different regions would be seized by different groups so different weapons might fall into the hands of some group, sure. but as the central state collapses, which is what balkanization implies, it’s not going to suddenly turn around and start nuking within it’s own borders. chemical and biological weapons? sure. but you can’t really hide nukes going off. it would literally invite invasion.

if it’s some splinter group that gets its hands on a nuclear weapon, they have to do so with knowledge of how to unlock, arm, and fire the things, so outside of splinter factions of the US military, that’s also out.

that said, we’re also likely to get invaded as the whole world has a stake in the outcome, at which point the state using nukes seems more reasonable, but they’re much more likely to try to hit the invading countries back home as to try and hit american soil.

so all in all, I don’t think nukes are likely if we balkanize due to the onset of a civil war.