How a Pile of Uranium Changed the World

On December 3rd, 1942 – 75 years ago – under the stands at the University of Chicago’s Stagg Field, scientists produced a breakthrough that would change the course of history. There, on that day, physicist Enrico Fermi and his team from the university initiated the first controlled, self-sustaining nuclear reaction, bringing forth the dawn of the nuclear age.

a portrait of Enrico Fermi from the Manhattan Project
Enrico Fermi

Building Chicago Pile One Under Stagg Field

Stagg Field was where the University of Chicago’s football team once dominated the Big Ten. But by 1942, the football team had left, and the stadium had largely fallen into disuse, making it the perfect place to test the new science of nuclear physics. In the old squash courts under the west stands, Fermi’s team built Chicago Pile Number One, or CP-1, the world’s first nuclear reactor. The reactor was simple and nondescript, perfect disguise for a top-secret project. Even Fermi didn’t put much thought into its aesthetic, naming it “pile” after what it looked like: a pile of wood and brick. CP-1 was was composed of 40,000 graphite blocks, arranged by scientists at the University’s Metallurgical Laboratory in a 24-foot-square wooden frame, which they filled with 9,000 pieces of uranium metal, composed of uranium-235 and uranium oxide fuel.

How it Worked

Uranium-235 is a highly radioactive element, meaning it breaks apart really easily. When individual atoms like this split, they release a lot of energy. In the case of uranium-235, a highly unstable isotope of uranium, it splits into stable forms of barium and krypton (two lighter elements), and spits out some extra neutrons. Meanwhile, these neutrons, along with the energy the unstable uranium-235 originally contained, are released into the environment.

With enough of this energy, the United States government thought, physicists could create the most destructive bomb the world would ever know. The implications of this research aside, Fermi’s had one goal in mind with CP-1: to achieve criticality, a nuclear status that causes radioactive elements, such as uranium-235, to decay reliably and consistently and release a ton of energy.

Highly radioactive elements are always splitting, or fissuring, on their own, letting go of subatomic particles like neutrons to reach a more stable, balanced state. But if there are other radioactive elements nearby, these subatomic particles will cause its neighbors to fissure as well. Criticality is when there are enough radioactive elements close together to release enough subatomic particles to sustain a nuclear chain reaction, where the fissure of one atom causes the fissure of another, and another, and so on, in a chain reaction. Fermi believed that if he concentrated enough uranium in one place, it would reach criticality, releasing enough neutrons to spontaneously initiate a chain reaction that would release a significant amount of energy.

The Results and Implications

The experiment worked, to a degree. Ultimately, CP-1 only released about 200 watts energy, just enough energy to power a light bulb, before they stopped the reaction by dropping control rods on the pile (these rods were composed of cadmium, a metal that easily absorbs neutrons.) But CP-1 taught the scientific community two groundbreaking lessons: 1) it is possible to artificially initiate a self-sustaining nuclear reaction, and 2) that it is possible to control it.  The scientists working on CP-1 knew at the time that they were creating something that would change the world – Fermi himself was recorded saying that CP-1 “meant that release of atomic energy on a large scale would be only a matter of time.” (Fermi). Many of the scientists had mixed feelings about the implications of their research, especially after the development of nuclear weapons. One of leaders of the project, Leo Szilard, predicting the use of nuclear technology in weaponry, commented that the day they first reached criticality at Stagg Field would be considered “a black day in the history of mankind.”

Peaceful Uses for Nuclear Energy

After the War, Fermi hoped that the world’s use of nuclear energy would turn to one of peace. In many respects, his wish came true.

For instance, CP-1 directly led to the discovery of how to use radiation for cancer therapy, a technique that was also pioneered at the University of Chicago. Radiation therapy works by damaging the DNA in cells that are actively dividing. Specifically, it causes DNA strands to break in two, a type of damage that cancer cells have trouble repairing. As cancer cells divide, these mutations accumulate, eventually killing the tumor.

The principles behind CP-1 also helped physicists learn how to wield the power of nuclear fission to create electricity. Nuclear power is a green energy, producing zero carbon emissions. Also, it is very efficient: it produces about 2.5 million more times the energy than an equal amount of coal. However, nuclear power produces tons of radioactive waste, which we still have not determined how to dispose of.

A nuclear power plant. The clouds coming out of the vents are steam (from water). Nuclear fission releases heat, which heats up water in the plant, which turns into steam and causes a turbine to turn and generate electricity.

The Nuclear Legacy

Even 75 years later, we do not fully understand the full power of nuclear energy, and it will still take time to develop methods to safely wield it. But in the meantime, the technology exists, and it is up to us to figure out what to do with the knowledge that Enrico Fermi and his team created at the University of Chicago. As the science of nuclear physics continues to progress, the number one scientific priority will be to learn new ways to contain this power and to use it safely as an alternative source of fuel. Nevertheless, it will take many lengthy discussions between physicists and policy-makers to determine the best way to handle our nuclear legacy.



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