If they launch a nuclear missile at us, first it will have to be of adequate nuclear warhead throw weight to destroy a significant target, then it will have to get here, then it will have to be accurate enough to hit said target, and finally it will have to get through the defenses already in place.  All of this is not so easily done, but they may get there some day, maybe sooner than we think. 
Radioactive contamination of the entire Hawaiian Islands is unlikely, unless they launch multiple successful missiles and the winds blow up and down the chain.   As I said before, nuclear retaliation and then contamination of North Korea and the surrounding areas will not be acceptable to adjacent countries, especially our adversaries, such as China and Russia and could lead to WWIII.  A conventional invasion could also lead to the same result, as China and Russia would not accept us being on their borders militarily.  The only solution is to keep building an impenetrable ABM defense.