Fermi Creates the First Controlled Nuclear Fission Chain Reaction

Physicist Enrico Fermi and his team demonstrated that nuclear energy could be released in a sustained chain reaction, leading to the development of both the atomic bomb and nuclear power plants.

Summary of Event

In December, 1938, Enrico Fermi, a professor of physics in Rome, took advantage of his 1938 Nobel Prize in Physics Nobel Prize in Physics;Enrico Fermi[Fermi] to leave his native Italy and escape Adolf Hitler’s increasing domination of Benito Mussolini’s Italy. With his family, Fermi arrived in New York City and settled down to continue his research at Columbia University. [kw]Fermi Creates the First Controlled Nuclear Fission Chain Reaction (Dec. 2, 1942)
[kw]Nuclear Fission Chain Reaction, Fermi Creates the First Controlled (Dec. 2, 1942)
[kw]Fission Chain Reaction, Fermi Creates the First Controlled Nuclear (Dec. 2, 1942)
[kw]Reaction, Fermi Creates the First Controlled Nuclear Fission Chain (Dec. 2, 1942)
Nuclear fission
Nuclear physics
Atomic piles
Nuclear fission
Nuclear physics
Atomic piles
[g]North America;Dec. 2, 1942: Fermi Creates the First Controlled Nuclear Fission Chain Reaction[00690]
[g]United States;Dec. 2, 1942: Fermi Creates the First Controlled Nuclear Fission Chain Reaction[00690]
[c]Physics;Dec. 2, 1942: Fermi Creates the First Controlled Nuclear Fission Chain Reaction[00690]
[c]Science and technology;Dec. 2, 1942: Fermi Creates the First Controlled Nuclear Fission Chain Reaction[00690]
[c]Engineering;Dec. 2, 1942: Fermi Creates the First Controlled Nuclear Fission Chain Reaction[00690]
Fermi, Enrico
Zinn, Walter Henry
Anderson, Herbert L.
Compton, Arthur Holly
Szilard, Leo
Bohr, Niels

Fermi and his associates in Rome had been studying the new nuclei produced when various chemical elements were bombarded by neutrons. In 1934, experiments on uranium produced a new radioactive isotope Radioisotopes . Fermi and his collaborators demonstrated chemically that the new isotope did not belong to any of the elements immediately below uranium on the periodic table. The team concluded that they had produced the first element ever found that was heavier than uranium. Transuranic elements The idea of a transuranic element caught the imagination of the scientific community and the popular press. When German chemist Ida Noddack Noddack, Ida published an article suggesting that Fermi had not ruled out the possibility that their new radioactivity came from a lighter chemical element produced when a uranium nucleus split into two parts, she was ignored.

Fermi’s team and other groups, including Irène Joliot-Curie and Paul Savitch in Paris, and Otto Hahn Hahn, Otto , Lise Meitner Meitner, Lise , and Fritz Strassmann Strassmann, Fritz in Berlin, continued to study the effects of irradiating uranium with neutrons. Fermi’s group demonstrated that neutrons that had been slowed down by passing them through a material containing hydrogen, such as paraffin, were more effective in producing radioactivity than were more energetic neutrons. The experimenters gradually compiled a list of several different radioactive species that were produced when uranium was bombarded.

In December, 1938, Hahn wrote to Meitner and informed her that he and Strassmann had incontrovertible evidence that bombardment of uranium with neutrons produced lighter elements and not transuranic elements. Meitner, who was Jewish, had been forced to leave Berlin for Sweden. She and her nephew, Otto Robert Frisch Frisch, Otto Robert , a young physicist working with Niels Bohr in Copenhagen, concluded that when a uranium nucleus absorbed a neutron, it split or fissioned into two lighter nuclei and some extra neutrons, releasing a hundred million times as much energy as was released in a typical chemical reaction between two atoms.

Enrico Fermi.

(Gift of Laura Fermi, Courtesy AIP Emilio Segré Visual Archives)

Frisch reported the discovery of nuclear fission to Bohr. Bohr was leaving Denmark for the United States and announced the discovery to the Fifth Washington Conference on Theoretical Physics Fifth Washington Conference on Theoretical Physics (1939) on January 26, 1939. Within days, Fermi and other American physicists had confirmed the discovery of nuclear fission. Fermi and Leo Szilard, a Hungarian physicist also driven into exile by Hitler’s advance in Europe, realized immediately that if the neutrons from one fission could be used to trigger a second fission, the resulting chain reaction could be used to produce energy. If the multiplication could be made geometric, the chain reaction would be a powerful explosive. Szilard feared that Hitler’s Germany would construct a superweapon based on these principles. He persuaded his American colleagues, including Fermi, to delay publication of their experimental results on fission.

Meanwhile, the physics community measured the energy released in uranium fission, the new nuclei produced, and the number of neutrons released during each fission. In August of 1939, Szilard and fellow Hungarian émigré Eugene P. Wigner Wigner, Eugene P. persuaded Albert Einstein Einstein, Albert to send a letter to President Franklin D. Roosevelt urging a research program into the possibility of a superweapon. The government hesitated while the physicists determined that only the rare isotope of uranium—U-235—underwent fission, while the isotope U-238, which constituted 99.3 percent of naturally occurring uranium, did not.

Fermi and his colleagues calculated that it should be possible to sustain a chain reaction in a matrix of pure uranium oxide with graphite between blocks of uranium to slow down the neutrons. Both the graphite and the uranium would have to be free of chemical impurities that would absorb neutrons and cut off the chain reaction. In July, 1941, Fermi and his group were funded to begin experiments in constructing a graphite-uranium “pile” designed to sustain a chain reaction. In December, Arthur Holly Compton, the American Nobel laureate in physics, was placed in charge of the project and moved the experiments to Chicago in early 1942.

By July, 1942, the Chicago group had completed sufficient preliminary work to design a pile that would sustain a chain reaction and to begin its construction. Thirty preliminary atomic piles of graphite and uranium had been built before the pile that sustained the first chain reaction was started. Construction of the pile began in November in a squash court since this was the only area available that was large enough to hold the 771,000 pounds of graphite, 80,590 pounds of uranium oxide, and the 12,400 pounds of uranium metal that were to compose the pile. The uranium metal packed uranium nuclei closer together, increasing the likelihood that a neutron would be captured by a uranium nucleus and cause a fission. Since Fermi could not obtain enough metal, the available supply was to be built into the center of the pile.

The pile was constructed inside an enormous balloon, because Fermi feared he might have to evacuate the pile to reduce neutron absorption by air. Construction crews headed by Walter Henry Zinn and Herbert L. Anderson worked around the clock machining and stacking the graphite and uranium blocks. All the scientists slid on the graphite dust, which permanently blackened the skin of their faces and hands. Control rods that absorbed neutrons were built into the pile to be withdrawn in order to start the chain reaction. Each day the control rods were withdrawn and measurements were taken to see how close the system was to sustaining a chain reaction.

On the evening of December 1, 1942, Anderson and Zinn realized that the layer of uranium and graphite that the night crew had placed on the pile should be sufficient to sustain a chain reaction. The crew went home for a few hours of sleep and reassembled at 8:30 the following morning. Fermi ordered the main control rods withdrawn, and the final control rod was moved foot by foot out of the pile as the assembled physicists, including Compton, who had fought bureaucratic battles for them, gathered to watch the neutron counters. If a chain reaction had been initiated, the counters were expected to spin as the neutron counting rate increased without sign of leveling off. At 11:35 a.m., the automatic safety control rods slammed back into the pile because they were set to operate on too low a neutron flux. According to his invariable custom, Fermi announced a lunch break.

At 2:00 p.m., the group gathered around the pile, and the withdrawal of the rods continued. At 3:25 p.m. (Anderson claims it was 3:36 p.m.), the control rod was removed the final foot. The counting rate climbed exponentially. A controlled fission chain reaction had been achieved and was sustained until Fermi ordered the control rods back into the pile at 3:53 p.m. As the group celebrated, they realized that the success of their experiment had inaugurated a new age.


The successful operation of the atomic pile provided physicists with a tool for studying the behavior of nuclear fission chain reactions. These studies were essential for the design and construction of an atomic bomb, since details of critical mass and neutron absorption by materials could be easily measured using atomic piles.

Second, atomic piles produced a second fissionable isotope, plutonium 239, Plutonium and the design of large-scale piles for the production of plutonium was soon under way. Plutonium was to prove more efficient as a fuel for bombs than highly enriched uranium 235.

Finally, the first atomic pile demonstrated that it was possible to produce a sustained energy source from nuclear fission, the basic result that leads to the construction of nuclear electric generating plants. Compton reported the successful initiation of a chain reaction to a colleague in Washington with the famous words, “the Italian navigator has just landed in the new world.”

In 1942, Fermi’s success produced surprisingly little excitement. The scientists were elated that their experiments had worked as planned and used the result to demonstrate that they were able to produce the technologies they promised. However, preliminary results for the thirty piles built before the first chain reaction was achieved had been so encouraging that the establishment of the Manhattan Project Manhattan Project
Nuclear weapons;invention to construct an atomic bomb were already well under way. It was important that Compton had brought the representative of the Du Pont Corporation Du Pont Corporation[Dupont Corporation] , Crawford Greenewalt Greenewalt, Crawford , to witness the start-up of the pile. The success of the pile may have been instrumental in persuading Greenewalt and Du Pont to undertake the large-scale production of plutonium for nuclear weapons at reactors in Hanford, Washington.

Certainly, the success of the pile removed the remaining uncertainties in anyone’s mind that nuclear fission existed and could produce a chain reaction. Fermi’s success gave the military confidence in the predictions of civilian scientists that they could produce a working nuclear fission weapon, even when the experimental work on the Manhattan Project went badly.

After 1942, the major work on the atomic bomb shifted to the materials production centers at Oak Ridge, Tennessee, and Hanford, Washington, and to the weapons design center at Los Alamos, New Mexico. Fermi’s success in sustaining a chain reaction in the atomic pile in Chicago was the final step in the preliminary physics experiments before the actual design of an atomic weapon began. It served as a technological benchmark in the march toward peaceful energy and the beginning of the final stages of construction of an atomic weapon.

The public remained ignorant of Fermi’s work until the end of World War II. When classification was lifted, the experiments leading to the bomb became scientific legend. Fermi’s atomic pile in the center of Chicago has become a major element of that story. Nuclear fission
Nuclear physics
Atomic piles

Further Reading

  • Bernardini, Carlo, and Luisa Bonolis, eds. Enrico Fermi: His Work and Legacy. New York: Springer, 2004. A laudatory history of Fermi, his work, and his place in the development of nuclear physics. Includes the chapters “The Birth of Nuclear Energy: Fermi’s Pile” and “From the Chicago Pile 1 to the Next-Generation Reactors.”
  • Fermi, Laura. Atoms in the Family: My Life with Enrico Fermi. Chicago: University of Chicago Press, 1954. A very readable account by Fermi’s wife of the events surrounding the development of the first sustained chain reaction from a nontechnical point of view.
  • Graetzer, Hans G., and David L. Anderson. “From Nuclear Fission to Chain Reaction, 1939-1942.” In The Discovery of Nuclear Fission: A Documentary History. New York: Van Nostrand Reinhold, 1971. This history uses original documents to describe the discoveries in nuclear fission surrounded by careful explanations for general readers and a clear historical narrative.
  • Libby, Leona Marshall. The Uranium People. New York: Crane, Russak, 1979. The autobiography of the only female physicist working on the atomic pile experiment, this volume provides a very readable eyewitness account of the work on the first nuclear reactor.
  • Nuclearfiles.org. An excellent resource for students studying the history of the atomic age. The site, a project of the Nuclear Age Peace Foundation, includes links to primary sources, time lines, study guides, suggested readings, and much more.
  • Rhodes, Richard. “The New World.” In The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. This excellent history of nuclear fission provides a detailed description of the Chicago pile experiment in the context of other work on nuclear physics taking place at the time.
  • Segrè, Emilio. Enrico Fermi: Physicist. Chicago: University of Chicago Press, 1970. A biography of Fermi written by a physicist who worked with him in Rome and the United States that provides insight into the way Fermi developed his physical ideas and conducted experiments.
  • United States Office of the Assistant Secretary for Nuclear Energy. The First Reactor. Springfield, Va.: National Technical Information Service, 1982. Reissued for the fortieth anniversary of the first chain reaction, this slim volume contains a reprint of Fermi’s account of the experiment and a concise summary of the work that led up to it.

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