IRC a more realistic scenario is that they do an airburst strike (limits fallout) above an unoccupied (no civlians left) part of Ukraine as a show of force. Official Russian doctrine:

Most military analysts believe that, in this case, Russia would pursue an ‘escalate to de-escalate’ strategy, initiating limited nuclear exchange to bring adversaries to the negotiating table. Russia will also threaten nuclear conflict to discourage initial escalation of any major conventional conflict

They think this will bring their enemies to the table.

I honestly don’t think it will, after all the atrocities they’ve committed. More likely, they’ll be fully ostracized, the west will give Ukraine even more weapons, Europe will enter the war in Ukraine, and the US will come with a conventional but devastating response.