I'm going to get downvoted for this but I cannot believe what I'm reading... we're talking about a nuclear attack involving inter-continental missiles. I'm far from an expert on this subject but I'm pretty sure you cannot build one in your garage on your own. (and especially not with a simple schema on a white board).
Of course, which is why the whole thing is ridiculous - but that would be true even if the person was planning an actual nuclear attack - in a free country, the only reasonable course of action would be to "let them try" - obviously there is no chance of success.
If they actually do get anywhere, AFAIK possession of radioactive material is illegal, so that's when you could actually arrest them.
