The two masses of U-235 had to combine with one another quickly enough to avoid the spontaneous fission of the atoms, which would cause the bomb to fizzle, and thus fail to explode. This combination created a critical mass that set off a fission chain reaction to eventually detonate the bomb. Once enough U-235 was obtained to power the bomb, Little Boy was constructed using a gun-type design that fired one amount of U-235 at another to combine the two masses. In order to avoid wasting time on one new method that could later prove insufficient to produce enough U-235 to allow the atomic bomb to reach critical mass, General Leslie Groves consulted with lead scientists of the project and agreed to investigate simultaneously four separate methods of separating and purifying the uranium-235: gaseous diffusion, centrifuge, electromagnetic separation and liquid thermal diffusion. The first challenge of the project was thus to determine the most efficient way to separate and purify uranium-235 from the overly-abundant uranium-238 – standard methods of separation could not be used due to the strong chemical similarity between the two isotopes. When a neutron bombards U-238, the isotope often captures the neutron to become U-239, failing to fission, and thus failing to instigate a chain reaction that would detonate a bomb. Most uranium found naturally in the world exists as uranium-238, leaving only 0.7% of naturally existing uranium as the U-235 isotope. Little Boy was powered by the uranium isotope U-235 in a process that didn’t come easily to the many Manhattan Project scientists working on the uranium extraction and enrichment process. Little Boy detonated due to a fission chain reaction involving the isotope U-235 of uranium, while Fat Man used plutonium’s Pu-239 form. Little Boy and Fat Man utilized different elements and completely separate methods of construction in order to function as nuclear weapons. Critical mass is defined as the amount of material at which a neutron produced by a fission process will, on average, create another fission event. The more fissionable material you have, the greater the odds that such an event will occur. This means you need enough U-235 or Pu-239 to ensure that neutrons released by fission will strike another nucleus, thus producing a chain reaction. In order to detonate an atomic weapon, you need a critical mass of fissionable material. For more on this topic, see Nuclear Fission. Both of those neutrons collide with uranium-235 atoms, each of which fission and release between one and three neutrons, and so on. However, one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. Two neutrons do not continue the reaction because they are lost or absorbed by a uranium-238 atom. When a uranium-235 atom absorbs a neutron and fissions into two new atoms, it releases three new neutrons and some binding energy. This is known as a chain reaction and is what causes an atomic explosion. The fission process becomes self-sustaining as neutrons produced by the splitting of atom strike nearby nuclei and produce more fission. Fission occurs when a neutron strikes the nucleus of either isotope, splitting the nucleus into fragments and releasing a tremendous amount of energy. The isotopes uranium-235 and plutonium-239 were selected by the atomic scientists because they readily undergo fission. The second weapon, dropped on Nagasaki, was called Fat Man and was an implosion-type device with a plutonium core. The first, Little Boy, was a gun-type weapon with a uranium core. developed two types of atomic bombs during the Second World War. The immense destructive power of atomic weapons derives from a sudden release of energy produced by splitting the nuclei of the fissile elements making up the bombs’ core.
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