[meursault]

How do the Savior races come to have such long lifespans? And are the Tamed Humans independent, or are they bred by the Saviors?

PS. Don't tell if they're spoilers.

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The best single resource for nuclear war effects is the book Nuclear War Survival Skills, which you can download for free. The mods don't let me post links, so search for it.

The most useful bit for OP's purposes is the first chapter. Search for "The Dangers from Nuclear Weapons; Facts versus Myths" and it should be the first result on Google.

Now, nuclear was is a really bad thing. Hundreds of millions will die in a full-fledged exchange. There will be widespread famine. Huge economic dislocations, and perhaps a modest degree of technological regression.

But it will almost certainly not wipe out human civilization or even modern industrial civilization. Here is one quote from aforementioned chapter:

Myth: Overkill would result if all the U.S. and U.S.S.R, nuclear weapons were used meaning not only that the two superpowers have more than enough weapons to kill all of each other's people, but also that they have enough weapons to exterminate the human race.

Facts: Statements that the U.S. and the Soviet Union have the power to kill the world's population several times over are based on misleading calculations. One such calculation is to multiply the deaths produced per kiloton exploded over Hiroshima or Nagasaki by an estimate of the number of kilotons in either side's arsenal. (A kiloton explosion is one that produces the same amount of energy as does 1000 tons of TNT.) The unstated assumption is that somehow the world's population could be gathered into circular crowds, each a few miles in diameter with a population density equal to downtown Hiroshima or Nagasaki, and then a small (Hiroshima-sized) weapon would be exploded over the center of each crowd. Other misleading calculations are based on exaggerations of the dangers from long-lasting radiation and other harmful effects of a nuclear war.

Of course, that's all assuming you portray nuclear war realistically. If not, feel free to have it blow absolutely everything to smithereens. As there are so many myths about nuclear war, and since nuclear wars are conventional and commonly accepted explanations for post-apocalyptic environments in pop culture, about 99% of people won't notice any difference.

PS. One more thing for OP. There are some places in the world that preserve seeds, knowledge, etc. in reinforced, remote bankers. One example is the Svalbard Global Seed Vault. These can play a big part in such a story.