Three Mile Island accident Summary

  • Last updated on November 10, 2022

The nation’s worst commercial nuclear accident, the core meltdown at Three Mile Island led first to a complete halt in the construction of nuclear power plants, then to serious economic and public-relations consequences for the United States’ nuclear power industry.

At 4:00 a.m., a stuck valve in the cooling system of unit 2 of the Three Mile Island Nuclear Generating Station set off a chain of reactor malfunctions that resulted in pandemonium and nearly a total meltdown. Nuclear power industryThe incident began when one of the plant’s main feed-water pumps failed; because water was no longer flowing, the steam generators were not removing heat from the reactor. Pressure began to increase, and a valve that should have closed after the excessive pressure was released did not do so. Plant operators assumed the valve had closed, so they were unaware that pressure was continuing to rise. Cooling water poured through the stuck valve, causing the core of the reactor to overheat. Since no instrument measured the water level in the core, operators did not realize the water level had fallen, uncovering the reactor core.Three Mile Island accident

Radiation alarms, activated by contaminated water, sounded, but they were initially ignored. Not realizing the plant was experiencing a loss of coolant, operators initiated a series of actions that made conditions worse by further reducing the coolant through the core. Consequently, the nuclear fuel overheated to a point at which the zirconium cladding (tubes holding the nuclear fuel pellets) ruptured and began to melt. It was later determined that about one-half the core melted during the early stages of the accident. Radiation levels were around three hundred times expected levels, and the plant was seriously contaminated. Although the plant suffered a severe core meltdown, the building’s containment wall was not breached. Such a breach could have caused a massive release of radiation into the environment.

The Three Mile Island nuclear plant in 1999, with the damaged unit at left.

(AP/Wide World Photos)

The incident at Three Mile Island caught federal and state authorities unprepared for such an emergency. Plant personnel initially had difficulty establishing communication with the Nuclear Regulatory Commission’s regional office, and there was no planned evacuation route. Governor of Pennsylvania Richard Thornburg advised that pregnant women and young children within a five-mile radius of the plant be evacuated, but no general evacuation was ordered. The crisis continued through April 1. Although the damage to the reactor was serious, the radiation was contained, and the amount released had a negligible effect on the physical health of people in the area or on the environment. The reactor was permanently shut down, and the damage caused by the meltdown was estimated at $500 million. Cleanup of the site continued through 1993.

Despite the relatively benign outcome of the accident, the fear it generated continued to haunt the nuclear power industry. Immediately after March, 1979, public approval of nuclear power fell, as did construction orders for nuclear reactors. Three Mile Island became an iconic symbol of the dangers of nuclear power, later joined by Chernobyl. The disasters in these two locations are often invoked by opponents of nuclear power seeking to build public distrust and defeat proposals for new nuclear power facilities.

Further Reading
  • Stephens, Mark. Three Mile Island. New York: Random House, 1980.
  • Walker, Samuel. Three Mile Island: A Nuclear Crisis in Historical Perspective. Berkeley: University of California Press, 2004.
  • Whitford, David. “Going Nuclear.” Fortune 6 (August, 2007): 42-54.

U.S. Department of Energy

Energy crisis of 1979

Environmental movement

Nuclear power industry

Occupational Safety and Health Act

Public utilities

Tennessee Valley Authority

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