By 1939 nuclear scientists had begun to delve seriously into the fission of uranium atoms causing a chain reaction, particularly in the U235 isotope. As a result, vast progress was made in the fields of neutron bombardment, the comparative efforts of slow neutrons versus fast neutrons in sustaining chain reactions, and the possible methods of separating U235 from U238 in natural uranium. Moreover, the possibility of an immense atomic explosion was becoming common knowledge, and calculations for a "critical mass" were just around the corner. As early as the spring of 1942, scientists and leaders across the United States were being exposed to plutonium, courtesy of Glen Seaborg’s neutron bombardment of U238. With two unique isotopes of fissionable material at the disposal of the country’s greatest minds, it was fitting that two unique atomic weapon designs emerged. The “gun bomb” focused around driving together two sub-critical masses, and the “implosion bomb” relied on a uniform shockwave to compress a plutonium center. Although both designs created the desired bang, they contained very unique features and lent themselves to plutonium and uranium in different ways.
The primary research and design center for the bombs w
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August 9, 1945 500 meters above Nagasaki (Young). Robert Oppenheimer said that the required supercritical mass would be about 100 kilograms or 220 lbs. The charges were all carefully shaped and positioned so the shock waves converged at the center of the bomb. When the bomb was detonated, the sphere would implode, or collapse inward, causing all the plutonium to fuse together, reach supercritical mass, and start the chain reaction. The "Mike" test of Operation Ivy, on November 1, 1952, was the first explosion of a true two-stage thermonuclear device (Brock). Richard Feyman calculated the amount of uranium needed to achieve the critical mass to be about 50 kilograms (110 lb.
Since the supply of plutonium in 1942 was minute, initial design work was based primarily around creating a U235 bomb. Moreover, it was said at the time that this area would be devoid of vegetation for 75 years. In the uranium bomb the deflectors would keep stray neutrons from the center section away from the larger mass- avoiding an early critical mass. Because Uranium-238 is not easily fissionable, great at reflecting neutrons, and available in large quantities, it was made into neutron deflectors for both bombs. A conventional explosive would be used to propel the center section into the large section. However, if more than the required mass is in place to start the reaction- termed a supercritical mass- the reaction would take place faster and grow exponentially. Unfortunately the uranium obtained was seldom pure, so a large amount would be needed. The theory was that two sub-critical pieces of uranium or plutonium are driven together with great force and speed. Elaborate timing circuits and vast sets of wirings set off explosive charges, once again, within the span of a few micro-seconds.
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