Three Mile Island Nuclear Generating Station
| Company type | Decommissioned nuclear power plant |
|---|---|
| Industry | Nuclear power generation |
| Headquarters | Harrisburg , United States |
Area served | Pennsylvania, United States |
Key people | CEO of Constellation Energy (name not specified) |
| Products | Electricity generation (nuclear) |
| Owner | Constellation Energy (Unit 1), EnergySolutions (Unit 2) |
Three Mile Island Nuclear Generating Station (TMI) is a decommissioned nuclear facility situated on Three Mile Island in Pennsylvania, United States, adjacent to the Susquehanna River near Harrisburg. The complex consists of two reactors: Unit 1, which is under the ownership of Constellation Energy, and Unit 2, held by EnergySolutions.
This site is historically notable for hosting the most severe accident in the history of commercial nuclear power in the U.S., when Unit 2 experienced a partial core meltdown on March 28, 1979. According to the U.S. Nuclear Regulatory Commission (NRC), no fatalities or injuries resulted from this event, and later research found no evidence linking the incident to increased cancer rates among nearby populations. However, there was one fatality related to decommissioning activities.
Although the core from Unit 2 has been extracted, the facility remains only partially dismantled. In July 1998, Amergen Energy (now Exelon Generation) acquired Unit 1 for $100 million. The plant was initially constructed by General Public Utilities Corporation (later GPU Incorporated) and operated by its subsidiary Metropolitan Edison Company. GPU merged with FirstEnergy Corporation in 2001. On December 18, 2020, following NRC approval, FirstEnergy transferred the license for Unit 2 to TMI-2 Solutions, a subsidiary of EnergySolutions.
Since 2015, Exelon operated Unit 1 at a loss and contemplated shutting it down due to high operational expenses without federal assistance. The unit ceased operations officially on September 20, 2019. The estimated cost for decommissioning Unit 1 was $1.2 billion with completion anticipated by 2079. Nevertheless, in September 2024, Constellation Energy revealed plans to invest $1.6 billion to reactivate Unit 1 as the Crane Clean Energy Center (CCEC), aiming for a restart by 2028. All power produced will be supplied to Microsoft under a two-decade contract supporting its expanding data center infrastructure.
Unit 2 has remained inactive since the accident and is planned to be closed permanently by 2052.
Emergency zones and nearby population
The NRC establishes two emergency planning zones around nuclear facilities: a 10-mile radius plume exposure pathway zone addressing airborne radioactive hazards, and a broader 50-mile ingestion pathway zone concerning contaminated food and water supplies.
In the year 2010, the population residing within a 10-mile radius of Three Mile Island was recorded at 211,261 people, marking an increase of 10.9% since 2000. Within a 50-mile radius, the population reached approximately 2,803,322 individuals, reflecting a growth of 10.3%. Nearby urban centers include Harrisburg (12 miles away), York (13 miles), and Lancaster (24 miles).
Electricity production
During its last full operational year in 2018, Three Mile Island produced 7,355 gigawatt-hours (GWh) of electricity. Nuclear energy contributed roughly 39% to Pennsylvania’s total electricity generation that year—83.5 terawatt-hours (TWh) out of an overall 215 TWh—with Three Mile Island accounting for about 4% of the state’s total output. By 2021, Pennsylvania's total electricity generation had increased to nearly 241 TWh.
| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Total Annual Generation |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2001 | 598,586 | 558,484 | 617,640 | 540,691 | 309,720 | 427,684 | 591,460 | 588,699 | 562,563 | 130,208 | 0 | 491,028 | 5,416,763 |
| 2002 | 628,077 | 569,814 | 631,547 | 606,413 | 622,103 | 575,870 | 609,816 | 609,997 | 595,565 | 619,838 | 610,530 | 633,949 | 7,313,519 |
| 2003 | 631,984 | 572,521 | 631,971 | 605,628 | 624,685 | 597,862 | 610,219 | 609,588 | 592,306 | 309,494 | |||
| 418,815 | 6,197,031 | ||||||||||||
| 2004 | 628,992 | 592,136 | 630,817 | 606,796 | 595,895 | 590,383 | 593,006 | 609,964 | 594,931 | 628,330 | 606,727 | 595,257 | 7,273,234 |
| 2005 | 634,556 | 574,716 | 633,072 | 603,671 | 622,371 | 590,245 | 606,729 | 606,110 | 590,523 | 431,976 | 227,852 | 633,640 | 6,755,461 |
(Additional annual data omitted for brevity)
Three Mile Island Unit 1
The Three Mile Island Unit 1 is a pressurized water reactor engineered by Babcock & Wilcox, boasting a net output of 819 MWe. Its original construction expenditure amounted to , which corresponds to $ billion when adjusted to 2018 values. Unit 1 commenced operation on September 2, 1974, initially holding a 40-year license that was subsequently extended by two decades, now valid until April 19, 2034.
TMI-1 operated with a closed-loop cooling system featuring two natural draft cooling towers. It sourced makeup water from the adjacent river to compensate for evaporation losses. Additionally, river water was employed to cool auxiliary equipment and dissipate decay heat during reactor shutdown periods. After temporarily ceasing operations for refueling on February 17, 1979, TMI-1 restarted on October 9, 1985, overcoming public resistance, federal injunctions, and regulatory hurdles.
The planned shutdown for Unit 1 was set for September 2019 after Exelon was unable to obtain state subsidies, rendering continued operation financially impractical. The reactor officially went offline on September 20, 2019. Nevertheless, the CEO of Constellation Energy, the current proprietor, indicated that the reactor remains in "excellent condition" and could be reactivated within three years if a decision to restart is made. As of 2024, Constellation is performing evaluations to determine the feasibility of restarting the unit.
Incidents
In February 1993, an individual with a documented history of mental illness bypassed a security checkpoint at the TMI facility, breached an entry gate, and collided with the Unit 1 turbine building. The person was detained after remaining concealed there for four hours.
Following the events of September 11, 2001, concerns emerged that United Airlines Flight 93 might have targeted Three Mile Island. The NRC responded by raising security levels at all U.S. nuclear plants to the maximum tier. Flight 93 ultimately crashed in Stonycreek Township, Pennsylvania—approximately 135 miles west of Three Mile Island—with Washington, D.C., believed to have been its intended destination.
On November 21, 2009, a minor radioactive release occurred inside the containment structure of TMI-1 during pipe-cutting activities. Exelon reported detecting a slight increase in radiation at a temporary opening used to introduce new steam generators. Around 20 employees received treatment for low-level exposure. No radioactive material escaped containment, and no threat was posed to the public. The airborne contamination resulted from an internal air pressure fluctuation that dislodged irradiated particles within the reactor piping system. Ventilation systems were subsequently adjusted to prevent similar incidents, and work resumed the following day. TMI-1 returned to operation on January 24, 2010.
Material handling accident
On September 10, 2021, a contractor from Alabama suffered fatal injuries while unloading equipment from a truck. Emergency responders from Londonderry Township declared the individual dead on arrival. The Nuclear Regulatory Commission confirmed that this work-related incident occurred outside the radiological controlled zone.
Three Mile Island Unit 2
The Three Mile Island Unit 2 was also a pressurized water reactor constructed by B&W and bore similarity to Unit 1 but featured a slightly higher net generating capacity of 906 MWe, compared with TMI-1’s 819 MWe. It obtained its operating license on February 8, 1978, and began commercial operations on December 30 of that year. Unit 2 was permanently shut down following the Three Mile Island accident in 1979.
Incident
On March 28, 1979, a failure in the cooling system led to a partial nuclear meltdown within the reactor core. This loss-of-coolant accident resulted in the release of approximately 43,000 curies (1.59 PBq) of radioactive krypton-85 gas and less than 20 curies (740 GBq) of iodine-131 into the surrounding environment.
Radiation exposure affected close to 2 million individuals. An assessment conducted by the World Nuclear Association determined that no fatalities or negative health consequences arose from the event, a conclusion supported by epidemiologist Maureen Hatch from Columbia University. The Pennsylvania Department of Health maintained a registry tracking over 30,000 residents living within proximity of TMI for nearly two decades, finding no unusual health outcomes. Additional epidemiological research has not shown elevated cancer incidence. Nonetheless, nearly $25 million was disbursed through insurance settlements to claimants who agreed to confidentiality regarding their injuries.
Since the incident, Unit 2 has remained offline.
By August 1993, cleanup operations were finalized with the removal of 2.3 million gallons of contaminated wastewater, according to the NRC.
The event significantly influenced public perception and nuclear energy policy across the United States. The movie The China Syndrome, which premiered shortly before the accident, achieved blockbuster success amid increased public apprehension.
Unit 2 Generator
On January 22, 2010, the NRC revealed plans to repurpose the electrical generator from the damaged Unit 2 reactor at TMI for use at Shearon Harris Nuclear Plant located in New Hill, North Carolina. The 670-ton generator was transported in two segments, refurbished, and installed during a scheduled refueling outage at Shearon Harris NPP in November 2010.
Post-accident
Exelon Corporation was established in October 2000 through the merger of PECO Energy Company and Unicom, which owned Commonwealth Edison. Exelon acquired PECO’s stake in AmerGen later that year and subsequently purchased British Energy’s share in 2003, thereby consolidating ownership of Three Mile Island Unit 1 under its Exelon Nuclear division. Despite its legacy, Unit 1 continues to produce electricity sufficient to power approximately 800,000 homes. However, Exelon has highlighted economic pressures stemming from low natural gas prices—$44/MWh for the plant compared to $25/MWh for natural gas and a regional average electricity price of $39/MWh as of 2016—raising concerns about the plant’s sustainability without governmental intervention.
Closure
On June 20, 2017, Exelon Generation, owner of Three Mile Island Unit 1, informed the Nuclear Regulatory Commission of its plan to retire the plant by September 30, 2019, unless legislative action in Pennsylvania provided support for the struggling nuclear industry amid suppressed electricity prices caused by natural gas competition. Exelon’s Senior Vice President Bryan Hanson confirmed that Unit 1’s closure was driven by unprofitability, with losses exceeding $300 million over five years despite robust operational performance.
Approximately 70 state legislators joined an industry-supported Nuclear Caucus but did not extend financial assistance.
In April 2019, Exelon projected that decommissioning Unit 1 would cost $1.2 billion over an estimated period approaching six decades. Unit 1 officially ceased generating power on September 20, 2019.
In 2022, ownership of Unit 1 transferred to Constellation Energy following its separation from Exelon. Meanwhile, Unit 2 was acquired by TriArtisan ES Partners, LLC after their purchase of EnergySolutions.
Decommissioning
Active site remediation at TMI-2 Solutions commenced in 2023. A provisional storage facility will temporarily house radioactive waste prior to its final disposal. Given the elevated radiation levels, much of the dismantling process will be conducted using remotely controlled machinery. Although initially anticipated to conclude by 2037, the decommissioning timeline has been extended, with completion now expected by 2052.
Following the 1979 incident at TMI-2, approximately 99% of the nuclear fuel and damaged core fragments were extracted and transported to the Idaho National Laboratory. Since the cleanup's conclusion in 1993, TMI-2 has remained in Post Defueling Monitored Storage (PDMS), continuously monitored for safety. The removal of the remaining 1% of residual fuel is planned for completion by 2029.
The projected expenditure for dismantling the reactor and associated structures at Three Mile Island is estimated at $918 million.
Reopening and Microsoft partnership
On September 20, 2024, Constellation Energy revealed intentions to restart operations at Three Mile Island Unit 1 by 2028, renaming it the Christopher M. Crane Clean Energy Center in honor of former Exelon CEO Chris Crane. Crane began his career in nuclear energy as a Reactor Operator with ComEd, retired in December 2022, and passed away in April 2024.
Microsoft has entered into a 20-year power purchase agreement to supply energy for its growing AI data center network. Constellation plans to invest $1.6 billion in plant modernization efforts, subject to regulatory approval.
Since TMI-1's shutdown in 2019, critical infrastructure components such as safety diesel fuel tanks and transformers have required significant replacement efforts, including a $100 million investment for new transformers scheduled by 2025. Inspections confirmed that steam generators remain in satisfactory condition. No substantial delays in major component availability have been identified, supporting the target reopening date of 2028, with a possibility of an earlier restart in 2027.
Seismic risk
According to a 2010 assessment by the Nuclear Regulatory Commission, the annual probability of an earthquake causing core damage at Three Mile Island is estimated at one in 25,000.
See also
External links
- Three Mile Island operator's official website
- Three Mile Island – Three Mile Island Alert