Operators shut down one of the reactors at a Montgomery County nuclear plant due to a problem with the system that controls the flow of steam to the turbine.
Officials said the Unit 1 control room at Exelon Nuclear's Limerick Generating Station began lowering reactor power following alarms late Tuesday.
They later ``manually scrammed'' the reactor, inserting all control rods to shut it down, the first time that has happened since July 2012 at the plant about 20 miles northwest of Philadelphia.
Plant spokeswoman Dana Melia said there is no threat to the public and service to customers will be unaffected.
She said the valves will be repaired and tested before the unit is put back into service.