Three Mile Island Nuclear Disaster


On March 28, 1979, a partial nuclear meltdown occurred at the Three Mile Island nuclear power plant near Middletown, Pennsylvania. During the accident small amounts of radioactive gases and radioactive iodine were released into the atmosphere. Although there were no deaths or injuries to plant workers or community members, it remains the most serious commercial nuclear power plant accident in U.S. nuclear power plant operating history.


The accident at the Three Mile Island Unit 2 (TMI-2, one of two power plants at the site) began around 4:00 A.M. on March 28, 1979, as the plant experienced a failure in the secondary (non-nuclear) system of the plant. The main feedwater pumps stopped running, caused by either a mechanical or electrical failure that prevented the steam generators from removing heat. When the pumps stopped, the turbine and the reactor automatically shut down, which resulted in the pressure in the primary (nuclear) system of the plant increasing. A relief valve opened to release the excess pressure; however, this valve should have closed again when the pressure decreased, but it stayed open. Signals should have been sent to the operator to indicate that the valve was still open, but no such signal was received.

Three Mile Island Nuclear Generating Station, in Londenderry Township near Middletown, Pa., is shown from across the Susquehanna River near Etters, Pa., Wednesday, May 24, 2017. On March 28, 1979,

Three Mile Island Nuclear Generating Station, in Londenderry Township near Middletown, Pa., is shown from across the Susquehanna River near Etters, Pa., Wednesday, May 24, 2017. On March 28, 1979, TMI Unit 2, right set of cooling towers, suffered a partial meltdown.
(AP Photo/John Zeedick)

The U.S. Nuclear Regulatory Commission (NRC) reacted within a few hours and sent inspectors and response teams to the site, although they did not know that the core had melted. Other agencies also responded, such as the U.S. Department of Energy, the U.S. Environmental Protection Agency, and Brookhaven National Laboratory. By 11 A.M. on March 28th, all nonessential personnel left the plant, and monitoring of radiation levels in the atmosphere above the plant was being conducted using helicopters. By the evening of March 28th, the reactor appeared to be stable, with the core flooded again and the temperature under control.

However, on March 30th (later known as Black Friday), confusion and rumors arose on the amount of radiation that had or could be released from the plant, which resulted in discussions between the NRC and the Pennsylvania governor, Richard Thornburgh, on whether the population near the plant should be evacuated. The decision was made to recommend that members of the population vulnerable to radiation, including pregnant women and pre-school-age children who lived within a 5-mile radius of the plant, leave the area. In response to the potential threats, about 140,000 people evacuated the area. Ninety-eight percent of the evacuees returned to their homes within three weeks.

Additional concerns arose on March 31st because of the presence of a large hydrogen bubble in the dome of the pressure vessel, the container that holds the reactor core. There were fears that the hydrogen bubble might burn or explode and rupture the pressure vessel. The core would then fall into the containment building and perhaps cause a breach of containment. On Sunday, April 1, it was determined that the bubble could not burn or explode because of the absence of oxygen in the pressure vessel. In addition, the operators had significantly reduced the size of the bubble.

On April 2, 1979, five days after the meltdown, the crisis at Three Mile Island was officially declared to be over. TMI- 2 was too badly damaged and contaminated to resume operations; the reactor was gradually deactivated and permanently closed. The reactor coolant system was drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site to an appropriate disposal site, reactor fuel and core debris shipped off-site to a Department of Energy facility, and the remainder of the site was being monitored. When the operating license for the TMI-1 plant expires, both plants will be decommissioned. TMI-1 is licensed to operate until April 19, 2034.


Based on many studies conducted by the NRC, the U.S. Environmental Protection Agency, the U.S. Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the State of Pennsylvania, the estimated average dose to about 2 million people in the area was only about 1 millirem. Exposure from a chest x-ray is about 6 millirem, and the natural background radiation dose in the area is about 100 to 125 millirem per year. The maximum dose to a person at the site boundary would have been less than 100 millirem.

In the aftermath of the accident, no possible adverse effects from radiation on human, animal, and plant life in the surrounding area could be directly correlated to the accident, although thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected and analyzed by various groups monitoring the area. The U.S. Environmental Protection Agency remained in the area for eight years, maintaining a field office to monitor the air. Only very low levels of radionuclides were found as a result of the accident.

Comprehensive investigative and assessment reports by several well-respected organizations concluded that in spite of serious damage to the reactor, most of the radiation was contained and that the radioactive release had little effect on the physical health of individuals or the environment. However, other studies reported that lung cancer and leukemia rates were two to ten times greater downwind of TMI than upwind and that infant mortality increased in downwind communities two years after the accident.


The accident was apparently caused by a combination of personnel error, design deficiencies, and component failures, although the definite proximate (or legal) cause of the meltdown remains unknown and no proof of negligence was ever uncovered. Examples of problems that occurred included reactor operators not trained to deal with accident conditions, and the NRC not having established effective communication channels with utilities. Once the accident occurred, the lines of authority were shown to be ill defined. The public received conflicting reports that caused panic and evacuations.


Dr. Helen Caldicott was born in Australia but moved to the United States to practice pediatric medicine in Boston. After the Three Mile Island nuclear accident in 1980, Caldicott left medicine. She felt strongly about the world's dependence on nuclear energy and devoted her career to researching and raising awareness on the dangers of nuclear power.

Caldicott was the founding president of the Physicians for Social Responsibility, a group devoted to educating people on the dangers of nuclear energy. She also created the Women's Action for Nuclear Disarmament (WAND), which works to divert government funding from nuclearrelated items to social causes.

Dr. Caldicott's lecture to SUNY college students about the threat of nuclear weapons became a 1982 Academy-Award-winning short documentary film, entitled If You Love This Planet.

Helen Caldicott (right) and Lily Tomlin (left) attend Kat Kramer's Films That Change The World: Fallout 5th Anniversary Think-Tank“ in Hollywood, California, November 14, 2013.

Helen Caldicott (right) and Lily Tomlin (left) attend Kat Kramer's Films That Change The World: Fallout 5th Anniversary Think-Tank“ in Hollywood, California, November 14, 2013.
(WENN Ltd / Alamy Stock Photo)

The Kemeny Commission, the presidential commission tasked with conducting a comprehensive investigation of the TMI accident, noted the type of valve that had stuck open had previously failed on eleven occasions in different nuclear power plants, nine of them in the open position, allowing coolant to escape.

Public health role and response

In the days after the accident, engineers, scientists and mechanics worked to minimize further release of radiation and to prevent a total meltdown of the core, while state and federal government officials tried to come up with emergency response measures. Initially reports indicated no threats to the public, but later it was reported that the situation was more complex than initially thought. Two days after the accident, Governor Richard Thornburgh recommended that preschool children and pregnant women within five miles of the plant evacuate the area. Residents within a ten-mile radius were asked to stay at home, turn off their air-conditioners, and close their windows. Farmers were told to keep their animals under cover and eating stored feed.

The Kemeny Commission concluded that the major health effect of the accident appeared to have been on the mental health of the people living in the region of Three Mile Island and of the workers at TMI. There was immediate, short-lived mental distress produced by the accident among certain groups of the general population living within 20 miles of TMI. The highest levels of distress were found among adults living within 5 miles of TMI or with preschool children; and among teenagers living within 5 miles of TMI, with preschool siblings, or whose families left the area. Workers at TMI experienced more distress than workers at another plant studied for comparison purposes. This distress was higher among the nonsupervisory employees and continued in the months following the accident.

This accident highlighted the lack of preparedness for a nuclear power plant incident, and the resulting heightened awareness led to better preparedness plans by agencies responsible for public health and safety. These plans include:

Background radiation—
The radiation in the natural environment, including cosmic rays and radiation from the naturally radioactive elements, both outside and inside the bodies of humans and animals. The usually quoted average individual exposure from background radiation is 300 millirem per year.
The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents the corrosion of the fuel by the coolant and the release of fission products in the coolants. Aluminum, stainless steel and zirconium alloys are common cladding materials.
The gas-tight shell or other enclosure around a reactor to confine fission products that otherwise might be released to the atmosphere in the event of an accident.
A substance circulated through a nuclear reactor to remove or transfer heat. The most commonly used coolant in the U.S. is water. Other coolants include air, carbon dioxide, and helium.
The central portion of a nuclear reactor containing the fuel elements, and control rods.
The reduction or removal of contaminating radioactive material from a structure, area, object, or person. Decontamination may be accomplished by (1) treating the surface to remove or decrease the contamination; (2) letting the material stand so that the radioactivity is decreased by natural decay; and (3) covering the contamination to shield the radiation emitted.
Water supplied to the steam generator that removes heat from the fuel rods by boiling and becoming steam. The steam then becomes the driving force for the turbine generator.
Nuclear reactor—
A device in which nuclear fission may be sustained and controlled in a self-supporting nuclear reaction. There are several varieties, but all incorporate certain features, such as fissionable material or fuel, a moderating material (to control the reaction), a reflector to conserve escaping neutrons, provisions for removal of heat, measuring and controlling instruments, and protective devices.
Pressure vessel—
A strong-walled container housing the core of most types of power reactors.
Primary system—
The cooling system used to remove energy from the reactor core and transfer that energy either directly or indirectly to the steam turbine.
Particles (alpha, beta, neutrons) or photons (gamma) emitted from the nucleus of an unstable atom as a result of radioactive decay.
Secondary system—
The steam generator tubes, steam turbine, condenser and associated pipes, pumps, and heaters used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation.
Steam generator—
The heat exchanger used in some reactor designs to transfer heat from the primary (reactor coolant) system to the secondary (steam) system. This design permits heat exchange with little or no contamination of the secondary system equipment.
A rotary engine made with a series of curved vanes on a rotating shaft. Usually turned by water or steam. Turbines are considered to be the most economical means to turn large electrical generators.


The TMI-2 cleanup took 11 years and cost about $1 billion. There were serious public relations consequences for nuclear power. Following the accident, the number of reactors under construction in the U.S. declined every year. At the time of the TMI accident, 129 nuclear power plants had been approved. Of those, only 53 (which were not already operating) were completed. Internationally, declines in nuclear power plant construction came as a result of the catastrophic Chernobyl nuclear power plant accident in 1986. In February 2012, the NRC approved construction and licensing of two new nuclear reactors at Plant Vogtle in Georgia, the first such approval in the U.S. since 1978.

The accident at Three Mile Island changed both the nuclear industry and the NRC. Public fear and distrust increased, NRC's regulations and oversight became broader and stronger, and management of the plants was scrutinized more carefully. The problems identified from careful analysis of the events during those days and the changes made in response to the analysis has reduced the risk to public health and safety.


Changes to the U.S. nuclear power plant program to provide protection against future accidents that were made in response to the Three Mile Island accident, according the the NRC, include:

See also Background radiation ; Chernobyl nuclear power station disaster ; Radiation ; Radiation exposure .



Derkins, Susie. The Meltdown at Three Mile Island. New York: Rosen Publishing Group, 2003.

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

Geigenbaum, Aaron. Emergency at Three Mile Island. New York: Bearport, 2007.

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.

Osif, Bonnie, A.; Anthony J. Baratta; and Thomas W. Conkling. TMI 25 Years Later: The Three Mile Island Nuclear Power Plant Accident and its Impact. University Park: Pennsylvania State University Press, 2006.

Walker, J. Samuel. Three Mile Island: A Nuclear Crisis in Historical Perspective. Berkeley: University of California Press, 2004.

WEBSITES, Dickinson College.

Judith L. Sims

  This information is not a tool for self-diagnosis or a substitute for professional care.