| CRITICAL MASS
A small sphere of pure fissile material, such as uranium-235, about the size of a golf ball, would not sustain a chain reaction. Too many neutrons escape through the surface area, which is relatively large compared with its volume, and thus are lost to the chain reaction. In a mass of uranium-235 about the size of a baseball, however, the number of neutrons lost through the surface is compensated for by the neutrons generated in additional fissions taking place within the sphere. The minimum amount of fissile material (of a given shape) required to maintain the chain reaction is known as the critical mass. Increasing the size of the sphere produces a supercritical assembly, in which the successive generations of fissions increase very rapidly, leading to a possible explosion as a result of the extremely rapid release of a large amount of energy. In an atomic bomb, therefore, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. A heavy material, called a tamper, surrounds the fissile mass and prevents its premature disruption. The tamper also reduces the number of neutrons that escape. If every atom in 0.5 kg (1.1 lb) of uranium were to split, the energy produced would equal the explosive power of 9.9 kilotons of TNT. In this hypothetical case, the efficiency of the process would be 100 percent. In the first A-bomb tests, this kind of efficiency was not approached. Moreover, a 0.5-kg (1.1-lb) mass is too small for a critical assembly. |