Assembling an "improvised" bomb is relatively easy. So is delivering it. Assembling a nuclear bomb is an entirely different deal. Firstly, only recognized "governments" possess the fissionable materials needed, let alone the conventional explosives necessary, as well as the ability to construct successful "explosive lenses" needed to "assemble" the fissionables in such a way that they actually work.
As far as I'm concerned, and someone may pose a counter-thought here, IF the various terrorist factions had been hitherto able to do so, a nuclear attack would have by now been carried out. Now, if a terrorist government, having all those pre-requisites available, saw fit to deliver, that's another story. Possibly the Iran story, but somehow I don't think so. If such an act, traceable to any country's government, were to occur, the elimination of that country's government, would be assured. They are not complete fools; they know that. Better to kill dozens or hundreds conventionally, than assure going to meet Allah, the other way. imp