It happened at 4am. A pressure valve in the Unit-2 reactor at Three Mile Island failed to close. Cooling water, contaminated with radiation, drained from the open valve into adjoining buildings, and the core began to dangerously overheat.

Emergency cooling pumps then went into operation but human interference, caused by misleading control room gauges, led to the cooling mechanism being manually shut down. Residual heat was still being released and the core heated to 4,000 degrees, 1,000 degrees below, or within about one hour, of complete meltdown (which could lead to a dangerous radiation cloud drifting across the countryside potentially killing many).

By 8am those on the outside had been let in on the unfolding crisis. By 8pm plant operators started to move water over the core again and the temperature began to drop. The core never broke through its protective shell...

Two days later, however, a bubble of highly flammable hydrogen gas was discovered within the reactor building. It exploded and a small amount of radioactive material went into the atmosphere.

The crisis began again as it was uncertain if a larger explosion was about to occur. The Governor ordered "pregnant women and preschool age children to leave the area within a five-mile radius of the Three Mile Island facility until further notice." There was panic. 100,000 people fled.

In the event, no-one outside the plant was affected by the problems, although those in the plant were exposed to unhealthy radiation doses.

Ironically The China Syndrome, starring Jane Fonda, was released around the time of this accident. The movie follows a group of journalists who discover a covered up nuclear accident.