Russia would be hardest hit though because most of their population is confined to two or three cities. In most of the rest of the world the population is more distributed.
It would be devastating to all sides of course, assuming Russia's oligarchs haven't stolen the nukes, but Russia would come off worse. Anyway Putin wants to be in charge he isn't going to risk getting the entire country destroyed, if only because it's not fun ruling over a crater riddled radioactive moonscape.