I think there are several reasons Putin won't use tactical nukes or any nukes.
1. Russia want to control Ukraine, making it radioactive would not help that effort.
2. What international support they have would dry up, outside of China. India would likely be very unhappy, Turkey too. US would likely put sanctions of China if they didn't stop all support.
3. The US will respond with proportional non-nuclear weapons, from ships or by plane. The only reason they're holding back is they don't want nukes in play. Once that line is crossed it could easily lead to NATO troops joining in to just vaporize every Russian target in Ukraine from the air and by sea.
4. Putin is a cold, calculating gangster, he likes wealth, he's a hedonist. He's been able to nuke the world for decades, but his violence had always been about increasing his power.