Because of the presence of stray neutrons in the atmosphere or the possibility of their being generated in various ways, a quantity of a suitable isotope of uranium (or plutonium) exceeding the critical mass would be likely to melt or possibly explode. It is necessary, therefore, that before detonation, a nuclear weapon should contain no piece of fissionable material that is as large as the critical mass for the given conditions.
In order to produce an explosion, the material must then be made “supercritical,” i.e., larger than the critical mass, in a time so short as to preclude a sub-explosive change in the configuration, such as by melting.
Two general methods have been described for bringing about a nuclear explosion, that is to say, for quickly converting a sub-critical system into a supercritical one. In the first method, two or more pieces of fissionable material, each less than a critical mass, are brought together very rapidly in order to form one piece that exceeds the critical mass (Fig. 1.52).
This may be achieved in some kind of gun-barrel device, in which an explosive propellant is used to blow one sub-critical piece of fissionable material from the breech end of the gun into another sub-critical piece firmly held in the muzzle end.
The second method makes use of the fact that when a sub-critical quantity of an appropriate isotope of uranium (or plutonium) is strongly compressed, it can become critical or supercritical as indicated above. The compression may be achieved by means of a spherical arrangement of specially fabricated shapes (lenses) of ordinary high explosive.
In a hole in the center of this system is placed a sub-critical sphere of fissionable material. When the high explosive lens system is set off, by means of a detonator on the outside of each lens, an inwardly-directed spherical “implosion” wave is produced. A similar wave can be realized without lenses by detonating a large number of points distributed over a spherical surface. When the implosion wave reaches the sphere of uranium (or plutonium), it causes the latter to be compressed and become supercritical (Fig. 1.53). The introduction of neutrons from a suitable source can then initiate a chain reaction leading to an explosion.