First Nuclear Chain Reaction Changed the World 75 Years Ago Today

In 1942, the abandoned squash court located underneath the disused football stadium at the University of Chicago was little more than an eye sore. But where students saw remnants of squash games past, physicist Enrico Fermi saw an ideal place for an experiment, the results of which would change the trajectory of World War II and usher in a new, fraught geopolitical era.

The reinforced brick room was perfectly sized to hold a neatly stacked pile of 40,000 graphite bricks, some containing uranium, others drilled with holes designed to fit 14-foot long cadmium-coated tubes.

A worker stands next to graphite blocks that formed the backbone of Chicago Pile-1, a primitive nuclear reactor.

On December 2, Fermi and nearly 50 fellow scientists piled into the bleachers. Geiger counters in hand, they watched the readings skyrocket as the neutron-absorbing tubes were removed one by one. Without the cadmium buffers, neutrons from splitting uranium atoms were unrestrained, free to crash into other uranium atoms, releasing even more neutrons that caused even more collisions.

When the last tube was removed at 3:25 pm, the pile was sustaining a steady stream of atomic energy. This was no longer a squash court. This was home to the world’s first manmade nuclear reactor and the provenance of the Atomic Age.

Today marks the 75th anniversary of the Chicago Pile-1 chain reaction, a scientific breakthrough that made nuclear power and weaponry possible. It also opened up entire new avenues of research in medicine, engineering, and aeronautics. Though that initial reaction only generated about half a watt of power, the event marked a turning point. Later developments would give humankind access to unprecedented levels of power while forcing us confront whether and how it should be used.

“They had basically created an entirely new energy source,” says Rachel Bronson, president and CEO of the Bulletin of the Atomic Scientists. “They had created fire in some ways.”

Scientists working on Chicago Pile-1 looked on as control rods were removed from the reactor, seen here in a painting of the event.

In the process, the minds behind the Chicago Pile-1 broke cultural and political barriers, she adds. Fermi was an Italian immigrant, and Hungarian refugees played crucial roles in the project, including Leo Szilard, who came up with the idea of a nuclear chain reaction, and Eugene Wigner, who would later share a Nobel Prize for his contributions to atomic research.

“So many of the big issues that we’re grappling with—how to manage nuclear power, what kind of funding should go into research and development, what should our immigration policy be, this was all swirling around the Manhattan Project in 1942,” Bronson says.

While those questions loomed in the background of the Chicago Pile experiments, Fermi’s team stayed focused on two immediate goals—one, figure out how to control nuclear energy before Germany, and two, prevent the reaction from spiraling out of control. Given that the safety controls were primitive by today’s standards and mostly relied on a few cadmium tubes to prevent a nuclear explosion, the risk was very real.

“We could have very easily lost Chicago,” says Peter Kuznick, director of the Nuclear Studies Institute at American University in Washington, D.C.

Chicago Pile-1 was build beneath the stands of Stagg Field at the University of Chicago, located in the heart of the city.

Fermi’s team was well aware of the destructive potential of their research. Even while constructing the Chicago Pile, Szilard believed that the experiments “would go down as a black day in the history of mankind.” Their experiments also helped usher in an era in which scientists were more outspoken about how their work was used. Following World War II and into the Cold War, physicists routinely argued for the restriction or elimination of nuclear arms. Such activism around nuclear issues is another legacy of Fermi’s chain reaction, Kuznick says.

Fermi’s team probably never envisioned that their radioactive pile of graphite bricks would lead to cancer-spotting imaging technologies or devices that can help find hidden tombs in ancient Egyptian pyramids. But as they sat in those University of Chicago bleachers, listening to the ever-increasing clicks of their Geiger counters, they knew that something big was happening, says Alex Wellerstein, assistant professor of science and technology studies at the Stevens Institute of Technology.

“They definitely thought they were on the cusp of a new world with their experiment,” he says. “They knew it was just the beginning.”