March 28, 1979; at the Three Mile Island nuclear power plant, on the Susquehanna River, 10 miles downstream of the state capital in Harrisburg, Pennsylvania, occurred the worst nuclear accident in U.S. history, when cooling water leaked from a malfunctioning valve and caused the reactor to come within an hour of a full meltdown before the cooling system was restarted.
They were doing maintenance, had the feed system locked out, lifted a PORV (pilot activated relief) and it did not reseat. They received so many alarms during the transient it took everyone's attention to try to silence them for a loooonng time where they missed critical indications. once they started drawing a steam bubble in the core they saw rising pressurizer level on the meter - but it was due to the displaced water in the core. They secured emergency feed, still not realizing they had a high temp alarm on the relief drain and the primary catch tank was filling over. At that point there was nothing to do but run.