Three Mile Island Nuclear Generating Station (commonly abbreviated as TMI) is a closed nuclear power plant on Three Mile Island^{[lower-alpha 1]} in Pennsylvania on the Susquehanna River just south of Harrisburg. It has two separate units, TMI-1 (owned by Constellation Energy) and TMI-2 (owned by EnergySolutions).^[6]

The plant was the site of the most significant accident in United States commercial nuclear energy when, on March 28, 1979, TMI-2 suffered a partial meltdown. According to the Nuclear Regulatory Commission (NRC) report, the accident resulted in no deaths or injuries to plant workers or in nearby communities.^[7] Follow-up epidemiology studies did not find causality between the accident and any increase in cancers.^{[8][9][10][11]} One work-related death has occurred on-site during decommissioning.^[12]

The reactor core of TMI-2 has since been removed from the site, but the site has not been fully decommissioned.^[13] In July 1998, Amergen Energy (now Exelon Generation) agreed to purchase TMI-1 from General Public Utilities for $100 million.^[14]

The plant was originally built by General Public Utilities Corporation, later renamed GPU Incorporated.^[15] The plant was operated by Metropolitan Edison Company (Met-Ed), a subsidiary of the GPU Energy division. In 2001, GPU Inc. merged with FirstEnergy Corporation.^[16] On December 18, 2020 - FirstEnergy transferred Unit 2's license over to EnergySolutions' subsidiary TMI-2 Solutions after receiving approval from the NRC.^[17]

Exelon was operating Unit 1 at a financial loss since 2015.^[18] In 2017 the company said it would consider ceasing operations at Unit 1 because of high costs unless there was action from the Pennsylvania government.^[19][20] Unit 1 officially shut down at noon on September 20, 2019.^[21]

Unit 1 decommissioning is expected to be completed in 2079 and will cost $1.2 billion.^[22][23] Unit 2, which has been dormant since the accident in 1979, is expected to close in 2052.^[24]

The NRC defines two emergency planning zones around nuclear power plants: a plume exposure pathway zone with a radius of 10 miles (16 km), concerned primarily with exposure to, and inhalation of, airborne radioactive contamination, and an ingestion pathway zone of about 50 miles (80 km), concerned primarily with ingestion of food and liquid contaminated by radioactivity.^[25]

The 2010 U.S. population within 10 miles (16 km) of Three Mile Island was 211,261, an increase of 10.9 percent in a decade, according to an analysis of U.S. Census data. The 2010 U.S. population within 50 miles (80 km) was 2,803,322, an increase of 10.3 percent since 2000. Cities within 50 miles include Harrisburg (12 miles to city center), York (13 miles to city center), and Lancaster (24 miles to city center).^[26]

During its last full year of operation in 2018, Three Mile Island generated 7,355 GWh of electricity. In that same year, electricity from nuclear power produced approximately 39% of the total electricity generated in Pennsylvania (83.5 TWh nuclear of 215 TWh total), with Three Mile Island Generating Station contributing approximately 4% to the statewide total generation. In 2021 the state of Pennsylvania generated approximately 241 TWh total electricity.^[27]

The Three Mile Island Unit 1 is a pressurized water reactor designed by Babcock & Wilcox with a net generating capacity of 819 MW_e. The initial construction cost for TMI-1 was US$400 million, equal to $2.47 billion in 2018 dollars.^[29] Unit 1 first came online on April 19, 1974, and began commercial operations on September 2, 1974.^[30] TMI-1 is licensed to operate for 40 years from its first run, and in 2009, was extended 20 years, which means it could have operated until April 19, 2034.^[31][32]

TMI-1 had a closed-cycle cooling system for its main condenser using two natural draft cooling towers. Makeup water was drawn from the river to replace the water lost via evaporation in the towers. Once-through cooling with river water is used for the service water system which cools auxiliary components and removed decay heat when the reactor was shut down. On February 17, 1979, TMI-1 went offline for refueling. It was brought back online on October 9, 1985, after public opposition, several federal court injunctions, and some technical and regulatory complications - more than six years after it initially went offline.^[33]

Unit 1 was scheduled to be shut down by September 2019 after Exelon announced they did not receive any commitments for subsidies from the state, rendering Exelon unable to continue operating the reactor.^[34][35] TMI-1 was shut down on September 20, 2019.^[36]

The Three Mile Island Unit 2 was also a pressurized water reactor constructed by B&W, similar to Unit 1. The only difference was that TMI-2 was slightly larger with a net generating capacity of 906 MW_e, compared to TMI-1, which delivers 819 MW_e. Unit 2 received its operating license on February 8, 1978, and began commercial operation on December 30, 1978. TMI Unit 2 has been permanently shut off after the Three Mile Island accident in 1979.^[44]

Exelon Corporation was created in October 2000 by the merger of PECO Energy Company and Unicom, of Philadelphia and Chicago respectively.^[54] Unicom owned Commonwealth Edison. The PECO share in AmerGen was acquired by Exelon during late 2000. Exelon acquired British Energy's share in AmerGen in 2003,^[55] and transferred Unit 1 under the direct ownership and operation of its Exelon Nuclear business unit.^[56][57] According to Exelon Corporation, "many people are surprised when they learn that Three Mile Island is still making electricity, enough to power 800,000 households" from its undamaged and fully functional reactor unit 1.^[58] Exelon viewed the plant's economics of $44/MWh as challenging due to the low price of natural gas at $25/MWh. As of 2016, the average price of electricity in the area was $39/MWh.^[59]

On June 20, 2017, Exelon Generation, the owners of Three Mile Island's Unit 1, sent to the Nuclear Regulatory Commission a formal notice of its intention to shut down the plant on September 30, 2019,^[60] unless the Pennsylvania legislature rescued the nuclear industry, which was struggling to compete as newfound natural gas resources drove down electricity prices.^[61] Exelon Generation's Senior Vice President Bryan Hanson noted that once Three Mile Island was closed, it could never be reopened for use again.^[60] Hanson explicitly stated the reason for the shutdown is because of the unprofitability of Unit 1. Unit 1 has lost the company over $300 million over the last half-decade despite it being one of Exelon's best-performing power plants.

Shut down of Unit 1 can go in two possible directions, the first being the immediate dismantlement after the radioactive fuel has been moved away from the plant. The dismantlement can proceed after the spent fuel is removed from the pool, put into storage casks, and the casks are transferred to the ISFSI pad for storage until the DOE takes them away to a DOE repository. Dismantling the plant this way will take anywhere from 8 to 10 years.^[62] The second option Exelon could take is the long-term storage, which involves mothballing the plant and letting the radiation decay for up to 60 years on its own to a harmless level before completely dismantling the buildings. The advantage to the long-term storage is the lack of radiation when the dismantlement would begin but the disadvantage would be the possible lack of qualified workers at the time of dismantlement. Exelon would also have to pay for limited maintenance and security of the plant over the potential 60 years.^[62] The entirety of the spent fuel will be moved to the Londonderry Township facility, which is another process that could take decades to complete.^[60]

About 70 state legislators signed the industry-inspired Nuclear Caucus but made no financial commitments.^[61]

In April 2019, Exelon stated it would cost $1.2 billion over nearly 60 years to completely decommission Unit 1.^[63] Unit 1 closed on September 20, 2019.

In 2022, Unit 1 was transferred to Constellation Energy following separation from Exelon. Unit 2 was also transferred to TriArtisan ES Partners, LLC - following their acquisition of EnergySolutions.

Following the TMI-2 accident in 1979, approximately 99% of the fuel and damaged core debris was removed from the reactor vessel and associated systems and shipped to the Idaho National Laboratory near Idaho Falls, Idaho. Since 1993, when the initial cleanup of the plant was completed, TMI-2 has been in a condition known as Post Defueling Monitored Storage (PDMS) and is under constant monitoring to ensure the plant's safety and stability.

The cost of decommissioning a closed nuclear reactor and related structures at Three Mile Island is estimated at $918 million.^[64]

The Nuclear Regulatory Commission's estimate of the risk each year of an earthquake intense enough to cause core damage to the reactor at Three Mile Island was 1 in 25,000, according to an NRC study published in August 2010.^[65][66]

• Chernobyl disaster – 1986 nuclear accident in the Soviet Union
• Fukushima nuclear accident – 2011 nuclear disaster in Japan