On this day March 28, 1979

A partial core meltdown of the Three Mile Island Nuclear Generating Station near Harrisburg, Pennsylvania, USA resulted in the release of an estimated 43,000 curies (1.59 PBq) of radioactive krypton to the environment.

President Jimmy Carter at Three Mile Island

President Jimmy Carter at Three Mile Island

By comparison, up to 600 times more iodine-131 (between 7,000 and 12,000 curies) were intentionally released into the atmosphere by the Hanford Site in the state of Washington in 1949 as part of a military experiment to test air force monitoring equipment.

The accident of 1979 was the most significant accident in the history of the American commercial nuclear power generating industry.

The accident began at 4 a.m on Wednesday, March 28, 1979, with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve (PORV) in the primary system, which allowed large amounts of reactor coolant to escape.

The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss of coolant accident due to inadequate training and ambiguous control room indicators.

The scope and complexity of the accident became clear over the course of five days, as employees of Metropolitan Edison (Met Ed, the utility operating the plant), Pennsylvania state officials, and members of the U.S. Nuclear Regulatory Commission (NRC) tried to understand the problem, communicate the situation to the press and local community, decide whether the accident required an emergency evacuation, and ultimately end the crisis.

In the end, the reactor was brought under control, although full details of the accident were not discovered until much later, following extensive investigations by both a presidential commission and the NRC.

The “Kemeny Commission Report” concluded that “there will either be no case of cancer or the number of cases will be so small that it will never be possible to detect them. The same conclusion applies to the other possible health effects.”

Several epidemiological studies in the years since the accident have supported the conclusion that radiation releases from the accident had no perceptible effect on cancer incidence in residents near the plant, though these findings have been contested by one team of researchers.

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