Three Mile Island Accident
A partial nuclear meltdown at Unit 2, the worst accident in US commercial nuclear history, caused by equipment failure and operator confusion.
The Accident
A combination of equipment malfunction, design deficiencies, and operator errors led to a loss of coolant and partial core meltdown. A stuck-open pressure relief valve drained cooling water for over two hours while operators, misreading their instruments, believed the reactor was safely filled with coolant.
The Hydrogen Bubble
As the situation became clearer over the following days, concern grew about a hydrogen gas bubble inside the reactor vessel. Fears that it might explode dominated news coverage and caused widespread panic, though later analysis showed explosion was not possible given the lack of oxygen in the reactor.
Response and Confusion
The NRC and the Carter administration struggled to communicate clearly with the public. Governor Richard Thornburgh recommended that pregnant women and young children evacuate a 5-mile radius as a precaution. An estimated 140,000 people left the area voluntarily.
Impact on Nuclear Power
TMI fundamentally changed the US nuclear industry. It led to better operator training, improved emergency response procedures, upgraded reactor design requirements, and stricter NRC oversight. It effectively halted new nuclear plant construction in the US for decades, a position that is only now reversing.
📅 Timeline
Feedwater pumps fail, triggering reactor shutdown
Pressure relief valve opens but fails to reclose
Operators unaware coolant is draining from reactor
Core partially uncovered and fuel begins to melt
Hydrogen bubble concern prompts wider public alarm
President Carter visits plant; situation stabilizes
Cleanup and decontamination of Unit 2 completed