Sorry if this question is inappropriate for this SE or perhaps too technical but it is frankly a genuine concern for me.
Basically, the question is, how much should the claims of NK be believed? My specific reasons for skepticism are:
Hans Bethe, a very great physicist who was key to the Manhattan Project, mentioned the need to machine plutonium as being a reason that non-state organizations like terrorists would have great difficulty making an a-bomb and I would guess other aspects like the initiator are non-trivial. Uranium gun-type bombs are problematic as I understand it for other reasons, probably isotope separation. Now, NK is a state of course but it lacks resources that other countries possess.
The range 15K kilometers which I believe make every site in the world a potential target, but is not such a range unlikely? I do not fully understand ballistic missiles: can such a missile accurately reach targets from its initial thrust alone or are in-flight corrections needed and also I think the missile has to know when it is near the target. How is navigation accomplished? I assume the GPS system can selectively deny info to anyone we choose.