Also called: Mark III (Show more) Fat Man, atomic bomb dropped on the city of Nagasaki, Japan, on August 9, 1945. Its use was the second and last time that nuclear weapons were employed in war. Fat Man was a plutonium implosion-type bomb. It had a spherical core of plutonium-239 surrounded by high explosives, the force of which pushed inward rather than outward on detonation. The explosive blast caused a shock wave that compressed the plutonium core into a supercritical state (in the case of Fat Man, from the size of a softball to that of a tennis ball). The energy generated by the resulting chain ...(100 of 844 words) Access the full article Help support true facts by becoming a member. Subscribe today!