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March 28, 1979: Three Mile Island Unit 2, on line for just three months and operating at full power, automatically shuts down when a pump bringing cooling water to the reactor core stops functioning. Unknown to operators, valves for backup pumps have been closed. A pressure relief valve in the reactor opens as designed, but fails to close when pressure returns to normal. Water needed to cool the reactor core continues to gush through the open valve and eventually the core is uncovered.
The accident lasts about five hours, as operators repeatedly misread control signals and shut off new cooling water. By the third hour, parts of the core have begun to melt from their own heat.
March 29: Although the accident is over, the drama is not. Metropolitan Edison, TMI's operator, says that less than one percent of the fuel has been damaged in the accident. But "bubbles" of gas have formed in the reactor's coolant system and traces of radioactive iodine have been detected in nearby communities. Gov. Dick Thornburgh directs local residents to stay indoors, with windows closed.
March 30: On the advice of Nuclear Regulatory Commission Chairman Joseph Hendrie, Thornburgh advises evacuation for an estimated 3,500 pregnant women and children living within five miles of the plant. Nearly 200,000 people flee their homes, some for several weeks.
April 9: NRC officials declare that the "crisis is over." Thornburgh lifts order closing nearby schools and says it is safe for pregnant women and young children to return.
May: The reactor is declared in "cold shutdown."
Oct. 25: The NRC fines Metropolitan Edison, TMI's operator, $155,000.
Oct. 30: The President's Commission on the Accident at Three Mile Island ("the Kemeny Commission") releases its report, pointing to poor training as a key factor in the accident and recommending dozens of changes in the operation and regulation of nuclear plants.
July 1980: More than 40,000 curies of radioactive gases are vented from the reactor building in preparation for cleanup crews.
May 1982: The first television inspection of the reactor vessel shows that damage is worse than expected. Dozens of fuel rods have shattered and melted, forming a molten mass below a void in the center of the vessel.
Nov. 7, 1983: Metropolitan Edison is indicted for falsifying leak rate data at TMI-2 and for destroying documents before the accident.
Feb. 29, 1984: Metropolitan Edison pleads guilty to one count and no contest to six counts of the 11-count indictment.
May 29, 1985: NRC votes 4 to 1 to restart TMI-1, the reactor's undamaged sister, which was down for refueling at the time of the accident. The plant is restarted in October.
September 1985: Still probing the damage inside the reactor, workers find a mound of rubble at the bottom of the vessel. The discovery leads to new estimates of the accident's severity: Core temperatures reached 5,000 degrees Fahrenheit; as much as 50 percent of the fuel melted.
October 1985: The slow process of "defueling" the reactor begins. Workers start removing the fuel mass and packaging it for shipment to the federal nuclear reservation near Idaho Falls.
July 1986: General Public Utilities-Nuclear (Metropolitan Edison's new subsidiary for nuclear plants) proposes to dispose of 2.3 million gallons of mildly radioactive water from the accident by evaporating it. The request is pending.
December 1986: General Public Utilities-Nuclear announces it intends to put TMI-2 in monitored storage at the end of the cleanup