An Unplanned Meltdown at America’s First Nuclear Power Reactor

In 1951, the U.S. government built the country's first breeder reactor, an experiment to show the country that nuclear power could be used for a peaceful purpose- generating electricity. And it worked, as far as generating power goes, but it was an experiment. As such, the scientists who operated Experimental Breeder Reactor No. 1 in Idaho fiddled with the equipment in 1955 to figure out why the reactor did not respond to coolant flow "in the most stable way." That led to a meltdown of the nuclear core, but the effects were not what we'd expect today after seeing all those movies. Engineer Ray Haroldsen was in his office, and did not know about the meltdown until a technician told him.   

Since the reactor was nearing the end of its useful life, the scientists decided to conduct an experiment that was riskier than they’d normally have tolerated. They decided to turn the coolant off while slowly turning the power up, in the hopes of determining what made the reactor act the way it did. They knew there was a risk the core could be destroyed, but they planned to proceed slowly and back off at the first sign of danger.
The experiment ended more quickly than they thought it would. The power produced by the reactor started rising and rapidly went off the scales. Haroldsen’s boss yelled to the technician to shut the reactor down.

Haroldsen tells the story of what happened that day, and the fallout (so to speak), in a video at Atlas Obscura.