More sanctions. A strongly worded condemnation from the UN. That's it. Here is how nuclear-armed nations would react to a Russian use of a nuclear weapon in Ukraine:
Pakistan: Our nuclear weapons are only for killing Indians, so we don't care.
India: Our nuclear weapons are only for killing Pakistanis, so we don't care.
Israel: Our nuclear weapons are only for killing Muslims, so we don't care.
China: Bad Putin! Bad boy! However, it doesn't affect us, so we don't care.
North Korea: Doesn't affect us, so we don't care. (But can we have some of the scientific data from that?)
France: Bad Putin! Bad boy! We care, but we're not going to risk a nuclear war with Russia over Ukraine.
UK: Bad Putin! Bad boy! We care, but we're not going to risk a nuclear war with Russia over Ukraine.
US: Bad Putin! Bad boy! We care, but we're not going to risk a nuclear war with Russia over Ukraine. (Fuck, that was cool, dude!)
Iran: Can we have one of those, please?
Russia risks no actual military repercussions for using nuclear weapons in Ukraine. The only POSSIBLE exception to this would be if a NATO country was impacted by direct radiation or fallout, and even then I don't think it's likely.
The overall general global nuclear threat is rising, even if there's no indication that something is imminent. Personally, my own DEFCON level hasn't dropped below 4 since Russia invaded.
