The Manhattan Project refers to the project to develop the first nuclear weapons during World War II by the United States, the United Kingdom and Canada. Formally designated as the Manhattan Engineering District (MED), it refers specifically to the period of the project from 1942-1946 under the control of the U.S. Army Corps of Engineers, under the administration of General Leslie R. Groves, with its scientific research directed by the American physicist J. Robert Oppenheimer.
The project succeeded in developing and detonating three nuclear weapons in 1945: a test detonation on July 16 (the Trinity test) near Alamogordo, New Mexico; an enriched uranium bomb code-named "Little Boy" detonated on August 6 over Hiroshima, Japan; and a plutonium bomb code-named "Fat Man" on August 9 over Nagasaki, Japan.
The project's roots lay in scientists' fears since the 1930s that Nazi Germany was also investigating such weapons of its own. Born out of a small research program that began in 1939, the Manhattan Project would eventually employ more than 130,000 people and cost a total of nearly $2 billion USD ($20 billion in 2004 dollars based on CPI), and result in the creation of multiple production and research sites operated in secret.
The three primary research and production sites of the project were the plutonium-production facility at what is now the Hanford Site, the uranium-enrichment facilities at Oak Ridge, Tennessee, and the weapons research and design laboratory which is now Los Alamos National Laboratory. Project research took place at over thirty different sites spread across the United States, Canada, and the United Kingdom. The MED maintained control over U.S. weapons production until the formation of the Atomic Energy Commission in January 1947.
In the first decades of the twentieth century, numerous changes in the understanding of physics occurred which resulted both in the recognition of nuclear fission as a potential energy source and the belief by many that it could be used as a weapon. Chief among these developments were the discovery of a nuclear model of the atom, which by 1932 was thought to consist of a small, dense nucleus containing most of the mass of the atom in the form of protons and neutrons, surrounded by a shell of electrons. Work on the phenomena of radioactivity, first discovered in uranium ores by Henri Becquerel in 1896 and followed up by the work of Pierre and Marie Curie on radium, seemed to promise that atoms, previously thought to be ultimately stable and indivisible, actually had the potential of containing and releasing immense amounts of energy. In 1919 Ernest Rutherford observed the first artificial nuclear disintegrations by bombarding nitrogen with alpha particles emitted from a radioactive source.
Movement along the road towards nuclear fission accelerated in the 1930s. In 1932, John Cockcroft and Ernest Walton were the first to "split the atom" (cause a nuclear reaction) by the use of artificially accelerated particles. In 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles. The same year Enrico Fermi reported similar results when bombarding uranium with neutrons, but he did not immediately appreciate the consequences of his results.
In December 1938, Germans Otto Hahn and Fritz Strassmann published experimental results about the bombardment of uranium with neutrons, showing that it somehow produced an isotope of barium as a result. Their Austrian co-worker Lise Meitner (a political refugee in Sweden at the time) and her nephew Otto Robert Frisch correctly interpreted the results as the splitting of the uranium nucleus after the absorption of a neutron—nuclear fission—which released a large amount of energy and additional neutrons. In 1933 Hungarian physicist Leó Szilárd had proposed that if a neutron-driven process released more neutrons than those required to start it, an expanding nuclear chain reaction might result. Upon experimentation, he found that the fission of uranium indeed released two or more neutrons on average, though he kept this secret for the time being, fearing its use as a weapon by fascist governments. Identical results were soon published by the Joliot-Curie group, however, to the dismay of Szilárd.
That such mechanisms might have implications for civilian power or military weapons was perceived by a number of scientists in many different countries around the same time. While all of these developments in science were occurring, however, many great political changes were happening in Europe. Adolf Hitler was appointed chancellor of Germany in January 1933. His anti-Semitic ideology caused all Jewish civil servants, which included many physicists at universities, to be fired from their posts. Consequently many European physicists who would later make key discoveries went into exile in the United Kingdom and the United States. After Nazi Germany invaded Poland in 1939, World War II began, and many scientists in the United States and the United Kingdom became anxious about what Germany might do with nuclear technology.
The idea that nuclear fission could be used for both the production of commercial energy as well as potentially for military purposes occurred to many scientists around the world almost simultaneously. Because of the escalating military conflict in Europe, many scientists discontinued publication on the subject for fear of aiding enemy scientists with their research. The primary difficulty, it was soon determined by Niels Bohr and John Wheeler, was that only one isotope of uranium, uranium-235 underwent fission, and only 0.7% of all uranium found in nature is uranium-235. The majority of uranium is uranium-238, the presence of which would actually inhibit a fission chain reaction. To make a uranium fission bomb would require the separation of the two almost identical atoms with a relatively high degree of accuracy — a massive amount of effort, depending on how much uranium-235 (enriched uranium) was needed for a bomb, which had not yet been determined.
In the United States, a group of three Hungarian Jewish refugee physicists, Leó Szilárd, Edward Teller, and Eugene Wigner believed that the energy released in nuclear fission might be used in bombs by the Germans. Germany had made many early discoveries in the physics of fission and still had a number of formidable physicists, including Werner Heisenberg, despite the expulsion of Jewish academics. These refugee scientists were desperate to encourage further research in the United States. Politically marginalized, however, they sought the assistance of Albert Einstein, easily the world's most famous physicist at the time and a Jewish refugee himself, in drafting a letter which they would attempt to have delivered to President Franklin D. Roosevelt. The Einstein-Szilárd letter was written on August 2, 1939, mostly by Szilárd, warning that "extremely powerful bombs of a new type may thus be constructed" by means of nuclear fission and urging the President to establish funds for further research in the U.S. to determine its feasibility.
The letter eventually made it to Roosevelt over a month later, who authorized the creation of an ad hoc Uranium Committee under the chairmanship of National Bureau of Standards chief Lyman Briggs. It began small research programs in 1939 at the Naval Research Laboratory in Washington, where physicist Philip Abelson explored uranium isotope separation. At Columbia University Enrico Fermi, who had emigrated because his wife was Jewish, built prototype nuclear reactors using various configurations of graphite and uranium. Work, however, proceeded at a relatively slow and uncoordinated pace, in part because the U.S. was not yet involved officially in World War II, and because Briggs was somewhat uncomfortable in pursuing the research. In 1940, the Uranium Committee became a section of the newly-established National Defense Research Committee (NDRC), run by the scientist-administrator Vannevar Bush, but was still a relatively small effort.
While the U.S. research was pursued at a leisurely pace, work in the United Kingdom was occurring as well. In March 1940 in Birmingham UK, Austrian Otto Frisch and German Rudolf Peierls calculated that an atomic weapon only needed 1 kilogram (2.2 pounds) of uranium-235, a far smaller amount than most scientists had originally expected, which made it seem highly possible that a weapon could be produced in a short amount of time. They sent their report, the Frisch-Peierls memorandum, to Henry Tizard, chairman of the Committee for the Scientific Survey of Air Warfare, the most important scientific committee in the British war effort. Tizard set up a sub-committee, the MAUD Committee, to investigate the feasibility in more depth, and after commissioning further research, the MAUD Committee produced their first report in March 1941. The committee confirmed that a uranium bomb could be produced using 25 pounds of uranium-235, and would produce an explosion equivalent to that of 1,800 tons of TNT. The research had also shown that isotopic separation of the required quantity of uranium-235 was technically feasible. In contrast, German physicist Werner Heisenberg had operated under the assumption that each neutron must split another atom to keep the chain reaction going, which resulted in a grave miscalculation of the mass of uranium-235 that was needed to start the chain reaction and keep it going (He calculated that it would take 130 tons of uranium to do just that).
Meanwhile in the U.S., the Uranium Committee had not made comparable progress. The first MAUD Report was sent from Britain to the USA in March 1941 but no comment was received from the USA. A member of the MAUD Committee and Frisch's and Peierl's professor, Mark Oliphant, flew to the US in August 1941 in a bomber to find out what was being done with the MAUD reports, and was horrified to discover that Lyman Briggs had simply locked them in his safe, telling nobody, not even the other members of the Uranium Committee, which had since become part of the Office of Scientific Research and Development in the summer of 1941, because the US was "not at war". There was little urgency elsewhere until Oliphant visited Ernest Lawrence, James Conant, chairman of the NDRC, and Enrico Fermi and told them of the MAUD Report. Lawrence also contacted Conant and Arthur Compton, a physicist and Nobel laureate at the University of Chicago, convincing them that they should take Frisch's and Peierl's work very seriously, and collectively, along with Vannevar Bush, an aggressive campaign was made to wrest the weapons research out of the hands of Briggs and to encourage an all-out program.
The National Academy of Sciences then proposed an all-out effort to build nuclear weapons. On October 9, 1941, Bush impressed upon Roosevelt at a meeting the need for an accelerated program, and by November Roosevelt had authorized an "all-out" effort. A new policy committee, the Top Policy Group, was created to inform Roosevelt of bomb development, and allow Bush and his colleagues to guide the project. The first meeting of the group, which discussed the reorganization of the S-1 committee research, took place on December 6, 1941 — the day before the Japanese attack on Pearl Harbor and the entrance of the United States into World War II.
Having begun to wrest control of the uranium research from the National Bureau of Standards, the project heads began to accelerate the bomb project under the OSRD. Arthur Compton organized the University of Chicago Metallurgical Laboratory in early 1942 to study plutonium and fission piles (primitive nuclear reactors), and asked theoretical physicist Robert Oppenheimer of the University of California to take over research on fast neutron calculations, key to calculations about critical mass and weapon detonation, from Gregory Breit. John Manley, a physicist at the Metallurgical Laboratory, was assigned to help Oppenheimer find answers by coordinating and contacting several experimental physics groups scattered across the country.
During the spring of 1942, Oppenheimer and Robert Serber of the University of Illinois worked on the problems of neutron diffusion (how neutrons moved in the chain reaction) and hydrodynamics (how the explosion produced by the chain reaction might behave). To review this work and the general theory of fission reactions, Oppenheimer convened a summer study at the University of California, Berkeley in June 1942. Theorists Hans Bethe, John Van Vleck, Edward Teller, Felix Bloch, Emil Konopinski, Robert Serber, Stanley S. Frankel, and Eldred C. Nelson (the latter three all former students of Oppenheimer) quickly confirmed that a fission bomb was feasible. There were still many unknown factors in the development of a nuclear bomb, however, even though it was considered to be theoretically possible. The properties of pure uranium-235 were still relatively unknown, as were the properties of plutonium, a new element which had only been discovered in February 1941 by Glenn Seaborg and his team. Plutonium was the product of uranium-238 absorbing a neutron which had been emitted from a fissioning uranium-235 atom, and was thus able to be created in a nuclear reactor. But at this point no reactor had yet been built, so while plutonium was being pursued as an additional fissile substance, it was not yet to be relied upon. Only microgram quantities of plutonium existed at the time and its properties were still largely unknown.
The scientists at the Berkeley conference determined that there were many possible ways of arranging the fissile material into a critical mass, the simplest being the shooting a "cylindrical plug" into a sphere of "active material" with a "tamper"—dense material which would focus neutrons inward and keep the reacting mass together to increase its efficiency (this model "avoids fancy shapes", Serber would later write). They also explored designs involving spheroids, a primitive form of "implosion" (suggested by Richard C. Tolman), and explored the speculative possibility of "autocatalytic methods" which would increase the efficiency of the bomb as it exploded.
Considering the idea of the fission bomb theoretically settled until more experimental data was available, the conference then turned in a different direction. Hungarian physicist Edward Teller pushed for discussion on an even more powerful bomb: the "Super", which would use the explosive force of a detonating fission bomb to ignite a fusion reaction in deuterium and tritium. This concept was based on studies of energy production in stars made by Hans Bethe before the war, and suggested as a possibility to Teller by Enrico Fermi not long before the conference. When the detonation wave from the fission bomb moved through the mixture of deuterium and tritium nuclei, these would fuse together to produce much more energy than fission could. But Bethe was skeptical. As Teller pushed hard for his "superbomb" —now usually referred to as a hydrogen bomb— proposing scheme after scheme, Bethe refuted each one. The fusion idea had to be put aside in order to concentrate on actually producing fission bombs.
Teller also raised the speculative possibility that an atomic bomb might "ignite" the atmosphere, due to a hypothetical fusion reaction of nitrogen nuclei. Bethe calculated, according to Serber, that it could not happen. In his book The Road from Los Alamos, Bethe says a refutation was written by Konopinski, C. Marvin, and Teller as report LA-602, showing that ignition of the atmosphere was impossible, not just unlikely. In Serber's account, Oppenheimer unfortunately mentioned it to Arthur Compton, who "didn't have enough sense to shut up about it. It somehow got into a document that went to Washington" which led to the question "never [being] laid to rest".
The conferences in the summer of 1942 provided the detailed theoretical basis for the design of the atomic bomb, and convinced Oppenheimer of the benefits of having a single centralized laboratory to manage the research for the bomb project, rather than having specialists spread out at different sites across the United States.
Though it involved over thirty different research and production sites, the Manhattan Project was largely carried out in three secret scientific cities and one public site that were established by power of eminent domain: Los Alamos, New Mexico; Oak Ridge, Tennessee; and Hanford, Washington. The latter two sites were chosen to gain access to the vast quantities of cheap (hydro-)electrical power available locally, which were necessary for the production of uranium-235 and plutonium.
The Los Alamos National Laboratory was built on a mesa that previously hosted the Los Alamos Ranch School. The site was chosen primarily for its remoteness from prying eyes. In addition to being the main 'think-tank', Los Alamos was responsible for final assembly of the bombs, mainly from materials and components produced by the other sites. Manufacturing undertaken at Los Alamos included casings, explosive lenses, and reduction and smelting of fissile materials into bomb cores.
Oak Ridge facilities covered more than 60,000 acres (243 km²) of several former farm communities in the Tennessee Valley area. Some Tennessee families were given two weeks' notice to vacate family farm lands that had been their home for generations. So secret was the site during WW2 that even the state governor was unaware that Oak Ridge (what was to become the fifth largest city in the state) was being built. At one point Oak Ridge plants were consuming 1/7th of all the electrical power being produced in the USA, more than the whole of New York City. Oak Ridge mainly produced uranium-235.
Hanford Site, which grew to almost 1000 square miles (2,600 km²), took over irrigated farm land, fruit orchards, a railroad, and two active farming communities, Hanford and White Bluffs, in a sparsely populated area adjacent to the Columbia River. Hanford was the plutonium production center.
The existence of these sites and the secret cities of Los Alamos, Oak Ridge, and Hanford were not made public until the announcement of the Hiroshima explosion and remained officially secret until the end of WWII.
Chicago pile - 1 As the Manhattan project progressed, Fermi and his crew worked on what was to be known as the very first nuclear chain reaction. The reactor was to be called CP-1 or Chicago Pile - 1. When this reactor was completed, it was a feat of engineering. And amazingly it worked, and they were able to control the Fission of uranium and create some plutonium.
Major Manhattan Project sites and subdivisions included:
The measurements of the interactions of fast neutrons with the materials in a bomb were essential because the number of neutrons produced in the fission of uranium and plutonium must be known, and because the substance surrounding the nuclear material must have the ability to reflect, or scatter, neutrons back into the chain reaction before it is blown apart in order to increase the energy produced. Therefore, the neutron scattering properties of materials had to be measured to find the best reflectors.
Estimating the explosive power required knowledge of many other nuclear properties, including the cross section (a measure of the probability of an encounter between particles that result in a specified effect) for nuclear processes of neutrons in uranium and other elements. Fast neutrons could only be produced in particle accelerators, which were still relatively uncommon instruments in physics departments in 1942.
The need for better coordination was clear. By September 1942, the difficulties involved with conducting preliminary studies on nuclear weapons at universities scattered throughout the country indicated the need for a laboratory dedicated solely to that purpose. An even greater need was the construction of massive industrial plants to produce uranium-235 and plutonium - the fissionable materials that would provide the nuclear explosives.
Vannevar Bush, the head of the civilian Office of Scientific Research and Development (OSRD), asked President Roosevelt to assign the large-scale operations connected with the quickly growing nuclear weapons project to the military. Roosevelt chose the Army to work with the OSRD in building production plants. The Army Corps of Engineers selected Col. James Marshall to oversee the construction of factories to separate uranium isotopes and manufacture plutonium for the bomb.
Marshall and his deputy, Col. Kenneth Nichols, had to struggle to understand the various proposed processes and the scientists with whom they had to work. Thrust suddenly into the new field of nuclear physics, they felt unable to distinguish between technical and personal preferences. Although they decided that a site near Knoxville would be suitable for the first production plant, they did not know how large the site had to be and so put off its acquisition. There were other problems, too.
Because of its experimental nature, the nuclear weapons work could not compete with the Army's more-urgent tasks for top-priority ratings. The selection of scientists' work and production-plant construction often were delayed by Marshall's inability to get the critical materials, such as steel, that also were needed in other military productions.
Even selecting a name for the new Army project was difficult. The title chosen by Gen. Brehon Somervell, "Development of Substitute Materials," was objectionable because it seemed to reveal too much.
Vannevar Bush soon became dissatisfied with Marshall's failure to get the project moving forward expeditiously and made this known to Secretary of War Stimson and Army Chief of Staff George Marshall, who then directed General Somervell to find a more energetic officer to direct the program, replacing Marshall. In the summer of 1942, Col. Leslie Groves was deputy to the chief of construction for the Army Corps of Engineers and had overseen construction of the Pentagon, the world's largest office building. He was widely respected as a highly intelligent, hard driving, though very brusque officer who got things done in a hurry. Hoping for an overseas command, Groves vigorously objected when Somervell appointed him to take charge of the weapons project. His objections were overruled and Groves resigned himself to leading a project he thought had little chance of succeeding. Groves appointed Oppenheimer as the project's scientific director, to the surprise of many. (Oppenheimer's radical political views were thought to pose security problems).
The first thing Groves did was rename the project The Manhattan District. The name evolved from the Corps of Engineers practice of naming districts after its headquarters' city (Marshall's headquarters were in New York City). At the same time, Groves was promoted to brigadier general, which gave him the rank thought necessary to deal with the senior scientists in the project.
Within a week of his appointment, Groves had solved the Manhattan Project's most urgent problems. His forceful and effective manner was soon to become all too familiar to the atomic scientists.
The first major scientific hurdle of the project was solved on December 2, 1942 beneath the bleachers of Stagg Field at the University of Chicago, where a team led by Enrico Fermi initiated the first self-sustaining nuclear chain reaction in an experimental reactor named Chicago Pile-1. A coded phone call from Compton saying, "The Italian navigator [referring to Fermi] has landed in the new world, the natives are friendly" to Conant in Washington, DC, brought the news that the experiment was a success. This was a major turning point.
The Hiroshima bomb, Little Boy, was made from uranium-235, a rare isotope of uranium that has to be physically separated from the more prevalent uranium-238 isotope, which is not suitable for use in an explosive device. Since U-235 is only 0.7% of raw uranium and is chemically identical to the 99.3% of U-238, various physical methods were considered for separation.
One method of separating uranium 235 from raw uranium ore was devised by Franz Simon and Nicholas Kurti, two more Jewish émigrés, at Oxford University. Their method using gaseous diffusion was scaled up in large separation plants at Oak Ridge Laboratories and used uranium hexafluoride (UF6) gas as the process fluid. This method eventually produced most of the U-235.
Another method - electromagnetic isotope separation, was developed by Ernest Lawrence at the University of California Radiation Laboratory at the University of California, Berkeley. This method resulted in devices known as calutrons which were effectively mass spectrometers. Initially the method seemed promising for large-scale production, but it proved to be expensive and it could not produce enough material. It was later abandoned. Other techniques were also tried, such as thermal diffusion. Most of this separation work was performed at Oak Ridge.
The uranium bomb was a gun-type fission weapon. One mass of U-235, the "bullet," is fired down a more or less conventional gun barrel into another mass of U-235, rapidly creating the critical mass of U-235 that results in a huge explosion.
In contrast, the bombs used in the first test at Trinity Site, New Mexico (the gadget of the Trinity test), and also in the Nagasaki bomb, Fat Man, were made primarily of Plutonium-239. Plutonium is a synthetic element.
Although uranium-238 is useless as fissile material for an atomic bomb, U-238 is used to produce plutonium. The fission of U-235 produces relatively slow neutrons which will be absorbed by U-238, which after a few days of decay, turns into plutonium-239. The production and purification of plutonium used techniques developed in part by Glenn Seaborg while working at Berkeley and Chicago. Beginning in 1943, huge plants were built to produce plutonium at Hanford (Site W) outside of Richland, Washington.
From 1943-1944, development efforts were directed to a gun-type fission weapon with plutonium, called "Thin Man". Once this would be achieved, the uranium version "Little Boy" would require a relatively simple adaptation, it was thought.
Initial tests of the properties of plutonium were done using cyclotron-generated plutonium-239, which was very pure but in very small amounts. On April 5, 1944, Emilio Segrè at Los Alamos received the first sample of Hanford-produced plutonium. Within ten days, he discovered a fatal flaw: reactor-bred plutonium was far less pure than cyclotron-produced plutonium, and as a result had a much higher spontaneous fission rate than uranium-235. The isotope responsible for this high fission rate was plutonium-240, formed from plutonium-239 by capture of an additional neutron. Unlike the cyclotron, the plutonium breeding reactors had a much higher neutron flux, thus increasing the proportion of plutonium-240 as compared to the cyclotron bred plutonium. The implications of this made a "gun" detonation mechanism unsuitable: because of the relatively slow speed of the gun mechanism, a plutonium bomb would "fizzle" (i.e. blow itself apart before it developed a substantial chain reaction).
Therefore, in July 1944, the difficult decision was made to cease work on the plutonium gun method; there would be no "Thin Man." The gun method was further developed for uranium only, which as expected, gave few complications. Most efforts were now directed to a different method for plutonium.
Ideas of using alternative detonation schemes had existed for some time at Los Alamos. One of the more innovative had been the idea of "implosion" — a sub-critical sphere of fissile material could, using chemical explosives, be forced to collapse in on itself, creating a very dense critical mass. Initially it had been entertained as a possible, though unlikely method. But after it was discovered that it was the only possible solution for using plutonium in a nuclear weapon, it received the highest project priority. By the end of July 1944, the entire project had been re-organized around solving the implosion problem. It eventually involved using shaped charges with many explosive lenses in order to produce the perfectly-spherical explosive wave needed for proper compression of the plutonium sphere.
Because of the complexity of detonating an implosion-style weapon necessary for the plutonium bomb, it was decided that in spite of the waste of expensive fissile material, a test would be required in order to have any confidence that it would work in practice. After much preparation, the first nuclear test took place on July 16, 1945, near Alamogordo, New Mexico, under the supervision of Grove's deputy Brig. Gen. Thomas Farrell. This test was dubbed by Oppenheimer as "Trinity".
A similar effort was undertaken in the USSR in September 1941 headed by Igor Kurchatov (with a specific difference in that some of Kurchatov's World War II investigations came secondhand from Manhattan Project countries, thanks to spies, including at least two on the scientific team at Los Alamos, Klaus Fuchs and Theodore Hall, unknown to each other).
After the MAUD Committee's report, the British and Americans exchanged nuclear information, but initially did not pool their efforts. A separate British project, code-named TUBE ALLOYS, was started but did not have American resources. Consequently the British bargaining position worsened and their motives were mistrusted by the Americans. Collaboration therefore lessened markedly until the Quebec Agreement of August 1943, when a large team of British and Canadian scientists joined the Manhattan Project.
The question of Axis efforts on the bomb has always been an issue of contention. It is believed that token efforts in Germany, headed by Werner Heisenberg, and in Japan, were also undertaken during the war but made little progress. It was initially feared that Hitler was very close to developing his own bomb. Many Nazi scientists in fact expressed surprise to their allied captors when the bombs were detonated in Japan; they had been convinced that talk of atomic weapons was merely propaganda. Incidentally Niels Bohr and Heisenberg discussed the possibility of the atomic bomb prior to and during the war. Bohr had recalled that Heisenberg was unaware that the supercritical mass could be achieved with U-235. In fact, Bohr, Heisenberg and Fermi were all colleagues who were key figures in developing the quantum theory together with Wolfgang Pauli prior to the war.
Together with the cryptographic efforts centered at Bletchley Park and also at Arlington Hall, the development of radar and computers in the UK and later in the USA, and the jet engine in the UK and Germany, the Manhattan Project represents one of the few massive, secret, and outstandingly successful technological efforts spawned by the conflict of World War II.