If that were to happen, the US has enough nuclear-armed subs with the equivalent of 25,000 Hiroshimas in fire power to clear North Korea in about 10 minutes. It would only take a few dozen warheads to do the job. Hopefully, the prevailing winds would blow the fallout out to sea.
But if you blow nukes on north korea, the chinese would consider that an act of war. It would be like the chinese blowing a nuke in mexico or canada.