March 28, 1979: Reactor Meltdown at Three Mile Island

1979: Equipment malfunction and human error lead to a partial reactor meltdown at the Three Mile Island nuclear power plant near Middleton, Pennsylvania. It is the most serious accident ever involving a U.S. commercial nuclear facility.

Things started going wrong at 4 a.m. when a malfunction caused the main feed-water pumps to stop running, leading to an overheating of the plant’s TMI-2 reactor. Although the reactor shut down automatically, a relief valve — which should have closed as the pressure decreased — failed to operate, causing coolant to leak out and the reactor’s core to overheat.

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