Celebrating the first self-sustaining nuclear chain reaction from 1942

Chicago Pile-1 paved the way for nuclear science and the Manhattan Project

By Brye Steeves | December 6, 2021

Cp1 Scientists Opt
Members of the Chicago Pile-1 team met at the University of Chicago on the experiment’s fourth anniversary in 1946. Of note, future Los Alamos Lab Director Harold Agnew is on the far left of the middle row; the experiment’s only female physicist Leona Marshall is to the right of center; and Nobel Laureate Enrico Fermi is on the far left of the first row. Courtesy of NSRC

On a bitter-cold winter day, 43 scientists gathered at an abandoned squash court at the University of Chicago where they would ultimately enable a secret lab in Los Alamos to change the world just years later.

It was Dec. 2, 1942. The group, led by Italian physicist and Nobel laureate Enrico Fermi, stacked graphite bricks, piling 57 layers that totaled more than 770,000 pounds. 

Later named Chicago Pile-1, their goal was to create the world’s first self-sustaining, controlled nuclear chain reaction. 

Inside the approximately 20-feet-tall pile were smaller blocks of uranium and control rods that, when removed, would cause the reaction to go critical – meaning create a nuclear chain reaction. It was roughly $1 million worth of materials, equivalent to nearly $16 million today, and a concept that a nuclear chain reaction would allow the weaponization of the atom. 

“Its success would be the crucial proof needed to know it would be possible to create an atomic bomb,” said LANL Historian Roger Meade (C-NR). “This was the precursor to the Lab we have today, nearly 80 years later.”

But no Eeyore

Fermi watched alongside his team – all men and just one woman, 23-year-old physicist Leona Marshall – from a balcony. They were monitoring the experiment on instruments named after fictional children’s storybook characters: Pooh, Piglet, and Tigger, according to the Department of Energy.

Shortly after 3:30 p.m., rods were removed one at a time. One was controlled from a balcony, one was an emergency safety rod and another would be withdrawn by scientist George Weil to cause criticality.

“Although Fermi was confident that he could control his experiment,” Meade said, “he nonetheless stationed three graduate students, known as the suicide squad, on top of the reactor to pour buckets of a cadmium solution over the experiment if the safety mechanism failed. The cadmium (a chemical element) solution would soak up neutrons and quash the fission process.” 

Luckily for the graduate students, their role was in vain. As the final rod came out, Fermi reportedly said, “This is going to do it. Now it will become self-sustaining.”

He was right. 

On to Los Alamos

A coded message, “The Italian navigator has just landed in the new world,” notified government officials of the experiment’s success and the team celebrated with paper cups of Chianti (Italian red wine). 

Many of the scientists signed the label on the wine bottle, which ended up being the only written record of attendees, according to the Department of Energy.

Many of the scientists from the Chicago Pile-1 team, including Fermi, would go on to work for the Manhattan Project’s secret wartime lab in Los Alamos. There, the results of the experiment were used to develop the world’s first nuclear explosive device during the Trinity Test in July 1945. Two atomic bombs were released weeks later in combat against Japan; World War II ended shortly thereafter. 

Want to see a piece of history?

Visit the Bradbury Science Museum to see a bar of graphite that was stacked near the center of the core of the Chicago-Pile 1 reactor. Argonne National Laboratory in Illinois sent the bar to Los Alamos in 1992 from its inventory of the Chicago Pile-1 graphite bars.