The commission sent investigators to the central Pennsylvania plant after a small amount of radiation was detected.
About 150 employees were sent home Saturday afternoon, but officials say there is no public health risk.
Exelon Nuclear spokeswoman Beth Archer says the radiation was quickly contained.
The unit has been shut for refueling and maintenance since Oct. 26. Workers are being tested for radiation exposure.
A partial meltdown occurred in Three Mile Island's Unit 2 reactor in 1979.