Partial meltdown at Three Mile Island

On 28 March 1979, a reactor at the Three Mile Island Nuclear Generating Station near Middletown, Pennsylvania, partially melted down. The accident remains the most serious to occur at a US nuclear power plant. At about 4:00am on 28 March, a valve in the cooling system of the Unit 2 reactor malfunctioned, causing coolant to stop circulating in the reactor core. Although the core shut down as it was supposed to, a series of equipment malfunctions and bad decisions by the plant’s workers exacerbated the problem, resulting in a loss of water coolant from the core and a partial meltdown. Despite the fact that no one was injured and the radiation exposure appeared to be minimal, the incident strengthened public opposition to nuclear power and crippled the industry for years. Today the Unit 1 reactor is still in use at Three Mile Island (right in above photo). The melted down reactor (left) has been shut down permanently.

For a more detailed account of what went wrong at Three Mile Island, see the news story in the June 1979 issue of Physics Today and the backgrounder from the US Nuclear Regulatory Commission. (Photo credit: Z22, CC BY-SA 3.0 )

Date in History: 28 March 1979