For both the Trinity device and the Fat Man , the Nagasaki bomb , nearly identical plutonium fission through implosion designs were used . The Fat Man device specifically used , about in volume , of Pu-239 , which is only 41 % of bare-sphere critical mass . ( See Fat Man article for a detailed drawing . ) Surrounded by a U-238 reflector/tamper , the Fat Man 's pit was brought close to critical mass by the neutron-reflecting properties of the U-238 . During detonation , criticality was achieved by implosion . The plutonium pit was squeezed to increase its density by simultaneous detonation , as with the " Trinity " test detonation three weeks earlier , of the conventional explosives placed uniformly around the pit . The explosives were detonated by multiple exploding-bridgewire detonators . It is estimated that only about 20 % of the plutonium underwent fission ; the rest , about , was scattered
Trinity device
Trinity (nuclear test)
Nuclear weapon design