☛ National Archives: President Jimmy Carter leaving [Three Mile Island] for Middletown, Pennsylvania., 04/01/1979. Identifier:540021. Public domain.
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On March 28, 1979, the Three Mile Island Unit 2 (TMI-2) reactor suffered a partial meltdown. The incident was rated at Level 5 on the International Nuclear and Radiological Event Scale. Although the exact health impact is hard to determine (and still subject to debate), it is recognized that radioactive gases were release in the environment. The U.S. NRC states the releases “had no detectable health effects on plant workers or the public”). Nonetheless, it was qualified as “the worst accident in the history of commercial nuclear power generation” in the official Report Of The President’s Commission On The Accident at Three Mile Island (p. 1; PDF). The events also had an important impact both on the nuclear industry and the public opinion. It fuelled anti-nuclear fear and opposition. On April 9, 1979, the TIME ran a cover with the words “Nuclear Nightmare”. (The film The China Syndrome opened just a few days prior to the incident: see IMDb).
In comparison, both the Chernobyl disaster (April 26, 1986) and the Fukushima Daiichi nuclear disaster (March 11, 2011) (previously here) are classified as Level 7 events on the INRE scale. At the time of writing, they are the only two events to be classified at that level.
There are plenty of resources about the Three Mile Island accident online. In 1999, PBS produced a 50-min documentary for its American Experience television program explaining the events: “Meltdown at Three Mile Island”. The video can be watched in full on YouTube or below:
Color photos from the President’s Commission on the Accident at Three Mile Island are available at the National Archives. The Dickinson College’s Three Mile Island web site has plenty of original documents (audio, video, PDF: see especially their Resource Center).