Fat Man: Definition and Links by Encyclopedian.com

The nuclear weapon nicknamed "Fat Man" was detonated over Nagasaki, Japan, on August 9, 1945. It was the second and, as of 2003, the last of the nuclear weapons that were ever used in warfare.

The 10-foot 8-inch (3.25 metres) long, five-foot (1.52 metres) diameter, 10,000-pound (4545 kg) weapon detonated at an altitude of about 1,800 feet over the city. It was dropped from the B-29 bomber Bock's Car, commanded by Frederick C. Bock. The bomb had a yield of about 20 kilotons, i.e. 8.4 x 10¹³ joule = 84 TJ (terajoule), slightly more than the bomb known as "Little Boy" dropped on Hiroshima three days earlier. Due to Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 40,000 people died in the nuclear explosion at Nagasaki.

"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.

This mechanism was necessary for a plutonium weapon in contrast to a uranium weapon (like "Little Boy") because the gun mechanism used in "Little Boy" (firing two sub-critical masses together into one super-critical mass) would have been less effective. Plutonium has a higher spontaneous neutron emission rate than uranium, and so two masses fired together would begin chain reactions before they formed a supercritical mass, resulting in a lower-yield weapon.

Earlier there had been one test explosion with this type of weapon, on July 16, 1945 at the Trinity site, due to worries about how the mechanism would perform in practice.