Site of the First Self Sustaining Nuclear Reaction
U.S. National Register of Historic Places
U.S. National Historic Landmark
Chicago Landmark
Drawing of the reactor
Location	Chicago, Cook County, Illinois, USA
Coordinates	41°47′32″N 87°36′3″W / 41.79222°N 87.60083°W / 41.79222; -87.60083Coordinates: 41°47′32″N 87°36′3″W / 41.79222°N 87.60083°W / 41.79222; -87.60083
Built	1942[2]
Governing body	Regenstein Library
NRHP Reference #	66000314[1]
Significant dates
Added to NRHP	15 October 1966 66000314[1]
Designated NHL	18 February 1965[2]
Designated CL	27 October 1971[3]

Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor.^[4][5] The construction of CP-1 was part of the Manhattan Project, and was carried out by the Metallurgical Laboratory at the University of Chicago. It was built under the west viewing stands of the original Stagg Field. The first man-made self-sustaining nuclear chain reaction was initiated in CP-1 on 2 December 1942, under the supervision of Enrico Fermi. Fermi described the apparatus as "a crude pile of black bricks and wooden timbers." It was made of a large amount of graphite as a neutron moderator and natural uranium fuel, with "control rods" of cadmium, indium, and silver. Unlike most subsequent nuclear reactors, it had no radiation shield or cooling system as it only operated at very low power.

The site is now a National Historic Landmark and a Chicago Landmark.

Origins[edit]

The idea of chemical chain reactions was first put forth in 1913 the German chemist Max Bodenstein for a situation in which two molecules react to form not just the molecules of the final reaction products, but also some unstable molecules which can further react with the parent molecules to cause more molecules to react.^[6] The concept of a nuclear chain reaction was first hypothesized by the Hungarian scientist Leo Szilard on 12 September 1933.^[7]

Szilard realized that if a nuclear reaction produced neutrons or dineutrons, which then caused further nuclear reactions, the process might be self-perpetuating. The neutron had been recently discovered by James Chadwick in 1932; the dineutron would not be until 2012. Szilard, however, did not propose nuclear fission as the mechanism for his chain reaction, since this had not yet been discovered or even suspected. Instead, Szilard proposed using mixtures of lighter known isotopes which produced neutrons in copious amounts, although he did entertain the possibility of using uranium as a fuel.^[8] He filed a patent for his idea of a simple nuclear reactor the following year,^[9] and assigned it to the British Admiralty in 1936 to ensure its secrecy.^[10]

Over the next two years, Szilard attempted to create a nuclear chain reaction using beryllium by bombarding it with X-rays,^[11][12] and then with indium, but with no success.^[13] The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938,^[14][15] followed by its theoretical explanation (and naming) by Lise Meitner and Otto Frisch,^[16][17] opened up the possibility of creating a nuclear chain reaction with uranium.^[18]

Research[edit]

In order for this to be the case, additional neutrons had to be emitted from fissioning uranium atoms. At Columbia University in New York, John Dunning, Herbert L. Anderson, Eugene T. Booth, Enrico Fermi, G. Norris Glasoe, and Francis G. Slack conducted the first nuclear fission experiment in the United States on 25 January 1939.^[19][20] Subsequent work soon showed that fast neutrons were indeed produced by fission.^[21][22]

Szilard obtained permission from the head of the Physics Department at Columbia, George B. Pegram, to use a laboratory for three months, and persuaded Walter Zinn to become his collaborator.^[23] They conducted a simple experiment on the seventh floor of Pupin Hall at Columbia, using a radium-beryllium source to bombard uranium with neutrons. Initially nothing registered on the oscilloscope, but then Zinn realized that it was not plugged in. When this was done, they discovered significant neutron multiplication in natural uranium, proving that a chain reaction might be possible.^[24]

While this demonstrated that the fission of uranium could produce more neutrons than it consumed, it was not a chain reaction. Szilard persuaded Fermi and Anderson to try a larger experiment using 500 pounds (230 kg) of uranium. To maximize the chance of fission, they needed a neutron moderator to slow the neutrons down. Hydrogen was a known moderator, so they used water. The results were disappointing. It became apparent that hydrogen slowed neutrons down, but also absorbed them, leaving fewer for the chain reaction.^[25]

Szilard then suggested Fermi use carbon, in the form of graphite. He felt he would need about 50 tonnes (49 long tons; 55 short tons) of graphite and 5 tonnes (4.9 long tons; 5.5 short tons) of uranium. As a back-up plan, Szilard also considered where he might find a few tons of heavy water; deuterium would not absorb neutrons like ordinary hydrogen, but would have the similar value as a moderator. Such quantities of materiel would require a lot of money.^[25] Fermi and Szilard still believed that enormous quantities of uranium would be required for an atomic bomb, and therefore concentrated on producing a controlled chain reaction.^[26] Fermi determined that fissioning uranium atom produced 1.73 neutrons on average. It was enough, but a careful design was call for to minimize losses.^[27][28]

Szilard drafted a confidential letter to the President, Franklin D. Roosevelt, explaining the possibility of nuclear weapons, warning of German nuclear weapon project, and encouraging the development of a program that could result in their creation. With the help of Wigner and Edward Teller, he approached his old friend and collaborator Einstein in August 1939, and convinced him to sign the letter, lending his fame to the proposal.^[29] The Einstein–Szilard letter resulted in the establishment of research into nuclear fission by the U.S. government, and ultimately to the creation of the Manhattan Project.^[30]

An Advisory Committee on Uranium was formed under Lyman J. Briggs, a scientist and the director of the National Bureau of Standards. Its first meeting on 21 October 1939, was attended by Szilard, Teller and Wigner, who persuaded the Army and Navy to provide $6,000 for Szilard to purchase supplies for experiments—in particular, more graphite.^[31] Fermi and Szilard met with representatives of National Carbon Company, who manufactured the graphite, where Szilard made another important discovery. By quizzing them about impurities in their graphite, he found that it contained boron, a neutron absorber. He then had graphite manufacturers produce boron-free graphite.^[32] Had he not done so, they might have concluded, as the Germans did, that graphite was unsuitable for use as a neutron moderator.^[33]

In a nuclear reactor, criticality is achieved when the rate of neutron production is equal to the rate of neutron losses, including both neutron absorption and neutron leakage. Thus, in the simplest case of a bare, homogeneous, steady state nuclear reactor, the neutron leakage and neutron absorption must be equal to neutron production in order to reach criticality. The critical radius of an unreflected, homogeneous, spherical reactor was calculated to be:^[34]

where M is the migration area and k is the medium neutron multiplication factor.

In order for a self-sustaining nuclear chain reaction to occur, they needed the neutron multiplication factor k to be higher than 1. For a practical reactor configuration, it needed to be at least 3 or 4 percent more.^[34]

Reactor[edit]

The reactor was a "pile" of metalic uranium pellets and graphite blocks, assembled under the supervision of the renowned physicist Enrico Fermi, in collaboration with Szilard, discoverer of the chain reaction, and assisted by Martin D. Whitaker, Walter Zinn, and George Weil. It contained a critical mass of fissile material (when moderated by the graphite), together with control rods. The shape of the pile was intended to be roughly spherical, but as work proceeded Fermi calculated that critical mass could be achieved without finishing the entire pile as planned.^[35]

CP-1 was originally to be built in Red Gate Woods, a forest preserve outside the city, but a labor strike prevented this. So Fermi built the "pile" under the west stands of Stagg Field, the University's disused football stadium, in a space that had been used as a rackets court.^[36] In the pile, the neutron-producing uranium pellets were separated from one another by graphite blocks. Some of the free neutrons produced by the natural decay of uranium would be absorbed by other uranium atoms, causing nuclear fission of those atoms and the release of additional free neutrons. The graphite between the uranium pellets was a neutron moderator; it slowed the neutrons, increasing the chance they would be absorbed.

The controls were rods made of cadmium, indium, and silver. Cadmium and indium absorb neutrons; silver becomes radioactive when irradiated by neutrons, which is used for measuring their flux. When the rods were inserted into the pile, the cadmium absorbed free neutrons, preventing the chain reaction. As the rods were withdrawn, more neutrons would strike uranium atoms, until a self-sustaining chain reaction developed. Re-inserting the rods would dampen the reaction.

The pile required an enormous amount of graphite and uranium. At the time, there was a limited source of pure uranium. Frank Spedding of Iowa State University was able to produce only two short tons of pure uranium. Westinghouse Lamp Plant supplied another three short tons of uranium metal, which it produced in a rush with a makeshift process. A large square balloon was constructed by Goodyear Tire to encase the pile.^[37][38]

First nuclear chain reaction[edit]

On 2 December 1942, CP-1 was ready for a demonstration. Before a group of dignitaries, George Weil worked the final control rod while Fermi carefully monitored the neutron activity. The pile "went critical" (reached a self-sustaining reaction) at 15:25. Fermi shut it down 28 minutes later.

After the chain reaction was observed, Arthur Compton, head of the Metallurgical Laboratory, notified James Conant, chairman of the National Defense Research Committee, by telephone. The conversation was in an impromptu code:

Compton: The Italian navigator has landed in the New World.

Conant: How were the natives?

Compton: Very friendly.^[39]

Unlike most reactors that have been built since, CP-1 had no radiation shielding and no cooling system of any kind. Fermi had convinced Arthur Compton that his calculations were reliable enough to rule out a runaway chain reaction or an explosion. There were insufficient enrichment levels for an explosion to be possible. But, as the official historians of the Atomic Energy Commission later noted, the "gamble" remained in conducting "a possibly catastrophic experiment in one of the most densely populated areas of the nation!"^[40]

Later operation[edit]

Operation of CP-1 was terminated in February 1943. The pile was then dismantled and moved to Red Gate Woods. There it was reconstructed using the original materials, plus an enlarged radiation shield, and renamed Chicago Pile-2 (CP-2). CP-2 began operation in March 1943 and was later buried at the same site, now known as the Site A/Plot M Disposal Site. CP-2 and other activities. including Chicago Pile 3 the first "heavy water" reactor, at the Red Gate Woods site led to it becoming the first site of Argonne National Laboratory.^[35]

Henry Moore's Nuclear Energy

Significance and commemoration[edit]

The site of CP-1 was designated as a National Historic Landmark on 18 February 1965.^[2] When the National Register of Historic Places was created in 1966, it was immediately added to that as well.^[1] The site was named a Chicago Landmark on 27 October 1971.^[3]

The site of the old Stagg Field is now occupied by the University's Regenstein Library. A Henry Moore sculpture, Nuclear Energy, stands in a small quadrangle just outside the Library, to commemorate the nuclear experiment.^[2]

A small graphite block from CP-1 can be seen at the Bradbury Science Museum in Los Alamos, New Mexico; another is currently on display at the Museum of Science and Industry in Chicago.^[41]

See also[edit]

Nuclear technology portal

• F-1 (nuclear reactor), its Soviet equivalent, built in 1946 and still operational
• ZEEP, its Canadian equivalent.

Notes[edit]

References[edit]

• Anderson, Herbert L. (1975). "Assisting Fermi". In Wilson, Jane. All In Our Time: The Reminiscences of Twelve Nuclear Pioneers. Chicago: Bulletin of the Atomic Scientists. pp. 66–104. OCLC 1982052. 
• Hewlett, Richard G.; Anderson, Oscar E. (1962). The New World, 1939–1946 (PDF). University Park: Pennsylvania State University Press. ISBN 0-520-07186-7. OCLC 637004643. Retrieved 26 March 2013. 
• Lanouette, William; Silard, Bela (1992). Genius in the Shadows: A Biography of Leo Szilard: The Man Behind The Bomb. New York: Skyhorse Publishing. ISBN 1-62636-023-5. OCLC 25508555. 
• Rhodes, Richard (1986). The Making of the Atomic Bomb. London: Simon & Schuster. ISBN 0-671-44133-7. 
• Weinberg, Alvin (1994). The First Nuclear Era: The Life and Times of a Technological Fixer. New York: AIP Press. ISBN 1-56396-358-2. 
• Zug, J. (2003). Squash: A History of the Game. New York: Scribner. ISBN 978-0-7432-2990-6. OCLC 52079735. 

External links[edit]

• CP-1 Goes Critical Describes in detail the construction and activation of CP-1. US Department of Energy, Office of History and Heritage Resources.
• Photos of CP-1 The University of Chicago Library Archive. Includes photos and sketches of CP-1.
• Video Showing the Met Lab, Fermi, and an active experiment using CP-1
• The First Pile 11 page story about CP-1
• "First-Hand Recollections of the First Self-Sustaining Chain Reaction". Department of Energy. Retrieved 23 September 2015.  Video of two of the last surviving CP-1 pioneers, Harold Agnew and Warren Nyer.

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