Nuclear Chemistry- The Atom Bombs Used in the Second World War

The United States developed two types of atomic bombs during the Manhattan Project in World War Two. The first was a bomb codenamed Little Boy. Little Boy was a gun-type weapon, meaning that it was developed to cause more casualties than its counter part, Fat Man. Fat Man was an implosion-type weapon, which means that it was developed to cause immense damage to the area that it was dropped on. Little Boy was dropped first and was dropped on Hiroshima, while Fat Man was dropped  second and was dropped on Nagasaki. Little Boy had a uranium core, while Fat Man had a plutonium core. The isotopes used were uranium-235 and plutonium-239, because both readily undergo fission. In atomic bombs, fission occurs when a neutron strikes the nucleus of an atom of a radioactive isotope, such as uranium-235 and plutonium-239, which splits the nucleus into separate fragments, releasing a massive amount of energy. This becomes a self-sustaining process inside of the atomic bomb, creating more fission amongst the atoms, because neutrons produced by the the fission, strike nearby nuclei. This is a chain reaction and is what caused the atomic explosions in Hiroshima and Nagasaki. The longest part of the Manhattan Project was the scientists trying to find the critical mass need of both uranium-235 and plutonium-239, in order for the atomic bomb to successfully create large enough explosions. This means that the scientists had to find the correct amount of the isotopes, that would allow for enough neutrons to be produced by the fission and strike the nuclei of other atoms, creating the chain reaction needed to create a chain reaction inside of the atomic bombs. Little Boy was constructed in a way so that there were two separate masses of uranium-235, and these two masses would quickly combine to avoid spontaneous fission of the atoms, which would cause the bomb to fizzle and fail to detonate. Spontaneous fission was one of the main issues that the scientists who were involved in the Manhattan Project encountered. Fat Man needed a different design, than that of Little Boy, because the plutonium isotope used for the core had a fission rate that was too high for the gun-type design. The isotope's high fission rate would cause the atoms to undergo spontaneous fission before the gun-type design could bring the two masses of plutonium together. Scientists created a new design that would quickly consolidate the plutonium, increasing the pressure and density, in turn allowing for the plutonium to reach its critical mass. For both bombs, once the critical mass was reached, a chain reaction was formed. Once the chain reactions were formed inside of the atomic bombs, they could be successfully detonated.

Little Boy (gun-type design)


























Fat Man (implosion type)



Photos from:
http://www.atomicheritage.org


Sources:
http://www.atomicheritage.org/history/science-behind-atom-bomb
http://www.ucsusa.org/nuclear-weapons/how-do-nuclear-weapons-work#.WQaF4GQrI1g

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