A coolant leak at the Three Mile Island's Unit 2 nuclear reactor outside Harrisburg, Pennsylvania leads to the core overheating and a partial meltdown.

Three Mile Island Nuclear Generating Station, is a shut-down nuclear power plant on Three Mile Island in Pennsylvania, US, on the Susquehanna River just south of Harrisburg. It has two separate units, Unit 1 (TMI-1) and Unit 2 (TMI-2).

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. Fissile nuclei absorb single neutrons and split, releasing energy and multiple neutrons, which can induce further fission. Reactors stabilize this, regulating neutron absorbers and moderators in the core. Fuel efficiency is exceptionally high; low-enriched uranium is 120,000 times more energy dense than coal.

Harrisburg is the capital city of the U.S. commonwealth of Pennsylvania. It is the ninth-most populous city in the state, with a population of 50,099 at the 2020 census, while the Harrisburg–Carlisle metropolitan statistical area has an estimated 615,000 residents and is the fourth-most populous metropolitan area in Pennsylvania. Harrisburg is situated on the east bank of the Susquehanna River 83 miles (134 km) southwest of Allentown and 107 miles (172 km) northwest of Philadelphia. It is the county seat of Dauphin County.

The Three Mile Island accident was a partial nuclear meltdown of the Unit 2 reactor (TMI-2) of the Three Mile Island Nuclear Generating Station, located on the Susquehanna River in Londonderry Township, Dauphin County near Harrisburg, Pennsylvania. The reactor accident began at 4:00 a.m. on March 28, 1979, and released radioactive gases and radioactive iodine into the environment. It is the worst accident in U.S. commercial nuclear power plant history. On the seven-point logarithmic International Nuclear Event Scale, the TMI-2 reactor accident is rated Level 5, an "Accident with Wider Consequences".