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The Unit 1 nuclear reactor at Exelon’s Limerick Generating Station shut down Wednesday morning following an electrical fault in a transformer, Nuclear Regulatory Commission (NRC) officials said.

Operators manually shut down the reactor from the power station’s control room following the fault, according to the NRC.

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“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,” said NRC spokesperson Neil Sheehan.

The NRC has two inspectors permanently assigned to the facility.

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Sheehan said Exelon 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, said Exelon Nuclear spokesperson Dana Melia.

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

The company said the Limerick, PA-based 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 trip in a circuit breaker.

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 the location of electrical switchgear rooms, battery rooms and other equipment rooms. There was no impact on the control room from this event,” Sheehan said.

“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.

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