Chicago Pile-1

Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor. Its construction was part of the Manhattan Project, the Allied effort to create atomic bombs during World War II. It was built by the Manhattan Project's Metallurgical Laboratory at the University of Chicago, 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, who described the apparatus as "a crude pile of black bricks and wooden timbers."

The reactor was made from 45,000 graphite blocks weighing 400 ST used as a neutron moderator, and was fueled by 6 ST of uranium metal and 50 ST of uranium oxide. 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. Unlike most subsequent nuclear reactors, it has no radiation shield or cooling system as it only operated at very low power.

The reactor was assembled in November 1942 under the supervision of Fermi, in collaboration with Leo Szilard, discoverer of the chain reaction, and assisted by Herbert L. Anderson, Walter Zinn, Martin D. Whitaker, and George Weil. It contained a critical mass of fissile material (when moderated by the graphite), together with cadmium 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 1943, CP-1 was moved to Red Gate Woods, and reconfigured to become Chicago Pile-2 (CP-2). There, it was operated until 1954, when it was dismantled and buried. The stands at Stagg Field were demolished in August 1957, but the site is now a National Historic Landmark and a Chicago Landmark.

Origins
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 a chain reaction to occur, 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 t of graphite and 5 t 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.

Government support
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 nuclear chain reaction by January 1943, and to have an atomic bomb by January 1945.

Development
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:
 * $$R_{crit} = \frac{\pi M}{\sqrt{k - 1}}$$

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 ft long, 8 ft wide and 11 ft 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 graphite blocks and tinplate iron cans of uranium oxide. The cans were 8 by cubes. When filled with uranium oxide, each weighed about 60 lb. 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 at his own location, the University of 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. In contrast, Columbia University 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 to dry it out, was pressed into cylindrical holes 3 in long and 3 in 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 ft long, 30 ft wide and 26 ft 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 ft 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 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 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 ST 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 1025 acres 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."

Compton informed Groves of his decision at the 14 November meeting of the S-1 Executive Committee. Although Groves "had serious misgivings about the wisdom of Compton's suggestion", he did not interfere.

Construction
Chicago Pile 1 was encased within a balloon so that the air inside could be replaced by carbon dioxide. Anderson had a dark gray balloon manufactured by Goodyear Tire and Rubber Company. While a 25 ft cube-shaped balloon was somewhat unusual, the Manhattan Project's high priority rating ensured prompt delivery with no questions asked. A block and tackle was used to haul it into place, with the top secured to the ceiling and three sides to the walls. The remaining side, the one facing the balcony from which Fermi directed the operation, was furled like an awning. A circle was drawn on the floor, and the stacking of graphite blocks began on the morning of 16 November 1942. The first layer placed was made up entirely of graphite blocks, with no uranium. Layers without uranium were alternated with two layers containing uranium, so the the uranium was enclosed in graphite. Unlike later reactors, it has no radiation shielding or cooling system, as was only intended to be operated at very low power.

The work was carried out in twelve-hour shifts, with a day shift under Zinn and a night shift under Anderson. For a work force they hired some thirty high school dropouts that were eager to earn a bit of money before being drafted into the Army. They machined 45,000 graphite blocks enclosing 19,000 pieces of uranium metal and uranium oxide. The graphite arrived from the manufacturers in 4.25 by bars of various lengths. They were cut into standard lengths of 16.5 in, each weighing 19 lb. A lathe was used to drill 3.25 in holes in the blocks for the control rods and the uranium. A hydraulic press was used to shape the uranium oxide into pseudospheres, cylinders with rounded ends. Drill bits had to be sharpened after each 60 holes, which worked out to be about once an hour. Graphite dust soon filled the air and made the floor slippery. Another group, under Volney C. Wilson, was responsible for instrumentation. They also fabricated the control rods, which were cadmium sheets nailed to flat wooden strips, cadmium being a potent neutron absorber, and the scram line, a Manila rope that when cut would drop a control rod into the pile and stop the reaction.

About two layers were laid per shift. Woods' boron trifluoride neutron counter was inserted at the 15th layer. Thereafter, readings were taken at the end of each shift. Fermi divided the square of the radius of the pile by the intensity of the radioactivity to obtain a metric that counted drown as the pile approached criticality. At the 15th layer, it was 390; at the 19th it was 320; at the 25th it was 270 and by the 36th it was only 149. The original design was for a spherical pile, but as work proceeded, it became clear that this would not be necessary. The graphite was now more pure than hitherto, and 6 ST of very pure metallic uranium began to arrive from Iowa State University, where a team under Frank Spedding had developed a new process to produce uranium metal. Westinghouse Lamp Plant supplied some 3 ST, which it produced in a rush with a makeshift process. The 2.25 in metallic uranium cylinders, known as "Spedding's eggs", were dropped in the holes in the graphite in lieu of the uranium oxide pseudosheres. The process of filling the balloon with carbon dioxide would not be necessary, and some twenty layers could be dispensed with. According to Fermi's new calculations, the countdown would reach 1 between the 56th and 57th layers. The resulting pile was therefore flatter on the top than on the bottom. Anderson called a halt after the 57th layer was placed. When completed, the wooden frame supported an elliptical-shaped structure, 20 ft high, 6 ft wide at the ends and 25 ft across the middle. It contained 6 ST of uranium metal, 50 ST of uranium oxide and 400 ST of graphite, at an estimated cost of $2.7 million.

First nuclear chain reaction
The next day, 2 December 1942, everybody assembled for the experiment. There were 49 scientists present, 48 men and one woman. Although most of the S-1 Executive Committee was in Chicago, only Crawford Greenewalt was present, at Compton's invitation. Other dignitaries present included Szilard, Wigner and Spedding. Fermi, Compton, Anderson and Zinn gathered around the controls on the balcony, which was originally intended as a viewing platform. Samuel Allison stood ready with a bucket of concentrated cadmium nitride, which he was to throw over the pile in the event of an emergency. The start up began at 09:54. Walter Zinn removed the zip, the emergency control rod, and secured it. Norman Hilberry stood ready with an ax to cut the scram line, which would allow the zip to fall under the influence of gravity. While Leona Woods called out the count from the boron trifluoride detector in a loud voice, George Weil, the only one on the floor, withdrew the last remaining control rod. At 10:37 Fermi ordered Weil to remove all but 13 ft of the last control rod. He then witdrew 6 in at a time, with measurements being taken at each step.

At 11:25, Fermi ordered the control rods reinserted. He then announced that it was lunch time.

The experiment resumed at 14:00. Weil worked the final control rod while Fermi carefully monitored the neutron activity. Fermi announced that the pile had gone critical (reached a self-sustaining reaction) at 15:25. Fermi switched the scale on the recorder to accommodate the rapidly increasing current. He wanted to test the control circuits but after 28 minutes, the alarm bells went off to notify everyone that the flux had passed the preset safety level, and he ordered Zinn to release the Zip, and the reaction rapidly halted. The pile had run for about 4.5 minutes at about 0.5 watts. Wigner opened a bottle of Chianti, which was drank from paper cups.

Compton notified James Conant, chairman of the NDRC, 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."

Later operation
Operation of CP-1 was terminated in February 1943. The pile was then dismantled and moved to Site A in the Argonne Forest, today known as Red Gate Woods. There it was reconstructed using the original materials, plus a radiation shield, and renamed Chicago Pile-2 (CP-2). CP-2 began operation in March 1943. CP-2 was joined by Chicago Pile 3, the first heavy water reactor, which went critical on 15 May 1944. During the war Zinn allowed CP-2 to be run around the clock, and its design made it easy to conduct experiments.

While they were not used to produce plutonium for weapons, the reactors were used to investigate research related to weapons, such as investigations of the properties of tritium. Wartime experiments included measuring the neutron absorption cross-section of elements and compounds. Albert Wattenberg recalled that about 10 elements were studied each month, and 75 over the course of a year. An accident involving radium and beryllium powder caused a dangerous drop in his white blood cell that lasted for three years. As the dangers of things such as inhaling uranium oxide became more apparent, experiments were conducted on the effects of radioactive substances on laboratory test animals.

The Red Gate Woods later became the original site of Argonne National Laboratory, which replaced the Metallurgical Laboratory on 1 July 1946, with Zinn as its first director. CP-2 and CP-3 operated for ten years before they outlived their usefulness, and Zinn ordered them shut down on 15 May 1954. Their remaining usable fuel was transferred to Chicago Pile 5 at the Argonne National Laboratory's new site in DuPage County, and the CP-2 and CP-3 reactors were dismantled in 1955 and 1956. High-level nuclear waste such as fuel and heavy water were shipped to Oak Ridge for disposal. The rest was encased in concrete and buried in a 40 ft deep trench in what is now known as the Site A/Plot M Disposal Site. It is marked by a commemorative boulder.

By the 1970s there was increased public concern about the levels of radioactivity at the site, which was used by the local residents for recreational purposes. Surveys conducted in the 1980s found strontium 90 in the soil at Plot M, trace amounts of tritium in nearby wells, and plutonium, technetium, caesium, and uranium in the area. In 1994, the United States Department of Energy and the Argonne National Laboratory yielded to public pressure and earmarked $24.7 million and $3.4 million respectively to rehabilitate the site. As part of the clean up, some 500 cuyd of radioactive waste was removed and sent to the Hanford Site for disposal. By 2002, the Illinois Department of Public Health had determined that the remaining materials posed no danger to public health.

Significance and commemoration
A commemorative plaque was unveiled at Stagg Field in December 1952, on the occasion of the tenth anniversary of CP-1 going critical. It read: "On December 2, 1942 man achieved here the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy" The plaque was removed when the West Stands were demolished in August 1957. 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 also named a Chicago Landmark on 27 October 1971.

Today the site of the old Stagg Field is occupied by the University's Regenstein Library, which was opened in 1970, and the Joe and Rika Mansueto Library, which was opened in 2011. A Henry Moore sculpture, Nuclear Energy, stands in a small quadrangle just outside the Regenstein Library, to commemorate the nuclear experiment. It was dedicated on 2 December 1967, to commemorate the 25th anniversary of CP-1 going critical. The commemorative plaques from 1952 and 1967 are nearby. A graphite block from CP-1 can be seen at the Bradbury Science Museum in Los Alamos, New Mexico; another is on display at the Museum of Science and Industry in Chicago.