Depends solely on the response from NATO.
From what little I've observed, I've come to several conclusions. If Russia does invade Ukraine, it won't be for occupying greater Ukraine. Therefore the conflict will be localized, away from Kiev (excluding severing communications), and not a matter of a total invasion of Ukraine--something which would more strenuously pressure NATO. Thus far, the US hasn't declared support for Ukraine in the event of invasion, leading me to believe the likely response to come from the US is strained diplomacy and further sanctions (with the possibility of arms shipments and sharing logistics). As of now, the US and EU are only focused on intimidation--a show of force--the usual.
Of course, with an invasion of this magnitude, there always comes the amplified risk of miscalculation, which could lead to a NATO intervention in some capacity. There will still be a potential scenario where nuclear war can become a reality. Although, I still find that likelihood negligible.
I don't presently believe the Russia-Ukraine conflict will go nuclear.