Three Mile Island Nuclear Generating Station (abbreviated as TMI), is a shut-down nuclear power plant on Three Mile Island in Pennsylvania, US, on the Susquehanna River just south of Harrisburg. It has two separate units, Unit 1 (TMI-1) (owned by Constellation Energy) and Unit 2 (TMI-2) (owned by EnergySolutions).
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 U.S. Nuclear Regulatory Commission (NRC) report, the accident resulted in no deaths or injuries to plant workers or in nearby communities. Follow-up epidemiology studies did not find causality between the accident and any increase in cancer cases. One work-related death had occurred in September 2021, on-site during decommissioning.
The reactor core of TMI-2 has since been removed from the site, but as of 2009 the site has not been fully decommissioned. In July 1998, Amergen Energy (now Exelon Generation) agreed to purchase TMI-1 from General Public Utilities for $100 million.
The plant was originally built by General Public Utilities Corporation, later renamed GPU Incorporated. 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. On December 18, 2020, FirstEnergy transferred Unit 2's license to EnergySolutions' subsidiary, TMI-2 Solutions, after receiving approval from the NRC.
Exelon was operating Unit 1 at a financial loss since 2015. 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. Unit 1 officially shut down at noon on September 20, 2019.
Unit 1 decommissioning was expected to be completed in 2079 and would have cost $1.2 billion, but in September 2024, Constellation Energy, the owner of the Unit, announced plans to invest $1.6 billion to bring the facility back online. The plant is expected to resume operations in 2027 as the Crane Clean Energy Center (CCEC). The entirety of the plant's energy output will be sold to Microsoft Corporation. Microsoft entered into a 20-year agreement to purchase as much electricity as possible from the plant, which will support the company’s growing energy needs for its expanding network of data centers.
Unit 2, which has been dormant since the accident in 1979, is expected to close in 2052.
Emergency zones and nearby population
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.
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).
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 electrical generating facilities in the state of Pennsylvania generated approximately 241 TWh of total electricity.
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 $2.61 billion in 2018 dollars. Unit 1 first came online on April 19, 1974, and began commercial operations on September 2, 1974. TMI-1 was licensed to operate for 40 years from its first run, and in 2009, its license was extended by 20 years, which means it could have operated until April 19, 2034.
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 cooling towers. Once-through the cooling towers, river water was used in the service water system, cooling auxiliary components and removing 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.
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 financially unable to continue operating the reactor. TMI-1 was shut down on September 20, 2019, but may be brought back. The CEO of Constellation Energy, the owner of Unit 1, says the reactor is in "excellent shape," and estimated that it would go online within three years of committing to a restart. As of 2024 Constellation is conducting tests for a potential decision to reopen.
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.
During and following the September 11, 2001 attacks, there was a concern that United Airlines Flight 93 was headed towards Three Mile Island. On that day, the NRC placed all of the nation's nuclear power plants into the highest level of security. United Flight 93 crashed into a field (present-day Flight 93 National Memorial) about 135 miles (217 km) west of Three Mile Island in Stonycreek Township, just outside Shanksville, Pennsylvania, with its actual target believed to have been Washington, D.C.
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." As of November 22, 2009, it was believed that no radiation escaped the containment building and the public was 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 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 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.
On September 10, 2021, a contractor from Alabama was fatally injured while unloading equipment from a truck. Fire and emergency medical personnel from Londonderry Township were dispatched and declared the contractor dead on arrival. The Nuclear Regulatory Commission said the injury was work-related, and the contractor was outside the radiological controlled area.