It's possible Putin just won't give a sh+t.
But Russia is not North Korea. They still have access to the internet, and I don't think turning Kiev into a smouldering pile of rubble, as was done in Grozny, could be kept hidden forever. Russian soldiers are also ultimately going to tell their family and friends what they saw.
I have a lot of concerns about Chernobyl in particular but also the other nuclear power plants. If the Russians fuck that up and radiation is released, I suspect that the world -- and the NATO countries -- will see it as an attack by Russia. They'll have no choice but to go in then. Of course, the civilians will pay the price as always.