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QUESTION: How long would it take for radiation to clear out to current levels after nuclear war?
In video game Fallout 3 the year is 2277 and the game takes place in Washington D.C which was hit by multiple nukes. In the game most of the land is clear from radiation with only around 10% of the land emitting 1-10 rads/sec and a few hotspots, that were the areas where the nukes exploded, where rads are in the hundreds to thousands. And how long would it take for the fallout to settle. As far as I know it is the most lethal radiation spreader because it travels far and becomes mixed in air and food and water we consume. So how long does it take for fallout to settle down? So how would you guess if a nuclear war occured in 2077 (the year in Fallout games when the nuclear war occured) to clear out? How much would the land have restored in two centuries?

ANSWER: 1-10 rads/second?!!?!!  That's a fatal dose in a few minutes to under a minute!  Hundreds to thousands are fatal doses instantly (you wouldn't die instantly at all, but you'd get a fatal dose in under a second at thousands of Rad/sec).

You're asking a question with no answer.  How many nukes?  How big?  How many worked?  How many never got deployed because people just couldn't fire them?  What kind of nukes?  Neutron bombs, for example, are designed to not make almost any fallout...primary fission weapons (this is well known and not classified or anything) make tons of fallout.  "Salted" bombs are designed to maximize nasty fallout.  The worst stuff goes away pretty fast, and rain washes a lot of the rest into total dilution faster than people think...but this question of "how long" depends on so many parameters that it just can't be answered that simply without a ton of further input.  It's like people asking me the range on a radiation detector without telling me how hot the source they're trying to find is, or what kind of radiation it's emitting.

---------- FOLLOW-UP ----------

QUESTION: Well in the game the nuclear war was fought with an arsenal equivalent to what we had at the height of thr cold war so does this information help at all in my question? Several cities in the game qith population of millions were hit by dozens of warheads.

This doesn't help at all with the choice of warheads or size, but the parameters of the game are unreasonable and your character would be dead in hours at the levels you mention.  It would take years after that to clear out most of the radioactive contamination, depending on weather, but not centuries.  You can walk around in Hiroshima right at ground zero nowadays.  Centuries later?  Stuff clears out, and the most highly radioactive components are the things with the shortest half-lives (basic nuclear physics, the hotter something is per number of atoms the faster it goes away).  Even tritium and 137Cs would have had a significant amount of time to wash away and decay to stability in a couple of centuries.  There might be hot spots from big nukes, but nothing that bad.

Physics

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#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.