The minimum mass sufficient to sustain a chain reaction is known as the "critical mass." This is the mass for which the neutrons produced in fission just balance the neutrons that are absorbed or escape. For an effective nuclear weapon, the critical mass must be assembled quickly. Two general approaches are used to achieve this, as pictured in Figure 17.1:
♦ Gun assembly. In the gun-type approach, two subcritical masses of fissionable material are brought together by firing one into the other, or by firing them both. This can be done in a straightforward way using a chemical propellant. The assembly speed depends on the velocity imparted to the mass or masses. As discussed in Section 17.4.1, the gun assembly method is too slow for use with 239Pu bombs and is only used with 235U bombs.
♦ Implosion technique. In the implosion approach, a given mass is changed from subcritical to critical by a very rapid compression, produced by a carefully placed set of chemical explosives surrounding the mass. The arrangement of the explosives and the timing of their firing require a more sophisticated design than is necessary in the gun assembly. The motivation for making this extra effort is speed of assembly. The implosion technique has been used for both 235U and 239Pu bombs.
Many of the nuclear weapons developed since World War II obtain a major fraction of their explosive energy from the fusion of hydrogen, in conjunction with fission. However, such "hydrogen bombs" are more complicated to build than are simple fission bombs, and although they may be more dangerous in terms of the size of the explosion produced, they are less immediate threats from the standpoint of weapons proliferation.3
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