[benlivengood]

I think you have to be pretty close to the actual fireball to die within 20ms, and most fireballs would be air bursts 1km altitude or higher.

As I understand it the main reason there isn't instant disintegration out to hundreds or thousands of meters is that as soon as enough initial gamma and X-rays turn surrounding material into plasma most of the energy released goes into fireball formation because the plasma is virtually opaque to all EM and the fireball grows in volume as a plasma until expansion reaches equilibrium with compressed surrounding air, everything at the plasma/gas interface is incandescent and radiates as a black body of ~10,000C which transfers a lot of heat but not sufficient to atomize many centimeters thick objects unless they are very close.

Portions of the towers that suspended initial nuclear tests survived, for example.