Basic Principles of Implosion Based on excerpts from Section 2 of Nuclear Weapons Frequently Asked Qustions by Carey Sublette. The key idea in implosion assembly is to compress a subcritical spherical, or sometimes cylindrical, fissionable mass by using specially designed high explosives. Implosion works by initiating the detonation of the explosives on their outer surface, so that the detonation wave moves inward.
Careful design allows the creation of a smooth, symmetrical implosion shock wave.
This shock wave is transmitted to the fissionable core and compresses it, raising the density to the point of supercriticality. Implosion can be used to compress either solid cores of fissionable material, or hollow cores in which the fissionable material forms a shell. It is easy to see how implosion can increase the density of a hollow core - it simply collapses the cavity. Solid metals can be compressed substantially by powerful shock waves also though. The convergent shock wave of an implosion can compress solid uranium or plutonium by a factor of 2 to 3.