That would not work because it would render the newly acquired territories fairly useless (you can't sell radiated crops). Moreover, dropping one or two tactica

That would not work because it would render the newly acquired territories fairly useless (you can't sell radiated crops). Moreover, dropping one or two tactical nuclear bombs may not substantially change the situation on the battlefield. On the contrary, there are reasons to assume that with Western help Ukrainian troops could deal with the radiation hazards much better and with better equipment than Russia. Putin would end up having to nuke the new territories themselves if the Ukrainian army continues to advance. However, that's not realistic either. What's realistic is a sort of showing-off gesture, a nuclear attack on a city in Western Ukraine. The political consequences for Russia would devastating, though, and it would serve almost no military purpose.