The industrial problem was centered around the production of sufficient fissile material, of sufficient purity. This effort was two-fold, and is represented in the two bombs that were dropped.
The Hiroshima bomb, Little Boy, was uranium-235, a minor isotope of uranium that has to be physically separated from more prevalent uranium-238, which is not suitable for use in an explosive device. The separation was effected mostly by gaseous diffusion of uranium hexafluoride (UF6), but also by other techniques. The bulk of this separation work was done at Oak Ridge.
The Nagasaki bomb, Fat Man, in contrast, consisted primarily of plutonium-239, a synthetic element which could be induced to supercriticality only by implosion. The design of an implosion device was at the center of the efforts by physicists at Los Alamos during the Project. The property of uranium-238 which makes it less suitable directly for use in an atomic bomb is used in the production of plutonium -- with sufficiently slow neutrons, uranium-238 will absorb neutrons and transmute into plutonium-239. The production and purification of plutonium was at the center of wartime, and post-war, efforts at the Hanford Site, using techniques developed in part by Glenn Seaborg.
The choice of civilian instead of military targets has often been criticized. However, the U.S. already had a policy of massive incendiary attacks against civilian targets in Japan. These dropped 20% explosives, to break up wooden structures and provide fuel, and then dropped 80% (by weight) small incendiary bombs to set the cities on fire. The resulting raids completely destroyed many Japanese cities, including Tokyo, even before atomic weapons were deployed. The allies performed such attacks because Japanese industry was extremely dispersed among civilian targets, with many tiny family-owned factories operating in the midst of civilian housing.
In the years between World War I and World War II, the United States had risen to pre-eminence in nuclear physics, driven by the work of recent immigrants and local physicists. These scientists had developed the basic tools of nuclear physics -- cyclotrons and other particle accelerators - and many new substances using these tools, including radioisotopes like carbon-14.
Enrico Fermi recalled the beginning of the project in a speech given in 1954 when he retired as President of the APS.
I remember very vividly the first month, January, 1939, that I started working at the Pupin Laboratories because things began happening very fast. In that period, Niels Bohr was on a lecture engagement in Princeton and I remember one afternoon Willis Lamb came back very excited and said that Bohr had leaked out great news. The great news that had leaked out was the discovery of fission and at least the outline of its interpretation. Then, somewhat later that same month, there was a meeting in Washington where the possible importance of the newly discovered phenomenon of fission was first discussed in semi-jocular earnest as a possible source of nuclear power.
US President Franklin D. Roosevelt was presented with a letter signed by Albert Einstein (transcribed by Leo Szilard) on October 11, 1939, which urged the United States to rapidly develop an atomic bomb program. The president agreed. The Navy awarded Columbia University the first Atomic Energy funding of $6,000, which grew into the Manhattan Project under Oppenheimer and Enrico Fermi's work.
Scientists in Germany discovered nuclear fission in late 1938. Refugee scientists Leo Szilard, Edward Teller and Eugene Wigner believed that the energy released in nuclear fission might be used in bombs by the Germans. They persuaded Albert Einstein, America's most famous physicist, to warn President Franklin Roosevelt of this danger in an August 2, 1939, letter. In response to the warning, Roosevelt ordered increased research in nuclear physics.
Under the auspices of National Bureau of Standards chief Lyman Briggs, small research programs had begun in 1939 at the Naval Research Laboratory in Washington, where physicist Philip Abelson explored uranium isotope separation. At Columbia University Italian nuclear physicist Enrico Fermi built prototype nuclear reactors using various configurations of graphite and uranium.
Vannevar Bush, director of the Carnegie Institution of Washington, organized the National Defense Research Committee in 1940 to mobilize the United States' scientific resources in support of the war effort.
New laboratories were created, including the Radiation Laboratory at the Massachusetts Institute of Technology, which aided the development of radar, and the Underwater Sound Laboratory at San Diego, which developed sonar.
The National Defense Research Council (NDRC) also took over the uranium project, as Briggs' program in nuclear physics was called. In 1940, Bush and Roosevelt created the Office of Scientific Research and Development to expand these efforts.
The uranium project had not made much progress by the summer of 1941, when word came from Britain of calculations by Otto Frisch and Fritz Peierls that showed that a very small amount of the fissionable isotope of uranium, U-235 - could produce an explosion equivalent to that of several thousand tons of TNT.
The National Academy of Sciences proposed an all-out effort to build nuclear weapons. Bush created a special committee, the S-1 Committee, to guide the effort. No sooner was this decision made than the Japanese bombed Pearl Harbor on December 7th, 1941. The war had begun for the United States.
At the University of Chicago Metallurgical Laboratory, the University of California Radiation Laboratory and Columbia University's physics department, efforts to prepare the nuclear materials for a weapon were accelerated-. Uranium 235 had to be separated from uranium ore and plutonium made by neutron bombardment of natural uranium. Beginning in 1942, huge plants were built at Oak Ridge (Site X) in Tennessee and Hanford (Site W) outside of Richland, Washington, to produce these materials.
When the United States entered World War II in December 1941, several projects were under way to investigate the separation of fissionable uranium 235 from uranium 238, the manufacture of plutonium, and the feasibility of nuclear piles and explosions.
Physicist and Nobel laureate Arthur Holly Compton organized the Metallurgical Laboratory at the University of Chicago in early 1942 to study plutonium and fission piles. Compton asked theoretical physicist J. Robert Oppenheimer of the University of California to study the feasibility of a nuclear weapon.
In 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, Richard Tolman and Emil Konopinski concluded that a fission bomb was feasible. The scientists suggested that such a reaction could be initiated by assembling a critical mass - an amount of nuclear explosive adequate to sustain it - either by firing two subcritical masses of plutonium or uranium 235 together or by imploding (crushing) a hollow sphere made of these materials with a blanket of high explosives. Until the numbers were better known, this was all that could be done.
Teller saw another possibility: By surrounding a fission bomb with deuterium and tritium, a much more powerful "superbomb" might be constructed. This concept was based on studies of energy production in stars made by Bethe before the war . When the detonation wave from the fission bomb moved through the mixture of deuterium and tritium nuclei, they would fuse together to produce much more energy than fission, in the process of nuclear fusion, just as elements fused in the sun produce light and heat.
Bethe was skeptical, and as Teller pushed hard for his "superbomb" and proposed scheme after scheme, Bethe refuted each one. When Teller raised the possibility that an atomic bomb might ignite the atmosphere, however, he kindled a worry that was not entirely extinguished until the Trinity test, even though Bethe showed, theoretically, that it couldn't happen.
The summer conferences, the results of which were later summarized by Serber in "The Los Alamos Primer" (LA-1), provided the theoretical basis for the design of the atomic bomb, which was to become the principal task at Los Alamos during the war, and the idea of the H-bomb, which was to haunt the Laboratory in the postwar era. Seldom has a physics summer school been as portentous for the future of mankind.
With the prospect of a long war, a group of theorists under the direction of J. Robert Oppenheimer met at Berkeley during the summer of 1942 to develop preliminary plans for designing and building a nuclear weapon. Crucial questions remained, however, about the properties of fast neutrons. John Manley, a physicist at the University of Chicago Metallurgical Laboratory, was assigned to help Oppenheimer find answers to these questions by coordinating several experimental physics groups scattered across the country.
The measurements of the interactions of fast neutrons with the materials in a bomb are 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. The need for it, however, was overshadowed by the demand for 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 Franklin 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.
OSRD scientists had explored several methods to produce plutonium and separate uranium-235 from uranium, but none of the processes was ready for production - only microscopic amounts had been prepared.
Only one method - electromagnetic separation, which had been developed by Ernest Lawrence at the University of California Radiation Laboratory at the University of California, Berkeley - seemed promising for large-scale production. But scientists could not stop studying other potential methods of producing fissionable materials, because it was so expensive and because it was unlikely that it alone could produce enough material before the war was over.
Marshall and his deputy, Col. Kenneth Nichols, had to struggle to understand both the 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, Tenn., would be suitable for the first production plant, they didn't 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.
The Manhattan District
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. Hoping for an overseas command, Groves 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.
The first thing he did was rechristen 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. This forceful and effective manner was soon to become all too familiar to the atomic scientists.
The first major scientific hurdle of project was solved on December 2, 1942 Below the bleachers of Stagg Field at the University of Chicago. Then and there a team led by Enrico Fermi initiated the first self-sustaining nuclear chain reaction. A coded message, "The Italian navigator has landed in the new world" was then sent to President Roosevelt to tell him that the experiment was a success.
... not done yet ...
see also the history section of Los Alamos National Laboratory
With the cryptology and cryptographic efforts centered at Bletchley Park and Arlington Hall and the development of microwave radar at MIT's Radiation Lab, the Manhattan Project represents one of few massive, secret, and outstandingly successful technological efforts spawned by the conflict of World War II.