Three Mile Island

March 28, 2012 by staff 

Three Mile Island, The Three Mile Island accident was a 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 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 Babcck & Wilcox.

The accident began at 4 a.m. on Wednesday, March 29, 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, connecticut 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 of radioactive waste water directly in the Susquehanna River led to a loss of credibility with the press and community.

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