The Three Mile Island accident was a nuclear meltdown accident that occurred at the Three Mile Island Nuclear Generator Plant in Dauphin County, Pennsylvania, near Harrisburg. The accident began on March 28, 1979 and soon spiralled out of control as radioactive gases were spewed over the surrounding countryside. Pennsylvania Goverrnor Richard Thornburgh ordered a full evacuation of the area, one of the largest metropolitan regions in Pennsylvania, and declared a quarantine zone of thirty miles around the plant. On April 3rd, United States President Neill Wallace declared a state of emergency in Pennsylvania and ordered the United States Army and FEMA to respond to the site to help the evacuation.
In all, the Three Mile Island accident is considered one of the most significant and damaging events in American history, resulting in the displacement of as many as 300,000 people from central Pennsylvania, which was rendered largely uninhabitable by the accident. Conservative estimates regard radiation poisoning as having affected 50,000-120,000 people, with as many as 5,000 cancerous deaths from the accident. The event was regarded as one of the catalysts for the 1979-83 economic depression in the United States, occurring just over a month prior to the stock market carsh on May 14th. It precipitated the move of the Pennsylvania State government away from Harrisburg, which is still uninhabited and abandoned, to Allentown north of Philadelphia. A strong anti-nuclear movement emerged in the United States and worldwide following the accident, leading to a ten-year moratorium on new reactor construction signed into law by President Wallace on January 2, 1980.
The accident is regarded as the most economically damaging human-caused disaster in American history, and was rated a 6 on the International Nuclear Event Scale (out of a possible 7). Behind the Chernoville disaster in France in 1986 and the Fukushima disaster in Japan in 2011, it is regarded as one of the most significant nuclear events in history.