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There are some pieces of media in which a city borders a wasteland that remains after nuclear apocalypses. The border is usually some kind of thick-tall-double-reinforced-anti-radiation-concrete™ wall.

enter image description here

Does it help?

My thoughts so far are below.

The city exists, so there was no impact there. It means that there are no explicit sources of radiation inside. It means the threat is from outside.

The worst thing that could happen is radioactive dust. The wall helps but just a bit. The wind can transfer dust to the city regardless of the wall.

Radiation can travel through contaminated water. A stream or rain, for example. Wall does not help.

Radiation can travel via building materials. Wall helps prevent people from obtaining such materials, but radiation is trackable. One can detect affected bricks or steel and discard them. If one can bring them in, then another can bring them out.

Radiation can travel via animals. Wall helps, but how much radiation exposure will it prevent? Not very much compared with dirty rain or dust storm.

Am I missing something or the wall should be much less fundamental?

Instead of a monumental obstacle, it should be more like a fence with barbed wire.

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    $\begingroup$ Zombies. Walls are very good protection against radiation-induced zombies. Don't forget the zombies. Zombies. $\endgroup$
    – JBH
    Commented Aug 22, 2022 at 1:56
  • $\begingroup$ @ooak, I think these are more like variables. Which wall™ makes more sense given the dust storm frequency and its radioactivity? Does contaminated water falling out of the sky affect effective wall height? How much does it cost in labor? $\endgroup$
    – FrogOfJuly
    Commented Aug 22, 2022 at 15:05
  • $\begingroup$ @FrogOfJuly A wall of any height and thickness is ultimately worthless for protection against contaminated dust. It might slow things down for gentle breezes, but any good wind (10mph or more) will bring dust over the wall just fine. The only way such a wall could help against dust is if it were combined with a very active effort to control the dust through replanting (e.g., weeds). $\endgroup$
    – JBH
    Commented Aug 22, 2022 at 16:13
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    $\begingroup$ You ask about keeping radiation out but for information your image is Mega City One from Dredd and in the sources the primary purpose of that wall is to keep mutants out. A secondary purpose is likely to keep the citizens in since it's harder to control them if they can easily leave and re-enter. It's not a "hard-science" setting. $\endgroup$
    – Eric Nolan
    Commented Aug 22, 2022 at 17:01
  • $\begingroup$ not unless the walls are unrealistically high, mountain valley are protected from much radiation because the mountains alter wind patterns. $\endgroup$
    – John
    Commented Aug 22, 2022 at 23:51

8 Answers 8

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The wall is not for the radiation!

You can't keep out radiation with a wall. Plus the wasteland is only a little bit radioactive these days. Makes your hair curly. No, that wall is because you have got good stuff in your city of survivors, like hotties, and food, and gasoline.

Out in the wasteland are desperate folks. Weirdos in metal masks on motorcycles. Mutants. Strange mystics with pygmy goats. People with cars that have spoilers so huge that they glide when they go over jumps. Yeah, those people.

road warrior

https://www.sceen-it.com/movie/196/Mad-Max-2

The wall is to keep them out and keep your good stuff in where you and your fellow curly-haired hotties can savor it undisturbed.

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    $\begingroup$ You're forgetting Mohawk-haired, knife-licking weightlifters, those can really crash your fabulous tea party ☕🏰. Little joke aside, you don't really answer about radioactivity protection, but at the same time, we have to remember that such walls can protect your anti-radioactive uniforms from unwanted burglars :). $\endgroup$ Commented Aug 22, 2022 at 12:31
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    $\begingroup$ Also mutant giant scorpions and things like that. $\endgroup$ Commented Aug 22, 2022 at 13:26
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    $\begingroup$ Uhh, a wall would have an effect, assuming the radiation is mostly stationary and not blowing around as dust. $\endgroup$
    – stix
    Commented Aug 22, 2022 at 18:23
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    $\begingroup$ @Tortilena - I was thinking maybe big ingots of radioactive material. Maybe carved into glowing statues. That would be a cool look for a video game. $\endgroup$
    – Willk
    Commented Aug 23, 2022 at 18:19
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    $\begingroup$ @Tortliena Absolutely. It depends on the local environmental conditions, not the least of which is the amount of rainfall, and whether any mitigation efforts were undertaken. $\endgroup$
    – stix
    Commented Aug 23, 2022 at 18:24
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Walls have never been useful against Mother Nature

It's certainly true that on a calm day with no breeze a thick concrete or lead-lined wall can have value. In that one case radioactive particles are sitting on the ground outside the wall and the emitted radiation would be stopped by the wall.

Now, I know we're comparing apples and oranges here, but to give you an idea of how little concrete alone is worth against radiation...

To reduce typical gamma rays by a factor of a billion, according to the American Nuclear Society, thicknesses of shield need to be about 13.8 feet of water, about 6.6 feet of concrete, or about 1.3 feet of lead. (Source)

Whether or not you need to reduce the radiation a billion-fold depends on how much radiation we're dealing with, which you don't explain. The odds are good that you don't need that much protection, but you're still needing to either line the wall with lead or have a stout, thick wall. Granted, if you put enough concrete in place, you'll eventually stop the radiation... but that's a LOT of concrete. There comes a point where the people of your apocalypse simply can't afford the wall.

But keep in mind, we're only talking about a calm day. On any other kind of day a wall is worthless.

That's because radioactive particles are carried on wind. And no matter how strong the wind, the particles being heavier than air, they fall. The greater the diameter of space the wall is protecting, the less valuable it becomes (or the higher it must be to make any difference whatsoever). You don't tell us how much area you're trying to protect, either.

But, because the time of radiation exposure is more important than the fact of radiation exposure, the question is what's easier, clean-up that you must do regardless of the wall, building a higher wall, or protecting individual buildings?

If you're trying to protect your people against radiation...*

...what you're going to have is a fundamentally underground society. From a suspension-of-disbelief perspective, you could claim that a forest of leaded glass skylights to provide natural lighting to your underground community. But underground it will be where it can be shielded with lead-lined rooftops.

Or, for the discerning homeowner, you could be half-underground, half-above ground with lead-lined (or unreasonably thick) walls and leaded glass. Thus, when the dust is blown in (and it will be blown in), it doesn't kill everyone. And yet that lovely view of the post-apocalyptic wasteland goes unhindered with your favorite Chardonnay.

To answer your question: No, it doesn't help

But that shouldn't stop you from using a frequently-used post-apocalyptic story trope. As I said in my comment: zombies. And people who might want to steal your lead.

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  • $\begingroup$ "about 6.6 feet of concrete, or about 1.3 feet of lead." In most cases, six feet of concrete will be vastly cheaper and easier to obtain than one foot of lead, so I disagree with your conclusion that the wall needs to be lead-lined. $\endgroup$
    – Skyler
    Commented Aug 23, 2022 at 14:35
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    $\begingroup$ @Skyler Wow! Did I make a mistake! I misread that quote to say "and" and not "or." Thanks for pointing that out! $\endgroup$
    – JBH
    Commented Aug 23, 2022 at 15:46
  • $\begingroup$ Quoting resistant to gamma rays is misleading. The only real danger is radioactive particles making it into the city, not rays being aimed at the city. $\endgroup$
    – Stef
    Commented Aug 24, 2022 at 9:34
  • $\begingroup$ @Stef, from my answer: Now, I know we're comparing apples and oranges here... and then I spend pretty much the rest of my answer talking about particles getting over the wall.... $\endgroup$
    – JBH
    Commented Aug 24, 2022 at 11:33
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Have three walls.

Have a tree wall.

Dust storms can be blocked by plants. Plants hold soil together with roots and block the movement of wind across land, making dust storms less effective. Trees are more resistant to radiation than animals, and this would serve as a first line of defense.

Have a concrete wall.

This wall stops raiders getting in, keeps civilians from wandering out, and most importantly serves as a mount for the third wall.

Have a soft wall.

You can use plastic tarp, leather, wood and oil, or a bunch of things. The key is that when there's a dust storm, or rad counters detect too much radiation in the wind, you cover up the city.

The concrete pillars serve as mounts allowing you to quickly pull up covers. This means that most dust will either blow over you, or accumulate on the covers where expendable poor people can clear it off later.

Most of the populace can hide behind thick concrete walls away from gamma radiation, while more risk taking people can ensure the covers stay strong and not much radiation gets in. They can pull back the covers and shake it clean it after the storm ends.

A large central pillar can ensure that most of the dust will slide off, and not accumulate on the soft wall.

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The wall is to keep people in.

As other answers have pointed out, a wall wouldn't keep radiation out, and walls generally are much more useful against people (or animals) than against nature. Of course, one way you could do your story is to have the radioactive wasteland full of zombies, or mutants, or reavers, and have the wall be protection for your citizens against them. But if you don't want to do that, here's another option.

If your wasteland is literally a wasteland, without ravening armies or monsters, then there's nothing to keep out, and the only reason to have a wall would be to keep people in. Now you might think, why would people need to be kept in? Wouldn't it be obvious that they shouldn't go out into the glowing wasteland? But in practice, people are stupid. If there wasn't a properly secure wall around the city, you'd have:

  • Adolescents venturing outside on a dare to impress each other. Who cares that there's a rule against going into the wasteland and they could die - it's cool to break rules and take risks.
  • People who don't believe in the danger of the wasteland. Scientists and authority figures all say you could die if you go out, but that's just fake news - what are they really hiding out there that makes them so desperate to keep people inside?

The second one of these could even be something to build a story around. Alternatively, the wall could actually be an unnecessary prison built by an authoritarian dictatorship to keep the people inside and under their control - that could also be something to build a story around.

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  • $\begingroup$ The second point is basically the plot of (spoiler) this movie. $\endgroup$
    – Trang Oul
    Commented Aug 24, 2022 at 11:19
  • $\begingroup$ @TrangOul Pedantic note, but that's a poor way to do spoilers, as I don't know what's being spoiled (hence don't know whether I've seen the film or not, whether it would be a spoiler for me or not) before actually receiving the spoiler :-) $\endgroup$ Commented Aug 24, 2022 at 12:42
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Only with an incompetent enemy

When the enemy were told to drop atomic bombs on the city they dutifully complied. However no one told anyone to turn the bombs on. So when they dropped they just made a thunk sound and crashed into the ground without detonating. So now your city is surrounded by a large number of highly radioactive objects that mostly kept themselves together (thus there is no dust to worry about, but maybe some small leaks in the casing). These are highly concentrated sources of radiation, so much radiation in fact that no one has been able to get near enough to them to move them away from the city.

One option would be just dig a hole, bury the bomb, then surround them in a lot of concrete, but again no one can get close enough. So people put big lumps of concrete/lead around the city, concentrated near each bomb site to cut the radiation that is being directed towards the city.

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You are exactly spot on

Anti-radiation concrete is useless against material exchange between inside and outside.

If the wall's radiation-absorbing properties are relevant, anybody going outside the city will need equivalent protection - probably more because they might get nearer to radiation sources.
So if you need a 20-cm concrete wall, anybody venturing outside will need a protective suit. Typical walled city near radioactive site stories don't follow that.
The suit would need to be the equivalent of 20 cm concrete if they plan to spend years of their lifetime outside. For occasional excursions, they'll still need breathing masks to protect them from inhaling radioactive dust.
Unless, of course, it's okay if the bold explorers typically die of cancers - again, not typical for the usual stories.

Walls don't protect from dust. Unless you make them so high that they prevent all air exchange, in which case a dome would be much easier to construct and maintain.
Animals are even harder to keep out, rodents and insects will use any hole you leave.
An airtight dome would be challenged to provide fresh air to inhabitants though; you'd need full climate control and air regeneration. Or some place from the outside that you can buy unpolluted air from, which would be an Achilles' heel for the city.

The most relevant effect of a wall would indeed be for keeping out other humans, or possibly keeping in our humans.
Depending on what kind of resources these humans have, fence & barbed wire might be enough or not.

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    $\begingroup$ "if you have a 20-cm concrete wall, anybody venturing outside will need 20 cm of concrete on every limb just to avoid getting irradiated more than in the city" Radiation damage depends on length of exposure as well as the strength of the radiation. It's totally plausible to want a very low level of radiation where people live every day but be OK with a higher level for a shorter exposure. $\endgroup$ Commented Aug 22, 2022 at 20:37
  • $\begingroup$ Mmm... right, you can get away with less protection when departing. $\endgroup$
    – toolforger
    Commented Aug 23, 2022 at 8:42
  • $\begingroup$ The dome doesn't need to be fully airtight, just to keep most dust out. Ideally the dome should be able to open and close; and meteorologists should predict when a more-dangerous-than-average cloud of radioactive dust is approaching. Then the dome can be closed when it is most dangerous, and open when it is less dangerous. $\endgroup$
    – Stef
    Commented Aug 24, 2022 at 9:37
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Actually for sandstorms, but the median citizen thinks its about radiation.

The old city was destroyed in the 5 hour war. Some of the suburbs survived and slowly rebuilt while the ecosystem collapsed around them.

With all the new deserts about the new city gets sandstorms. A big wall lets the sand all pile up in one place. Then it gets "swept" away using diggers. Maybe the typical citizen thinks the sand is kept out because it is radioactive. But its really more about the inconvenience of sand piling up everywhere and hitting you in the face when the wind blows. Voters are scared of the sand's supposed radioactivity and frankly a giant wall is a better vote-winner than a public information campaign. (Besides, what would the campaign say? "Don't worry about the sand, since the fallout hit the water table our municipal water supply is 1000 times more radioactive").

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The wall is to keep people from wandering outside because some people will disregard the hazard. Also future technology can have an invisible wall on top of the visible wall like a forcefield so the wall is there to remind people of old technology.

Even if the wall stops 1 person from going into the radioactive zone its worth it. No reference to fifth harmony.

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