At 3:25 p.m. on December 2, 1942, in a converted squash court beneath the west stands of Stagg Field, physicist George Weil pulled a cadmium-plated control rod out of a lattice of graphite bricks and uranium slugs, and for 28 minutes the pile behind him ran as the first self-sustaining nuclear chain reaction in history. There was no flash. No bang. Just the quickening click of a boron trifluoride counter and 49 people standing very still in a cold room in Hyde Park.
The pile produced about half a watt of power. Less than a flashlight bulb. It was enough to change the century.
A squash court under a football stadium
The room was buried under the disused west grandstand of the University of Chicago’s football field. The floor was concrete. The ceiling was low. The air smelled of graphite dust, which coated the scientists’ skin and clothes.
Fermi had wanted to build the pile elsewhere, but decided he could not wait. He chose the squash court instead, in the middle of a residential neighborhood, a mile from the Chicago Loop.
Arthur Compton, who ran the University of Chicago’s Metallurgical Laboratory and had approved the location, later said he made the judgment himself that Fermi’s calculations were sound enough to bet a city on.
The pile itself
Chicago Pile-1 was, by the standards of what came after it, comically crude. It was built from graphite blocks machined by hand, stacked into a rough ellipsoid, with a wooden frame holding the whole thing together. Inside the graphite, at carefully calculated intervals, sat pieces of uranium metal and uranium oxide, packed into pseudospheres and pressed into the blocks.
The scientists called it a pile because that is exactly what it was — a pile of black bricks. The word reactor did not yet exist in this sense.
Graphite was the moderator, slowing the neutrons emitted by fissioning uranium-235 down to speeds at which they would be captured by other uranium nuclei and cause them to split in turn. Cadmium was the brake. Long strips of wood clad in cadmium foil slid into channels through the pile and swallowed neutrons wholesale, keeping the chain reaction from beginning until Fermi decided it should.
Three lines of defense
Fermi did not entirely trust the mathematics. Or rather, he trusted the mathematics and did not trust the world. He built three separate shutdown systems into the pile.
The first was the main control rod that Weil operated by hand, sliding it out inch by inch on Fermi’s orders. The second was an automatic safety rod, spring-loaded and rigged to a neutron monitor, set to slam back into the pile if the count rose too fast. The third was called zip — a rod held above the pile by a rope. A physicist stood ready with an axe: if everything else failed, he was to cut the rope and let gravity do the rest.
Above the pile, on a platform, scientists stood with buckets of concentrated cadmium sulfate solution. If the axe failed, they were to pour the solution directly into the pile and poison the reaction with dissolved cadmium. None of them expected to need it. All of them stood ready anyway.
3:25 p.m.
Inside the squash court, 49 people gathered on a balcony overlooking the pile. Fermi stood at the railing with his slide rule. Weil stood at the base of the pile, next to the last remaining control rod.
Fermi worked through the morning in steps, ordering Weil to withdraw the rod six inches at a time and then watching the neutron counters find a new steady rate. Each pull brought the pile closer to criticality — the point at which each generation of neutrons would produce exactly one more generation, and the reaction would sustain itself.
Fermi called a break for lunch. The pile was left with control rods fully inserted and locked.
Work resumed in the afternoon. At 3:25 p.m. Fermi told Weil to withdraw the rod one more foot. Fermi watched the counter for several minutes, worked the slide rule, and said, quietly, to Compton next to him: According to historical accounts, Fermi observed that the reaction had become self-sustaining and was following an exponential curve.
No one cheered. The counter kept clicking. The pile was now producing neutrons faster than it was losing them. Left alone, it would have kept increasing until something in the room caught fire or melted. Fermi let it run for 28 minutes at a peak power of about half a watt, then ordered zip dropped. The rods slammed home. The clicking slowed and died. The world’s first reactor had been shut down.
The coded telegram
Compton walked to a phone and called James Conant at Harvard, president of the National Defense Research Committee. The two men spoke in a code they had made up on the spot. Compton reportedly used coded language in his call to Conant, comparing Fermi (the Italian navigator) to having reached the New World. Conant asked how he had found the natives. Compton reportedly replied that the natives were friendly, continuing the coded metaphor.
Someone produced a bottle of Chianti in a straw basket. Forty-nine people drank the wine out of paper cups, in silence, without a toast. Then they signed the straw basket and went back to work. Physicist Leona Woods, the only woman in the room, later said the mood was closer to sober recognition than celebration. Several people wrote afterward that they knew they had just opened a door that could not be closed.
Fermi himself
Fermi had left Italy four years earlier, boarding a ship for the United States directly after collecting the 1938 Nobel Prize in Stockholm rather than returning to Rome, where Mussolini’s racial laws had put his Jewish wife Laura in danger.
His Nobel had been awarded for experiments in which he bombarded uranium with neutrons. What he had actually produced, without recognizing it, was evidence of nuclear fission, years before Otto Hahn and Fritz Strassmann identified the process in Berlin. Fermi had split the atom without knowing it, and won a Nobel for the wrong reason, and then done the thing correctly on a squash court in Chicago.
Half a watt
The peak thermal power of Chicago Pile-1 that afternoon was about 0.5 watts. The pile was later dismantled and moved to a site outside the city.
Half a watt is nothing. A single AA battery drives a small LED at more power. And yet the entire logic of what followed — the plutonium production reactors at Hanford, the Trinity test, Hiroshima and Nagasaki, the reactor fleets that today supply a significant portion of the world’s electricity — followed from the confirmation that the equations were right and the reaction could be held steady.
The Manhattan Project would eventually cost about $2 billion in wartime dollars.
What sat above them
The nearest human beings to the pile, when it went critical, were the 49 scientists themselves. The next-nearest were students walking to class on the quads a few hundred feet away, and the residents of Hyde Park’s apartment buildings on 57th Street. None of them knew.
The pile had no radiation shielding, no containment building, no cooling system, no emergency plan beyond the axe and the buckets. Fermi’s confidence in his own arithmetic was the containment.
The site is now marked by Henry Moore’s bronze sculpture Nuclear Energy, a rounded 12-foot form that some viewers see as a mushroom cloud and others as a human skull. It sits on a paved plaza on Ellis Avenue, on the exact footprint of the west stands, dedicated on December 2, 1967, exactly 25 years after the experiment. Students walk past it on the way to organic chemistry.
The lineage of the room
The Metallurgical Laboratory that ran the experiment became Argonne National Laboratory on July 1, 1946, the first of the U.S. national labs. The physics that Fermi’s team confirmed on that afternoon is the same physics that runs every operating reactor on Earth in 2026, from the AP1000s at Vogtle in Georgia to the VVERs on the Dnipro at Zaporizhzhia — a plant whose fate Nuclear Power Daily has followed closely through the war in Ukraine and its repeated grid disconnections.
The uranium slugs Fermi used were fashioned by hand at the Metallurgical Laboratory’s machine shop and at Mallinckrodt Chemical Works in St. Louis. The graphite came from National Carbon in Cleveland. The cadmium sheets came from a local supplier. Nothing about the pile was exotic in its ingredients. Everything about it was exotic in its arrangement.
Twenty-eight minutes
Fermi kept the pile critical for 28 minutes. He could have kept it running longer. He could have taken it to a higher power. He did neither. He shut it down at the moment he had proven the point he needed to prove, because every additional minute increased the risk of a runaway, and because the equations had already told him what he needed to know.
The neutron flux inside the pile at the moment of shutdown was still tiny by the standards of a modern reactor. A commercial pressurized water reactor today runs at around 3,000 megawatts thermal, six billion times the power CP-1 reached. But the shape of the reaction — the way one fission triggers roughly one more, generation after generation, held in balance by a moving absorber — is identical.
Eighty-three years later, in the same city, ground has been broken on the former U.S. Steel South Works site for what will be a fault-tolerant quantum computer campus, cryogenic plants cooling photonic chips within a few degrees of absolute zero. The lineage runs directly back to the squash court. Chicago has spent almost a century turning theoretical physics into infrastructure and then turning the infrastructure into a neighborhood.
The pile itself does not survive. It was dismantled in 1943, moved to Argonne, reassembled as Chicago Pile-2, and eventually buried in a shallow grave in the Palos Forest Preserve southwest of the city, where it remains — a small mound of graphite and uranium under trees, fenced off, quietly radioactive, marked by a granite stone that reads simply that the world’s first nuclear reactor was buried here.
On December 2, 1942, at 3:53 p.m., Fermi gave the order. The counter slowed. Somebody uncorked the Chianti. Outside, the sun was already going down over Lake Michigan, and none of the streetcar riders on 55th Street had any idea that the century had just turned over beneath their feet.
