Three Mile Island returned to service this week.
Operators connected the plant's turbine generator to the regional power grid early Tuesday morning, a spokesman said.
The unit had been taken offline Aug. 22 to repair a heater element on the plant's pressurizer tank, according to spokesman Ralph DeSantis.
Electric customers were not affected, officials said.
"Our personnel performed the work safely and efficiently," said Rick Libra, TMI site vice president.