More than anything, I think China doesn't want 20M+ refugees to become their problem in the world's eyes. And like I said, nobody can invade NK because 30 million civilian casualties is not an acceptable consequence to anyone.
Look at the density map, and look at where you see big cities on the SatMap. Anything within 50 miles of the border is gone, anything within 200 miles is vulnerable.
http://map.primorye.ru/raster/maps/a...a_pop_1973.jpg
http://maps.google.com/maps?f=q&sour...911377&t=h&z=9