Race to the first nuclear chain reaction

Beginning in October 1942, members of the classified Metallurgical Laboratory project worked as many as 90 hours a week, often smoking cigarettes in place of eating. UChicago students as well as local laborers worked side by side with the scientists to build a lattice of graphite bricks and uranium into a 20-foot-tall, beehive-shaped structure known as Chicago Pile-1—hoping it would create a chain reaction that would grow from the splitting of a single uranium nucleus.

Graduate student Albert Wattenberg, PhD’47, recalled one morning when physicist Alvin Graves came into work unexpectedly, complaining he couldn’t sleep. “He felt the Nazis were working, that they were pushing ahead to get there before us,” Wattenberg wrote in 1982 in the Bulletin of the Atomic Scientists. “We were in a real race, and he felt he shouldn’t be taking a day off.”

As UChicago spends the fall commemorating the Dec. 2, 1942 breakthrough, a look back at the months leading up to the experiment recalls the intense pressures and pioneering science that produced the first nuclear chain reaction.

The Plan B decision to build at UChicago

The University wasn’t the original site for the historic experiment though. In early 1942, Compton identified a promising plot of land while on a horseback ride in a forest preserve about 25 miles southwest of Chicago. But by late October, workers constructing the buildings in the so-called Argonne Forest went on strike, and it soon became clear that the site wouldn’t be ready until year’s end. 

Fermi suggested to Compton that he could demonstrate the controlled chain reaction safely on campus—under Stagg Field, the long-abandoned, crumbling home of the former Big Ten football powerhouse. And if something were to go wrong, “I will walk away—leisurely,” Fermi once wrote. As a safeguard, a series of control rods would be installed to prevent a runaway reaction.

“According to Fermi’s calculations, which I carefully checked…it should take some minutes for the reaction to double its power," Compton wrote in his memoir. “If this proved correct, there would be ample time for adjustments, and the reaction would be under full control.”

Compton at the outset had predicted a nuclear chain reaction would be achieved by Jan. 1, 1943. With time of the essence, Compton told Fermi to proceed without informing UChicago President Robert Maynard Hutchins. Compton felt Hutchins, a trained jurist and former Law School dean, “was in no position to make an independent judgment of the hazards involved.”

“As a responsible officer of the University, according to every rule of organizational protocol, I should have taken the matter to my superior. But that would have been unfair,” wrote Compton. “Based on considerations of the University’s welfare, the only answer he could have given would have been—no. And this answer would have been wrong. So I assumed the responsibility myself.” 

Path to criticality

The self-assured Fermi gave Compton little cause for concern. In September, Fermi began a series of multi-hour weekly lectures at Eckhart Hall on the UChicago campus, where he described the measurements that would determine when the pile would go critical.

When he started building chain-reacting piles at Columbia University after his arrival in January 1939, Fermi would don a lab coat and worked alongside football players enlisted to move the 50- to 100-pound bricks of graphite. “With Fermi, it was the work that made the physics worthwhile. He wanted to wrestle with nature himself, with his own hands,” wrote physicist Herbert Anderson, who ran the night shift in Chicago’s pile program, in a 1974 Bulletin essay. “He liked to have someone to work with. He liked the companionship; the work went faster that way.”

There were no blueprints for the Chicago pile. Instead, machinists and scientists reported on the daily progress of construction to Fermi. Two crews formed: One pressed uranium oxide power into 22,000 spheres the size of baseballs. The other used a wood planer to mill about 400 tons of graphite into rectangles, which were then drilled to create holes to hold the uranium.

“We found out how coal miners feel,” wrote Wattenberg in the Bulletin. “One shower would remove only the surface graphite dust. About a half-hour after the first shower, the dust in the pores of your skin would start oozing.”

By late fall, dozens of smaller test piles had provided proof-of-concept for Fermi’s larger experiment. But CP-1, 20 times larger than its predecessors, would require even larger amounts of uranium and graphite in purer forms.

On Nov. 16, two  12-hour shifts began to construct the pile—work that would continue non-stop over the next 15 days. By the evening of Dec. 1, they had constructed the reactor, which resembled a 57-layer graphite cake, wrapped in wood and studded with hundreds of uranium raisins toward the center that would serve as the nuclear fuel for the reaction.

By the morning of Dec. 2, Chicago Pile-1 was ready. 

Unlocking a giant

Nearly a year of hard work culminated on a cold, snowy Wednesday, when at 3:25 p.m. on Dec. 2, the feverish clickety-clack of Geiger counters proved to the 49 people huddled inside the balcony of the squash racquets courts that they had sustained and controlled a nuclear chain reaction for the first time in history.

Although at full strength it produced only enough energy to power a lightbulb, the discovery marked the dawn of the Atomic Age—forever changing the world. “The event was not spectacular: No fuses burned, no lights flashed,” Fermi later conceded. But the UChicago scientists had succeeded, beating Compton’s original prediction by nearly a month.

To mark the occasion, scientist Eugene Wigner presented Fermi with a small bottle of Chianti wine. “No toast, nothing, and everyone had a few memorable sips,” wrote Leona Woods, a 23-year-old UChicago graduate student, in her memoir.

“There was a greater drama in the silence than if the words had been spoken. Everyone was thinking—if we did it, haven’t the Germans already achieved the chain reaction?”

For Wigner, one of the lead scientists on the project who would later win a Nobel in physics, the experiment was the culmination of years of scientific work, which began when he and fellow Hungarian physicist Szilard convinced Albert Einstein to sign a letter to President Roosevelt in 1939, urging the United States to fund nuclear research before the Nazis developed a bomb.

“For some time, we had known that we were about to unlock a giant; still, we could not escape an eerie feeling when we knew we had actually done it,” Wigner wrote in The New York Times Magazine in 1963. “We felt as, I presume, everyone feels who had done something he knows will have very far-reaching consequences which he cannot foresee.”

—UChicago doctoral candidate Gregory Montoya-Mora contributed archival research to this report.