Nuclear Bombs Essays - Nuclear Weapon Design, Actinides,

Nuclear Bombs THE FIRST DESIGN of a nuclear weapon in the United States was a gun-barrel assembly, in which two sub-critical masses of very highly enriched uranium (HEU), were brought together by normal artillery propellant in a short gun barrel into a single over-critical configuration. (Criticality defines the minimum amount of a fissionable material in a particular configuration and density capable of a self-sustaining chain reaction). The second type of fission weapon is the implosion assembly, in which a high explosive (with a much faster detonation speed than the propellant used in a gun-type weapon) compresses fissile material so that it reaches a super-critical mass. Less fissile material is required for an implosion assembly because the critical mass varies inversely as the square of density. A nuclear explosion requires an exponentially growing fission chain reaction in which a neutron causes fission, producing energy and liberating two or three neutrons, more than one of which on average goes on to cause another fission, and so on. This chain breeding of neutrons and consequent fission is terminated by the disassembly of the system caused by the rapid energy release resulting from the fission process. In both the gun-barrel and implosion-type assemblies, neutron sources were devised that would emit neutrons at the appropriate time, and rapidly enough so that the chain reaction would, with high probability, be initiated before the material disassembled mechanically at speeds similar to that with which it was assembled. In the fissionable materials used in nuclear weapons (U-235 and plutonium-239), the fission is caused mainly by fast neutrons, which travel only a distance of seven to 10 centimeters before colliding with a nucleus, so that each doubling of the neutron population occurs in about 0.01 microseconds (one-hundred millionth of a second). The power of compound interest is such that beginning with a single fission, the time required 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 Weapon The 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 microseconds. In the years following 1945, innovations were made to reduce the amount of costly fissionable material needed for nuclear weapons and to improve their safety. With the initial configuration much farther from criticality, the weapon was safer against undesired nuclear explosion. Nevertheless, one could conceive of accidents in which the high explosive would detonate at one point by, for instance, the impact of a rifle bullet on the explosive or the accidental dropping of the nuclear bomb. Almost from the beginning of the U.S. program, nuclear weapons were required to be safe against such undesired nuclear explosions. For