Unless there was some sort of super huge lead shield.....or other material in the mountain they supposedly detonated the bomb in....
I could forsee radiation climing up through the soil and into the air in as short as a few hours of the initial blast.
but then again, i am not a nuclear scientist, so i could be wrong.
i am willing to bet that north korea didnt even come close to detonating a nuke.
they just want to scare the rest of the world. they prolly used a few tons of TNT and make a big boom.