at this doubling interval to cause fission of 1 kilogram of fissionable material is the time required for 80 such doublings, or less than 1 microsecond (one millionth of a second). This corresponds to an energy release equivalent to about 17 kilotons (17,000 tons) of high explosive. The gun-type weapon used at Hiroshima, which contained approximately 60 kilograms of HEU, produced an energy release equivalent to about 15 kilotons of high explosive.The Acquisition of a WeaponThe separation of U-235 from the 140-times-as-abundant isotope uranium-238 (U-238) in natural uranium is a costly and difficult process, which originally could not be counted on to provide fissile material as rapidly as was thought to be necessary in the U.S. weapon program during World War II. Accordingly, with the discovery of the new element plutonium (in particular, the Pu-239 isotope that is produced in natural-uranium nuclear reactors by the parasitic capture of neutrons by U-238), production reactors were built at Hanford, Washington. A reactor with a thermal power of 250 megawatts produces about 250 grams of plutonium per day. Approximately 6 kilograms of plutonium was used in the world's first nuclear explosion--the “Trinity” test conducted at Alamogordo, New Mexico, on July 16, 1945--and an identical weapon detonated over Nagasaki three days after Hiroshima.Plutonium cannot be used in a gun-assembly weapon because the components are moved too slowly. Pu-239 is accompanied by the isotope Pu-240, which has a “spontaneous fission” decay that injects neutrons continuously into any mass of plutonium. The relatively slow assembly of metallic blocks in a plutonium gun (measured in milliseconds) would allow time for such neutrons to start the chain reaction when the assembly is barely super-critical, leading to a much reduced yield. Thus, for the plutonium weapon, assembly is achieved through implosion, which occurs on a time scale of microse...
