Ukraine has 15 nuclear reactors that are prime targets for non-nuclear weapons and which can cause a nuclear disaster at each of them or all of them.
How? Russian could bomb the hell out of these nuclear power plants and cause loss of offsite power, loss of cooling water supplies, loss of electrical switchyards, and lots of damage to reactors buildings, control buildings and diesel generator buildings.
Each nuclear power reactor has much more radioactive material that can be released to the environment than a nuclear bomb. Each reactor could become a multiple Chernobyl disasters or Fukushima disasters if Putin wanted to cause the most damage and devastation in Ukraine.
Not a pretty thought - but with a mad man Putin you never know.