History of Three Mile Island Nuclear Generating Station in Timeline

The Three Mile Island Nuclear Generating Station, located near Harrisburg, Pennsylvania, is a decommissioned nuclear power plant. It comprises two distinct units, TMI-1 and TMI-2. The plant gained notoriety due to a partial meltdown incident at TMI-2 on March 28, 1979, considered the most severe nuclear accident in U.S. history. This event led to significant changes in nuclear plant regulations and emergency response protocols.

April 19, 1974: Unit 1 Begins Operation

Unit 1 of the Three Mile Island nuclear power plant first came online on April 19, 1974.

September 2, 1974: Unit 1 Begins Commercial Operations

Unit 1 of the Three Mile Island nuclear power plant commenced commercial operations on September 2, 1974.

February 8, 1978: TMI-2 Receives Operating License

On February 8, 1978, Three Mile Island Unit 2, a pressurized water reactor, was granted an operating license.

December 30, 1978: TMI-2 Begins Commercial Operation

Three Mile Island Unit 2 commenced commercial operation on December 30, 1978.

February 17, 1979: Unit 1 Goes Offline for Refueling

Unit 1 of the Three Mile Island nuclear plant was taken offline on February 17, 1979 for refueling.

March 28, 1979: Three Mile Island Accident

The Three Mile Island nuclear plant experienced a partial meltdown on March 28, 1979, which is considered the most significant accident in U.S. commercial nuclear energy history.

1979: Fuel Removal and Cleanup Begins at TMI-2

Following the accident in 1979, the process of removing fuel and damaged core debris from Three Mile Island Unit 2 began.

1979: Three Mile Island Accident

The Three Mile Island accident occurred in 1979, leading to the permanent shutdown of Unit 2.

1979: Unit 2 Dormant Since Accident

Unit 2 of the Three Mile Island nuclear power plant has remained inactive since the accident in 1979.

October 9, 1985: Unit 1 Restarts After Public Opposition

After being offline for over six years due to public concerns and legal battles, Unit 1 of the Three Mile Island nuclear plant was brought back online on October 9, 1985.

February 1993: Security Breach at TMI

In February 1993, an individual with a history of mental illness bypassed security checkpoints at the TMI nuclear plant, drove into the facility, and entered the Unit 1 turbine building, raising concerns about security protocols.

August 14, 1993: Cleanup of Three Mile Island Declared Complete

On August 14, 1993, the New York Times reported the completion of the Three Mile Island cleanup, 14 years after the accident.

1993: TMI-2 Enters Post Defueling Monitored Storage

After the initial cleanup in 1993, Three Mile Island Unit 2 entered a state of Post Defueling Monitored Storage (PDMS).

1997: Pennsylvania Department of Health Concludes TMI Health Registry

In 1997, after nearly 20 years of monitoring, the Pennsylvania Department of Health concluded its registry of residents living near Three Mile Island, finding no evidence of unusual health effects from the 1979 accident.

July 1998: Amergen Energy Purchases TMI-1

In July 1998, Amergen Energy (now Exelon Generation) bought TMI-1 from General Public Utilities.

October 2000: Exelon Corporation Formed, Acquires TMI Unit 1

Exelon Corporation was formed in October 2000 through a merger, acquiring ownership of Three Mile Island Unit 1.

2000: Population Data Used for Comparison

The 2000 U.S. Census data was used as a benchmark to analyze the population growth around Three Mile Island in 2010.

September 11, 2001: National Security Alert Impacts TMI

Following the September 11, 2001 attacks, there were concerns that United Airlines Flight 93 was targeting Three Mile Island, leading to heightened security measures at nuclear plants nationwide.

2001: GPU Inc. Merges with FirstEnergy Corporation

GPU Inc., the parent company of Three Mile Island, merged with FirstEnergy Corporation in 2001.

2003: Exelon Assumes Full Ownership of TMI Unit 1

Exelon acquired full ownership of Three Mile Island Unit 1 in 2003, bringing it under the direct operation of its Exelon Nuclear business unit.

November 21, 2009: Radioactivity Release at TMI-1

On November 21, 2009, a release of radioactivity occurred inside the TMI-1 containment building during pipe-cutting work, prompting an investigation and raising safety concerns.

November 22, 2009: Public Safety Assurances Following Release

Following the radioactivity release on November 21, 2009, assurances were given on November 22, 2009, that no radiation had escaped the containment building and the public was not at risk.

2009: Operating License for Unit 1 Extended

In 2009, the operating license for Unit 1 of the Three Mile Island nuclear plant was granted a 20-year extension.

January 22, 2010: TMI-2 Generator to be Reused

On January 22, 2010, the NRC announced that the electrical generator from the damaged Unit 2 at Three Mile Island would be refurbished and reused at the Shearon Harris Nuclear Plant in North Carolina.

January 24, 2010: TMI-1 Returns to Service

On January 24, 2010, TMI-1 was brought back online after an incident involving the release of radioactivity.

August 2010: NRC Study Estimates Earthquake Risk at TMI

An NRC study published in August 2010 estimated a 1 in 25,000 chance of an earthquake severe enough to cause core damage to the reactor at Three Mile Island.

November 2010: Refurbished TMI-2 Generator Installed at Shearon Harris NPP

The refurbished generator from Three Mile Island Unit 2 was installed at the Shearon Harris Nuclear Power Plant in November 2010.

2010: Population Growth Around Three Mile Island

In 2010, the U.S. population within a 10-mile radius of Three Mile Island saw a 10.9% increase over the previous decade, reaching 211,261.

2015: Exelon Operating Unit 1 at Financial Loss

Exelon began operating Unit 1 of the Three Mile Island nuclear plant at a financial loss starting in 2015.

2016: Exelon Notes Economic Challenges of TMI Unit 1

In 2016, Exelon highlighted the economic challenges facing Three Mile Island Unit 1 due to the low price of natural gas.

June 20, 2017: Exelon Announces Intention to Shut Down TMI Unit 1

On June 20, 2017, Exelon announced its plan to shut down Three Mile Island Unit 1 on September 30, 2019, citing economic challenges due to competition from natural gas.

2017: Exelon Considers Ceasing Operations at Unit 1

In 2017, due to high costs, Exelon considered shutting down operations at Unit 1 unless the Pennsylvania government provided support.

2018: Three Mile Island's Electricity Generation

During its last full year of operation in 2018, Three Mile Island generated 7,355 GWh of electricity.

2018: Construction Cost of TMI-1 Adjusted for Inflation

The initial construction cost of TMI-1, which was US$400 million, is equivalent to $2.47 billion in 2018 dollars.

April 2019: Exelon Estimates Decommissioning Cost for TMI Unit 1

In April 2019, Exelon estimated a cost of $1.2 billion over 60 years to fully decommission Three Mile Island Unit 1.

September 2019: Scheduled Shutdown of Unit 1

Unit 1 of the Three Mile Island plant was scheduled to be shut down by September 2019 following Exelon's announcement that they had not secured any commitments for state subsidies.

September 20, 2019: Unit 1 Officially Shuts Down

Unit 1 of the Three Mile Island nuclear plant was officially shut down on September 20, 2019, due to financial losses and lack of government subsidies.

September 30, 2019: Planned Shutdown Date for TMI Unit 1

September 30, 2019, was the planned shutdown date for Three Mile Island Unit 1, as announced by Exelon.

December 18, 2020: FirstEnergy Transfers Unit 2 License

FirstEnergy transferred the license for Unit 2 to EnergySolutions' subsidiary, TMI-2 Solutions on December 18, 2020, following approval from the NRC.

September 10, 2021: Fatal Accident at TMI

On September 10, 2021, a contractor tragically died at the Three Mile Island nuclear plant while unloading equipment, marking a somber event in the plant's history.

2021: Total Electricity Generation in Pennsylvania

In 2021, electrical generating facilities across Pennsylvania produced approximately 241 TWh of electricity.

2022: Transfer of Ownership for TMI Units 1 and 2

In 2022, ownership of Three Mile Island Units 1 and 2 was transferred to Constellation Energy and TriArtisan ES Partners, LLC, respectively.

September 2024: Constellation Energy Announces Plans to Reopen Unit 1

Constellation Energy revealed plans to invest $1.6 Billion in September 2024, aiming to bring the Three Mile Island facility back online.

September 20, 2024: Constellation Energy Announces Plans to Reopen TMI Unit 1

On September 20, 2024, Constellation Energy announced plans to reopen Three Mile Island Unit 1, renamed the Christopher M. Crane Clean Energy Center, in 2028.