The National Nuclear Security Administration is scrambling to keep up with demands for new nuclear weapons without having to actually invent a new nuclear weapon. The problem is that in 1994, Congress blocked any development of new nuclear weapons. The desire for a new penetrating nuclear bomb (to destroy the 140 deep bunkers built in potentially enemy countries, mostly over the last decade) can only be fulfilled under these rules by adapting an existing warhead to a new casing. There are other considerations as well. Nuclear warheads have always been built as light as possible, which makes them relatively fragile as machinery goes. The Pentagon wants to replace some of the systems in existing warheads with more robust (but heavier) designs so that a warhead will be available to use in a hard-shell penetrating nuclear weapon if one is needed.--Stephen V Cole