Have there be two main factions in post-nuclear Russia. The Moskovians and the Peterbyrgians. Both have an extensive system of fallout shelters designed for this exact purpose. Sure, the cities are gonna get nuked, but the underground will survive. Both cities have a high proportion of "elite" people such as scientists, politicians, doctors, lawyers, etc.
The Moscow region is the "old" government, meaning it wants to re-consolidate all of Russia under itself again. St. Petersburg is the "old-old" monarchist capital, where they have re-instated a Grand Duke, and they're totally willing to let Siberia and the Far East go. Both sides have recruiting parties, advanced technology, and good secure home-bases. If I were to use Fallout terminology, think Enclave vs. Enclave.
Additionally, if you want to include the other countries that were part of the USSR, have them organize under Kiev, it being one of the first cities there (or Novgorod, yeah), and the high amount of fallout shelters. These guys would be more like the Brotherhood, because in Russia, the tendency to centralize means that the good professionals go to either Petersburg or Moscow, so the Kiev guys would just have to rely on technology already there, without making any new ones.
But the main question is- how the hell do you have enough food for all. I mean, yeah, a fuckton of people will die to starvation, but crops would be failing worldwide. In a couple of years post-war there will be no more food.