Three Mile Island Nuclear Generating Station

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Three Mile Island Nuclear Generating Station
Three Mile Island (color)-2.jpg
The Three Mile Island NPP on Three Mile Island, circa 1979
CountryUnited States
Coordinates40°09′07″N 76°43′30″W / 40.15205°N 76.72491°W / 40.15205; -76.72491
Construction began1968–1970
Commission dateSeptember 2, 1974
Operator(s)Exelon Nuclear
Nuclear power station
External links
WebsiteThree Mile Island
CommonsRelated media on Commons

Three Mile Island Nuclear Generating Station (TMI) is a civilian nuclear power plant (NPP) located on Three Mile Island in the Susquehanna River, south of Harrisburg, Pennsylvania. It has two separate units, known as TMI-1 and TMI-2. The plant is widely known for having been the site of the most significant accident in United States commercial nuclear energy, on March 28, 1979, when TMI-2 suffered a partial meltdown. According to the US Nuclear Regulatory Commission, the accident resulted in no deaths or injuries to plant workers or members of nearby communities.[1] The reactor core of TMI-2 has since been removed from the site, but the site has not been decommissioned.[2]

Three Mile Island is so named because it is located three miles downriver from Middletown, Pennsylvania. The plant was originally built by General Public Utilities Corporation, later renamed GPU Incorporated.[3] 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, through the selling of its outstanding common stock.[4]

Surrounding population

The Nuclear Regulatory Commission 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.[5]

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), Lancaster (24 miles to city center).[6]

Three Mile Island Unit 1

The Three Mile Island Unit 1 is a pressurized water reactor designed by Babcock and Wilcox with a net generating capacity of 802 MWe. The initial construction cost was 400 million US$, equal to $2,073,684,211 today. Unit 1 first came online on April 19, 1974, and began commercial operations on September 2, 1974.[7] 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.[8][9] 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.[10]

On November 21, 2009, a radiation leak 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."[11] 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.[12]


Exelon Corporation was created in October 2000 by the merger of PECO Energy Company and Unicom, of Philadelphia and Chicago respectively.[13] 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,[14] and transferred the plant under the direct ownership and operation of its Exelon Nuclear business unit.[15][16] 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.[17]

June 2010 Photo of Three Mile Island nuclear power plant with deactivated Unit 2 located on the left.

Three Mile Island Unit 2

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 802 MWe. Unit 2 received its operating license on February 8, 1978, and began commercial operation on December 30, 1978.[18]


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 a significant 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.[1]

The nuclear power industry claims that there were no deaths, injuries or adverse health effects from the accident,[19] and a report by Columbia University epidemiologist Maureen Hatch agrees with this finding.[20] Another study by Steven Wing of the University of North Carolina found that lung cancer and leukemia rates were 2 to 10 times higher downwind of TMI than upwind.[21] The Radiation and Public Health Project, an anti-nuclear organization, reported a spike in infant mortality in the downwind communities two years after the accident.[22][23]

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 incident, which was released just 12 days before the disaster, became a blockbuster hit.[24]

Unit 2 Generator

On January 22, 2010 officials at the Nuclear Regulatory Commission 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. Preliminary work is under way to move the generator. It will be transported in two parts, weighing a combined 670 tons. TMI's Unit 2 reactor has been shut down since the partial meltdown in 1979.[25]

Seismic risk

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.[26][27]

See also


  1. ^ a b "Fact Sheet on the Three Mile Island Accident". US Nuclear Regulatory Commission. Retrieved 2008-12-18.
  2. ^ "Accident at Three Mile Island". Policy Almanac. Retrieved 2008-12-18.
  3. ^ "GPU, Inc. -- Company History". Retrieved 2009-04-01.
  4. ^ "FirstEnergy — Company history". Retrieved 2009-04-01.
  5. ^
  6. ^ Bill Dedman, Nuclear neighbors: Population rises near US reactors,, April 14, 2011 Accessed May 1, 2011.
  7. ^ "Power Generation: Three Mile Island Unit 1". Amerigen — Exelon Corporation. Retrieved 2008-12-15.
  8. ^ "Three Mile Island 1 - Pressurized Water Reactor". Nuclear Regulatory Commission. Retrieved 2008-12-15.
  9. ^ DiSavino, Scott (2009-10-22). "NRC renews Exelon Pa. Three Mile Isl reactor license". Thomson Reuters. Retrieved 2009-10-23.
  10. ^ Stephanie Cooke (2009). In Mortal Hands: A Cautionary History of the Nuclear Age, Black Inc., p. 299.
  11. ^ "Three Mile Island radiation leak investigated -". CNN. 2009-11-22. Retrieved 2010-05-25.
  12. ^ "Radiation leak at Three Mile Island". ABC News — WPVI Philadelphia. Retrieved 2009-11-22.
  13. ^ "Exelon — Merger Filing". Retrieved 2009-04-01.[dead link]
  14. ^ "A Corporate History of Three Mile Island — Three Mile Island Alert". Retrieved 2009-04-01.
  15. ^ "Exelon — Three Mile Island Unit - 1". Retrieved 2009-04-01.
  16. ^ "Three Mile Island: About TMI — About Us". Retrieved 2009-04-01.
  17. ^ 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
  18. ^ "Three Mile Island — Unit 2". Nuclear Regulatory Commission. Retrieved 2008-12-15.
  19. ^ "Three Mile Island: 1979". World Nuclear. Retrieved 2008-12-18.
  20. ^ The Washington Post. 1999-03-30 Retrieved 2010-05-25. Missing or empty |title= (help)
  21. ^ David Williamson (1997-02-24). "Study suggests Three Mile Island radiation may have injured people living near reactor". UNC-CH News Services. Retrieved 2008-12-29.
  22. ^ David Teather (2004-04-13). "US nuclear industry powers back into life". The Guardian. London. Retrieved 2008-12-29.
  23. ^ Andy Newman (2003-11-03). "In Baby Teeth, a Test of Fallout; A Long-Shot Search for Nuclear Peril in Molars and Cuspids". The New York Times. Retrieved 2009-03-31.
  24. ^ "FAQ for the China Syndrome". IMDB. Retrieved 2008-12-29.
  25. ^ "Three Mile Island generator moving to Shearon Harris". WRAL. Retrieved 2010-01-22.
  26. ^ Bill Dedman, "What are the odds? US nuke plants ranked by quake risk,", March 17, 2011 Accessed April 19, 2011.
  27. ^

External links