The worst accident in U.S. commercial nuclear history, it reshaped nuclear safety regulations and accelerated the decline of new reactor construction.
Key Facts
- Date of accident
- March 28, 1979, at 4:00 a.m.
- Reactor affected
- Unit 2 (TMI-2), Three Mile Island Nuclear Generating Station
- INES rating
- Level 5 — Accident with Wider Consequences
- Cleanup cost
- ~$1 billion (approx. $2 billion in 2025 dollars)
- Cleanup duration
- August 1979 to December 1993
- Detected health effects
- None detected among plant workers or the public
By the Numbers
Location
Cause → Event → Consequence
The accident originated in failures of the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve (PORV) in the primary system. This allowed coolant to escape the pressurized loop. Operators, inadequately trained for a loss-of-coolant accident (LOCA), failed to recognize the situation, and design flaws including poor control layout and ambiguous alarms compounded the mechanical failures.
Beginning at 4:00 a.m. on March 28, 1979, the TMI-2 reactor at Three Mile Island suffered a partial nuclear meltdown. Large quantities of water escaped the primary coolant loop through the stuck-open PORV, and the core was significantly damaged. Radioactive gases and radioactive iodine were released into the environment, marking the largest release of radioactive material in U.S. history at that time.
The accident intensified public concern about nuclear power and prompted new regulatory requirements across the industry. It accelerated the slowdown in construction of new nuclear plants in the United States. Cleanup cost approximately $1 billion and lasted until 1993. Epidemiological studies yielded mixed results regarding cancer rates in the surrounding area, and TMI-1, the companion reactor, was eventually retired in 2019 before being slated for restart to power Microsoft data centers.