I think the MAD would be enough. Russia has such a vast space (not possible to effectively cover with air defense), even if just European part, with so many legitimate targets without any need for bunker busters/etc., that just a test underground explosion showing that Ukraine got it would be enough for Russia to hit the brakes screeching.

Btw, according to the Russian nuclear doctrine nukes can be used when the country is invaded even with just conventional weapons if the invasion presents severe threat to the country. Thus according to the Russian nuclear doctrine Ukraine is allowed to use nukes.