[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 U.S. Nuclear Regulatory Commission (NRC) report, the accident resulted in no deaths or injuries to plant workers or in nearby communities.

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.

[29] The Three Mile Island Unit 1 is a pressurized water reactor designed by Babcock & Wilcox with a net generating capacity of 819 MWe.

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.

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.

[39] In February 1993, a man drove his car past a checkpoint at the TMI nuclear plant, then broke through an entry gate.

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.[41][42] 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.

The air pressure change occurred when inside building ventilation fans were started to support outage activities.

[48] A review by the World Nuclear Association concluded that no deaths, injuries or adverse health effects resulted from the accident,[49] and a report by Columbia University epidemiologist Maureen Hatch confirmed this finding.

The China Syndrome, a movie about a nuclear disaster, which was released 12 days before the incident and received a glowing reception from the movie-going public, became a blockbuster hit.

[55] 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.

[56] Exelon Corporation was created in October 2000 by the merger of PECO Energy Company and Unicom, of Philadelphia, Pennsylvania and Chicago, Illinois respectively.

[59][60] 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.

[62] 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,[63] unless the Pennsylvania legislature rescued the nuclear industry, which was struggling to compete as newfound natural gas resources drove down electricity prices.

[64] Exelon Generation's Senior Vice President Bryan Hanson noted that once Three Mile Island was closed, it could never be reopened for use again.

[67] On September 20, 2024, citing a resurgence in the need for nuclear generated carbon free electricity, Constellation Energy announced plans to reopen Three Mile Island Unit 1 in 2028.

Crane, who started his career in nuclear energy as a Reactor Operator with ComEd, retired from Exelon in December 2022 and died on April 13, 2024.

[68][69] As part of this plan, Microsoft entered into a 20-year agreement to purchase power from the facility once it resumes operation, aiming to support the energy demands of its expanding AI data centers.

[70] 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.