conds.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 some years, this was accomplished by systems in which some of the fissile core of the weapon would be kept separate from the explosive and inserted only during the flight of the aircraft on an actual mission. But because this impeded military readiness and flexibility, later weapons were designed with internal mechanical safing devices, or so that they were “inherently” one-point safe.In 1951, the United States first tested the “boosting” concept, in which a small amount of thermonuclear fuel was added to the ordinary fission bomb. This is currently accomplished by the use of a gas mixture of deuterium and tritium within the hollow “pit” of an implosion weapon. At the temperatures reached in the incipient nuclear explosion, a fraction of the tritium nuclei react with the deuterium nuclei to form helium nuclei and a neutron of 14 million-volt energy; these neutrons are extremely effective at causing fission in the now compressed fissionable material. While the thermonuclear reaction produces a relatively small amount of the total energy, it does result in a substantial number of neutrons that steps up, or boosts, the fission reaction to a higher level. Boosting further increases the safety of such an explosive, because a larger amount of fissionable material would otherwise be req...
