...it really doesn't matter.

337 cities in the US with a population over 100,000. 80% of the US population lives in 3% of the area; that's 264 million Americans in cities and surprise! that's where the bases tend to be. Just spitball it - that's a nuke for every 200,000 people, more or less. San Antonio is looking at seven or so of those.

Presume two thirds of the arsenal is duds. Russia still hits the US with around 500 550kt warheads for 275 megatons. The US arsenal is not duds. That's around 1700 warheads which, doctrinally, are tuned to about 150kt for another 255 megatons. Nature figures a 400MT exchange is good for 150 teragrams of soot kicked up. They figure it'll take about five years to come back down. Here's their calculated calorie reduction.

Now - the population of San Antonio after between half and a whole megaton of airburst is going to go down. Prolly 70-75%. Something like 35-40% immediately, the rest not many days after that. But the population outside of San Antonio is mostly going to have to worry about (1) no electricity (2) no heat (3) no water (4) no food long before they care about radiation. 'cuz agriculture is going down by 99% or more. For five years.

So. If you've got a mountain cabin with really good air filters, an endless freshwater supply and six years of food per person, you'll probably survive until things can grow again.

But if you don't? You're done.

Remember us, AstroFrank. Our tin foil didn't save us.