Plant officials and government regulators said the shutdown at the plant, where a partial meltdown of a reactor in 1979 is considered the worst commercial nuclear power plant accident in U.S history, posed no threat to public health or safety.
The shutdown occurred shortly after 2:15 p.m. when the failure of a coolant pump tripped the computerized system that shuts downs the reactor in the event of any safety-related problems.
Once the reactor has cooled down enough, plant workers will be able to access the containment building and troubleshoot the problem, Nuclear Regulatory Commission spokesman Neil Sheehan said.
The plant is named after the island where it's located in the Susquehanna River, south of Harrisburg. It automatically shut down on Aug. 22 while operators were manually shutting it down for repairs, said Ralph DeSantis, a spokesman for operator Exelon Generation.
DeSantis said the release of steam Thursday made a loud noise audible to nearby residents but no detectable levels of radiation were released.
Three Mile Island is located about 12 miles south of Harrisburg, Pa. The plant can generate 852 megawatts of carbon free power - enough electricity for about 800,000 homes.