I believe that the worry is not a nuclear explosion (of the atom bomb type, or indeed the neutron bomb which kills living organisms but leaves buildings standing), but instead a dirty bomb -- an explosion which vaporises radioactive waste all over the place. In theory it's a lot easier to do this, but I'm far more sceptical about the actual threat one of those poses... You'd need a really large amount of rather radioactive material for it to become a big problem... I think the far more serious threat is the panic and fear that one of these would create, rather than any real radioactivity...