[mikeash]

The possibility of taking an existing bomb and using it to build a new nuclear bomb without being an actual military is absurd. You'll not get an inefficient bomb, you'll get a dud.

Dirty bombs are a different threat altogether. One that's almost entirely psychological. No doubt effective at that, but a completely different scenario than the one being discussed.

[jlgreco]

It is fairly trivial to do with uranium, but with plutonium you are correct; you would never get anything beyond a fizzle (if you were lucky).

If the discussion is "what if terrorists steal a bomb", then the discussion is implicitly about dirty bombs. No other discussion really makes sense.