It depends on exactly how much skill these guys have. It's trivially easy, compared to getting the weapons-grade materials in the first place. It's also impossi

It depends on exactly how much skill these guys have. It's trivially easy, compared to getting the weapons-grade materials in the first place. It's also impossibly difficult, compared to building suicide bomb vests and such. You do need enough of a science and engineering base to be able to build something new, based entirely on theoretical calculations, when you've never seen an example before. If ISIS starts turning out their own rifles, ammo, truck repair parts, maybe some drones or something, or otherwise demonstrates dedication at a societal level to designing and building things, then I'd start to worry. As it is now, it's pretty hard to buy. And speaking of potential damage, a nuclear blast is just the start of the potential issues. The real question, if somebody managed to pull that off, is how would the target of such an attack reply? How would the allies of that target think and expect, and how would any countries that are rivals to the target and/or allied/sympathetic to the suspected attackers react to the response?