Three Mile Island Nuclear Power Plant-near Harrisburg, Pennsylvania

May 27, 2007

On 28 March 1979 the unit 2 reactor at Three Mile Island nuclear power plant near Harrisburg Pennsylvania USA suffered a partial core meltdown. It started with a relatively minor failure of the secondary cooling system which led to the automatic opening of a safety valve in the primary cooling circuit for the reactor itself. That would not have been problematic, but the valve stayed stuck in the open state un-noticed by the reactor operators. Despite the nuclear fission process being shut down, the reactor remained very hot owing to the radioactive decay heat of the nuclear fuel. With insufficient coolant, parts of the core eventually dried out and finally melted. There was a build up of steam and some radioactive gas was vented to the outside. The radiation exposure to the public was extremely small and it is probable that no-one suffered adverse health effects as a result of the accident. It is certain that no-one was killed promptly. In many ways the design features of the reactor prevented what could have been a far more serious accident.