[eno_]

If a nuke is popped in the U.S., it will not be a backyard project. It will have come from Iraq or Iran. The same, for that matter, with any serious bioweapons attack.

[drjoe]

The problem is not in the domain of physics nor is it in the fundamental design. All that has been known for decades. A plutonium implosiondesign has problems associated with its implementations [the polonium initiator and the ignition devices] and this would take some doing. However a uranium gun design is almost trivial in construction [for at least some degree of explosion]. A terrorist might not get a large yield from a gun design [it might well fizzle before it REALLY goes BOOM, but then a terrorist only wants to do somethig along these lines. Remember that the gun design bomb dropped on Japan had NEVER been tested before it was used.

The real problem is in obtaining fissionable material and in the fabrication of this stuff. Probably better to steal one.