Science Fair Project Encyclopedia
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 were killed outright by the bombing at Nagasaki, and about 25,000 were injured. Many more thousands would die later from related injuries, poisoning and nuclear fallout/radiation.
"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 impractical. 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 'fizzle', or explosion with no fissile component. It is theoretically possible to build a plutonium gun-type device, but it would need to be 19 feet long in order to allow the sub-critical masses to be fused into a critical mass before a fizzle occurs. The mass of a plutonium gun-type device would have been beyond the payload of the B-29.
Earlier there had been one test explosion with this type of weapon (called the "gadget"), 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.
The United States produced a small stockpile of "Fat Man" bombs after the war, but they were highly idiosyncratic and extremely delicate. It was eventually re-worked in the MK 4 Fat Man bomb, which was similar in principle but was appropriate for long-term stockpiling, use by non-experts, and used a more efficient implosion system (with a 60 point implosion system, compared to the 32 point weapon used in the war).
Schematic cross-section of the "gadget"; some boundaries are approximate. From left to right (outside inward):
- dural casing, ~140 cm inner diameter
- exploding-bridgewire detonator (allows for instantaneous detonation of explosives)
- 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, improves efficiency, reduces the amount of fission material needed)
- the "pit"; plutonium-239–plutonium-240–gallium delta-phase alloy (96%–1%–3% by molality) (fissionable material)
- air gap
- beryllium–polonium-210 "initiator" (the "urchin"), neutron source
- 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
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