Three Mile Island Accident Summary

  • Last updated on November 10, 2022

The nuclear power plant accident at Three Mile Island exposed weaknesses in design, management, and operation and led to new safety measures designed to avoid a repetition elsewhere.

Summary of Event

On March 28, 1979, the Three Mile Island (TMI) nuclear power plant on the Susquehanna River near Harrisburg, Pennsylvania, nearly suffered a catastrophe as its Unit Two malfunctioned, setting into play events that resulted in the most serious accident to that time in the history of the commercial nuclear power industry. Had it not finally been contained, the malfunction would have resulted in devastation similar to that caused by the plant in Chernobyl, Ukraine, in 1986. Nuclear energy;power plants Three Mile Island nuclear plant accident Disasters;nuclear plant accidents Energy;nuclear [kw]Three Mile Island Accident (Mar. 28, 1979) [kw]Island Accident, Three Mile (Mar. 28, 1979) [kw]Accident, Three Mile Island (Mar. 28, 1979) Nuclear energy;power plants Three Mile Island nuclear plant accident Disasters;nuclear plant accidents Energy;nuclear [g]North America;Mar. 28, 1979: Three Mile Island Accident[03540] [g]United States;Mar. 28, 1979: Three Mile Island Accident[03540] [c]Disasters;Mar. 28, 1979: Three Mile Island Accident[03540] [c]Energy;Mar. 28, 1979: Three Mile Island Accident[03540] Thornburgh, Dick Scranton, William W., III Hendrie, Joseph M. Carter, Jimmy [p]Carter, Jimmy;Three Mile Island accident

The TMI accident exposed many weaknesses in nuclear power plant design, management, and operation. The ineffectiveness of the Nuclear Regulatory Commission Nuclear Regulatory Commission (NRC) and inadequacy of emergency preparedness were also exposed, leading to proposal of many changes by the many investigators deployed to study the event, including a presidential commission and congressional committees, the Nuclear Regulatory Commission, Pennsylvania governmental groups, and industrial organizations. In 1980, the comptroller general published a report to Congress that reviewed eight of the other reports and gave its own independent observations.

The Three Mile Island Unit Two, as well as its sister, Unit One, was a pressurized water reactor. It generated electric power by boiling water into steam, which then spun the blades of a turbine generator. The heat to convert the water to steam was produced by chain reaction fission of uranium in the reactor’s core. This core was covered with water as its primary coolant and encapsulated in a structure forty feet high with walls of steel eight inches thick. The coolant was radioactive and under pressure, which allowed it to be superheated to 575 degrees Fahrenheit without boiling. It then was pumped to a steam generator, where the coolant heated cooler water in a secondary system. Under less pressure, the water turned to steam and spun turbine blades, propelling a generator. The steam passed through a condenser, changing it back to water. It then began its circuit through to the boiler and back again through this secondary system, also called the “feedwater” circuit.

At 4:00 a.m. on Thursday, March 29, 1979, two pumps in this system shut down; the steam turbine followed a few seconds later. Its steam was released. What little coolant was left in the secondary system boiled. The primary coolant could not transfer its heat load, and it too began to boil, increasing pressure in the reactor and in the primary system. A relief valve opened, allowing radioactive water and steam to drain into a tank to prevent a primary coolant explosion. This valve should have shut off after thirteen seconds, but it remained open for more than two hours.

Less than a minute later, emergency backup pumps automatically engaged to add water to the secondary system. No water was added, however, because valves controlling the flow had been closed for maintenance two weeks earlier. According to Nuclear Regulatory Commission rules, the plant was to be shut down if these valves were closed for more than seventy-two hours.

Two minutes into the crisis, the emergency core coolant system kicked in to add water to the reactor core. Technicians, however, believed that the reactor was already full of water. They also assumed that the pressurizer relief valve was closed when it was not.

Four minutes later, with pressure in the primary cooling loop high, it was thought that the system was filling with water. Since additional increases in pressure could cause the system to blow, one emergency pump was stopped. Twelve and one-half minutes into the incident, the other was reduced to one-half speed. This was proper procedure, since the attendants believed the system was filling with water. This condition is known as “going solid” and must be avoided to lessen the possibility of the primary system’s breakdown. The reactor core in fact was not covered by water, and temperatures began rising toward the meltdown point of 5,000 degrees Fahrenheit. There were no meters that could measure the depth of water in the reactor core, so the operators could only guess about this critical information.

At eight and one-half minutes into the crisis, the closed valves on the feedwater system were opened, filling the secondary system with water. This helped to draw heat from the primary system. The relief valve allowed primary cooling water to drain into a tank that spilled its radioactive water onto the containment building’s floor. The water then was pumped into a tank in the nearby auxiliary building. Radioactivity was released in this final procedure at 4:38 a.m.

Pockets of steam collected in two sets of pumps for reactor cooling, resulting in vibrations that caused them to be turned off. With no cooling system in operation, the reactor suffered severe damage. The twelve-foot-tall fuel rods were only half covered with water. The shields around the rods themselves were destroyed by the intense, rising heat, releasing radioactive debris into the primary coolant, which itself was spilling onto the floor. Hydrogen and radioactive gases from the coolant collected in the containment building. Radiation levels rose within the buildings and radioactive gases were released into the atmosphere. At 6:50 a.m., a general emergency was declared.





Early Wednesday afternoon, hydrogen that had accumulated in the containment building exploded. Hydrogen continued to be created by the uncovered core, fueling fears of a catastrophic explosion.

Another scenario envisioned was the so-called China Syndrome. China Syndrome In the scenario, the core would become so hot (about 5,200 degrees Fahrenheit) that it would melt. This superheated material would bore its way through the bottom of the plant and down through the ground until it hit water. The water would become high-pressure steam and would erupt from the earth, spewing radioactivity into the air all around the plant. A typical nuclear reactor could release about the same radiation as would a thousand bombs of the size used at Hiroshima.

A 1975 study estimated that a plant slightly larger than Three Mile Island could cause thirty-three hundred deaths and forty-five thousand radiation injuries immediately. Forty-five thousand cancer and forty thousand thyroid tumor fatalities would result in the longer term. Fourteen billion dollars of damage to property would also occur.

Controlled and uncontrolled radiation leaks from the plant continued through Wednesday, March 28, and Thursday, March 29. On Friday, Governor Dick Thornburgh ordered an evacuation of pregnant women and small children within five miles of the facility. A hydrogen bubble began to grow in the reactor vessel. It was thought that it could self-ignite in five to eight days, resulting in a possible meltdown. A general evacuation was considered by Thornburgh and Lieutenant Governor William W. Scranton III but was not ordered. It was thought that such an evacuation might set off an evacuation panic and result in more injuries than it might prevent.

On Saturday morning, John Herbein, Herbein, John the Metropolitan Edison vice president for generation, said that the bubble had decreased in size by two-thirds and that the danger was over. Harold Denton Denton, Harold of the NRC disagreed and said that the bubble actually had increased in size. Lack of information, poor communication among the numerous people from the varied agencies involved, incorrect wire service reports, and alarmist news reports fueled the mounting alarm on the part of the public, both locally and nationally. More than half of the families within a twelve-mile radius of the plant evacuated at least one member.

Later on Saturday, Harold Denton told Thornburgh that the size of the hydrogen bubble had been reduced. Joseph M. Hendrie, a commissioner of the NRC, had a group working on the same problem. They reported that the bubble could be explosive in six or seven days.

On the afternoon of Sunday, April 1, President Jimmy Carter visited the facility. At about the same time, the hydrogen bubble shrank, eliminating the possibility of explosion, and the crisis wound down.


Numerous changes were made in the operation of nuclear power plants as a result of the Three Mile Island incident. This was a contingency for which there had been no plan, since it was thought to have a negligible probability of happening.

Until TMI, nuclear plants were constructed with three levels of safety built in, known as “defense in depth.” The first level involved using quality construction standards and emergency practices to prevent accidents. It is inevitable that mistakes will be made, accidents will happen, and equipment will break down. These factors required another level to prevent or control their effects. These were built into the original design. The last level of safety assumed that special design features would fail. The containment building could mitigate or slow the release of radioactive particles from the plant should that happen.

In a complete meltdown, the core would eat through the floor of the plant, contaminating the groundwater supply. Radiation might also quickly breach the containment building and result in many deaths and injuries. Because it might be impossible to contain a meltdown, design features to delay the release of radioactivity were suggested. These designs provided more time to evacuate the area. They included core “catchers” to slow the core melting through the floor, a filtering system to provide for filtering and release of gases in the containment building to prevent overpressurization, and hydrogen control systems to prevent or minimize the formation of a hydrogen bubble, which was so potentially dangerous at TMI. Control room design changes were adopted that made controls more recognizable and accessible to operators in emergency situations.

Prior to 1979, nuclear plants were located close to major population areas. It was thought that the probability of radiation exposure to the public was quite small. After the accident, it became apparent that anything made by human beings is subject to failure. A return to the policy of constructing plants far from populated areas was thought to be prudent.

The inadequate qualifications and training of operators contributed to the severity of the accident. Training programs had been geared toward running the plant under normal conditions rather than under stressful emergencies. Supervisory and management personnel knew little about actual operations and were not able to help the operators mitigate problems.

The Nuclear Regulatory Commission now requires more operators, who are better qualified and have passed a more stringent licensing examination. Supervisors need engineering expertise, training is more rigorous, and simulators are used to prepare operators to deal with emergency situations.

Studies found that initial situations similar to that at TMI had occurred at other plants, but operators were able to react before a major emergency developed. There was no system in place at the NRC or within the nuclear power industry to collect or distribute information to other operators about the problems encountered. A system to review and analyze information was implemented to collect data on American and foreign nuclear reactors. The Office for Analysis and Evaluation of Operating Data was created to be the focal point of this effort.

At the time of the TMI accident, the quality assurance programs of both Metropolitan Edison and the NRC were deficient. Standards used in the construction and operation of power plants were to be monitored by an independent department within each utility to ensure compliance. The NRC reviewed the utilities’ efforts. These standards did not apply to equipment unrelated to safety or to radiation survey monitors. Equipment not related to safety had a significant involvement in the accident, and many of the radiation monitoring instruments at TMI did not work. An acknowledgment from the NRC that rigid quality assurance standards and their strict implementation were essential was expected to lessen the likelihood of a future similar event.

Emergency procedures on the part of the NRC and state and local governments were found to have been lax or nonexistent. The accident demonstrated that an emergency was possible, prompting emergency and evacuation plans to be implemented or upgraded for existing nuclear power plants. In addition, operating licenses would be granted to new nuclear generating plants only if state and local governments had federally approved emergency plans. The Federal Emergency Management Agency (FEMA), Federal Emergency Management Agency rather than the NRC, became responsible for evaluating emergency plans. Emergency planning

During the emergency, numerous TMI employees were assigned various emergency response duties. Many had received no training and did not understand what needed to be done. Additionally, half of the radiation dose rate monitors were not operable. The NRC thus became more rigorous in requiring emergency training and equipment maintenance.

Each of the five members of the Nuclear Regulatory Commission had equal responsibility and authority in all decisions in 1979. The chair had vaguely defined administrative and executive functions, but decision-making power lay with joint action of the commissioners and not with the chair. With no one ultimately in command, slow, inefficient management resulted.

After reorganization of the NRC in 1980, the chair had more power, although the commission as a whole still set the framework within which the chair could operate. The chair was allowed to act in the name of the commission in an emergency, determining policies, giving orders, and directing all actions concerning the emergency. The chair gained the ultimate responsibility for emergency decision making. This was expected to provide more timely responses instead of the delays involved with management by committee.

The TMI accident prompted reconsideration of nuclear power as a source of energy. Although relatively inexpensive, nuclear power posed the risk of disasters and the problem of nuclear-waste disposal. Regulators had to decide how many costly safety requirements to impose, and the federal government faced choices of which energy sources to promote and even whether to allow construction of new nuclear power plants. The 1986 nuclear disaster at Chernobyl renewed these concerns worldwide. Nuclear energy;power plants Three Mile Island nuclear plant accident Disasters;nuclear plant accidents Energy;nuclear

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Del Tredici, Robert. The People of Three Mile Island. San Francisco: Sierra Club Books, 1980. Interviews with local people and others connected with the event.
  • citation-type="booksimple"

    xlink:type="simple">Gray, Mike, and Ira Rosen. The Warning: Accident at Three Mile Island. New York: W. W. Norton, 2003. A very readable investigative report that dramatically pulls the reader through the complex events of the Three Mile Island disaster itself. Less technical than some of the other publications; fast and informative reading.
  • citation-type="booksimple"

    xlink:type="simple">Ramsey, Charles B., and Mohammad Modarres. Commercial Nuclear Power: Assuring Safety for the Future. New York: John Wiley & Sons, 1998. Written for the general reader, the book objectively looks at world energy demands and the role that nuclear power should play, and examines the lessons learned from the nuclear industry’s past accidents.
  • citation-type="booksimple"

    xlink:type="simple">Sorensen, John H., Jon Soderstrom, Emily Copenhaven, Sam Carnes, and Robert Bolin. Impacts of Hazardous Technology: The Psycho-social Effects of Restarting TMI-1. Albany: State University of New York Press, 1987. Reviews the background of TMI and projects the effects of starting the undamaged sister reactor, TMI-1.
  • citation-type="booksimple"

    xlink:type="simple">Starr, Philip, and William Pearman. Three Mile Island Sourcebook: Annotations of a Disaster. New York: Garland, 1983. This book is divided into three sections. The first provides a chronology of media coverage from TMI’s announcement of opening in 1966 until 1981. Three local newspapers, The New York Times, and Newsweek are surveyed. The next section is annotations of state and federal documents. The last section covers books, articles, and other publications written about TMI.
  • citation-type="booksimple"

    xlink:type="simple">Stephens, Mark. Three Mile Island. New York: Random House, 1980. Written by a staff member of the presidential commission. Recounts the immediate events of the incident and offers suggestions to avoid future problems.
  • citation-type="booksimple"

    xlink:type="simple">U.S. General Accounting Office. Three Mile Island: The Most Studied Nuclear Accident in History. Washington, D.C.: Author, 1980. This inquiry was made to determine whether the investigations done up to that time were thorough and accurate in their presentation of the facts and their conclusions as to the causes of the accident. Eight investigative reports, as well as other materials, were reviewed, and although reports varied as to depth and detail, the facts and conclusions were determined to be consistent. Equipment breakdowns, insufficient training of operators, poor design, and inadequate emergency and operating procedures were the chief culprits. Blame was also placed on the Nuclear Regulatory Commission with its poor structure, practices, and attitudes.
  • citation-type="booksimple"

    xlink:type="simple">Walker, J. Samuel. Three Mile Island: A Nuclear Crisis in Historical Perspective. Berkeley: University of California Press, 2004. Thoroughly researched scholarly account of the TMI accident. Includes photos, notes.

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