One of the reactor's at Exelon Nuclear's Limerick Generating Stations shut down unexpectedly late Tuesday night.
The shutdown was due to a problem with the system controlling the flow of steam to the turbines, WFMZ reported.
Alarms were sounded around 11:20 p.m. Tuesday, WMFZ said, and officials began lower reactor power. It later was shut down fully, for the first time since July 2012.
In an email to Patch, Diane Screnci, a senior public affairs officer for the Nuclear Regulatory Commission, wrote: "The plant responded as designed. It is in a stable and safe condition. There was no impact on the public."
"The NRC resident inspectors (who are assigned to the plant on a daily basis to carry out the agency’s inspection and oversight program) responded to the site to follow up on the company’s actions," Screnci continued. "They’ve been overseeing Exelon’s actions to understand the cause of the shutdown."