The Three Mile Island accident was a partial meltdown of the Three Mile Island, Unit 2 (TMI-2) reactor in Pennsylvania, United States. It began at 4 a.m. on March 28, 1979. It is the most significant accident in U.S. commercial nuclear power plant history. On the seven-point International Nuclear Event Scale, it is rated Level 5 - Accident with Wider Consequences.The accident began with failures in the non-nuclear secondary system followed by a stuck-open pilot-operated relief valve (PORV) in the primary system that allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident (LOCA). TMI training and procedures left operators and management ill-prepared for the deteriorating situation. During the event these inadequacies were compounded by design flaws, including inconveniently arranged instruments and controls, the use of multiple similar alarms, and a failure of the equipment to clearly indicate coolant inventory level or the position of the stuck-open PORV.The accident crystallized anti-nuclear safety concerns among activists and the general public, and resulted in new regulations for the nuclear industry. It has been cited as a contributor to the decline of a new reactor construction program, a slowdown that was already underway in the 1970s. The partial meltdown resulted in the release of radioactive gases and radioactive iodine into the environment.

Anti-nuclear movement activists expressed worries about regional health effects from the accident. Some epidemiological studies analyzing the rate of cancer in and around the area since the accident did not determine that there was a statistically significant increase in its rate, while others did. However, due to their nature, such studies cannot conclusively establish whether or not there was a causal connection linking the accident with these cancers. Cleanup started in August 1979, and officially ended in December 1993, with a total cleanup cost of about $1 billion (equivalent to $2 billion in 2021).

Three Mile Island Nuclear Generating Station (TMI) is a closed nuclear power plant on Three Mile Island in Londonderry Township, Pennsylvania on Lake Frederic, a reservoir in the Susquehanna River just south of Harrisburg. It had two separate units, TMI-1 (owned by Constellation Energy) and 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 Nuclear Regulatory Commission (NRC) report, the accident resulted in no deaths or injuries to plant workers or in nearby communities. Follow-up epidemiology studies have linked no incidents of cancer to the accident. Only one work-related death has occurred on-site during decommissioning.The reactor core of TMI-2 has since been removed from the site, but the site has not been decommissioned. In July 1998, Amergen Energy (now Exelon Generation) agreed to purchase TMI-1 from General Public Utilities for $100 million.Contrary to popular belief, Three Mile Island is named after the length of the island itself, not because it is three miles downriver from Middletown, Pennsylvania. 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. During 2001, GPU Inc. merged with FirstEnergy Corporation. On December 18, 2020 - FirstEnergy transferred the plant's license over to EnergySolutions subsidiary TMI-2 Solutions after receiving approval from the NRC.Exelon was operating Unit 1 at Three Mile Island at a 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. After no avail, Unit 1 officially shut down at noon on September 20, 2019.Unit 1 decommissioning is estimated to be finished in 2079 and will cost $1.2 billion.Unit 2, which has been dormant since the accident in 1979, is estimated to close in 2037.