Hundreds of nukes have been set off above ground since 1945. There was no nuclear winter. I suspect a nuclear war won't be the cataclysmic event feared. Of course millions will die and perhaps billions but not all 8 billion of us.
I suppose the difference would be in a nuke bomb radiation is incinerated. If you look at the Chernobyl, as in just leaked, the entire area is still nuclear contaminated and uninhabitable. Yet Japan is safe.
A dirty bomb would be more of delayed death as the exposure eats the body.
That plant in Ukraine id a very large dirty bomb if struck. As its blown into the air that active cloud would drift deep into Europe and years if not decades of all life/death issues. All depends on direction of wind and how much gets into the air.