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Three Mile Island Nuclear Reactor

Three Mile Island Nuclear Reactor

The 1970s were a decade of great optimism about the role of nuclear power in meeting world and national demands for energy. Warnings about the declining reserves of coal , petroleum , and natural gas , along with concerns for the environmental hazards posed by power plants run on fossil fuels , fed the hope that nuclear power would soon have a growing role in energy production. Those expectations were suddenly and dramatically dashed on the morning of March 28, 1979.

On that date, an unlikely sequence of events resulted in a disastrous accident at the Three Mile Island (TMI) Nuclear Reactor at Harrisburg, Pennsylvania. As a result of the accident, radioactive water was released into the Susque-hanna River, radioactive steam escaped into the atmosphere , and a huge bubble of explosive hydrogen gas filled the reactor's cooling system. For a period of time, there existed a very real danger that the reactor core might melt.

News of the accident produced near panic among residents of the area. Initial responses by government and industry officials downplayed the seriousness of the accident and, in some cases, were misleading and self-serving.

The first official study of the TMI accident was carried out by the President's Commission on the Accident at Three Mile Island. Chaired by John G. Kemeny, then president of Dartmouth College, the Commission attempted to reconstruct the events that led to the accident. It found that the accident was initiated during a routine maintenance operation in which a water purifier used in the system was replaced. Apparently air was accidentally introduced into the system along with the purified fresh water.

Normally the presence of a foreign material in the system would be detected by safety devices in the reactor. However, a series of equipment malfunctions and operator errors negated the plant's monitoring system and eventually resulted in the accident.

At one point, for example, operators turned off emergency cooling pumps when faulty pressure gauges showed that the system was operating normally. Also, tags which hung on water pumps indicating that they were being repaired blocked indicator lights showing an emergency condition inside the reactor.

The Kemeny Commission placed blame for the TMI accident in a number of places. They criticized the nuclear industry and the Nuclear Regulatory Commission for being too complacent about reactor safety. They suggested that workers needed better training. The Commission also found that initial reactions to the accident by government and industry officials were inadequate.

No one was killed in the TMI accident, but a 1990 study showed that there were no statistically significant increases in cancers for residents downwind of the accident. A study completed in the mid-1990s, however, refutes those findings stating that "cancer incidence, specifically, lung cancer and leukemia , increased more following the TMI accident in areas estimated to have been in the pathway of radioactive plumes than in other areas." This study also indicates that the radiation levels may have been higher than originally reported, and that cancer rates (lung/leukemia) were 210 times higher than upwind rates. Cleanup operations at the plant took over six years to complete, and with a $1 billion price tag, cost more than the plant's original construction.

Probably the greatest effect of the TMI accident was the nation's loss of confidence in nuclear power as a source of energy. Since the accident, not one new nuclear power plant has been ordered, and existing plans 65 others were eventually canceled.

See also Carcinogen; Radiation exposure; Radioactive waste; Radioactivity

[David E. Newton ]



Dresser, P. D. Nuclear Power Plants Worldwide. Detroit, MI: Gale Research, 1993.

Ford, D. Three Mile Island: Thirty Minutes to Meltdown. New York: Penguin, 1983.

Gray, M., and I. Rosen. The Warming: Accident at Three Mile Island. New York: W.W. Norton, 1982.

Moss, T. H., and D. L. Sills, eds. The Three Mile Island Nuclear Accident: Lessons and Implications. New York: Academy of Sciences, 1981.


Booth, W. "Post-Mortem on Three Mile Island." Science (December 4, 1987): 13421345.

Stranahan, S. Q. "Three Mile Island: It's Worse Than You Think." Science Digest (June 1985): 5457+.


President's Commission on the Accident at Three Mile Island. Report of the President's Commission on the Accident at Three Mile Island. Washington, DC: U.S. Government Printing Office, 1979.

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