Three Mile Island accident
The Three Mile Island accident was a 1979 nuclear accident at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania, in the United States.
The Three Mile Island accident occurred on March 28, 1979, at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania. It was a partial meltdown in Unit 2 caused by cooling-system and valve failures combined with operator errors. The event is widely considered the most serious accident in U.S. commercial nuclear power history.
Background
Three Mile Island is a nuclear power station on an island in the Susquehanna River near Harrisburg, Pennsylvania. The accident occurred in Unit 2, one of the plant's two reactors.
Accident sequence
A mechanical failure and a stuck-open pilot-operated relief valve led to loss of coolant, while plant operators misread the situation and delayed the correct response. The result was overheating and a partial core melt.
Effects and public impact
The accident released a small amount of radioactive material, but cited sources report no deaths, no injuries, and no demonstrable health effects in the surrounding public. The event nevertheless caused widespread concern about nuclear power.
Aftermath
Three Mile Island became a major reference point in nuclear safety policy. It led to improved training, regulation, and emergency planning in the U.S. nuclear industry.
Legacy
The accident is remembered as the worst commercial nuclear power accident in U.S. history and a turning point in public attitudes toward nuclear energy. It remains a key case study in reactor safety and human factors.
Key facts
- The accident began on March 28, 1979, due to equipment failure and operator error.
- It resulted in a partial core meltdown but no reported deaths or injuries.
- Small amounts of radioactive gas were released with no documented health effects.
- The accident is the most serious in U.S. commercial nuclear power history.
- It prompted major changes in nuclear regulation and safety procedures, influencing international practices including in Canada.
Canadian readers may know Three Mile Island as a landmark nuclear-safety event that influenced international reactor regulation, emergency planning, and public debate about nuclear power, including in Canada.
Frequently asked questions
What was the Three Mile Island accident?
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References
- PBS American Experience — https://www.pbs.org/wgbh/americanexperience/films/three/Supports: Date, location, and description of the accident as a major U.S. nuclear disaster.
- World Nuclear Association — https://world-nuclear.org/information-library/safety-and-security/safety-of-plants/three-mile-island-accidentSupports: Partial core melt, small radioactive release, and reported lack of public health harm.
- Encyclopaedia Britannica — https://www.britannica.com/event/Three-Mile-Island-accidentSupports: Accident sequence, partial meltdown, and historical significance.
- U.S. Department of Energy — https://www.energy.gov/ne/articles/5-facts-know-about-three-mile-islandSupports: Most serious U.S. commercial nuclear accident and no detectable health effects reported.
- U.S. Nuclear Regulatory Commission — https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.htmlSupports: Accident overview, causes, and regulatory impact.
- Idaho National Laboratory — https://www.inl.gov/trending-topics/three-mile-island/Supports: Cooling malfunction, partial core melt, and safety improvements after the accident.