m. Wednesday

The Unit 1 nuclear reactor at Exelon's Limerick Generating Station was shut down at 8:15 a.m. Wednesday following an electrical fault in a transformer, the Nuclear Regulatory Commission said in a statement sent to area media outlets.

The reactor was manually shut down from the power station's control room following the fault, according to the NRC.

"The NRC is closely monitoring the shutdown, with our Resident Inspectors assigned to the plant gathering information from the control room and sharing it with NRC staff. There do not appear to be any complications at this point. We will want to know more about Exelon’s root cause evaluation of what caused the event and about any and all corrective action," NRC spokesperson Neil Sheehan said.

The NRC has two inspectors permanently assigned to the facility.

Sheehan said Exelon had declared an "Unusual Event," which is the lowest of four levels of emergency classification used by the NRC.

An "electrical disturbance caused a loss of power to generator cooling equipment" at the plant, Exelon Nuclear spokesperson Dana Melia said in a statement.

"There is no threat to the health and safety of the public associated with this event," Melia said.

The company said the Unit 1 plant will remain offline until repairs, testing, and inspections are complete. The generating station's other power plant, Unit 2, continued to operate at full power.

Melia denied reports that there had been an explosion at the plant.

"It wasn't an explosion," Melia said, instead referring to the fault as an "electrical flash-out," comparable to a circuit breaker being tripped.

Sheehan said the failed transformer was in the building that also houses the plant's control room.

"The 13-kv (kilovolt) transformer that failed was located in the plant’s control structure, not the turbine building. The control structure in where the control room is housed but it usually also is location of electrical switchgear rooms, battery rooms and other equipment rooms. There was no impact on the control room from this event," Sheehan said via e-mail.

"There can be a loud noise when an electrical transformer experiences a failure. That may be what [the reports] are referring to," Sheehan said.

The plant, which is in the process of applying for license extensions that would keep it operating until 2049, experienced three unscheduled shutdowns between February and June of 2011.

Aric July 19, 2012 at 03:30 AM
Don't get me wrong, I agree this is not really anything to worry about re: Limerick. Middle (end? I hope...) of a heat wave, not surprising a transformer let go. Yet people are overreacting to the thought that a single person might be hiding in their back yard, when there's a potential for much worse up the river that never makes the news.
Tom Bartman July 19, 2012 at 03:09 PM
I agree. What happened seems to be what is known as an arc-flash which occurs when transformer insulation fails. It is nothing new. The downstream relays see the fault and trip offline. It worked as planned. There is also misconception about how the power grid works. Our power comes from the grid of many many generation plants. When one goes offline, electricity still finds where load is needed.
Tom Bartman July 19, 2012 at 10:09 PM
The reactor will be offline for 'weeks to months'
Stephen Eickhoff July 20, 2012 at 07:30 PM
Self-cooling houses? The closest we had to those in PA were the stone colonials the 17th and 18th century German and Dutch settlers built... and those required that you build them on top of a spring. Most people wouldn't really like having running water in their basement. Unless you want to live in a cave, a "self-cooling" house would still require energy to run a geothermally-coupled heat pump.
Aric July 20, 2012 at 08:18 PM
Perhaps I should have been clearer, Stephen... What I meant by "self cooling" is designed-in architectural features to deal with the heat passively, like high ceilings, transoms over the windows to allow air flow, chimneys to draw the heat up and out while replacing the air with cooler air from the north side of the building, ceiling fans, etc. Our 1905 Dutch Colonial does ok in this regard, and we end up only running the A/C on the hottest days (maybe 2 weeks total). The 1859 Colonial I'm restoring (with its 19" thick stone walls) can pretty much go all summer with the inside never going above 80 if you manage the windows properly at night. Chances of doing that with new construction? Basically nil.

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