I would imagine that even a small conventional bomb landing anywhere within a few miles of Chernobyl would unearth and spread contamination.

I remember reading somewhere that all the ground was full of small radioactive particles in the area surrounding Chernobyl. So basically what they had to do was dig up the first few feet of dirt and flip it over, burying the contaminated dirt. Any small bomb would undo that. Then the wind would carry it and we'd have a whole mess.