Site of the First Self Sustaining Nuclear Reaction
Drawing of the reactor
|Location||Chicago, Cook County, Illinois, USA|
|Governing body||Regenstein Library|
|NRHP Reference #||66000314|
|Added to NRHP||15 October 1966 66000314|
|Designated NHL||18 February 1965|
|Designated CL||27 October 1971|
|Chicago Pile-1 (CP-1)|
|Reactor concept||Research reactor (uranium/graphite)|
|Designed and build by||Metallurgical Laboratory|
|Operational||1942 to 1943|
|Main parameters of the reactor core|
|Fuel (fissile material)||Natural uranium|
|Fuel state||Solid (pellets)|
|Neutron energy spectrum||Information missing|
|Primary control method||Control rods|
|Primary moderator||Nuclear graphite (bricks)|
|Remarks||The Chicago Pile-1 (CP-1) is the world's first artificial nuclear reactor.|
Chicago Pile-1 (CP-1) is the world's first artificial nuclear reactor. 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 December 2, 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 has no radiation shield or cooling system as it only operated at very low power. In 1943, CP-1 was moved and reconfigured to become Chicago Pile-2 (CP-2).
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. The concept of a nuclear chain reaction was first hypothesized by the Hungarian scientist Leo Szilard on 12 September 1933. Szilard realized that if a nuclear reaction produced neutrons or dineutrons, which then caused further nuclear reactions, the process might be self-perpetuating. 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. He filed a patent for his idea of a simple nuclear reactor the following year. The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, followed by its theoretical explanation (and naming) by Lise Meitner and Otto Frisch, opened up the possibility of creating a nuclear chain reaction with uranium. and then with indium, but with no success.
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. Subsequent work confirmed that fast neutrons were indeed produced by fission. 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. 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.
Szilard 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. 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. 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.
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. Had he not done so, they might have concluded, as the Germans did, that graphite was unsuitable for use as a neutron moderator.
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 Eugene Wigner and Edward Teller, he approached his old friend and collaborator Albert Einstein in August 1939, and convinced him to sign the letter, lending his fame to the proposal. The Einstein–Szilard letter resulted in the establishment of research into nuclear fission by the U.S. government. 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.
In April 1941, the National Defense Research Committee (NDRC), created a special project headed by physicist, Arthur Compton, a Nobel-Prize-winning professor at the University of Chicago, to report on the uranium program. Compton's report, submitted in May 1941, foresaw the prospects of developing radiological weapons, nuclear propulsion for ships, and nuclear weapons using uranium-235 or the recently discovered plutonium. In October he wrote another report on the practicality of an atomic bomb. For this report, he worked with Fermi on calculations of the critical mass of uranium-235. He also discussed the prospects for uranium enrichment with Harold Urey.
Niels Bohr and John Wheeler had theorized that heavy isotopes with odd atomic numbers were fissile. If so, then plutonium-239 was likely to be. In May 1941, Emilio Segrè and Glenn Seaborg at the University of California produced 28 μg of plutonium in the 60-inch cyclotron there, and found that it had 1.7 times the thermal neutron capture cross section of uranium-235. At the time only such minute quantities of plutonium-239 had been produced, in cyclotrons, but it was not possible to produce a sufficiently large quantity that way. Compton discussed with Wigner how plutonium might be produced in a nuclear reactor, and with Robert Serber about how the plutonium produced in a reactor might be separated from uranium. His report, submitted in November, stated that a bomb was feasible.
The final draft of Compton's November 1941 report made no mention of using plutonium, but after discussing the latest research with Ernest Lawrence, Compton became convinced that a plutonium bomb was also feasible. In December, Compton was placed in charge of the plutonium project. Its objectives were to produce reactors to convert uranium to plutonium, to find ways to chemically separate the plutonium from the uranium, and to design and build an atomic bomb. The reactor project now became part of the effort to build an atomic bomb. It fell to Compton to decide which of the different types of reactor designs that the scientists should pursue, even though a successful reactor had not yet been built. He proposed a schedule to achieve a controlled chain reaction by January 1943, and to have a bomb by January 1945.
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:
where M is the migration area and k is the medium neutron multiplication factor. The first generation of the reaction will produce k neutrons, the second will produce k2, the third k3 and so on. In order for a self-sustaining nuclear chain reaction to occur, k must be greater than 1. For a practical reactor configuration, it needs to be at least 3 or 4 percent more.
Fermi christened his apparatus a "pile". Emilio Segrè later recalled that:
I thought for a while that this term was used to refer to a source of nuclear energy in analogy with Volta's use of the Italian term pila to denote his own great invention of a source of electrical energy. I was disillusioned by Fermi himself, who told me that he simply used the common English word pile as synonymous with heap. To my surprise, Fermi never seemed to have thought of the relationship between his pile and Volta's.
Another grant, this time of $40,000, was obtained from the S-1 Uranium Committee to purchase more materials, and in August 1941 Fermi began to plan for a new test. The pile he proposed to build was 8-foot (2.4 m) long, 8-foot (2.4 m) wide and 11-foot (3.4 m) high. This was too large to fit in the Pupin Physics Laboratories. Fermi recalled that:
We went to Dean Pegram, who was then the man who could carry out magic around the University, and we explained to him that we needed a big room. He scouted around the campus and we went with him to dark corridors and under various heating pipes and so on, to visit possible sites for this experiment and eventually a big room was discovered in Schermerhorn Hall.
The pile was built in September 1941 from 4-by-4-by-12-inch (10 by 10 by 30 cm) graphite blocks and tinplate iron cans of uranium oxide. The cans were 8-by-8-by-8-inch (20 by 20 by 20 cm) cubes. When filled with uranium oxide, each weighed about 60 pounds (27 kg). There were 288 cans in all, and each was surrounded by graphite blocks so the whole would form a lattice structure. The uranium oxide was heated to remove moisture, and packed into the cans while still hot on a shaking table. The cans were then soldered shut. For a workforce, Pegram secured the services of Columbia's American football team. It was the custom at the time for football players to perform odd jobs around the university. They were able to manipulate the heavy cans with ease. The final result was a disappointing k of 0.87.
Compton felt that having teams at Columbia University, Princeton University, the University of Chicago and the University of California was creating too much duplication and not enough collaboration, and he resolved to concentrate the work in one location. Nobody wanted to move, and everybody argued in favor of their own location. In January 1942, soon after the United States entered World War II, Compton decided to concentrate the work at the Metallurgical Laboratory in Chicago, where he knew he had the unstinting support of university administration. Other factors in the decision were that scientists, technicians and facilities were more readily available in the Midwest, where war work had not yet taken them away, and Chicago's central location. By contrast, Columbia was engaged in two other Manhattan Project efforts under Harold Urey and John Dunning, and was hesitant to add a third.
Before leaving for Chicago, Fermi's team made one last attempt. Since the cans had absorbed neutron, they were dispensed with. Instead, the uranium oxide, heated to 480 °F (249 °C) to dry it out, was pressed into cylindrical holes 3 inches (7.6 cm) long and 3 inches (7.6 cm) in diameter drilled into the graphite. The entire structure was then canned by soldering sheet metal around it, and the contents were then heated above the boiling point of water to remove moisture. The result was a k of 0.918.
Choice of site
In Chicago, Samuel K. Allison had found a suitable space 60 feet (18 m) long, 30 feet (9.1 m) wide and 26 feet (7.9 m) high, sunk slightly below ground level,in a space under the stands at Stagg Field that had originally built as a rackets court. Stagg Field had been unused since the University of Chicago had given up playing American football following a 89-0 thrashing by the University of Michigan's football team in 1939, but the courts under West Stands were still used for playing squash and handball. Leona Woods and Anthony L. Turkevich played squash there in 1940. Being intended for strenuous exercise, the area was unheated. The nearby North Stands had a pair of two ice skating rinks on the ground floor. Allison used the racket court area to construct a 7-foot (2.1 m) experimental pile before Fermi's group arrived in 1942.
The United States Army Corps of Engineers assumed control of the nuclear weapons program in June 1942, and Compton's Metallurgical Laboratory became part of what came to be called the Manhattan Project. Brigadier General Leslie R. Groves, Jr., became director of the Manhattan Project on 23 September 1942. He visited the Metallurgical Laboratory for the first time on 5 October. Between 15 September and 15 November 1942, groups under Herbert L. Anderson and Walter Zinn constructed 16 experimental piles there. Fermi designed a new pile, which would be spherical to maximize k, which was predicted to be around 1.04. Leona Woods completed her doctoral thesis and then was detailed to build boron trifluoride neutron detectors. She also helped Anderson locate the large number of 4-by-6-inch (10 by 15 cm) timbers required at lumber yards in Chicago's south side. Shipments of high-purity graphite arrived, mainly from National Carbon, and high-purity uranium dioxide from Mallinckrodt in St Louis, which was now producing 30 short tons (27 t) a month. Metallic uranium also began arriving in larger quantities, the product of newly-developed techniques.
On 25 June, the Army and the Office of Scientific Research and Development (OSRD) had selected a site in the Argonne Forest near Chicago for a plutonium pilot plant. This became known as Site A. Some 1,025 acres (415 ha) were leased from Cook County in August, but by September it was apparent that the proposed facilities would be too extensive for the site, and it was decided to build the pilot plant elsewhere. A building at Argonne to house Fermi's experimental pile was commenced, with its completion scheduled for 20 October. Due to industrial disputes, construction fell behind schedule, and it became clear the materials for Fermi's pile would be on hand before the new structure was completed. In early November, Fermi came to Compton with a proposal to build the experimental pile under the stands at Stagg Field. In a nuclear reactor, there are delayed neutrons. Making up about one percent of the total number of neutrons, they are emitted from radioactive fission products created by the reaction rather than directly by the uranium. Their appearance is therefore delayed by anything from milliseconds to minutes, hence the name. With a k close to one, this delay allows the reactor to be controlled, and gives time to shut it down.
Compton told Fermi to build Chicago Pile-1, the first nuclear reactor, at Stagg Field. What could possibly go wrong, apart from a catastrophic nuclear meltdown blanketing one of the United States' major urban areas in radioactive fission products? Compton later explained that:
As a responsible officer of the University of Chicago, according to every rule of organizational protocol, I should have taken the matter to my superior. But this would have been unfair. President Hutchins was in no position to make an independent judgment of the hazards involved. Based on considerations of the University's welfare, the only answer he could have given would have been—no. And this answer would have been wrong.
The reactor was a "pile" of metalic uranium pellets and graphite blocks, assembled under the supervision of Fermi, in collaboration with Szilard, discoverer of the chain reaction, and assisted by Zinn, Martin D. Whitaker, 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. 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. The pile was reconfigured and rebuilt multiple times before the final test. Its wood frame was 24 square feet, and required 40,000 graphite blocks enclosing 19,000 pieces of uranium metal and uranium oxide. 
First nuclear chain reaction
On December 2, 1942, CP-1 was ready for a demonstration. Before a group of dignitaries, 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, 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:
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!"
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 (1944) the first "heavy water" reactor, at the Red Gate Woods site led to it becoming the first site of Argonne National Laboratory.
Significance and commemoration
The site of CP-1 was designated as a National Historic Landmark on 18 February 1965. When the National Register of Historic Places was created in 1966, it was immediately added to that as well. The site was named a Chicago Landmark on 27 October 1971.
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.
- F-1 (nuclear reactor), its Soviet equivalent, built in 1946 and still operational
- ZEEP, its Canadian equivalent.
- Experimental Breeder Reactor I (Chicago Pile-4)
- Chicago Pile-5
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- Video Showing the Met Lab, Fermi, and an active experiment using CP-1
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