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