I'm writing a tale about a post apocalyptic world. I'm studying the timeline about the world history, I've planned that, using "clean" thermonuclear bombs, radioactivity should be quite lower after less than one year after the "day zero". Is that realistic enough? considering that real shelter and improvised ones couldn't store enough provisions for long time and they could shelter very few people in it. So i'm asking, how much survivors should wait before going out the shelter and starting to survive in the new world?

EDIT1: Some shelters are controlled by the governments and pretty safe, secluted and far from major cities, other are private shelters in the underground of the cities, some others, like the one who shelters the main characters (maybe, still workin on it) are caves very far from cities. I guess they are the less safe enviroments from nuclear fallout. But still, we are talkin about "cleaner nukes"

  • How close to the source of the detonation is the shelter? Also "Safe" is a relative term depending the conditions within the shelter. – suchafunkymonkey Aug 10 at 7:39
  • edited with more info – Dany Raven Aug 10 at 7:45
  • Welcome to WorldBuilding Dany Raven! If you have a moment please take the tour and visit the help center to learn more about the site. I would also like to point out that while your question is interesting, it lacks so much information users might ask to close it because it is too broad, you may want to post in this site first worldbuilding.meta.stackexchange.com/q/6168/28224 which is called Sandbox, where other users can help "build" your question to be of WBSE quality. I upvoted your question so that you have reputation to access the Sandbox. Enjoy and Good Day! – Mr.J Aug 10 at 8:08
  • @mr.j thank you – Dany Raven Aug 10 at 8:15
  • Important questions: #1 Where are you? (Bombs can be air burst, the cleanest and best for killing people; ground burst, very dirty; or penetrating, very dirty and good for underground targets) #2 Who launched the weapons? (Soviet/Russian bombs are known to be VERY dirty. – RonJohn 20 hours ago

You can make it as light and as bad as you like

There are so many factors to take into account here, especially depending what the aim of the war was, and how it was conducted. The notion that a nuclear war will necessarily be an all out affair where the goal is maximum destruction is not realistic. You do not need to invent any new kind of nuclear weapon for this, because what matters most is not what is detonated but where it is detonated. An air burst produces very little fallout, while a ground burst — especially with a "salted" weapon — can be extremely bad.

But this is all moot anyway because a nuclear war will not be such that the entire world is blanketed by nuclear weapons. The impact and fallout zones where there is any acute danger will be fairly local. So although some fiction franchises thrive on the notion of having to huddle in shelters CoughFalloutCough in reality the best thing to do will probably just to travel away from the impact areas and go to an area not affected by the war.

So I would turn the question around:

Can you have a post-apocalyptic scenario that involved nuclear war, where people do not have to huddle in shelters for a very long time?

Answer: yes... you can tailor the backstory that way. You do not even have to go into detail about it. you can make the situation as severe and as light as you want and not lose realism.

And in any case... radiation is not this terrible affair that people make it out to be. By the time you have absorbed so much radiation that you are now suffering acute radiation syndrome, you have only increased your lifetime probability of contracting cancer by about 10-20%, up from the normal 35-45%. And radiation is really easy to avoid.... even simple dosimeters can be used for that. No, people will be having far worse issues to contend with than that.... food, clean water, electricity, lawlessness.

  • Yes, my tale will be focused on the doubts you wrote in the final part. The radioactivity issue is marginal but i want it to be realistic enough, that's why I'm here and I really thank you for the help – Dany Raven Aug 10 at 7:56
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    @DanyRaven Well you are going to have a problem there because reality is not realistic... according to your readers. "Whaaaa'?! They survived a nuclear war but they are not constantly bothered radiation and mutants and galloping cancer?! So unrealistic!!!". But radiation is essentially the modern world's Boogey Man: something to frighten people with. The fear of radiation is killing much more people after Chernobyl and Fukushima than radiation itself. – MichaelK Aug 10 at 8:00
  • yes I'm aware of the risks of a possibly boring plot. But i want to descrive a postnuclear world where survivors have to strive to survive, with scarce resources and low tech – Dany Raven Aug 10 at 8:03
  • Also I really Hate Fallout because my plot ends to become too similar to the one from that vg franchise :D – Dany Raven Aug 10 at 8:09
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    @DanyRaven You are going to need something much worse than a nuclear war then. Do recall that a mere 300 years ago, the world's population was roughly 3/4 less than it is now. Kill 75% of all living humans and all you do is knock us back to the 1700's. And the world was by no means an apocalyptic place to live in by then. And a mere 50 years ago... we were not using computers and semiconductors to any great extent. 100 years ago, an Alexanderson Alternator was the best internet around. But the 1920s was not the apocalypse. – MichaelK Aug 10 at 8:10

TVTropes has a pretty good article about the detonation of a nuclear bomb (1 megaton) at Walt Disney Headquarters (for no reason in particular... probably just to be funny) in Los Angeles and it's affect in the LA area. It's a good read if you can find it.

Thermo-nuclear is a H-Bomb and basically uses a fission explosion to generate heat necessary to create a fusion explosion... which is far more destructive. A lot of this will be based on how close to the ground the bomb is. Most of the radioactive fallout of a nuclear blast is based on how much of the fireball intersects with the ground. The more intersection, the more radioactive material will be vaporized and then fall out. This isn't to say that it is all that will be destructive, as the overpressure or blast, and the radiant heat (the light) will still kill for some distance outside of ground zero. The greater the height, the more threat it makes to more stuff.

The radiant heat is pretty destructive as it will start a "Simba's Prideland" of fires... everything the light touches will burn. Famed negative shadows of Hiroshima are people shaped light patches on walls that were saved from charring because there was a person casting a shadow at the time of the radiant heat. In Nagasaki, despite the more powerful of the two bombs dropping, it was less destructive to the overall city because it detonated in the valley of two mountains... essentially blocking much of the radiant heat and over-pressure from destroying the rest of the city.

It's important to remember this is "Simba's Pridelands" so you can survive pretty close to the epicenter in the right conditions, Akiko Takakura entered the history books as being the closest survivor to ground zero in Hiroshima, at a distance of about 300 meters. She survived because at the time of the blast, she was in the vault of her bank and the over-pressure was weak enough not to blow apart the vault structure and the light had already burnt the building's exterior.

It's also important to note that it will be light first than over-pressure... for the same reason it's always lightning and thunder.

So with that out of the way, FEMA answers this with the 7-10 rule. For every measure of 7 hours of time, radiation levels decrease by 10%. If you know your initial rads per hour, this can be mathed out so that 7*.1X = y. This is also rendered as the 7 X 7 X 7 rule. After 7 hours, the initial rads have decreased by 90%. After 49 hours, the initial rads decrease by 99%. After 343 hours (a little over two full weeks) the rads decrease by 99.9%. Ground Zero of Hiroshima today is so safe, it's a tourist attraction.

It doesn't seem unrealistic to me. If the bomb is of a futuristic technology, which you have created, it doesn't seem unlikely that it would behave differently from atomic bombs of the past. This suggests that the nuclear fallout of out time is no longer fatal between 1-5 years after effect. (Which doesn't necessarily mean SAFE, but just not deadly.) I think your futuristic technology could easily account for differences in time spans.

  • Thank you, I still haven't planned how it should be the technology before the war, for now I've planned the discovery of a new chemical element, which would be used in the making of "clean" nuclear power. And we all know how it would be used x) I also planned the historical background but I guess it's off topic on this question. – Dany Raven Aug 10 at 7:49
  • I would maybe also recommend a new nuclear element. We know the decay rates of current elements, discovering a new radioactive element on the periodic table and making "clean bombs" out of this might easily account for the differences from known radiation today. – Sarah Stark Aug 10 at 7:52
  • @DanyRaven No... no no no.... you do not need to invent any new tech. What matters is not what kind of bomb it is (unless it is made deliberately "dirty") but where it is detonated. air bursts are very "clean" and will not produce much fallout at all. – MichaelK Aug 10 at 7:57
  • @michaelK thank you, but I've already planned the new element story because most of the subplot is based on that ;) – Dany Raven Aug 10 at 8:05

Oh, this cafe is closing in 10 minutes so I don't have time for a proper answer ...

How long depends on the half-life of the radioisotopes formed -- or the sort of time-averaged half-life, since there will be many. Very short half-lives create intense radiation, but don't last long. Very long half life isotopes can last centuries, but do relatively little damage -- unless there is a biological mechanism that concentrates them inside your body.

It is possible to design a thermonuclear bomb so that it creates high levels of the worst kind of isotopes over very wide areas (up to a thousand or so miles downwind, in the case of a multi-megaton device.)

However in the case of "clean" thermonuclear bombs that you specified, there isn't actually a very large amount of the most dangerous types. There is some, sure, but depending on height of burst and weather, it will tend to be either concentrated near ground zero -- avoid that spot, but elsewhere is ok -- or very widely dispersed -- in which case no-one place is extremely dangerous. (Cancer rate increased worldwide, but no-one is fallign down dead in a matter of weeks.)

In brief: it is very unlikely that they have to wait years. In most places, usually a couple of weeks to a month will be sufficient. This doesn't mean it will be safe to eat root vegetables that quickly, but to walk around (maybe with a dust mask on) is perfectly survivable.

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