The Pyongyang wind rose indicates that a lot of the fallout will be carried N, NW, and W -- i.e. to the Chinese territory. I doubt they would welcome nuking so close to their borders.
That's something I hadn't considered, but it strikes me as the most reasonable approach, and (I might be entirely off on this one!) one befitting a culture that really understands the concept of honor/'saving face'.
http://www.weatheronline.co.uk/weather/maps/city?FMM=1&FYY=2...
This leaves aside the question of how real these sanctions are, and how much China will keep helping NK behind the scenes.