page_title: History of nuclear weapons
section_title: From Los Alamos to Hiroshima
hierarchical_section_title: History of nuclear weapons / From Los Alamos to Hiroshima
caption_attribution_description: English: Large electromagnet called "Alpha 1 racetrack" used to separate the isotope uranium-235 from uranium-238 in natural uranium at the Y-12 plant in Oak Ridge, Tennessee, around 1945, part of the secret US World War 2 Manhattan Project to make an atomic bomb. It is divided into sections called alpha calutrons which operate on the principle of the mass spectrometer. In a large vacuum tank the magnetic field bends the path of lighter 235U ions in a beam of uranium ions more than the heavier 238U ions, so the two isotopes separate into separate beams and are collected by separate cups. The calutron tanks are spaced around the ring, located between the coils that generate the magnetic field. Since copper was desperately scarce during World War 2, the magnet's coils were made of thousands of tons of silver, borrowed from the U.S. Treasury. Eventually 14,700 tons were used for coil windings, returned to the Treasury after the war. The rectangular duct around the top of the machine contains the silver bussbar that supplies the current. This technique is no longer used for uranium enrichment. Caption: "View inside an Alpha 1 building, looking over one track toward the second. The view shows inside and outside tank fronts with enclosed bus running along the top of the tracks. Spare "tank units" may be seen on the floor between the tracks"
context_page_description: Nuclear weapons possess enormous destructive power from nuclear fission or combined fission and fusion reactions. Building on scientific breakthroughs made during the 1930s, the United States, Canada, free France and Great Britain collaborated during World War II, in what was called the Manhattan Project, to build a fission weapon, also known as an atomic bomb. In August 1945, the atomic bombings of Hiroshima and Nagasaki were conducted by the United States against Japan at the close of that war, standing to date as the only use of nuclear weapons in hostilities. The Soviet Union started development shortly after with their own atomic bomb project, and not long after, both countries were developing even more powerful fusion weapons known as hydrogen bombs. Britain and France built their own systems in the 1950s, and the list of states with nuclear weapons has gradually grown larger in the decades since.
context_section_description: With a scientific team led by J. Robert Oppenheimer, the Manhattan project brought together some of the top scientific minds of the day, including many exiles from Europe, with the production power of American industry for the goal of producing fission-based explosive devices before Germany. Britain and the U.S. agreed to pool their resources and information for the project, but the other Allied power, the Soviet Union (USSR), was not informed. The U.S. made an unprecedented investment in the project which at the time was the largest industrial enterprise ever seen, spread across more than 30 sites in the U.S. and Canada. Scientific development was centralized in a secret laboratory at Los Alamos.
  For a fission weapon to operate, there must be sufficient fissile material to support a chain reaction, a critical mass. To separate the fissile uranium-235 isotope from the non-fissile uranium-238, two methods were developed which took advantage of the fact that uranium-238 has a slightly greater atomic mass: electromagnetic separation and gaseous diffusion. Another secret site was erected at rural Oak Ridge, Tennessee, for the large-scale production and purification of the rare isotope, which required considerable investment. At the time, K-25, one of the Oak Ridge facilities, was the world's largest factory under one roof. The Oak Ridge site employed tens of thousands of people at its peak, most of whom had no idea what they were working on.
  Although uranium-238 cannot be used for the initial stage of an atomic bomb, when it absorbs a neutron, it becomes uranium-239 which decays into neptunium-239, and finally the relatively stable plutonium-239, which is fissile like uranium-235. After Fermi achieved the world's first sustained and controlled nuclear chain reaction with the creation of the first atomic pile, massive reactors were secretly constructed at what is now known as Hanford Site to transform uranium-238 into plutonium for a bomb.
  The simplest form of nuclear weapon is a gun-type fission weapon, where a sub-critical mass would be shot at another sub-critical mass. The result would be a super-critical mass and an uncontrolled chain reaction that would create the desired explosion. The weapons envisaged in 1942 were the two gun-type weapons, Little Boy (uranium) and Thin Man (plutonium), and the Fat Man plutonium implosion bomb.
  In early 1943 Oppenheimer determined that two projects should proceed forwards: the Thin Man project (plutonium gun) and the Fat Man project (plutonium implosion). The plutonium gun was to receive the bulk of the research effort, as it was the project with the most uncertainty involved. It was assumed that the uranium gun-type bomb could then be adapted from it.
  In December 1943 the British mission of 19 scientists arrived in Los Alamos.  Hans Bethe became head of the Theoretical Division.
  In April 1944 it was found by Emilio Segrè that the plutonium-239 produced by the Hanford reactors had too high a level of background neutron radiation, and underwent spontaneous fission to a very small extent, due to the unexpected presence of plutonium-240 impurities. If such plutonium were used in a gun-type design, the chain reaction would start in the split second before the critical mass was fully assembled, blowing the weapon apart with a much lower yield than expected, in what is known as a  fizzle.
  As a result, development of Fat Man was given high priority. Chemical explosives were used to implode a sub-critical sphere of plutonium, thus increasing its density and making it into a critical mass. The difficulties with implosion centered on the problem of making the chemical explosives deliver a perfectly uniform shock wave upon the plutonium sphere— if it were even slightly asymmetric, the weapon would fizzle. This problem was solved by the use of explosive lenses which would focus the blast waves inside the imploding sphere, akin to the way in which an optical lens focuses light rays.
  After D-Day, General Groves ordered a team of scientists to follow eastward-moving victorious Allied troops into Eur
caption: Electromagnetic U235 separation plant at Oak Ridge, Tenn. Massive new physics machines were assembled at secret installations around the United States for the production of enriched uranium and plutonium.
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