01 Overview
At 4:00 AM on March 28, 1979, cooling water stopped flowing to the reactor core of Unit 2 at Three Mile Island. A combination of equipment failures and operator errors led to the worst nuclear accident in U.S. history, though radioactive releases were small compared to later disasters.
02 Cause
A feedwater pump failure caused the reactor to shut down automatically. A pilot-operated relief valve opened to relieve pressure but failed to close - the control room indicator showed the valve as closed when it was actually open. Operators, unaware coolant was escaping, reduced emergency cooling water.
03 Impact
About 2 million people received an average radiation dose of 1 millirem - roughly equivalent to a chest X-ray. No direct health effects have been conclusively attributed. Governor Thornburgh advised evacuation of pregnant women and children within 5 miles; about 140,000 people voluntarily evacuated.
04 Response
Federal and state officials faced the challenge of communicating accurate information amid public panic and conflicting data. President Carter visited the plant. The Nuclear Regulatory Commission was criticised for its handling of information. Cleanup of the damaged reactor took 14 years.
05 Legacy
Three Mile Island fundamentally changed the US nuclear industry. It led to a complete overhaul of regulatory processes, emergency response planning, and operator training. No new nuclear power plants were ordered in the U.S. after 1979 for nearly 30 years. Unit 1 was restarted in 2024 for AI data centre power.