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Three Mile Island

Three Mile Island

Three Mile Island is a nuclear power plant located in central Pennsylvania about 10 miles south of Harrisburg in Londonderry Township.

Exelon owns and operates Unit 1 of the power plant which is built on an island in the Susquehanna River and began commercial operation on September 2, 1974. Three Mile Island Unit 1 is a pressurized water reactor designed by Babcock and Wilcox.  The unit is capable of generating 852 net megawatts (MW), enough electricity to power over 800,000 average American homes.

Three Mile Island Unit 2 is owned by First Energy Company of Akron which was damaged during an accident in 1979 and never reopened.

Committed to Safety

Three Mile Island, like all U.S. nuclear energy facilities, is based on a “defense-in-depth” design, which means there are redundant layers of safety.  There are multiple layers of safety systems to provide water to the reactor core.  These safety systems, and their backup safety systems, are powered by multiple and redundant power sources.  Nuclear energy plants are built with multiple physical barriers, including thick, steel-reinforced concrete walls around the reactor to contain radioactive materials.   

Our employees are personally committed to safety.  They are highly-skilled workers and continually receive training to make our safe work practices even safer.  We have a culture of continuous learning and implement lessons learned from operating experience to continue to operate safe nuclear energy facilities.  Each employee has 100 percent accountability.  We welcome strict, daily oversight by the Nuclear Regulatory Commission (NRC).  We work with the NRC to ensure that all regulations are complied with - and exceeded.

Learn more about the safety of Exelon's nuclear energy plants.


In 2009, Three Mile Island Unit 1 received a 20 year extension to the plant's operating license from the U.S. Nuclear Regulatory Commission. With license extension, Three Mile Island is well positioned to help meet the growing demand for energy well into the future as power needs across central Pennsylvania and the United States are projected to increase. The operation of Three Mile Island produces no greenhouse gas emissions and improves the air quality of the region.

Generating More Low-Carbon Electricity

As part of Exelon2020, Exelon Nuclear launched a series of planned power uprates across the company's nuclear fleet that will generate additional generation capacity within 8 years without turning a spade of earth. Uprate projects improve the efficiency and increase electricity output of a nuclear generating unit through upgrades to plant equipment. The projects take advantage of new production and measurement technologies, new materials and learning from a half-century of nuclear power operations.

Emergency Planning

Three Mile Island employs a sophisticated emergency response plan to protect public health and safety approved by the Nuclear Regulatory Commission (NRC) and the Commonwealth of Pennsylvania. The TMI Emergency Plan includes coordination with local counties, municipalities and school districts.

Supporting the Local Economy

The majority of Three Mile Island’s approximately 520 Exelon employees live in Dauphin and Lancaster Counties. The power plant’s annual payroll is about $50 million. During refueling outages, Three Mile Island employs several hundred temporary contractors who boost the local economy during their stay. 

Community Involvement

Three Mile Island aims to be a good neighbor and is very active in the local community.

  • Sponsorship.  The power plant sponsors several community events including the Middletown Music Fest and other community activities.
  • Charitable Giving & Volunteering.  Three Mile Island contributes annually to numerous community and charitable organizations.  Each year employees give generously to the community including local United Way chapters. The station also sponsors blood drives throughout the year, helping the Central Pennsylvania Blood Bank supply the needs of area hospitals. They also run Toys for Tots fund drives to support The Salvation Army. 

  • Number of units: 1
  • Total site net generation: 829 MW
  • 2014 net generation: 7.3 million MWh
  • Customers served:  800,000 homes
  • 2014 capacity factor: 99.4%

Site Vice President: Rick Libra

Plant Manager: Tom Haaf

Route 441S P.O. Box 480
Middletown, Pennsylvania

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Three Mile Island