Operators manually shut down a reactor at the Limerick Nuclear Power Plant in Montgomery County Wednesday morning after an electrical problem.
The electrical disturbance took place on the non-nuclear side of the plant, Exelon Corporation officials said.
The disturbance caused a loss of power to generator cooling equipment and the Unit 1 reactor will remain offline until repairs, inspections and testing can be completed.
Limerick Unit 2 continues to operate at full power.
In keeping with NRC procedures, company officials declared an "unusual event" at 8:39 a.m. It was terminated at 9:46 a.m.
An "unusual event" is the lowest of four NRC emergency classifications.
The commission says it is monitoring the situation, and is unaware of any complications. It's not immediately clear when the reactor will be back in operation.