Long-term ecological effects of a global (but strategic) thermonuclear war

Question

I'm currently toying with a post-nuclear apocalypse, with most, if not all, human survivors living in underground bunkers or vaults, and probably away from previous population centers—my test case is somewhere in rural Idaho. The details of the vaults, or of any humans that may have avoided the carnage by (for instance) fleeing into space, are not relevant here but may form another question or several later.

I am assuming/handwaving that the participants do not have enough tonnage to simply blanket their opponents in an attempt to zero population counts. Instead warheads are targeted at major population centers, seats of government, major military installations, et al. Due to its remoteness, I do not expect my characters to be living in the Hollywood ruin of Fallout or such, but I do expect that there was enough fallout from major targets to justify their bunker in the first place.

Naturally this will have major ecological impact, but I don't know what that might be, or how long (or how short) it might take the area to recover. On a timescale of up to, say, 400 years, what impacts will the effects of "standard" fallout have on the flora and fauna of the area? How long will it take the area to return to a state resembling its current one (2015), if it does at all? Will populations be decimated, will the food chain change, will evolutionary selection kick in?

In short, if my characters pop their heads out of their door at various points in the next couple centuries after a major nuclear war, and then go right back inside, what are they going to see?

Answer 1

I'm assuming a "sensible" use of nuclear weapons here, ie. most bombs are set for air-burst to maximize damage rather than ground-burst to maximize fallout, and that a large number of small-yield (100 KT) bombs are used rather than a small number of large-yield (10 MT) bombs. I'm also assuming your survivors are in the central/southern panhandle of Idaho, about equidistant from Spokane, Boise, and Butte.

The hazards from fallout change over time.

In the immediate aftermath (days to weeks) of a war, the risk is acute radiation poisoning: radiation damage building up faster than your body can repair it. Your survivors aren't near any militarily important targets, so they won't be at risk unless they're unlucky enough to be in a rainout hotspot or the winds are blowing from Spokane or Boise. Odds are, the bunker won't help them here because there's no risk to begin with.

After a few weeks, most of the radioisotopes involved have decayed. At this point, your survivors can travel just about anywhere without short-term risk; the only likely exception is ground zero -- stay out of blast-damaged areas.

Over the longer run, the risk is an increased cancer and birth defect rate from accumulated radiation dose. The bunker will help a bit here, by providing a living space with pre-war radiation levels, but most of the exposure will be from food, as longer-lived radioisotopes accumulate up the food chain.

A good model for a badly-hit area (say, New York City or Cheyanne Mountain) is the aftermath of the Chernobyl disaster: the isotope ratios are different, but the overall pattern of radiation over time should be similar. In the first decades after the war, there will be a dieoff of large, long-lived species, particularly those high on the food chain. You'll find, for example, lots of mice in the badly-hit areas because they don't live long enough to develop cancer, but few wolves or deer. In the longer run, though, things will get back to more-or-less normal, as species migrate back in from less-affected areas.

After 400 years, you're likely to need to perform a scientific survey to identify any but the most-damaged areas. The highest-threat radioisotopes are those with a half-life of around 30 years (eg. Caesium-137 or Strontium-90) because of their mix of relatively high radiation output and relatively long half-life, but after 13 half-lives, the remaining amount will be almost undetectable. Long-lived isotopes such as un-fissioned Plutonium-239 will still be around, but their long half-life also means they don't produce much radiation.