How many years should survivors wait before exiting a nuclear bomb shelter?

Question

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"

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.