Three Mile Island Unit 2

A minor fault stopped feedwater to the steam generators. The reactor scrammed correctly and a relief valve opened to relieve pressure, as designed — and then stuck open, which was not designed. Coolant escaped for over two hours.

The control room had no indicator for the valve's actual position, only for the signal sent to it. Meanwhile the pressuriser level read high, which operators were trained to treat as the reactor being full of water. It was not: steam voids were displacing the level while the core was uncovering.

Acting on that reading, they throttled back the emergency core cooling that had started automatically and was doing the right thing. About half the core melted.

The containment building held. Off-site releases were small and no one was hurt.

The industrial and political consequences were enormous. No new US reactor was ordered for thirty years. The reactor at TMI-2 was defuelled over more than a decade; Unit 1 next door ran until 2019 and is now being restarted.

TMI reshaped how the industry thinks about operators. The failure was not a component but the interface between people and machine: instrumentation that showed a demand rather than a state, and training that taught a rule rather than a mental model of the plant.

It produced the Institute of Nuclear Power Operations, simulator training against realistic degraded scenarios, symptom-based rather than event-based emergency procedures, and a fundamental redesign of control room information displays.