Three Mile Island is the site of a nuclear power plant in south central Pennsylvania. In March 1979, a series of mechanical and human errors at the plant caused the worst commercial nuclear accident in U.S. history, resulting in a partial meltdown that released dangerous radioactive gasses into the atmosphere. Three Mile Island stoked public fears about nuclear power—since the accident, construction has not begun at any new nuclear power plant sites in the United States.

The China Syndrome

Construction of the Three Mile Island nuclear plant began in 1968, in Londonderry Township, Pennsylvania, on a small island in the Susquehanna River just south of the state capital in Harrisburg. Construction ended in 1978 when the second of two nuclear reactors at the site came online to produce electricity.

A thriller film, called The China Syndrome, hit theaters in March of 1979. The film, starring Jane Fonda, Jack Lemmon and Michael Douglas, dealt with the aftermath of a fictional nuclear meltdown at a reactor outside of Los Angeles.

The nuclear industry, at the time, dismissed The China Syndrome plot as far-fetched. Many experts said nuclear meltdowns—where a nuclear reactor overheats causing radioactive fuel to melt—were almost impossible, calling them “black swan” events.

Three Mile Island Accident

In the early morning hours of March 28, 1979, a mechanical or electric failure set off an unlikely series of events that led to a partial meltdown at the Unit 2 reactor. Water pumps that helped to cool the radioactive fuel in the reactor core malfunctioned.

Plant staff didn’t realize the reactor was experiencing a loss of coolant and took a series of actions that made the problem worse. These further starved the reactor core of water flow and caused it to overheat.

The nuclear fuel began to melt through its metal container—about half the reactor core melted. Trace amounts of radioactive gasses escaped into the surrounding community as a geyser of steam erupted from the top of the plant.

The melting fuel created a large hydrogen bubble inside the unit that officials worried might cause an explosion, releasing even larger amounts of radioactive material.

Pennsylvania governor Dick Thornburgh advised pregnant women and pre-school-age children within a five-mile radius of the plant to evacuate the area. The crisis ended three days later when experts determined the hydrogen bubble could not burn or explode.

TMI Impact

After the Three Mile Island (or TMI) accident, public support for nuclear energy fell from an all-time high of 69 percent in 1977 to 46 percent in 1979.

An estimated two million people were exposed to small amounts of radiation as a result of the TMI accident. There are no known health impacts. Several government agencies and independent groups conducted studies, but no adverse effects could be found to correlate to these exposures.

Careful analysis of the accident led to sweeping changes in the way nuclear plants are regulated in the United States. Federal requirements for safety controls and emergency response planning became more stringent, and officials imposed a temporary moratorium on the licensing of all new reactors.

Required design changes after the Three Mile Island accident resulted in higher costs and longer construction times for new nuclear plants. As a result, construction of nuclear reactors steeply declined. No entirely new nuclear plants started after 1979 have been completed in the United States.

Three Mile Island Cleanup

The cleanup effort lasted 14 years and cost an estimated 1 billion dollars. The damaged reactor was permanently closed and entombed in concrete after the accident.

Radioactive fuel and water were removed, and workers eventually shipped 15 tons of radioactive waste to a nuclear waste storage facility in Idaho.

Anti-Nuclear Movement

The Three Mile Island incident helped to galvanize the anti-nuclear movement in the United States. The anti-nuclear movement emerged as a social movement against the global nuclear arms race in the early 1960s at the height of the Cold War.

High profile protests in response to the events at Three Mile Island took place around the country, including one in New York City in 1979 involving 200,000 people.

Three Mile Island Today

Three Mile Island Nuclear Generating Station today generates power from its Unit 1 reactor. The Unit 1 reactor is owned and operated by Exelon Corporation.

Exelon announced in 2017 that it would close the plant in 2019. Dismantling the remaining reactor could take up to 10 years.


Backgrounder on the Three Mile Island Accident. U.S. NRC.
14-Year Cleanup at Three Mile Island Concludes. The New York Times.
A brief history of the Three Mile Island nuclear plant known for 1979 reactor accident. ABC News.