Smithsonian - National Museum of American History, Behring Center
Three Mile Island
Unit 2 nuclear power plant
First looks inside the reactor

Three Mile Island: The Inside Story

Five Score Years of Nuclear Power

1896: Radioactivity of uranium discovered

1902: Recognition that radioactive atoms contain huge amounts of energy, far greater than that extracted through chemical reactions

1905: Einstein’s theory of relativity provides basis (E=mc²) for inference that all atoms contain such vast quantities of energy

1912: Nuclear structure of atoms discovered and atomic nucleus recognized as site of radioactivity and atomic energy

1932: Neutron discovered; recognition that nuclei are composed of neutrons and protons

1939: Fissioning of uranium nuclei by neutrons discovered; release thereby of large quantity of atomic energy observed

1942-45: World War II Manhattan Project builds first operating nuclear reactors—not to produce energy but to convert uranium atoms into the plutonium used in first and third atomic bombs (Alamogordo, Nagasaki)

1954: Nautilus, first nuclear powered submarine, launched—the first major application of nuclear reactors to power generation

1957: First civilian nuclear reactor for power (electricity) generation goes into operation at Shippingport in western Pennsylvania

1968: Construction begins on TMI-1, first nuclear power plant on Three Mile Island in the Susquehanna River below Harrisburg, Pa.

1969: Construction begins on second nuclear power plant on Three Mile Island, TMI-2

1974: Three Mile Island Unit 1 (TMI-1) begins operation

1978: Construction permit issued to Shearon Harris in North Carolina, the last U.S. commercial nuclear power plant licensed to begin construction

1978 April: TMI-2 begins operation

1979 March/April: The accident at TMI-2

1980 July: First entry into TMI-2 containment building

1981: Cleanup of TMI-2 begins with extraction of radioactivity from the hundreds of thousands of gallons of retained cooling water

1982 July: First TV inspection of interior of TMI-2 reactor vessel

1983 August: ‘Sonar’ mapping of interior of TMI-2 reactor vessel

1985: Removal of wreckage from TMI-2 reactor vessel and shipment to Idaho begins

1985 October: TMI Unit 1 permitted to return to service after many delays and despite many protests, one of about 100 commercial nuclear power plants presently operating in the United States

1986 April: Chernobyl nuclear power plant disaster in Ukraine (Eastern Europe) releases massive quantities of radioactive material

1987: Recognition that about half the fuel in TMI-2 reactor had melted

1989: Recognition that molten fuel had poured into bottom of reactor vessel

1993: TMI-2 reactor placed in Post-Defueling Monitored Storage

1996: Last commercial nuclear power plant to be completed in the U.S. begins operation at Watts Bar, Tenn.

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