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Three Mile Island Accident Timeline

(March 28, 1979 – Present)

The Three Mile Island Accident was a partial nuclear meltdown that occurred at Unit 2 (TMI-2) of the Three Mile Island Nuclear Generating Station near Harrisburg, Pennsylvania, on March 28, 1979. Triggered by a combination of equipment malfunctions, design flaws, and operator errors, the accident released small amounts of radioactive gases into the environment and led to no immediate deaths or injuries. However, it severely damaged public confidence in nuclear power in the United States, halted new plant construction for decades, and prompted sweeping reforms in nuclear safety regulations and emergency response planning. Less

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Health and Legal Aftermath

Wing finds increased cancers near TMI

1979 - 1985

Steven Wing found a significant increase in cancers between 1979 and 1985 among people who lived within ten miles of TMI.

Gofman predicts excess cancer deaths

1979

John Gofman used his own, non-peer reviewed low-level radiation health model to predict 333 excess cancer or leukemia deaths from the 1979 Three Mile Island accident.

Houser exceeds quarterly radiation dose limit

1979

Houser had gone past the NRC's quarterly dose limit for radiation exposure (3 rem/qtr in 1979) by one and was only admitted back to work the following quarter.

Evacuees return home within three weeks

Apr 1979

According to a survey conducted in April 1979, 98% of the evacuees had returned to their homes within three weeks.

Large anti-nuclear demonstration in New York City

Sep 1979

The largest demonstration was held in New York City in September 1979 and involved 200,000 people, with speeches given by Jane Fonda and Ralph Nader.

Kemeny Commission report released

Oct 31, 1979

After the TMI incident, President Carter commissioned a study, Report of the President's Commission on the Accident at Three Mile Island. It was instructed to produce a final report within six months, and after public hearings, depositions, and document collection, released a completed study on October 31, 1979.

Citizens' groups win $25 million settlement

1981

In 1981, citizens' groups succeeded in a class action suit against TMI, winning $25 million in an out-of-court settlement.

Counties vote to retire Unit 1 permanently

1982

In 1982, the citizens of the three counties surrounding the site voted overwhelmingly in a non-binding resolution to retire Unit 1 permanently.

Metropolitan Edison indicted on criminal charges

1983

In 1983, a federal grand jury indicted Metropolitan Edison on criminal charges for the falsification of safety test results prior to the accident.

Class action appeal over health effects fails

2002

According to Eric Epstein, chair of Three Mile Island Alert, the TMI plant operator and its insurers paid at least $82 million in publicly documented compensation to residents. However, a class action lawsuit alleging detrimental health effects was rejected by Judge Sylvia Rambo, and the final appeal failed in 2002.

Sovacool estimates $2.4 billion in property damages

2007

Sovacool, in his 2007 preliminary assessment of major energy accidents, estimated that the TMI accident caused a total of $2.4 billion in property damages.

Cultural Depictions

The China Syndrome premieres

Mar 15, 1979

Twelve days before the accident, the movie The China Syndrome premiered and was initially met with backlash from the nuclear power industry, which claimed it was "sheer fiction" and a "character assassination of an entire industry."

Meltdown: Three Mile Island released on Netflix

May 4, 2022

Meltdown: Three Mile Island is a four-part docuseries released by Netflix on May 4, 2022.

Radioactive: The Women of Three Mile Island released

2023

Radioactive: The Women of Three Mile Island is a 2023 documentary about the accident.

The Accident

Partial core meltdown begins at TMI-2

Mar 28, 1979

The main chain of events leading to the partial core meltdown began at 4:00:36 a.m. on Wednesday, March 28, 1979. The accident released radioactive gases and radioactive iodine into the environment.

Radioactive releases contaminate the environment

Mar 28, 1979

According to official figures compiled by the 1979 Kemeny Commission from Met Ed and NRC data, a maximum of 480 petabecquerels (13 MCi) of radioactive noble gases, primarily xenon, were released by the event.

President Carter visits Three Mile Island

Apr 1979

President Jimmy Carter visited the site in April 1979, on March 28, 1979, which would have provided the means for a high dose of radiation to occur had he been exposed during the crisis.

Gilinsky later reveals fuel temperature findings

2009

In a 2009 article, Gilinsky wrote that it took five weeks to learn that "the reactor operators had measured fuel temperatures near the melting point."

Cleanup and Recovery

Cleanup of TMI-2 begins

Aug 1979

Cleanup at TMI-2 started in August 1979 and officially ended in December 1993, with a total cost of about $1 billion (equivalent to $2 billion in 2025).

Television camera reveals damaged reactor interior

1985

In 1985, a television camera was used to see the interior of the damaged reactor.

Removal of radioactive fuel from the site begins

1985

Starting in 1985, almost 100 short tons (91 t) of radioactive fuel were removed from the site.

Core samples taken from reactor vessel corium

1986

In 1986, core samples and samples of debris were obtained from the corium layers on the bottom of the reactor vessel and analyzed.

NRC declares remaining radioactivity contained

1988

In 1988, the NRC announced that, although it was possible to further decontaminate the Unit 2 site, the remaining radioactivity had been sufficiently contained as to pose no threat to public health and safety.

First major cleanup phase completed

1990

The first major phase of the cleanup was completed in 1990, when workers finished shipping 150 short tons (140 t) of radioactive wreckage to Idaho for storage at the Department of Energy's National Engineering Laboratory.

Official end of TMI-2 cleanup

Dec 1993

Cleanup at TMI-2 started in August 1979 and officially ended in December 1993, with a total cost of about $1 billion (equivalent to $2 billion in 2025).

TMI-2 Solutions purchases the site

2020

In 2020 the site was purchased by TMI-2 Solutions, a subsidiary of EnergySolutions, with the intent of cleaning up the site for less money than is available in a dedicated fund.

99% of nuclear fuel cleaned up at TMI-2

May 8, 2023

On May 8, 2023, TMI-2 Solutions announced that 99% of the nuclear fuel has been cleaned up and that the site has entered the next phase of clean-up that will last until 2029. TMI-2 Solutions planned to finish clean-up and demolish the plant by 2052.

Plant Ownership and Restart

TMI-1 restarted

1985

TMI-1 was restarted in 1985, then retired in 2019 due to operating losses.

NRC vote allows TMI-1 to resume operations

1985

In 1985, a 4–1 vote by the Nuclear Regulatory Commission allowed TMI-1 to resume operations.

GPU shortens its name to GPU Inc

1996

In 1996, General Public Utilities shortened its name to GPU Inc.

GPU sells TMI-1 to AmerGen Energy Corporation

1998

In 1998, GPU sold TMI-1 to AmerGen Energy Corporation, a joint venture between Philadelphia Electric Company and British Energy. GPU was legally obliged to continue to maintain and monitor TMI-2.

Philadelphia Electric merges with Unicom to form Exelon

2000

In 2000, Philadelphia Electric merged with Unicom Corporation to form Exelon.

GPU acquired by FirstEnergy and dissolved

2001

In 2001, GPU was acquired by FirstEnergy Corporation and dissolved, and the maintenance and administration of Unit 2 was contracted to AmerGen.

Exelon buys remaining AmerGen shares

2003

In 2003, Exelon bought the remaining shares of AmerGen from British Energy.

NRC grants license extension for TMI-1

2009

In 2009, the NRC granted a license extension which allowed the TMI-1 reactor to operate until April 19, 2034.

Exelon Nuclear absorbs and dissolves AmerGen

2009

In 2009, Exelon Nuclear absorbed and dissolved AmerGen.

Announcement of planned shutdown by 2019

2017

In 2017, it was announced that operations would cease by 2019 due to financial pressure from inexpensive natural gas, unless lawmakers stepped in to keep it open.

TMI Unit 1 shuts down

Sep 20, 2019

TMI Unit 1 shut down on September 20, 2019, after being retired due to operating losses.

Constellation announces plans to restart the plant

Sep 2024

In September 2024, Constellation Energy announced plans to restart the Three Mile Island nuclear plant to sell the power to Microsoft for artificial intelligence data centers.

Unit 1 expected back online in 2027

2027

Constellation expects the Unit 1 reactor at Three Mile Island to come back online in 2027, subject to approval by the Nuclear Regulatory Commission. It is expected to return to service in either 2027 or 2028 as part of a deal to power Microsoft data centers. Constellation also plans to apply to extend the plant's operations to at least 2054.

Key Facts

  1. The accident began at approximately 4:00 a.m. on March 28, 1979, when a failure in the secondary, non-nuclear section of TMI-2 caused the reactor to shut down automatically.
  2. A stuck-open pilot-operated relief valve allowed coolant to escape from the reactor core, but misleading instrument readings led operators to believe the valve had closed, causing them to reduce emergency cooling water.
  3. Approximately half of the reactor's uranium fuel core melted during the incident, making it the most serious commercial nuclear accident in U.S. history, though containment structures prevented a major release of radiation.
  4. No deaths or immediate injuries resulted directly from the accident, and later studies found no measurable long-term health effects on residents near the plant, though public fear and evacuation of pregnant women and children caused significant social disruption.
  5. The accident cost roughly $1 billion to clean up, took nearly 14 years to complete, and led to major reforms by the Nuclear Regulatory Commission, including enhanced operator training, improved instrumentation, and stricter safety standards.

Source

This Three Mile Island Accident timeline was generated with the help of AI, using information found on the internet.

We work hard to keep these timelines accurate, but mistakes do get through. If you spot one, email us at [email protected] and we'll fix it for future visitors.

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