The Three Mile Island Accident (1979)
On 28 March 1979, a stuck-open relief valve and a series of operator misdiagnoses led to a partial meltdown of the Unit 2 reactor at Three Mile Island, Pennsylvania, the worst accident in US commercial nuclear power history. The event triggered an evacuation advisory, shook public confidence in nuclear energy, produced sweeping regulatory reform, and contributed to a decades-long stall in new US reactor construction. It remains the reference case for nuclear safety culture and human factors engineering.
Events
Nuclear Expansion and the TMI Plant
The Three Mile Island Nuclear Generating Station stood on an island in the Susquehanna River near Harrisburg, Pennsylvania, with two pressurized water reactors operated by Metropolitan Edison. Unit 2, rated at roughly 900 megawatts electrical, entered commercial service in December 1978 after only about ninety days of operation. The plant belonged to the largest reactor-building program in history, as the United States added dozens of plants through the 1970s in response to oil shocks and projections of rising electricity demand. Critics had already raised concerns about emergency planning and control-room complexity, but the industry's safety record was spotless. TMI-2 had experienced earlier malfunctions during its brief operating life, including trips and leaks. The stage was set for the first full test of a commercial reactor under severe accident conditions.
Location: Londonderry Township, Pennsylvania, United States
Feedwater Failure and the Stuck Relief Valve
At about 4:00 a.m. on 28 March 1979, the secondary feedwater pumps in TMI-2 stopped when water from a maintenance operation had leaked into an instrument air line, tripping the turbine. Reactor pressure surged to 2,255 psi (155.5 bar), opening the pilot-operated relief valve to vent coolant. The valve then stuck open, but its control-room indicator showed only that the command to close had been sent, not the valve's actual position. Operators, seeing pressurizer water level rise, misread the situation as too much rather than too little water and throttled back the emergency cooling injections. A loss-of-coolant accident therefore went unrecognized for hours. Design flaws compounded the errors: hundreds of alarms sounded simultaneously, and no instrument displayed coolant inventory directly.
Location: Londonderry Township, Pennsylvania, United States
Partial Meltdown of the Reactor Core
With coolant escaping and emergency injection curtailed, the water level in the reactor vessel fell and the core was left uncovered and heating. The fuel cladding reacted with steam to generate hydrogen, and by the afternoon of 28 March roughly half of the reactor core had melted, a fact not fully confirmed until examinations during the cleanup years later. Radioactive gases and traces of iodine-131 were released to the environment, and radioactive water was deliberately discharged into the Susquehanna River. The plant's operators and the utility's management struggled through 28 and 29 March with contradictory indications and shifting assessments. Off-site radiation measurements remained low, and no acute health effects were detected among workers or the public. The severity of the core damage, however, was far greater than anyone at the plant understood at the time.
Location: Londonderry Township, Pennsylvania, United States
Evacuation Advisory and Public Panic
On 30 March 1979, Governor Dick Thornburgh, acting on conflicting radiation information, advised pregnant women and preschool-age children within 5 miles (8 kilometers) of the plant to leave the area, and schools within the zone were closed. An estimated 144,000 people evacuated voluntarily amid days of confusing announcements from the utility, the Nuclear Regulatory Commission, and the White House. Journalists converged on the site, and the term "hydrogen bubble," referring to a feared hydrogen explosion inside the reactor vessel, dominated national headlines for days. The radiation scare became a national media event even as measured doses to the public remained modest. Anti-nuclear activists organized mass demonstrations, and a rally in New York City drew tens of thousands. The episode demonstrated how little federal, state, and utility communication plans were prepared for a real emergency.
Location: Harrisburg, Pennsylvania, United States
President Carter Visits the Plant
On 1 April 1979, President Jimmy Carter toured the Three Mile Island control room and met with officials, in part to reassure a frightened public. Carter, a naval officer trained in nuclear engineering under Admiral Hyman Rickover, was briefed on the hydrogen bubble concerns and the state of the reactor. His visit signaled that the federal government had taken direct control of the response, and it marked the first time a sitting president had visited the scene of a nuclear accident. The White House set up an interagency task force to coordinate monitoring and technical assessment. Public confidence, however, did not return with the president's departure. Polls taken after the accident showed a sharp rise in opposition to nuclear power, particularly among people living near reactor sites.
Location: Middletown, Pennsylvania, United States
The Kemeny Commission Report
On 30 October 1979 the commission appointed by President Carter and chaired by Dartmouth president John Kemeny delivered its report on the accident. It concluded that the fundamental problem was not equipment failure alone but "mindset" - the industry's belief that severe accidents were essentially impossible and therefore needed no serious planning. The report criticized operator training, control-room design, the utility's management, and the NRC's regulatory apparatus, and it found that the plant's emergency response had been improvised and poorly communicated. The Nuclear Regulatory Commission responded with a generation of reforms: revised operator licensing and simulator training, improved control-room instrumentation and alarm design, strengthened emergency planning requirements, and new severe-accident analysis. The International Nuclear Event Scale, created later, rated TMI-2 at Level 5, "Accident with Wider Consequences." The report became a foundational document of modern safety culture, whose principles spread to industries far beyond nuclear power.
Location: Washington, DC, United States
The Decade-Long Cleanup
Cleanup of the damaged TMI-2 reactor began in August 1979 and officially ended in December 1993, after fourteen years of remote-inspection, defueling, and decontamination work. The project cost about $1 billion, roughly $2 billion in 2025 dollars, and required novel robotics and video inspection to examine the melted core, which had flowed into the vessel's lower head in ways that earlier models had not predicted. Workers removed about 100 metric tons of damaged fuel and debris. The unit itself was permanently mothballed, while its undamaged sibling TMI-1 restarted in 1985 and ran until 2019, when it was retired for economic reasons. The engineering data gathered during cleanup informed the analysis of later accidents at Chernobyl and Fukushima. Epidemiological studies of cancer rates around the site produced mixed and contested findings, and no causal link to the accident has been established.
Location: Londonderry Township, Pennsylvania, United States
The Long Shadow on Nuclear Energy
Three Mile Island reshaped the trajectory of nuclear energy in the United States: no reactor ordered after 1978 reached completion for decades, orders were cancelled across the industry, and public opposition hardened in ways that outlasted the accident itself. The renewable economics of the 1980s and the industry's regulatory costs combined to end the first nuclear era, and the accident became shorthand for technological risk in public debate. Nearly half a century later, the site returned to the center of a new discussion: in September 2024 Constellation Energy announced a 20-year agreement with Microsoft to restart the retired TMI-1 unit as the Crane Clean Energy Center, targeting a 2027-28 return to service to power data centers. The plan testified to how completely the economics of nuclear power had inverted, from post-accident retreat to AI-driven demand. TMI thus spans both the fall and the possible rebirth of civilian nuclear energy in America.
Location: United States