North Korea would not have to hit the US. A high altitude nuclear blast between Japan and the USA, then let the winds blow the radioactive dust over the western states.
During world war II, the japanese released high altitude balloons carrying bombs. A few of the balloons with the bombs bombs were found in the deserts of California. The balloons had been carried from japan to California by the high altitude jet stream.
http://en.wikipedia.org/wiki/Fire_balloon
All north korea has to do is get nuke into the upper atmosphere, then let the winds and panic do the rest.