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Three Mile Island Nuclear Generating Station

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Three Mile Island Nuclear Generating Station (TMI) is a nuclear power plant located on Three Mile Island in Londonderry Township, Pennsylvania, on the Susquehanna River just south of Harrisburg. It has two separate units, TMI-1 (owned by Exelon Generation) and TMI-2 (owned by FirstEnergy Corp).[3] The plant is widely known for having been the site of the most significant accident in United States commercial nuclear energy, on 28 March 1979, when TMI-2 suffered a partial meltdown. As per the Nuclear Regulatory Commission (NRC) report, the accident resulted in no deaths or injuries to plant workers or members of nearby communities.[4] Follow-up epidemiology studies have linked no incidents of cancer to the accident.[5][6][7][8] The reactor core of TMI-2 has since been removed from the site, but the site has not been decommissioned.[9] In July 1998, Amergen Energy (now Exelon Generation) agreed to purchase TMI-1 from General Public Utilities for $100 million.[10]

Three Mile Island Nuclear Generating Station
The Three Mile Island nuclear power plant on Three Mile Island, circa 1979
The Three Mile Island NPP on Three Mile Island, circa 1979
CountryUnited States
LocationLondonderry Township, Dauphin County, Pennsylvania
Coordinates40°9′14″N 76°43′29″W / 40.15389°N 76.72472°W / 40.15389; -76.72472Coordinates: 40°9′14″N 76°43′29″W / 40.15389°N 76.72472°W / 40.15389; -76.72472
Construction beganUnit 1: May 18, 1968
Unit 2: November 1, 1969
Commission dateUnit 1: September 2, 1974
Unit 2: December 30, 1978
Decommission dateUnit 2: March 28, 1979
Construction cost$1.557 billion (2007 USD)[1]
($2.00 billion in 2016 dollars[2])
Owner(s)Unit 1: Exelon
Unit 2: FirstEnergy
Operator(s)Exelon Nuclear
Nuclear power station
Reactor typePWR
Reactor supplierBabcock & Wilcox
Cooling towers4 × Natural Draft
Cooling sourceSusquehanna River
Thermal capacity1 × 2568 MWth
Power generation
Units operational1 × 819 MW
Make and modelB&W LLP (DRYAMB)
Units decommissioned1 × 880 MW
Nameplate capacity819 MW
Capacity factor95.65% (2017)
73.25% (lifetime)
Annual net output6862 GWh (2017)
External links
WebsiteThree Mile Island
CommonsRelated media on Commons

Three Mile Island is so named because it is located three miles downriver from Middletown, Pennsylvania.[11] The plant was originally built by General Public Utilities Corporation, later renamed GPU Incorporated.[12] The plant was operated by Metropolitan Edison Company (Met-Ed), a subsidiary of the GPU Energy division. During 2001 GPU Inc. merged with FirstEnergy Corporation.[13]

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.[14]

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 for 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).[15]

Exelon has been operating Unit 1 at Three MIle Island at a loss since 2015.[16] On May 30, 2017, the company said it would consider ceasing operations at Unit 1 in 2019 due to high costs of operating the plant unless there was government action.[17][18]

Unit 2, which has been dormant since the accident in 1979, is still owned by FirstEnergy, and it is estimated to close in 2036.[19]


Three Mile Island Unit 1Edit

The Three Mile Island Unit 1 is a pressurized water reactor designed by Babcock & Wilcox with a net generating capacity of 819 MWe. The initial construction cost for TMI-1 was US$400 million, equal to $2.03 billion today.[20] Unit 1 first came online on April 19, 1974, and began commercial operations on September 2, 1974.[21] TMI-1 is licensed to operate for 40 years from its first run, and in 2009, was extended 20 years, which means it may operate until April 19, 2034.[22][23] TMI-1 has a closed-cycle cooling system for its main condenser using two natural draft cooling towers. Makeup water is 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 removes decay heat when the reactor is shutdown. When TMI-2 suffered its accident in 1979, TMI-1 was offline for refueling. It was brought back online in October 1985, after public opposition, several federal court injunctions, and some technical and regulatory complications.[24]


In February 1993, a man drove his car past a checkpoint at the TMI nuclear plant, then broke through an entry gate. He eventually crashed the car through a secure door and entered the Unit 1 turbine building. The intruder, who had a history of mental illness, hid in the turbine building and was apprehended after four hours.

Three Mile Island from Goldsboro, Pennsylvania in 2013
Three Mile Island from Middletown, Pennsylvania in 2014

On November 21, 2009, a release of radioactivity occurred inside the containment building of TMI-1 while workers were cutting pipes. Exelon Corporation stated to the public that "A monitor at the temporary opening cut into the containment building wall to allow the new steam generators to be moved inside showed a slight increase in a reading and then returned to normal. Approximately 20 employees were treated for mild radiation exposure."[25] As of November 22, 2009, it is believed that no radiation escaped the containment building and the public is not in any danger. The inside airborne contamination was caused by a change in air pressure inside the containment building that dislodged small irradiated particles in the reactor piping system. Some of the small particles became airborne inside the building and were detected by an array of monitors in place to detect such material. The air pressure change occurred when inside building ventilation fans were started to support outage activities. The site has modified the ventilation system to prevent future air pressure changes. Work continued on the project the following day. On January 24, 2010, TMI-1 was brought back online.[26]

Three Mile Island Unit 2Edit

Unit 2 during its time in operation, viewed from the west
June 2010 Photo of Three Mile Island nuclear power plant with deactivated Unit 2 located on the left.

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 MWe, compared to TMI-1, which delivers 819 MWe. 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.[27]


Three Mile Island in background behind Harrisburg International Airport, a few weeks after the accident.

On March 28, 1979, there was a cooling system malfunction that caused a partial melt-down of the reactor core. This loss-of-coolant accident resulted in the release of an amount of radioactivity, estimated at 43,000 curies (1.59 PBq) of radioactive krypton-85 gas (half life 10 yrs), but less than 20 curies (740 GBq) of the especially hazardous iodine-131 (half life 8 days), into the surrounding environment.[4]

Nearly 2 million people were exposed to a small amount of radiation from the accident.[28] A review by the World Nuclear Association concluded that no deaths, injuries or adverse health effects resulted from the accident,[29] and a report by Columbia University epidemiologist Maureen Hatch confirmed this finding.[5][30] Because of the health concerns, the Pennsylvania Department of Health kept a registry of more than 30,000 people that lived within 5 miles of TMI at the time of the accident. The registry was kept for nearly 20 years until 1997, when no evidence was found of unusual health effects.[31] Further epidemiology studies have not shown any increase in cancer as a result of the accident.[6][7][8] However, almost $25 million was paid in insurance settlements to people who then agreed not to discuss their injuries in ongoing litigation.[32][better source needed]

Unit 2 has not been operational since the accident occurred.[33]

The New York Times reported on August 14, 1993, 14 years after the accident, that the clean up had been finished. According to the United States NRC, 2.3 million gallons of waste water had been removed.[34]

The incident was widely publicized internationally, and had far-reaching effects on public opinion, particularly in the United States. The China Syndrome, a movie about a nuclear disaster, which was released just 12 days before the incident and received a tepid reception from the movie-going public, became a blockbuster hit.[35]

Unit 2 GeneratorEdit

On January 22, 2010, officials at the NRC announced the electrical generator from the damaged Unit 2 reactor at TMI will be used at Shearon Harris Nuclear Plant in New Hill, North Carolina. The generator was transported in two parts, weighing a combined 670 tons. It was refurbished and installed during a refueling outage at Shearon Harris NPP in November 2010. TMI's Unit 2 reactor has been shut down since the partial meltdown in 1979.[36]


Exelon Corporation was created in October 2000 by the merger of PECO Energy Company and Unicom, of Philadelphia and Chicago respectively.[37] 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,[38] and transferred Unit 1 under the direct ownership and operation of its Exelon Nuclear business unit.[39][40] 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.[41] Exelon views the plant's economics of $44/MWh as challenging due to the low price of natural gas at $25/MWh, and may close it around 2019. As of 2016, the average price of electricity in the area is $39/MWh.[42]


Aerial view

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,[43] unless the Pennsylvania legislature rescues the nuclear industry, which is currently[when?] struggling to compete in a world where newfound natural gas resources have driven down electricity prices.[44] Exelon Generation's Senior Vice President Bryan Hanson noted that once Three Mile Island is closed, it can never be reopened for use again.[43] 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 dollars over the last half-decade despite its 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 immediately after the radioactive fuel has been moved away from the plant. The dismantlement can proceed after the spent fuel is removed from the casks. Dismantling the plant this way will take anywhere from 8 to 10 years.[45] 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 sixty years.[45] The entirety of the spent fuel will be moved to the Londonderry Township facility, which is another process that could take decades to complete.[43]

Mike Pries, a Dauphin County Commissioner says the value of the property, and the $1 million a year Exelon pays in property taxes, would plummet the moment Unit 1 shuts down, which would harshly hurt Pennsylvania's economy.[44] The Unit 1 plant manager says that the 675 workers remain committed to operating the plant safely and efficiently even though their future employment is uncertain.[44]

It is currently[when?] unknown if Pennsylvania legislators are willing to pay for the costs of nuclear energy. About 70 legislators signed the industry-inspired Nuclear Caucus this year[when?] but have made no commitments.[44]

Seismic riskEdit

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.[46][47]

See alsoEdit


  1. ^ "EIA - State Nuclear Profiles". United States Energy Information Administration. Archived from the original on 2017-05-19. Retrieved 3 October 2017.
  2. ^ Thomas, Ryland; Williamson, Samuel H. (2018). "What Was the U.S. GDP Then?". MeasuringWorth. Retrieved January 5, 2018. United States Gross Domestic Product deflator figures follow the Measuring Worth series.
  3. ^ Maykuth, Andrew (2019-02-28). "Peach Bottom, other U.S. nuclear power plants could be running until 2054. Is it safe?". Philadelphia Inquirer. Retrieved 2019-03-25.
  4. ^ a b "Fact Sheet on the Three Mile Island Accident". US Nuclear Regulatory Commission. Retrieved 2008-12-18.
  5. ^ a b Maureen C. Hatch; et al. (1990). "Cancer near the Three Mile Island Nuclear Plant: Radiation Emissions". American Journal of Epidemiology. Oxford Journals. 132 (3): 397–412. PMID 2389745.
  6. ^ a b Levin RJ (2008). "Incidence of thyroid cancer in residents surrounding the three mile island nuclear facility". Laryngoscope. 118 (4): 618–628. doi:10.1097/MLG.0b013e3181613ad2. PMID 18300710. Thyroid cancer incidence has not increased in Dauphin County, the county in which TMI is located. York County demonstrated a trend toward increasing thyroid cancer incidence beginning in 1995, approximately 15 years after the TMI accident. Lancaster County showed a significant increase in thyroid cancer incidence beginning in 1990. These findings, however, do not provide a causal link to the TMI accident.
  7. ^ a b Hatch MC, Wallenstein S, Beyea J, Nieves JW, Susser M (June 1991). "Cancer rates after the Three Mile Island nuclear accident and proximity of residence to the plant". American Journal of Public Health. 81 (6): 719–724. doi:10.2105/AJPH.81.6.719. PMC 1405170. PMID 2029040.
  8. ^ a b
  9. ^ "Accident at Three Mile Island". Policy Almanac. Retrieved 2008-12-18.
  10. ^ STAFF, CBS21 WEB. "History of Three Mile Island". WHP. Retrieved 2017-12-07.
  11. ^ "Middletown – Three mile Island". Retrieved 2012-05-15.
  12. ^ "GPU, Inc. – Company History". Retrieved 2009-04-01.
  13. ^ "FirstEnergy — Company history". Retrieved 2009-04-01.
  14. ^
  15. ^ Bill Dedman, Nuclear neighbors: Population rises near US reactors,, April 14, 2011 Accessed May 1, 2011.
  16. ^ Thompson, Charles (2019-03-19). "Save Three Mile Island? What a difference 40 years makes". Retrieved 2019-03-25.
  17. ^ Bade, Gavin (30 May 2017). "After failing to clear PJM auction, Exelon says Three Mile Island nuke will close in 2019". Utility Dive. Retrieved 30 May 2017.
  18. ^ "Middletown – Three mile Island". Retrieved 2019-02-03.
  19. ^ "NRC: Three Mile Island - Unit 2". Retrieved 2019-03-25.
  20. ^ "A Corporate History of Three Mile Island | Three Mile Island Alert". Retrieved 2016-11-11.
  21. ^ "Three Mile Island". Amerigen — Exelon Corporation. Retrieved 2013-02-21.
  22. ^ "Three Mile Island 1 – Pressurized Water Reactor". Nuclear Regulatory Commission. Retrieved 2008-12-15.
  23. ^ DiSavino, Scott (October 22, 2009). "NRC renews Exelon Pa. Three Mile Isl reactor license". Thomson Reuters. Retrieved 2009-10-23.
  24. ^ Stephanie Cooke (2009). In Mortal Hands: A Cautionary History of the Nuclear Age, Black Inc., p. 299.
  25. ^ "Three Mile Island radiation leak investigated -". CNN. November 22, 2009. Retrieved 2010-05-25.
  26. ^ "Radiation leak at Three Mile Island". ABC News — WPVI Philadelphia. Retrieved 2009-11-22.
  27. ^ "Three Mile Island — Unit 2". Nuclear Regulatory Commission. Retrieved 2008-12-15.
  28. ^ "Three Mile Island - Facts & Summary -". Retrieved 2017-12-07.
  29. ^ "Three Mile Island: 1979". World Nuclear Association. Retrieved 2008-12-18.
  30. ^ The Washington Post. March 30, 1999 Retrieved 2010-05-25. Missing or empty |title= (help)
  31. ^ "Three Mile Island | TMI 2 |Three Mile Island Accident. - World Nuclear Association". Retrieved 2017-12-07.
  32. ^
  33. ^ "Three Mile Island - Unit 2". Archived from the original on 2006-10-04.
  34. ^ News, A. B. C. (2017-05-31). "A history of the Three Mile Island nuclear plant". ABC News. Retrieved 2017-12-07.
  35. ^ "FAQ for the China Syndrome". IMDB. Retrieved 2008-12-29.
  36. ^ "Three Mile Island generator moving to Shearon Harris". WRAL-TV. Retrieved 2010-01-22.
  37. ^ "Exelon — Merger Filing". Archived from the original on February 10, 2009. Retrieved 2009-04-01.
  38. ^ "A Corporate History of Three Mile Island — Three Mile Island Alert". Retrieved 2009-04-01.
  39. ^ "Exelon — Three Mile Island Unit – 1". Retrieved 2009-04-01.
  40. ^ "Three Mile Island: About TMI — About Us". Retrieved 2009-04-01.
  41. ^ Allen Abel, Life after a meltdown: Locals near Three Mile Island may be wary, but they aren't moving, The National Post, Saturday, March 19, 2011, p.A5
  42. ^ Barrett, Paul (2016-12-22). "States Are the Nuclear Industry's Best Hope". Retrieved 2017-01-12. unless the government intervenes to keep the plant running, the notorious facility's "long-term future past 2019" is in doubt.
  43. ^ a b c "Three Mile Island operator takes another step toward closing nuclear plant". 2017-06-23. Retrieved 2017-12-07.
  44. ^ a b c d "Three Mile Island fights once again for its nuclear survival". Retrieved 2017-12-07.
  45. ^ a b Writer, AD CRABLE | Staff. "After announcement of closure, what's next for Three Mile Island nuclear plant?". LancasterOnline. Retrieved 2017-12-07.
  46. ^ Bill Dedman, "What are the odds? US nuke plants ranked by quake risk,", March 17, 2011 Accessed April 19, 2011.
  47. ^ "Archived copy" (PDF). Archived from the original (PDF) on May 25, 2017. Retrieved May 5, 2017.CS1 maint: Archived copy as title (link)

External linksEdit