On 28 March, a chain of events began at the Three Mile Island nuclear power station in Pennsylvania, USA, which almost led to a massive nuclear disaster, the likes of which had not been seen on earth. An automatic valve closed wrongly, affecting the circulation of water in the core of the plant's Unit 2 reactor. The fusion process in the reactor was automatically shut down after the cooling system failed. However, instead of stabilising, a series of mistakes caused coolant to drain away from the core. When technicians finally realised what had happened, the intensely radioactive ("hot") fuel rods had been partially exposed, causing them to overheat and begin to break down.

Meltdown, which would release huge amounts of radiation and turn the surrounding area into a toxic no-man's land, became a very real possibility. Already, radioactive steam had escaped into the atmosphere. Even worse, a cloud of explosive hydrogen gas had formed in the reactor, making a catastrophic explosion more than likely. Still, after two days, no one had been officially evacuated from the area, although almost a million people in the surrounding counties were put on evacuation alert and thousands had fled the area.

The crisis lasted a total of twelve days, focusing media attention on the dangerous nature of nuclear power and leading many to conclude that nuclear technology was indeed a threat to humans. Eventually, however, the hydrogen gas was contained and coolant circulation was restored. Later inspections of the damaged core revealed the frightening conclusions that more than half the reactor's fuel had melted and meltdown might have been a reality in just another half hour.

In the aftermath of the incident, several similar nuclear reactors around the country temporarily suspended operations, and plans for new reactors were temporarily frozen. These government measures, together with rapidly rising public resistance crippled the nuclear power industry for years. Meanwhile, the ruined reactor situated on Three Mile Island on the Susquehanna River, just 16 kilometres (10 miles) south-east of the state capital, Harrisburg, took more than a decade to decontaminate.