A quick comment. It was actually Ian's remarks on this thread that caused me to re-think the problems discussed here.

I re-evaluated, after N. Korea tested a new nuclear device, estimated to be around 100 Kilotons. This is a big step up in destructive power. A-bombs of this size are "city killers". It's not easy to find comparative charts for the effects of nuclear weapons. But one thing stands out as a truly a problem for a 100-Kilton bomb ... retinal damage.

If I am reading the data correctly, a 100-Kiloton weapon is capable of damaging the human retina from a distance of more than 100 kilometers. If you look directly at the blast - which seems very possible - the destruction to the human eye is immense. This means that there would be a huge radius around the location of the explosion where people are partially blinded, or fully blinded. Permanently. This may be the most serious problem from a large-scale nuclear explosion, even more serious than the heat or radiation.

It is certainly true that widespread fires would also be a big problem.