How a Pile of Uranium Changed the World

On December 3rd, 1942,  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 achieved the primary objective of the top secret Mahattan Project: they initiated the first controlled, self-sustaining nuclear reaction, bringing forth the dawn of the nuclear age.

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, a 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 from their nuclei. Meanwhile, these neutrons, along with the energy originally contained within the unstable uranium-235, escape into the world. This is one type of radiation.

In the 1930s, physicists knew they could produce large amounts of energy by splitting atoms. And by the 1940s, while in the midst of a world war, the United States government thought physicists could use this energy to create the most destructive bomb the world would ever know. This would be a substantial yet secret, multi-year endeavor. But when it came to Fermi’s CP-1, he only had one goal in mind: to achieve “criticality,” a nuclear status that causes radioactive elements, such as uranium-235, to decay reliably and consistently on their own and release a ton of energy in the process.

Fermi and his team decided they’d take advantage of the natural properties of radioactive elements: 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. 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. ( They dropped neutron-absorbing control rods on the pile to end the reaction.) Despite its modest success, CP-1 taught the scientific community two groundbreaking lessons: 1) it is possible to artificially initiate a self-sustaining nuclear reaction, and 2) it is possible to control how far it goes.  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.” 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, the principles behind CP-1 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. Nuclear power produces tons of radioactive waste, though, which we still haven’t figured out how to dispose of in a sustainable way.

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 energy. Nevertheless, it will take many lengthy discussions between physicists and policy-makers to determine the best way to handle our nuclear legacy.

References and Further Reading:

http://www.atomicheritage.org/key-documents/fermi-chicago-pile-1

https://www.uchicago.edu/features/how_the_first_chain_reaction_changed_science/

http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/what-is-uranium-how-does-it-work.aspx