On March 28, 1979, and for several days thereafter -- as a result of technical malfunctions and human error -- Three Mile Island's Unit 2 Nuclear Generating Station was the scene of the nation's worst commercial nuclear accident. Radiation was released, a part of the nuclear core was damaged, and thousands of residents evacuated the area. Events here would cause basic changes throughout the world's nuclear power industry.

Prior to March 28, 1979, few people living outside of Central Pennsylvania had ever heard of Three Mile Island. After that date, however, just about anyone with access to a television, radio, or newspaper could take a map of Pennsylvania and point to a sandbar in the middle of the Susquehanna River and know that that was the site of the nation's worst commercial nuclear reactor accident.

The accident did not get as far as the ultimate nightmare of meltdown, in which a liquefied core of uranium drops through its shielding into the ground and spreads large amounts of radiation haphazardly across the countryside. And, although no one died as a direct result of the accident, it led to a major shake-up at the Nuclear Regulatory Commission and a host of better safety measures at nuclear power plants across the country. It also led communities near those plants to draw up detailed plans to deal with future emergencies.

A Timeline of the Accident

4:00 am Maintenance workers inadvertently shut off the water supply carrying heat from the reactor to the turbine that generates electricity. With no water being added, steam pressure drops and safety systems automatically shut down turbine and the generator it powers.
4:00:02 am Steam flow reduction raises temperature of reactor coolant, causing water to expand.
4:00:08 am Pressure continues to rise inside pressurizer. Emergency systems shut down reactor.
4:00:09 am Nuclear reaction stops, but decay heat causes water to continue to heat up inside the reactor. Because pumps that normally supply the steam generators shut down, three emergency pumps automatically start. Compressed steam in the reactor causes the pilot-operated relief valve to open. The valve, which should have closed when pressure decreased, remains open for 2 hours 22 minutes. Meanwhile, steam and water flow out of the reactor coolant system through a drain pipe to a relief tank.
~4:02:00 am Operators in the control room fail to recognize that the emergency relief valve was stuck in the open position. Their instruments seem to indicate that water is completely flooding the reactor, so they override the automatic system that had started the high-pressure water pumps, depriving the core of cooling water. Although the reactor has been shut down, atomic reactions inside it continue to produce heat. The reactor's temperature soars above 2000° F.
4:05:30 am Steam inside the reactor displaces water and eventually leaves core uncovered.

7:30 am Metropolitan Edison issues a "general emergency" at the plant.

8:15 am The Nuclear Regulatory Commission dispatches experts to the scene. Their monitors pick up signs of radioactivity in the air around the site, caused by vented steam.

by 8:00 pm Plant operators have managed to bring the core's temperature down to about 280° F, but the core has already been damaged, water has ruptured an overflow system, and radioactive steam has escaped into the atmosphere.

Pennsylvania Highways. www.pahighways.com/features/threemileisland.html
Three Mile Island Alert. www.tmia.com
"Three Mile Island: The Inside Story." National Museum of American History. americanhistory.si.edu/tmi/index.htm

Nuclear Regulatory Commission

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This page was last updated on 11/20/2012.