Three Mile Island Accident

The Three Mile Island accident was a partial nuclear meltdown which occurred at the Three Mile Island power plant in Dauphin County, Pennsylvania, United States on March 28, 1979. It was the worst accident in U.S. commercial nuclear power plant history, and resulted in the release of small amounts of radioactive gases and radioactive iodine into the environment.

The power plant was named after the island on which it was situated, and was owned and operated by General Public Utilities and Metropolitan Edison (Met Ed). The reactor involved in the accident, Unit 2, was a pressurized water reactor manufactured by Babcock & Wilcox.

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 nuclear 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 human factors, such as human-computer interaction design oversights relating to ambiguous control room indicators in the power plant's user interface. In particular, a hidden indicator light led to an operator manually overriding the automatic emergency cooling system of the reactor because the operator mistakenly believed that there was too much coolant water present in the reactor and causing the steam pressure release. The scope and complexity of the accident became clear over the course of five days, as employees of Met Ed, 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. The NRC's authorization of the release of 40,000 gallons (about 150,000 liters) of radioactive waste water directly in the Susquehanna River led to a loss of credibility with the press and community.

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 released from the accident had no perceptible effect on cancer incidence in residents near the plant, though these findings are contested by one team of researchers. Cleanup started in August 1979 and officially ended in December 1993, with a total cleanup cost of about $1 billion. The incident was rated a five on the seven-point International Nuclear Event Scale: Accident With Wider Consequences.

Communications from officials during the initial phases of the accident were confusing. There was an evacuation of 140,000 pregnant women and pre-school age children from the area. The accident crystallized anti-nuclear safety concerns among activists and the general public, resulted in new regulations for the nuclear industry, and has been cited as a contributor to the decline of new reactor construction that was already underway in the 1970s. Public reaction to the event was probably influenced by The China Syndrome, a movie which had recently been released and which depicts an accident at a nuclear reactor.