I did in fact already run this scenario. The fallout area is large if you set the threshold low enough. But from what I understand, the largest rings do not render the area uninhabitable by any means. Perhaps the smallest inner ring does. Fallout is also generally a short-term effect -- weeks, not decades.
Also be aware that fallout from an underground blast (as in a silo) is going to spread much differently from fallout from an airburst.
Contrary to what you assume, the fallout increases in the underground burst compared to the burst in the air as the surface is reached see: http://fas.org/nuke/intro/nuke/effects.htm
I'm not trying to assume. I'm trying to research. I read that page, and several others. From what I understand, the local fallout (ie, within a few mile radius) is much higher in subsurface bursts, but the longer-range fallout is substantially less.
Thus, I am unconvinced by the argument that the longer-range fallout would "wipe out" something the size of Arkansas.
Also be aware that fallout from an underground blast (as in a silo) is going to spread much differently from fallout from an airburst.