On March 28, 1979, a partial meltdown occurred of a nuclear reactor on Three Mile Island in Dauphin County, Pennsylvania. Due to a combination of both mechanical and human failures, nuclear coolant was allowed to escape into the environment sparking the accident. It was the worst accident in U.S. commercial nuclear power plant history.

Nobody died directly as a result of the accident; however the event crystallized the public’s perception of nuclear power at the time. Safety concerns among activists and the general public resulted in new regulations for the nuclear industry, and the accident has been cited as a contributor to the decline of a new reactor construction program that was already underway in the 1970s.

The cleanup of Three Mile Island started in August 1979 and officially ended in December 1993, with a total cost of about $1 billion.