Fat Man
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 known nuclear weapon to be used in anger (that is, used with intent to harm in warfare or otherwise).
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 (550 m) over the city. It was dropped from the B-29 bomber Bockscar, piloted by Major Charles Sweeney. The bomb had a yield of about 20 kilotons, or 8.4×1013 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. In the end, it gave somewhere around 20 kt, 2 to 4 times the expected yield.
Schematic cross-section of Gadget; some boundaries are approximate. From left to right (ouside inward):
- dural casing, ~140 cm inner diameter
- exploding-bridgewire detonator
- faster explosive, Composition-B; 60% RDX, 39% TNT, 1% wax
- slower explosive (Baratol)
- faster explosive, "amplifier"
- aluminum–boron "pusher"; absorbs stray neutrons and widens/smooths implosion pulse
- natural-uranium "tamper"; neutron reflector, inertial containment, and fission material
- the "pit"; plutonium-239?–plutonuim-240–gallium delta-phase alloy (96%–1%–3% by molality)
- air gap
- beryllium–polonium-210 "initiator"
External Links
- thorough descriptions of Gadget ("a great deal of tissue paper and scotch tape were used to make everything fit snugly") and Fat Man at Nuclear Weapons Archive
