EVENT
Three Mile Island
The March 1979 partial meltdown of Unit 2 at the Pennsylvania nuclear plant — the founding case study of Normal Accident Theory and the event that transformed Charles Perrow from organizational sociologist into risk theorist.
On March 28, 1979, a pressure relief valve stuck open in the Unit 2 reactor at Three Mile Island, Pennsylvania. Coolant drained from the reactor core. Automated systems responded correctly, shutting down the reactor and activating emergency cooling. But the control room instruments told the operators a story that was internally coherent and factually wrong. The operators, skilled professionals following their training, interpreted the instruments according to their mental models, made decisions consistent with those models, and made the situation catastrophically worse. The investigation initially blamed 'human error.' Perrow, serving on the President's Commission, looked at the same evidence and saw a system architecture that had guaranteed the failure regardless of who was operating it.
In The You On AI Field Guide
Three Mile Island became the founding case study of Normal Accident Theory because it displayed every element of the framework with diagnostic clarity. The stuck valve was a component failure; it was not the accident.