Schematic cross-section of an implosion nuclear weapon design showing the basic principle of the need to use high explosive lensing in order to focus the detonation waves (initially from the detonators, in red) into a symmetrical, spherical wave which compresses the core.
Own work. Drawn in Inkscape.
Based on the diagram used by Richard Rhodes in Making of the Atomic Bomb, as well as another graphic which was floating around the internet (on this page) which had the idea of putting them all around in a circle (fairly obvious, but I hadn't seen it done that way previously).
User:Fastfission