I don't fully understand where you're going with this question. Maybe you are implying a terrorist could get their hands on 1kg of U-235 and either make a dirt

I don't fully understand where you're going with this question. Maybe you are implying a terrorist could get their hands on 1kg of U-235 and either make a dirty bomb or a nuke. A dirty bomb would not really be more dangerous than a dirty bomb made with Cadmium. As for a nuke, 1kg is well below the minimum necessary to make a nuclear bomb, which is about 15 kg. But that's not the only thing. You need weapons grade Uranium for that, i.e. Uranium where U-235 constitutes at least 85% of the whole. Civilian reactors use Uranium that is enriched to much lower levels. Currently, the vast majority of the civilian reactors use Uranium enriched to 4.5%. A terrorist who gets their hands on that, can forget about building a nuclear bomb, except if they have means to further enrich that Uranium. In other words, if they have centrifuges, like Iran does. I might remind you that Iran is a state, not a single person. In any case, if you have centrifuges, you might as well start with Uranium ore, there's no point in stealing from an existing civilian reactor.