Exelon said in a statement that Unit 2 shut down early Sunday morning when a turbine control valve tripped after scheduled testing and maintenance on an electrical system in a non-nuclear section of the power plant.
Exelon said there were no injuries, no radiation was released and there was no danger to the public. Unit 1 is still operational.
Nuclear Regulatory Commission spokesman Neil Sheehan said the reactor remained off-line as of late Tuesday afternoon. Exelon is investigating to determine the cause of the problem.
The plant is about 20 miles northwest of Philadelphia.