At 3:25 in the afternoon of December 2, 1942, George Weil stood inside a converted squash court beneath the west stands of Stagg Field at the University of Chicago, gripping a wooden rod. The rod was cadmium-coated, roughly twelve feet long, and Weil held it by hand because the instruments in the room required a human being to pull it slowly, in precise increments, according to instruction. He was not a physicist. He was a machinist who had worked on the project for months.
He knew what the pile was; he had helped build it. He knew what the instruments measured, and he knew what those measurements meant. He did not need to be a physicist to understand that the number on the gauge before him was rising toward a point no number had ever reached in any laboratory in any country on earth. The pile behind him measured twenty feet across and twenty feet high: an oblate ellipsoid of graphite bricks and uranium blocks, assembled by hand over eighteen days in November, stacked in layers on a wooden frame.
The entire structure was covered with a ballooning fabric of rubberized cloth, inflated at first, then deflated when it proved unnecessary. Graphite dust from the bricks had settled on every surface in the squash court: the floor, the instruments, the clothes and skin and hair and lungs of everyone who had worked in the room during those eighteen days of stacking. There were 40,048 graphite bricks in the pile. There were 22,000 pieces of uranium and uranium oxide pellets distributed through the graphite at positions determined by calculation.
There were control rods: strips of cadmium mounted on wooden frames, absorbing the neutrons that uranium produced and preventing the reaction from sustaining itself. The rods could be inserted to slow the reaction; they could be drawn out to allow it to accelerate. The relationship between rod position and reaction rate was what the experiment was designed to prove. The experiment had been running since morning.
Each time a rod was pulled out another foot, the instruments rose. Each time the rod stopped, the instruments settled. Enrico Fermi, directing the experiment from a balcony above the squash court floor, read the numbers, made calculations with his slide rule, announced where the next rod position would be, and waited for the instruments to confirm his calculations. At 3:25, Fermi asked Weil to pull the rod to thirteen feet out of the pile.
The instruments rose. They did not settle. They kept rising. The crackle of the radiation counters, which had been a sequence of individual clicks one could count, became a continuous sound in which no single click could be distinguished.
Fermi watched the instruments for four and a half minutes. He turned to the men standing around him on the balcony and said, “The reaction is self-sustaining. ”
No one on that balcony had ever done this before. No one anywhere had ever done this before.
The number on the instrument was a number that had never existed on earth until the moment it appeared. Fermi told Weil to insert the rod. The crackling separated back into countable intervals. The experiment was over.
It had lasted four and a half minutes. A bottle of Italian Chianti appeared, procured from somewhere by Fermi’s colleague Herbert Anderson, wrapped in a paper bag. The men on the balcony drank from paper cups because no one had thought to bring glasses. George Weil, who had held the rod downstairs, did not appear in the photographs taken that evening.
He was not on the balcony. He was where the work was. This is the story of how the United States built the world’s first artificial nuclear reactor inside a university building in the middle of a major American city, without telling the city what was happening, in eighteen days, by men whose names do not appear on the commemorative plaque that now marks the site where the squash court stood. To understand why the world’s first nuclear chain reaction happened under a football stadium on Chicago’s South Side, you have to understand what 1939 produced and what 1942 demanded, and what the gap between those two years cost in time and in alternative sites considered and rejected.
In 1938, two German chemists named Otto Hahn and Fritz Strassmann bombarded uranium with neutrons in a laboratory in Berlin and produced barium. This was not what the experiment was supposed to produce. Barium is not an expected product when you bombard uranium with neutrons under the physics governing laboratories in 1938. Physicists who understood the result recognized it as evidence of uranium nuclear fission.
A nucleus that splits releases energy. A nucleus that splits also releases additional neutrons. Those additional neutrons can split additional nuclei. A dense enough mass of uranium can theoretically sustain this process indefinitely.
Each fission event produces the neutrons that caused the next fission event. The chain becomes self-sustaining without external input once the reaction starts. The energy released by a self-sustaining chain reaction in a sufficient mass of uranium is not a small amount. Calculations that followed the Berlin results—made within weeks in laboratories across Europe and the United States by physicists who grasped the implications immediately—produced numbers no conventional chemical explosive could approach.
A controlled chain reaction was a source of energy. An uncontrolled chain reaction was a weapon. The physicists who had emigrated from Europe to the United States during the 1930s, leaving behind governments moving toward war, knew what a weapon of that magnitude would mean in the hands of the government that now occupied the Berlin laboratory where Hahn and Strassmann had done their work. Enrico Fermi, Leo Szilard, Edward Teller, Eugene Wigner.
They had left. They were in the United States. Germany had the results and still had its physicists, those who had not left. The question of whether the German program would achieve a self-sustaining chain reaction before the American program was, by 1941, not a theoretical question.
It was a military question with a military deadline. The program that became the Manhattan Project began acquiring its institutional structure in 1941. It became an official military project under Army control in June 1942, led by General Leslie Groves, who was simultaneously overseeing construction of the Pentagon—which tells you something about the Army’s assessment of what Groves could manage. The scientific leadership of the Chicago portion of the program was in the hands of Arthur Compton.
The man Compton assigned to design and build the reactor was Enrico Fermi. Fermi had been working on the chain reaction problem since 1939, first at Columbia University in New York, then moving to Chicago in 1942 when the decision was made to consolidate the reactor work. The plan called for a dedicated facility at Palos Hills, southwest of Chicago, with concrete walls thick enough to serve as radiation shielding and enough space to build a reactor of the size the calculations indicated was necessary. The Argonne site, as it was called, was under construction in November 1942.
It was not finished in November 1942. Fermi’s calculations said he had enough material to achieve a self-sustaining chain reaction now, in the space he currently occupied: a squash court under the west stands of Stagg Field. He told Compton. Compton asked him how great the risk was.
Fermi said the risk of an uncontrolled reaction was almost zero. Compton asked again. Fermi said the same thing again. Compton approved running the experiment in the available space rather than waiting for the planned facility.
He did not ask permission from University of Chicago president Robert Hutchins. He told him afterward. The city of Chicago was not informed. The residents of the Hyde Park neighborhood surrounding the university were not informed.
The state of Illinois was not informed. The federal government knew about the experiment because it was funding it. But the information did not reach the city government, or any official with authority over the public safety of half a million people within the potential range of consequences that Fermi had assessed as essentially impossible—and for which no one in 1942 had any experimental basis for assessment whatsoever, because the experiment had never been run before. The graphite arrived at Stagg Field in the fall of 1942.
It came from the Speer Carbon Company in St. Mary, Pennsylvania, shipped by rail in blocks weighing about twenty pounds each. The graphite was of a purity specified by Fermi, higher than any commercial graphite had ever required, because impurities in graphite absorb neutrons, and neutron-absorbing impurities make the reaction harder to sustain. No graphite manufacturer had ever produced graphite to these specifications.
They produced it because the contract required it, and the contract was government, and the government was not explaining why the specifications were what they were. The graphite blocks were shaped to standard dimensions at a facility in Chicago before delivery to the site. Shaping produced graphite dust. In 1942, graphite dust was regarded as a nuisance: a material that coated every surface it touched with a black carbon layer that was hard to remove and stained skin and clothing on contact.
In 1942, it was not understood to pose a significant respiratory hazard at the concentrations produced by this shaping operation—a different question from the concentrations produced by coal mining, where the hazard was more obvious in its effects. The men who shaped the blocks and the men who handled the shaped blocks worked amid the graphite dust the operations produced. They did not wear respirators. Respirators were not standard equipment for graphite handling in 1942.
The relationship between inhaling graphite dust and lung disease did not exist in the safety literature governing industrial operations that year. The uranium came through a different supply chain. Uranium oxide pressed into pellets and uranium metal formed into ingots was obtained through a network of suppliers organized by the Manhattan Project specifically because uranium was not a commodity American industry had previously ordered in these quantities. Metal Hydrides, a metals company based in Cleveland that produced the metallic uranium ingots, was running a production process it had never run before, at a scale it had never attempted, on a schedule that left no room for process development.
The workers who ran those operations handled uranium metal and uranium oxide compounds under conditions that did not require, under the occupational health standards of 1942, more protection than the conditions governing other heavy metals in similar operations. Uranium is radioactive. The uranium processed for the reactor in 1942 was natural uranium, not enriched, meaning it contained the same isotopic ratios as uranium ore when extracted from the ground. Natural uranium is weakly radioactive.
It emits alpha particles stopped by a sheet of paper or the dead outer layer of human skin. It emits negligible gamma radiation. The acute radiation hazard of handling natural uranium externally is low. The hazard of natural uranium compounds when inhaled or ingested as dust is different, because uranium inside the body deposits in bone and kidney tissue and remains there.
The men shaping uranium ingots in Cleveland in 1942 were breathing uranium dust. Whether they breathed it at concentrations exceeding the 1942 safety limits—which were not designed for uranium specifically, because uranium shaping had not been an industrial process before 1942—is a question the occupational health records of that period do not fully answer. The graphite and uranium arrived at the University of Chicago. The assembly crew that built the reactor was a mix of physicists who worked on the project, machinists, and laborers hired for construction work.
Herbert Anderson, one of Fermi’s closest collaborators, coordinated construction directly. The machinists who stacked bricks and handled the ingots were not physicists. They were skilled craftsmen hired because the work required people capable of working precisely with physical materials and because the schedule required more hands than the physics group could provide. Some were local hires.
Some had followed the project from Columbia University. Their employment records were part of Manhattan Project personnel files, classified at the time of the project and released to varying degrees of completeness in the decades that followed. The squash court was under the west stands of Stagg Field, the football stadium built for the University of Chicago team—a team the university had discontinued in 1939 because Robert Hutchins believed organized collegiate football was incompatible with the university’s academic mission. The stadium was empty.
The squash courts under the stands were institutional space of the kind large universities accumulate without a specific purpose. The space was available. Fermi looked at it and saw a reactor. It was about thirty feet wide and sixty feet long, with enough ceiling height for the structure the calculations required.
The floor of the squash court was covered with a wooden frame on which the first layer of graphite was laid. The assembly crew worked from a scaffold that was raised as the pile grew. The work was manual. Each graphite brick weighed about twenty pounds.
A man lifting twenty-pound bricks and placing them in precise positions on a growing pile, working through a Chicago November in a room that graphite dust had turned black within the first hour of the first assembly day, was doing heavy labor under conditions the project’s wartime necessities defined as acceptable. The necessity was real. The German program was real. The men supervising assembly were aware of both.
The men stacking bricks understood the necessity and the secrecy, but did not always know the full details of what they were building, because the compartmentalization the Army imposed on Manhattan Project personnel meant people knew only what they needed to know to do their jobs, and no more, systematically. The pile grew at a rate of about one layer per day over the eighteen days of November. Each layer was a mix of solid graphite bricks and other bricks that had holes bored into them to accommodate the uranium blocks. The holes were drilled before assembly in the machine shop by workers who drilled tens of thousands of holes into graphite blocks according to drawings given to them without full context for what the drawings represented.
The uranium blocks were placed in the holes by hand. Uranium blocks were handled in 1942 without gloves for the same reason graphite bricks were handled without respirators: the industrial health protocols of 1942 had not been developed for these materials at these scales and concentrations. Graphite dust covered everything. It covered the tools.
It covered the wooden frame. It covered the control rods. It covered the skin and hair of everyone who worked in that room. Workers changed clothes when leaving at the end of the day, sometimes during the day.
Changing clothes removed visible contamination. It did not address what had been inhaled. Workers who left the squash court at the end of their shifts carried inside them whatever portion of the graphite and uranium dust they had breathed during the shift had settled in their respiratory tracts. The mass of that portion, measured in micrograms per shift, was not recorded.
No measurements were taken because they were not required by the standards of the time and because taking them would have slowed the work in ways the schedule did not permit. Assembly was completed on December 1. The pile contained 385 tons of graphite, 6 tons of uranium metal, and 34 tons of uranium oxide. It was 20 feet in diameter and about 20 feet high at its middle section, tapering toward the ellipsoidal shape the design had determined sufficient to reach criticality—the point at which the chain reaction becomes self-sustaining.
Based on neutron multiplication measurements Fermi’s team made during assembly, the control system consisted of three types of rods. Safety rods were attached to a weight and rope. If the rope was cut, the weight fell and the rods were pulled into the pile by gravity, halting the reaction. This was the emergency shutdown system.
A second set of rods was connected to an automatic system that monitored neutron flux and inserted the rods if the flux exceeded a set limit. The third rod was the one George Weil held on the morning of December 2: the manual control rod that Fermi would use to bring the pile step by step toward criticality under direct human observation. There was also a crew of three men standing above the pile with buckets of cadmium sulfate solution. Their job, in the event that the control rods failed, the automatic shutdown failed, the safety rods failed, and the reaction began to run away, was to pour the cadmium solution over the pile.
Cadmium absorbs neutrons. A pile drenched in cadmium solution would stop reacting. Whether three men with buckets could pour cadmium solution fast enough to halt a runaway reaction in a pile of this size was a question that had not been tested experimentally, because no pile of this size had ever existed. The men assigned to this task were called the “suicide squad.
” They stood on the pile, held their buckets, and waited. Fermi’s slide rule deserves mention. The calculations that guided the entire experiment—which said the pile would reach criticality when the control rods were at specified positions and not before—were performed on a six-inch bamboo slide rule Fermi carried in his shirt pocket. The instruments monitoring the reaction during the experiment were ionization chambers, a galvanometer, and a paper strip recorder.
The calculations the instruments were verifying were the calculations Fermi made with the slide rule. When Fermi asked Weil to draw the rod to thirteen feet and the instrument readings rose without stabilizing, the calculations were verified. The pile had reached criticality. The reaction was self-sustaining.
The chain was running. The graphite was doing precisely what Fermi had specified. The uranium was doing exactly what the calculations predicted. The number on the instruments was a number unprecedented on earth, and it was there because the calculations were accurate and because the men who built the reactor had built it to the exact standards those calculations required.
Four and a half minutes. Then the rod was reinserted. The men on the balcony at 3:25 that afternoon are recorded in the historical records. Arthur Compton was there.
Herbert Anderson was there. Walter Zinn, who operated the control instruments, was there. George Weil was downstairs. The bottle of Chianti that circulated among those on the balcony was signed by those present.
It is now preserved in the archives of Argonne National Laboratory. The signatures on the bottle are not the same names found in the machine shop records, the graphite handling records, and the uranium ingot production records from the plants that supplied the reactor with its components. The University of Chicago installed a commemorative plaque at the Stagg Field site in 1967, on the twenty-fifth anniversary of the experiment. The plaque was mounted on a Henry Moore sculpture the university commissioned to mark the location.
The sculpture was titled “Nuclear Energy. ” It stands at the intersection of Ellis Avenue and 57th Street, in the part of campus where the west stands of Stagg Field had been, where the squash courts had been, where the first reactor had been. The plaque records the date and the event. It records Fermi’s name.
It records the nature of what happened. The machine shop workers are not on the plaque. The workers who stacked the graphite bricks are not on the plaque. The men who shaped the uranium ingots in Cleveland are not on the plaque.
The people who drilled 22,000 holes into the graphite blocks in the shop are not on the plaque. The plaque records the achievement. It does not have room for the labor. That is how commemorative plaques work.
They are not designed to contain every name. They are designed to contain the names the institution responsible for the plaque text decided to include. The institution in this case was the University of Chicago in cooperation with Argonne National Laboratory. Both institutions had a clear vision of what the plaque should accomplish, and neither had an administrative process for determining which construction workers’ roles were central enough to include.
The Manhattan Project records are in the National Archives. The personnel files, payroll records, and construction records of the Chicago Metallurgical Laboratory—the code name for the Chicago branch of the Manhattan Project—were declassified in stages over the decades after the war ended. Some are fully available to researchers. Some are partially available.
The full picture of who worked in those squash courts during November 1942, under what conditions, at what wages, and with what understanding of what they were doing and what it might do to them, is a picture requiring a researcher, the National Archives, and a degree of patience, because it was never assembled into a widely published record during the years when the men who could describe it were alive. The Manhattan Project’s concern for its workers’ health was genuine, but it was also limited in the way any institutional concern for worker health is limited by the information available at the time and by the priority the institution assigns to the matter relative to other priorities it is managing. The physicists who designed the nuclear reactor knew it would produce radiation during operation. They also knew the reactor would be operated only briefly, that radiation levels during the December 2 experiment were low, and that the long-term health effects of the exposure received by construction workers during assembly were governed by the occupational health understanding of 1942—which did not include a complete accounting of what might ultimately result from weeks of construction exposure to low levels of uranium and graphite dust inside the bodies of the men so exposed.
The long-term health studies that could have produced that accounting would have required finding the men and following them over decades, and an institutional commitment to doing so beyond what was available after the war, when the Manhattan Project had achieved its objective and the men who worked on it had dispersed back to where they came from. Some were identified. Some were followed. The epidemiological record of the Chicago Pile-1 construction workforce is not the same record that would have existed had follow-up been systematic and fully funded from the moment the experiment was completed.
The reactor was dismantled in February 1943. Chicago Pile-1 was moved to the Palos Hills site—the Argonne site that had not been ready in November—and reassembled in a facility with concrete radiation shielding, operating under the name Chicago Pile-2 for research purposes until 1956. A subsequent reactor, Chicago Pile-3, was built at Argonne and operated during the postwar period. The Argonne research reactor program became the foundation of American nuclear research for decades.
The squash court beneath Stagg Field was decontaminated and returned to ordinary use. The 1943 decontamination removed what graphite and uranium could be removed. Whether the decontamination was complete by the standards later decades would apply to contaminated nuclear facilities is a question answered by the fact that later decades applied different standards to contaminated nuclear facilities than were available for 1943 to apply. The squash court was cleaned and the space reused.
The university demolished the west stands of Stagg Field in 1957. The site is now an outdoor plaza containing the Henry Moore sculpture. The workers who assembled the reactor in November 1942 were, by 1943, working in other Manhattan Project facilities or had returned to the industrial workforce they came from. The war was still on.
The Manhattan Project was expanding, not contracting. The production facilities at Oak Ridge, Tennessee, and Hanford, Washington, were under construction on a scale that made Chicago Pile-1 look like what it was: a laboratory demonstration of a scientific principle. The production reactors at Hanford were built to produce plutonium on an industrial scale. The enrichment facilities at Oak Ridge were built to separate uranium-235 from natural uranium on an industrial scale.
The workers who built those facilities were the inheritors of the construction workforce that built Chicago Pile-1—more numerous, working under more controlled conditions by 1943 and 1944 than the squash court had provided, but working with the same materials and the same fundamental unknowns about the long-term consequences of the work. The Pile-1 experiment proved one thing on which every nuclear reactor in the world has depended since December 2, 1942. It proved that a chain reaction could be started and stopped in a controlled medium under the supervision of people who understood what the instruments were telling them. That it could be sustained for a defined period and then halted.
That the calculations predicting the reactor would reach criticality at a specific rod position and not before were calculations confirmed by the physical world. Every commercial nuclear power plant generating electricity in the United States today operates on the physics confirmed by George Weil’s pull of a rod in a Chicago squash court in 1942. Boiling water reactors, pressurized water reactors, research reactors producing isotopes for medical imaging, naval reactors in submarines and aircraft carriers of the United States Navy—all descend from that ellipsoidal pile of graphite bricks and uranium ingots, assembled in November 1942 by a crew whose names are not usually fully recorded, using 40,048 pieces of graphite, 22,000 uranium ingots, and eighteen days of work. The Henry Moore sculpture was cast in bronze and installed in 1967.
It weighs twelve tons. Moore, a Briton who was not in Chicago in 1942, designed it to evoke simultaneously a human skull, a mushroom cloud, and the interior of a cathedral. Whether it evokes these things is a question that depends on the viewer. It is abstract enough that the evocation requires some assistance from the title and the identification plaque.
The identification plaque does its job. The sculpture does what abstract bronze sculptures on university campuses do in winter: it withstands the weather, acquires the patina that bronze acquires, and remains present in the space without needing to be understood by everyone who passes it. The squash court no longer exists. The wood of the frame has been gone for eighty years.
The graphite bricks were moved and ultimately disposed of through the radioactive waste management systems developed in later decades. The uranium blocks were reprocessed in the production reactors at Hanford. The building that housed the squash court was demolished. The space the building occupied is the outdoor plaza where the sculpture stands.
What exists at that site now is the sculpture, the plaque, and the knowledge, if you carry it with you, of what happened there. That knowledge is not self-evident in the sculpture. A person who passes it without knowing what the plaque says does not know what happened there. A person who reads the plaque learns that on December 2, 1942, at 3:25 in the afternoon, at a site now marked by a bronze abstraction, the first self-sustaining nuclear chain reaction in history was achieved.
The plaque does not mention who held the rod. It does not mention that a man named George Weil held a piece of cadmium-coated wood in his hands while forty-two scientists and officials watched from a balcony above him. That when Enrico Fermi ordered him to draw the rod to thirteen feet, he drew it to that distance and held it there while the instruments rose to a number that had never existed before. It does not mention that the graphite dust inhaled by the construction crew during those eighteen November days was never studied for its health effects in any systematic way that followed the workers who inhaled it over their subsequent lives.
It does not mention what George Weil did after he finished holding the rod: that he went where he went afterward, away from the balcony and the photographs and the Chianti, back toward the work the next day would require. The plaque says what plaques usually say. The men are in the records. The records are in the archives.
The sculpture stands in the weather at Ellis Avenue and 57th Street, twelve tons of bronze over the site of the squash court, over the site of the wooden frame, over the site of the first layer of graphite bricks that the assembly crew laid on that frame on a day in November 1942, when what they were building would succeed, and they would build it, and the calculations would be accurate, and George Weil would hold the rod. He held the rod. The reaction still runs by itself. Not in that squash court, which no longer exists, but in every reactor that descends from that pile he helped build, now operating in this country and in thirty other nations, producing the electricity the grids carry, the isotopes hospitals use, and the propulsion submarines use, sustained by the physics confirmed by the instruments at 3:25 on an afternoon in December 1942, while a man on a balcony read his slide rule and a man below held a piece of wood.
The reaction still runs by itself. The men who built that pile are in the records. The records are in the archives. The commemorative plaque is on Ellis Avenue at 57th Street.
The sculpture stands in the weather.
