Wikipedia: Fat Man

The atomic bomb nicknamed "Fat Man" was dropped over Nagasaki, Japan, on August 9, 1945. Released by the [B-29 Superfortress]? "Bock's Car," the 10-foot 8-inch long, five-foot diameter, 10,000-pound weapon detonated at an altitude of about 1,800 feet over the city. The bomb had an yield of about 20,000 tons of TNT, about the same as the bomb dropped on Hiroshima?. Because of Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima.

"Fat Man" was an implosion type weapon using plutonium. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Numerous detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the capsule, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear explosion.