For the Russians it doesn't make any tactical sense to use a dirty bomb really anywhere. They are basically a terror weapon that would communicate we don't have the ability or intent to ever take/hold Lviv, Kyiv, Kherson take your pick, but we're flipping you off as we retreat. And it would probably be more difficult for them to create and employ a dirty bomb when tactical nuclear weapons are available. Now, Sabotaging Enerhodar nuclear power plant creating a nuclear mess if they thought they couldn't hold it - that is probable - but currently Ukraine isn't pushing that way.
The Russians could assess that the Ukrainians may use one on Donetsk, Luhansk, Sevastopol, and maybe Melitopol to deny use of an occupied city not likely to be retaken in short term.