While it is possible, it is unlikely that they go to Sears&Roebuck and buy new initiators and explosive lenses. The greater number of some of these devices smuggled, the greater the probability of them being detected. For example, some years ago, a junked piece of radiotherapy equipment found its way to a scrap dealer in Mexico. Rolling around the back of a truck, with the scrap iron were pellets of cobat60. The iron was melted and cast into decorative table legs of the type used in outdoor restaurants. A shipment of these tables was on an 18-wheeler that had to make a delivery to TMA Eberline, and when it pulled up it set off every detector in the place. None of the material ever found its way to the public...and this is just one example that an insignificant worker-ant like me is aware of.
In addition, enough material to form a critical mass in not just radioactively "hot", it is thermally hot, and sufficiently so that the charges deteriorate from the heat, and from neutron and gamma bombardment, in particular the explosives and binders used.
But a "dud" would make a serious dirty bomb, plutonium being one of the worst carcinogens known.
What would such an attack accomplish except to remind us that we have twice as many nuclear warheads and the people who were killed on 9-11, and provide us a perfect license to use them?
The other consequence, knowing the American temper when finally, finally provoked would be the festive decoration provided by Muslims dangling from every lamp post.
Well, it that's what it takes.....
A very interesting vision! I must go get a few more lamp posts.