I had somewhat of an A-Bomb craze in my school time.
Given the first decade or so of possibility of nuclear war that was actually pretty sound advise.
Sure tons of people would die and sure if you were too close you were SOL, but for most it was pretty good advice.
You could have been protected from the initial flash, flying debris (especially glass), falling debris and in case the builduing collapsed you would have a small hollowed out protected space.
Overall scale of war would have ment a general survivability on a larger time frame especially for supply chains a goverment entities.
It would have been an inimaginably huge disaster, but at least with a way past it.
As long as we keep the bombs in the lower kiloton range and the numbers of bombs in the dozens (and the main delivery method as planes).
Gives you enough time and low enough total yieled to recover.
As we moved into the thermonuclear age, from their into the ICBM age, the whole thing changed.
Yes, hiding under your desk would still help, just not anywhere close to as many people.
Weapon yields suddenly scaled by a factor of 10, numbers of weapons in a case of war as well, and time went down by 100 maybe.
When you were 2 miles from the bomb in a school before, you had a decent chance of getting out if that school alive.
Now make that more like 10 miles, or more.
One of the horrible things is that as you scale up detonation yield, heat gets exproportionaly more prevalent as a source of damage.
A broken bone is usually not deadly, shards of glass in your face are often not deadly wounds. They are immensly painfull and horrible without medication, but you would get through.
Try a 70% skin area third degree burn without treatment and you would be lucky to get along a few days.
A 5 megaton airburst would inflict 3rd degree burns up to 15 miles from ground zero tonany unprotected skin.
For comparison, at 150 kilotons (that is still several times hiroshima) that's not even 2 miles.
At some point, just standing in the street and hoping you are close enough is the best way to go IMO...
That is the horrible part: Once they got the key to fusion, they got the key to easily and cheaply scale up the explosion or to make the bomb tiny with basicly no work.
With close to no limits.
These machines are perfection, just with a completly horrid purpose.