Pretty much the entire world is having a meltdown over Putin's actions so far. And rightly so... If he used nukes on a Ukrainian city that would put an end to him and Russia as it exists today. He'd also likely lose a major Russian city in the process. He has a lot to lose.

Nukes (and fear) are the only option the Russians have for projecting their military power around the world (by that I mean anything other than a land battle in Europe). Logistics have ALWAYS been a problem for the Russian military. They have a massive army but they can't move it and support it around the world like we can. Their navy isn't a match for the US navy and the combined navies of the major western countries. Putin won't resort to nukes over Ukraine. If it gets any worse for him they'll find a way to "settle" their differences and he'll withdraw forces in a way to save face. It'll all be BS but that's how it'll be marketed.