Unfortunately politicians are what they are. Read history of the "Cuban Missile Crisis", which was very very very near US /CCCP nuking each other. And for what reason?

However, Ukraine is next to Russia and any nuke there poses danger directly to Russia itself, including St.Petersburg, which is home town of the Putin he loves. Maybe they use tactical nukes if if desperate situation, but in a such case NATO doesn't respond. Because it's clear it most likely leads to full scale nuking - that would be the ultimate doomsday for the planet Earth.