DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY
A-bomb "Fat Man", an atomic fission bomb of the type dropped over Nagasaki, Japan on August 9th 1945. It was this act, along with a similar attack on Hiroshima, that forced the surrender of Japan on September 2nd of that year. The device consisted of a slightly subcritical mass of plutonium encased by explosives. The firing of these charges caused compression of the fissionable material and so rendered it supercritical. Hence this was called the 'implosion' method. The ensuing nuclear chain reaction released vast amounts of energy; despite weighing under 5 tonnes it had a yield equivalent to 20,000 tonnes of high explosive.
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