RU has said that they will use nuclear weapons if their territory is threatened and UA and RU have territorial disputes.
Personally I don't see a situation in which UA is able to take Crimea and RU doesn't launch a tactical nuke. Putin isn't going to allow Crimea to be taken, and I honestly suspect he'd rather risk his life than live with the shame of such a defeat.
But this assumes that UA would try to retake Crimea and that's something that will only happen if UA manages to push back RU. And while Zelensky has suggested he wishes to liberate Crimea, whether he will or not probably depends on US support. So there are at least a few conditionals here, but I personally don't think Putin launching a nuke is that unlikely in the event of an embarrassing defeat where contested territory is taken from them.