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 ...
