What happened at Three Mile Island?

Three Mile Island is, in hindsight, a reassuring story. In 1979 a partial meltdown occurred after equipment and operator errors — and the containment building did exactly its job. Only a tiny amount of radiation reached the environment, and study after study found no detectable rise in cancer or health effects in the surrounding population.

The worst US commercial reactor accident harmed no one's health, and it made the industry markedly better: it transformed operator training and control-room design. It's a powerful example of how nuclear's layered safety holds up even when things go wrong.