I don't think an all out Russia/US first or second strike nuclear conflict is likely. A far more plausible scenario in my opinion, is that a terrorist organization simultaneously detonates one or two nuclear devices in both counties and each country in turn launches a massive retaliatory strike ..blowing all of us away
That also seems unlikely. Both nations know that the other isn't completely insane and as long as they aren't seeing launches from the other side they aren't going to retaliate. The alert level would redline on both sides but a retaliatory strike for an accidental or terrorist detonation is unlikely.
For that matter even a terrorist nuke is unlikely to result in a nuclear strike on the terrorist nation. It would kill too many citizens of other nations who would happen to be in the target nation at the time. (Like when we hit the Chinese embassy in Belgrade)