Backgrounder on the Three Mile Island Accident

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Three Mile Island History (Video)

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The Three Mile Island Unit 2 reactor, near Middletown, Pa., partially melted down on March 28, 1979. This was the most serious accident in U.S. commercial nuclear power plant operating history, although its small radioactive releases had no detectable health effects on plant workers or the public. Its aftermath brought about sweeping changes involving emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations. It also caused the NRC to tighten and heighten its regulatory oversight. All of these changes significantly enhanced U.S. reactor safety.

A combination of equipment malfunctions, design-related problems and worker errors led to TMI-2’s partial meltdown and very small off site releases of radioactivity.

Summary of Events

The accident began about 4 a.m. on Wednesday, March 28, 1979, when the plant experienced a failure in the secondary, non-nuclear section of the plant (one of two reactors on the site). Either a mechanical or electrical failure prevented the main feedwater pumps—component (1) in the animated diagram)—from sending water to the steam generators (2) that remove heat from the reactor core (3). This caused the plant's turbine-generator (4) and then the reactor itself to automatically shut down. Immediately, the pressure in the primary system (the nuclear piping portion of the plant shown in orange) began to increase. In order to control that pressure, the pilot-operated relief valve (5) opened. It was located at the top of the pressurizer (6). The valve should have closed when the pressure fell to proper levels, but it became stuck open. Instruments in the control room, however, indicated to the plant staff that the valve was closed. As a result, the plant staff was unaware that cooling water in the form of steam was pouring out of the stuck-open valve. As alarms rang and warning lights flashed, the operators did not realize that the plant was experiencing a loss-of-coolant accident.

Other instruments available to plant staff provided inadequate or misleading information. During normal operations, the large pressure vessel (7) that held the reactor core was always filled to the top with water. So there was no need for a water-level instrument to show whether water in the vessel covered the core. As a result, plant staff assumed that as long instruments showed that the pressurizer water level was high enough, the core was properly covered with water too. That wasn’t the case.

Unaware of the stuck-open relief valve and unable to tell if the core was covered with cooling water, the staff took a series of actions that uncovered the core. The stuck valve reduced primary system pressure so much that the reactor coolant pumps (8) started to vibrate and were turned off. The emergency cooling water being pumped into the primary system threatened to fill up the pressurizer completely—an undesirable condition—and they cut back on the flow of water. Without the reactor coolant pumps circulating water and with the primary system starved of emergency cooling water, the water level in the pressure vessel dropped and the core overheated.

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Animated Diagram of the Sequence of Events

The following animated diagram graphically depicts the sequence of events associated with the accident at TMI-2.

an animated plant diagram showing the series of events which caused the TMI accident - as found in this article's Summary of Events section

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Health Effects

The NRC conducted detailed studies of the accident’s radiological consequences, as did the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the Commonwealth of Pennsylvania. Several independent groups also conducted studies. The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. To put this into context, exposure from a chest X ray is about 6 millirem and the area’s natural radioactive background dose is about 100 125 millirem per year for the area. The accident’s maximum dose to a person at the site boundary would have been less than 100 millirem above background.

In the months following the accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area, none could be directly correlated to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various government agencies monitoring the area. Very low levels of radionuclides could be attributed to releases from the accident. However, comprehensive investigations and assessments by several well respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment.

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Impact of the Accident

A combination of personnel error, design deficiencies, and component failures caused the TMI accident, which permanently changed both the nuclear industry and the NRC. Public fear and distrust increased, NRC’s regulations and oversight became broader and more robust, and management of the plants was scrutinized more carefully. Careful analysis of the accident’s events identified problems and led to permanent and sweeping changes in how NRC regulates its licensees – which, in turn, has reduced the risk to public health and safety.

Here are some of the major changes that have occurred since the accident:

  • Upgrading and strengthening of plant design and equipment requirements. This includes fire protection, piping systems, auxiliary feedwater systems, containment building isolation, reliability of individual components (pressure relief valves and electrical circuit breakers), and the ability of plants to shut down automatically;
  • Identifying the critical role of human performance in plant safety led to revamping operator training and staffing requirements, followed by improved instrumentation and controls for operating the plant, and establishment of fitness-for-duty programs for plant workers to guard against alcohol or drug abuse;
  • Enhancing emergency preparedness, including requirements for plants to immediately notify NRC of significant events and an NRC Operations Center staffed 24 hours a day. Drills and response plans are now tested by licensees several times a year, and state and local agencies participate in drills with the Federal Emergency Management Agency and the NRC;
  • Integrating NRC observations, findings, and conclusions about licensee performance and management effectiveness into a periodic, public report;
  • Having senior NRC managers regularly analyze plant performance for those plants needing significant additional regulatory attention;
  • Expanding NRC’s resident inspector program–first authorized in 1977–to have at least two inspectors live nearby and work exclusively at each plant in the U.S. to provide daily surveillance of licensee adherence to NRC regulations;
  • Expanding performance-oriented as well as safety oriented inspections, and the use of risk assessment to identify vulnerabilities of any plant to severe accidents;
  • Strengthening and reorganizing enforcement staff in a separate office within the NRC;
  • Establishing the Institute of Nuclear Power Operations, the industry’s own “policing” group, and formation of what is now the Nuclear Energy Institute to provide a unified industry approach to generic nuclear regulatory issues, and interaction with NRC and other government agencies;
  • Installing additional equipment by licensees to mitigate accident conditions, and monitor radiation levels and plant status;
  • Enacting programs by licensees for early identification of important safety related problems, and for collecting and assessing relevant data so operating experience can be shared and quickly acted upon; and
  • Expanding NRC’s international activities to share enhanced knowledge of nuclear safety with other countries in a number of important technical areas.

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Current Status

Today, the TMI 2 reactor is permanently shut down and 99 percent of its fuel has been removed. The reactor coolant system is fully drained and the radioactive water decontaminated and evaporated. The accident’s radioactive waste was shipped off site to an appropriate disposal area, and the reactor fuel and core debris was shipped to the Department of Energy’s Idaho National Laboratory. The company TMI-2 Solutions acquired the license for Unit 2 in 2020 and is responsible for remaining decommissioning activities there. Unit 1 permanently ceased operation in September 2019, and Constellation Energy Company (formerly Exelon Generation) is responsible for decommissioning activities there.

Below is a chronology of highlights of the TMI 2 cleanup from 1980 through 1993.

Date Event
July 1980 Approximately 43,000 curies of krypton were vented from the reactor building.
July 1980 The first manned entry into the reactor building took place.
Nov. 1980 An Advisory Panel for the Decontamination of TMI‑2, composed of citizens, scientists, and State and local officials, held its first meeting in Harrisburg, Pa.
July 1984 The reactor vessel head (top) was removed.
Oct. 1985 Fuel removal began.
July 1986 The off-site shipment of reactor core debris began.
Aug. 1988 GPU submitted a request for a proposal to amend the TMI‑2 license to a “possession‑only” license and to allow the facility to enter long‑term monitoring storage.
Jan. 1990 Fuel removal was completed.
July 1990 GPU submitted its funding plan for placing $229 million in escrow for radiological decommissioning of the plant.
Jan. 1991 The evaporation of accident-generated water began.
April 1991 NRC published a notice of opportunity for a hearing on GPU's request for a license amendment.
Feb. 1992 NRC issued a safety evaluation report and granted the license amendment.
Aug. 1993 The processing of accident-generated water was completed involving 2.23 million gallons.
Sept. 1993 NRC issued a possession-only license.
Sept. 1993 The Advisory Panel for Decontamination of TMI-2 held its last meeting.
Dec. 1993 Monitored storage began.

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Additional Information

Further information on the TMI 2 accident can be obtained from NUREG documents, many of which are on microfiche. They can be ordered for a fee from the NRC’s Public Document Room at 301 415 4737 or 1 800 397 4209; e mail The PDR is located at 11555 Rockville Pike, Rockville, Md.; however the mailing address is: U.S. Nuclear Regulatory Commission, Public Document Room, Washington, D.C. 20555. A glossary is also provided below.

Additional Sources for Information on Three Mile Island

  • NRC Annual Report - 1979, NUREG-0690

  • "Population Dose and Health Impact of the Accident at the Three Mile Island Nuclear Station," NUREG-0558

  • "Environmental Assessment of Radiological Effluents from Data Gathering and Maintenance Operation on Three Mile Island Unit 2," NUREG-0681

  • "Investigation into the March 28, 1979 Three Mile Island Accident by the Office of Inspection and Enforcement," NUREG-0600

  • "Three Mile Island; A Report to the Commissioners and to the Public," by Mitchell Rogovin and George T. Frampton, NUREG/CR-1250, 1980 (Vol. I, Vol. II Pt. 1, Vol. II Pt. 2, Vol. II Pt. 3)

  • "Lessons learned From the Three Mile Island - Unit 2 Advisory Panel," NUREG/CR-6252

  • "The Status of Recommendations of the President's Commission on the Accident at Three Mile Island," (A ten-year review), NUREG-1355

  • "NRC Views and Analysis of the Recommendations of the President's Commission on the Accident at Three Mile Island," NUREG-0632

  • "Environmental Impact Statement related to decontamination and disposal of radioactive wastes resulting from March 28, 1979 accident Three Mile Island Nuclear Station, Unit 2," NUREG-0683 (Vol. I, Vol. II)

  • "Answers to Questions About Updated Estimates of Occupational Radiation Doses at Three Mile Island, Unit 2," NUREG-1060

  • "Answers to Frequently Asked Questions About Cleanup Activities at Three Mile Island, Unit 2," NUREG-0732

  • "Status of Safety Issues at Licensed Power Plants" (TMI Action Plan Reqmts.), NUREG-1435

  • "Three Mile Island Accident of 1979 Knowledge Management Digest – Overview", NUREG/KM-0001

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Auxiliary feedwater ‑ (see emergency feedwater)

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.

Cladding ‑ 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.

Emergency feedwater system ‑ Backup feedwater supply used during nuclear plant startup and shutdown; also known as auxiliary feedwater.

Fuel rod ‑ A long, slender tube that holds fuel (fissionable material) for nuclear reactor use. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.

Containment ‑ 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.

Coolant ‑ 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.

Core ‑ The central portion of a nuclear reactor containing the fuel elements, and control rods.

Decay heat ‑ The heat produced by the decay of radioactive fission products after the reactor has been shut down.

Decontamination ‑ 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.

Feedwater ‑ 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.

Pressurizer - A tank or vessel that controls the pressure in a certain type of nuclear reactor.

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.

Radiation ‑ Particles (alpha, beta, neutrons) or photons (gamma) emitted from the nucleus of an unstable atom as a result of radioactive decay.

Reactor Coolant System ‑ (see primary system)

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.

Turbine ‑ 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.

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Plant Diagram

Sketch of the components which comprise the TMI-2 Plant showing the Reactor Building and its components; the Turnbine Building and its components, and the Cooling Tower along with arrows indicating the directional flow of cooling water

April 2022

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Page Last Reviewed/Updated Thursday, March 28, 2024