The fear is not that they will use nukes during the collapse, but that after the collapse, there will be multiple smaller nations, each with their own supply of nukes and unpredictable governments.

This was also a concern during the collapse of the Soviet Union, and the US government worked closely with the USSR government to make sure that all nuclear weapons remain within Russia. In particular, Ukraine was forced/convinced to give up its nuclear arsenal.

They didn't, but those nukes did end up in various member states. The last couple months people have talked a lot about the 1993 deal between the US, Russia and Ukraine that got Ukraine to give up its nukes in exchange of (now apparently worthless) guarantees of territorial integrity.

If Russia collapses, there's going to be mostly tribal territories in Siberia that have nukes.

Only barely