I don't think the groups that are warring for control of the Soviet Union/Russia would use nuclear weapons on what they would view as their own people. On the other hand if a terrorist group was able to somehow seize a nuclear weapon then a city is likely to get nuked. For example if radical Chechens were able to seize some nuclear weapons Moscow or some other major Russian city is likely to be attacked. However on the other hand it is possible that some radicalized ultra-communist elements of the Soviet Army would be able to seize a nuclear weapon that they would then use against their enemies of choice (possible targets include some of the Caucasian states that may rise up in rebellion during a civil war, a nation that has either left the Soviet Union or is attempting to leave, or even, in a more unlikely case, against a city controlled by pro-reform elements.