... it seems that a common myth about Nuclear Warfare is that victims are instantly vaporized. POOF! A hydrogen bomb is detonated and the entire population of a city is zorched out of existence. Perhaps it's a hold-over of the special effects from "The Day After".
The reality would be that, except for a small percentage of people at or near ground zero, the vast majority of casualties would be burned, cut and broken. Death would not be quick. Suffering would be terrible. The wounded would far exceed the dead. Many would linger for hours or days before death.
Aside from radiation; the blast of an atomic weapon, although huge, is not categorically different from a conventional explosion. The initial pulse of hard radiation caused by the nuclear reaction would cause instant death, or death within a few hours, only within the radius of the 'fireball'. Inside of that area death would be instantaneous from heat anyway.
Fallout; debris sucked-up and irradiated by the explosion takes some time to start falling back to earth. Radiation levels with-in the zone of blast destruction would be relatively low
for minutes- even hours.
These are important facts for consideration. The decision to take shelter and where, when and how has a lot to do with who is injured and who is not. It is fanciful and dangerous superstition to think choosing to not seek shelter means a brave and quick death.