Three Mile Island

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The most serious nuclear reactor accident to date in the United States occurred at 4 a.m. on March 28, 1979, at the Three Mile Island nuclear power plant outside Middletown, Pennsylvania. Operator errors in dealing with a pump that had shut down caused the Unit 2 pressurized-water reactor to lose coolant and overheat. The temperature of the reactor core then rose to the point at which some of the zirconium-alloy fuel cladding failed, fuel itself partially melted, and cladding reacted with steam to produce bubbles of vapor and hydrogen, which then escaped into the reactor building, along with fission products from the reactor core. As a result of the failure to close a backup valve that could be operated manually, coolant was not restored to the reactor core until more than six hours after the accident, by which time enough hydrogen had accumulated in the building to pose the treat of a low-level explosion. The building had been designed to seal automatically in the event of a pressure rise, but no rise occurred, and four hours were allowed to elapse before the building was sealed, during which time radioactive gases escaped into the atmosphere.

Within three hours after the first sign of trouble, elevated radiation levels were detected by monitors in the reactor auxiliary building. A site emergency was declared, and officials enlisted the aid of local, state, and federal emergency personnel. The presence of a large hydrogen bubble in the reactor vessel prompted widespread fear that the reactor might explode, a concern that experts failed to allay although they knew it to be a misapprehension. Adding to the fear, dosimeter readings made in a helicopter three hundred feet above the auxiliary building's ventilation stack were misinterpreted by officials to signify elevated ground levels of radiation, prompting the governor of Pennsylvania to recommend the evacuation of all pregnant women and preschool children residing within five miles of the plant, who then complied.

Although large amounts of radiation were released, the resulting exposure of the public was relatively slight, resulting mainly from xenon-133 that was present in the gaseous plume. The largest dose of radiation any member of the public may have received is estimated to have been smaller than his or her annual dose from natural background irradiation, and the average dose to those living within fifty miles of the reactor is estimated to have been 40 to 50 times smaller than that. Because of the small magnitude of the doses that were received, no demonstrable injuries from the radiation were expected, nor have any actually been observed. Nevertheless, the legacy of fear and resentment left by the accident has adversely affected the well-being of those living nearby, and it has heightened negative attitudes toward nuclear energy.

Arthur C. Upton

(see also: Energy; Environmental Determinants of Health; Nuclear Power )


Baum, A.; Gatchel, R.; and Schaeffer, M. (1983). "Emotional, Behavioral, and Psychological Effects of Chronic Stress at Three Mile Island." Journal of Consulting and Clinical Psychology 51:565572.

Kemeny, J.G. (1979). The President's Commission on the Accident at Three Mile Island. New York: Pergamon Press.

Moss, T. H., and Sills, D. L., eds. (1981). "The Three Mile Island Nuclear Accident: Lessons and Implications." In Annals of the New York Academy of Sciences 365. New York: New York Academy of Sciences.

Three Mile Island

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THREE MILE ISLAND, the site of the worst civilian nuclear power program accident in the United States, is located in the Susquehanna River near Harrisburg, Pennsylvania. In the early 1970s, Metropolitan Edison built two reactors on Three Mile Island for commercial energy production. On 28 March 1979, a faulty valve allowed water coolant to escape from Metropolitan Edison's second reactor, Unit 2, during an unplanned shutdown. A cascade of human errors and technological mishaps resulted in an overheated reactor core with temperatures as high as 4,300 degrees and the accidental release of radiation into the atmosphere. Plant operators struggled to resolve the situation. Press reporters highlighted the confusion surrounding the accident, while Governor Richard L. Thornburgh of Pennsylvania and President Jimmy Carter visited the stricken plant, urging the nation to remain calm. On 30 March, state officials evacuated pregnant women and preschool children from the immediate area as a safety measure. On 2 April, temperatures decreased inside the Unit 2 reactor, and government officials declared the crisis over on 9 April.

A commission authorized by President Carter investigated the calamity. Government analysts calculated that, at the height of the crisis, Unit 2 was within approximately one hour of a meltdown and a significant breach of containment. The lessons learned at Three Mile Island led to improved safety protocols and equipment overhauls at commercial reactors across the country. Three Mile Island also contributed to rising public anxiety over the safety of nuclear energy, anxieties fueled by the coincidental release of The China Syndrome, a fictional movie about the cover-up of a nuclear plant accident, just twelve days before the disaster at Three Mile Island. The Three Mile Island accident became a rallying cry for grassroots antinuclear activists. Wary of sizable cost overruns and public resistance, electrical utilities shied from constructing new nuclear plants in the years that followed. Over an eleven-year period, the cleanup of Three Mile Island's severely damaged reactor cost in excess of $1 billion.


Cantelon, Philip L., and Robert C. Williams. Crisis Contained: The Department of Energy at Three Mile Island. Carbondale: Southern Illinois University Press, 1982.

President's Commission on the Accident at Three Mile Island. Report of the President's Commission on the Accident at Three Mile Island: The Need for Change: The Legacy of TMI. Washington, D.C.: Government Printing Office, 1979.

Stephens, Mark. Three Mile Island. New York: Random House, 1980.

Robert M.Guth


See alsoElectric Power and Light Industry ; Nuclear Power ; Nuclear Regulatory Commission .

Three Mile Island

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Three Mile Island

Three Mile Island, situated near Harrisburg, Pennsylvania , is home to a nuclear power plant. On Wednesday, March 28, 1979, a cooling malfunction caused part of the core to melt in one of two reactors in the plant. The reactor was destroyed, and within a couple days, radioactive gas was released.

Prompted international headlines

What followed was five days of crisis management for the United States government and Three Mile Island officials. Even before breakfast on March 28, a state of emergency was declared. By that evening, however, the condition of the reactor seemed to be improving. It was not until evening of March 29 that experts realized how serious the damage was to the reactor. It was at that point officials were forced to acknowledge the possibility of a radioactive leak.

By Friday, mass hysteria had set in. Experts in Washington, D.C. , had, until that time, underestimated the damage caused by the meltdown. Unofficial reports of high levels of radioactivity were released, and the governor of Pennsylvania recommended that all pregnant women and children evacuate the area. The public grew concerned about radiation exposure, but the fear was based on rumor. The levels of radioactivity proved to be only minimal.

Fears of nuclear disaster

For various technical and mechanical reasons, no one was quite sure if the damaged reactor would explode. This being the main concern, inspectors suggested to the media that evacuations of people within a tenor twenty-mile radius from Three Mile Island might be necessary. By Saturday, March 31, area residents were in a panicked state.

With the assurance that a possible explosion would be days away, President Jimmy Carter (1924–; served 1977–81) and his wife toured the nuclear plant on Sunday, April 1. Later that day, experts declared that an explosion would not occur. At the same time, they declared the crisis to be over. But the local population continued to live in fear and doubt.

Long-term consequences

When all investigations had been completed and reports filed, the cause of the meltdown was judged to have been caused by two factors: human error and questionable construction of the reactor. The accident at Three Mile Island had a lasting impact. Nuclear power fell out of favor with the American public, and for decades after, it was considered dangerous. Cleanup of the plant spanned three decades and cost about $975 million. In addition, the local economy was hurt as tourists made a point of staying away from the surrounding Pennsylvania communities. Milk processed locally was considered unsafe to drink, so higher prices were paid for milk brought in from outlying regions. Locals had to pay more money for their energy bills because Three Mile Island had supplied about forty percent of the area's power. Without that forty percent, customers had to pay for higher-priced power brought in from the outside.

More than two thousand personal injury claims were filed by people who believed their health was negatively impacted by the accident. All lawsuits were summarily dismissed in 1996 for lack of evidence.

Three Mile Island

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Three Mile Island Island on the River Susquehanna near Harrisburg, Pennyslvania, USA. It is the site of a nuclear power-generating plant where a near-disastrous accident took place in March 1979. The accident involved the failure of the feedwater system, which picks up heat from the system that has circulated through the reactor core. The accident released radioactive water and gases into the environment.

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