The principal concept in an implosion assembly is to compress a subcritical usually spherical
or cylindrical mass of fissile material to the point where it becomes supercritical, by means
of specially designed high explosives.
Implosion works by means of initiating the detonation of the explosives on the outer surface,
in order that the detonation wave moves in. Shock waves instantly compress the fissile
material which rapidly increases the density. It compresses it and raises the density to the
supercriticality point. This leads to a very powerful nuclear explosion. High explosives are
needed to produce this large compression of the fissile material.