The world’s first nuclear reactor came with a manual off switch, and the off switch was a man holding an axe.

His name was Norman Hilberry. On the afternoon of 2 December 1942, he stood on a spectators’ balcony at the University of Chicago, watching a rope. Below him rose a lopsided stack of graphite bricks, packed into a disused squash court under Stagg Field’s west stand. Tied to the rope was a weighted cadmium rod, nicknamed zip. If the chain reaction climbed faster than Enrico Fermi had calculated, Hilberry’s job was to swing his axe, sever the rope and let the rod fall into the pile to smother it.

That was the contingency plan, in a city of more than three million people.

A pile of bricks under a football stadium

Fermi’s reactor was gloriously low-tech: something like 45,000 machined graphite bricks, as the Linda Hall Library describes it, drilled with holes, loaded with lumps of uranium metal and uranium dioxide, and stacked into a rough sphere about six metres tall.

Graphite was there as a moderator. Its job was to slow down the neutrons flying out of splitting uranium atoms, because a slow neutron is far more likely to split the next atom than a fast one. Get the geometry and the purity right and the reaction feeds itself.

Crews worked around the clock for a fortnight, laying 57 layers into precise positions, and finished on the evening of 1 December. Nobody told the mayor, and Arthur Compton, the physicist running the lab, said he did not tell the university president either, according to the University of Chicago.

No radiation shielding. No cooling system. Writing for the Lindau Nobel Laureate Meetings, Susanne Dambeck puts the ingredients at 360 tonnes of graphite, 45 tonnes of uranium oxide and 5.4 tonnes of uranium metal, with nothing between the pile and the people beside it.

The man with the axe

“I stood there with this axe to cut the rope,” Hilberry told an interviewer in 1965, in an oral history held by the Nuclear Museum. He was the backstop for the mechanical control rods. He had one of his own.

Three men, Harold Lichtenberger, Warren Nyer and Alvin Graves, stood on a platform above the bricks holding big glass bottles of cadmium salt solution. If the rods jammed, they were to tip the lot over the pile and poison the reaction. Nyer and the others became known, informally, as the suicide squad.

Where the word scram comes from

So why do engineers call an emergency reactor shutdown a “scram”? Folklore has it as an acronym coined for Hilberry: Safety Control Rod Axe Man. Tom Wellock, the NRC historian who ran down the suicide squad story, went looking for the word’s real origin. The acronym does not survive contact with the witnesses, Hilberry included, who only heard it second-hand years later. Nyer remembered a colleague wiring up a control panel with a big red button, and someone asking what you do once you have pressed it: you scram out of there.

Fermi broke for lunch

Just after 11am, Fermi had George Weil begin easing the last cadmium rod out, foot by foot. He tracked the neutron count on a slide rule, predicting each reading correctly before it landed. By midday the numbers pointed one way only.

Then he stopped and sent everyone to lunch. Oak Ridge National Laboratory‘s account picks up when the crew came back: Weil pulled the rod another foot, and the neutron counters ran up into a continuous clatter.

The pile went critical at 3.25pm. It ran for less than five minutes at about half a watt of heat, roughly a torch globe, with a neutron multiplication factor of 1.0006, as documented by the Atomic Heritage Foundation. Six ten-thousandths above break-even was the whole ballgame.

Eugene Wigner produced a bottle of Chianti he had bought for the occasion. Forty-nine people drank it from paper cups and signed the straw basket afterwards. That wrapper, held today by the National Archives at Chicago, became the only written record of who was in the room.

The reactor is buried in a forest

CP-1 did not stay under the stadium. It was pulled apart, trucked out to leased woodland roughly 30 kilometres south-west of downtown Chicago, and rebuilt with shielding as Chicago Pile-2, soon joined by a second reactor. When the site shut in 1954, both were pushed into a hole and covered over. Radioactive waste went into a fenced patch of ground roughly 45 metres on a side, as set out in a fact sheet from the Department of Energy, which still monitors the groundwater there.

Cleanup finished and the land went back to public recreation in 1991. Red Gate Woods is now a picnic and mountain biking spot run by the Forest Preserves of Cook County, and riders pedal past a granite marker that tells them what is underneath.

Every power reactor built since has inherited Hilberry’s rope. The rods are held up by electromagnets now, so a blackout releases them automatically, no volunteer required. Eighty years of engineering, and the last line of defence is still a heavy piece of metal falling down a hole.