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Thousands of years after a nuclear war that killed all life on the surface of earth, the descendants of the humans who fled to bunkers emerge...

How deep would these bunkers have to be to prevent radiation seep (radiation radiating down) from significantly harming the people in these bunkers? The radiation levels are just high enough to kill all life on the surface (except possibly cockroaches) fairly rapidly (1-2 hours). The bunker doesn't get a nuke dropped on it, and you don't need to worry about food or power. If lead or some other material would drastically help, feel free to include reasonable amounts of it covering the base (but not more than say, a few meters of it).

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    $\begingroup$ Less than you might think - 60-100mm of lead is likely to be enough. A couple metres of water - the same. $\endgroup$ – Tim Apr 10 '17 at 0:39
  • $\begingroup$ That's what the answers are telling me. It might need to be a bit more, because of water carrying radioactive particles seeping into the ground, but it wouldn't be the hundreds of metres I was envisioning. $\endgroup$ – Gryphon Apr 10 '17 at 0:43
  • $\begingroup$ you could perhaps look into the bunkers that the Swiss are required to build? $\endgroup$ – Tim Apr 10 '17 at 0:44
  • $\begingroup$ Never heard of these, could you link something? $\endgroup$ – Gryphon Apr 10 '17 at 0:46
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    $\begingroup$ Don't be taken in by the cockroaches' propaganda, most domestic cockroaches would be killed by a cold day if they weren't in a house. $\endgroup$ – Separatrix Dec 28 '18 at 14:13
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Comparatively not much, something around several meters thickness of rock/concrete (initial estimate was too high, thanks for Joe Kissling for the correction).

The currently worst thing humans have is the cobalt bomb: a hydrogen bomb with a casing of Cobalt-59. It could also be combined with a neutron bomb which has much better penetration for high-density materials (steel). The explosion of the bomb with the high neutron density transmutes the casing to Cobalt-60 which is a strong gamma radiator. It has a half-life of 5.2 years and to weaken the radiation to one half 11 mm lead is necessary. Having meters of small density (earth, stone) is sufficient to reduce radiation to normal levels, wet earth will also prevent neutrons to penetrate the shielding.

The problem is not really radiation seep, the problem is the radioactive dust that needs to be filtered out of the air and food. 5 years half-life will deplete your stocks, you need to produce food (probably using nuclear reactor and subterranean gardens).

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    $\begingroup$ The OP says thousands of years, so food issue must have been solved $\endgroup$ – Kilisi Apr 9 '17 at 21:00
  • $\begingroup$ 20 m seems overkill, even for Cobalt 60. Two meters of rock or concrete is enough according to this calculator. Even with outside rad sources a billion times greater than background levels. $\endgroup$ – Joe Kissling Apr 9 '17 at 21:15
  • $\begingroup$ @JoeKissling Thanks for pointing out the calculator. For 0.4 µS/hr (natural background radiation) as penetration and 10 S/hr contamination I get also 0.9 m of concrete, so I will adjust the value. Still I would advise to be cautious because the initial explosion will create a strong neutron flux which is far more penetrating than the gamma radiation, so I would err on the side of caution. $\endgroup$ – Thorsten S. Apr 9 '17 at 21:51
  • $\begingroup$ @ThorstenS. That initial neutron flux is a real killer alright but is only an issue of you are very close to the weapon when it detonates, OP said they did not have a nuke dropped on them. $\endgroup$ – Joe Kissling Apr 9 '17 at 21:54
  • $\begingroup$ the deeper measure may be better some of the radioactive dust will make it into a the groundwater and thus penetrate several meters unless it is solid rock not porous earth. $\endgroup$ – John Apr 9 '17 at 22:30
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You'll have to be a whole lot clearer about what you are asking about. "Radiation seep" isn't a defined technical term. Radon seeps up out of the ground. Radioactive (contaminated) water seeps from containment into the ocean. Radionuclei seep into ground water.

Even assuming you meant radioactive fallout, you haven't provided enough details of what type of radioactivity needs to "seep" down into their bunker. The deepest water I know of was found at a depth of 100 km. At 100 km, you'd expect the temperature to be 2500°C or 4500°F.

In other words, to get below where water might possibly "seep", you'd have to get to temperatures above what we could live at. (Temps increase about 25°C per km, which means we're not going to be able to live below 2 km. Perhaps you should reframe your question since it is the dose of radiation which causes damage and dose depends on concentration, type, and energy of the radiation.

By the way as you get deeper into the Earth, radon levels increase. Anyway, if we assume very long lived radioactive isotopes, with very energetic (lethal, damaging) decay modes, then the question naturally arises whether such isotopes will be transported downward at a sufficient rate to be a hazard to some sub-surface habitat.

Of course, the answer is it depends on the geology (and hydrology) at that specific location. There are several minerals which are natural barriers to water. Salt is one, another is certain types of clay. I suggest you take a look at the proposed design of the Yucca Mountain repository (defunct) because they attempted to design it to prevent water "seeping" in (and out). It would be a pretty good start for a design for your habitat's "shield".

You can't really build "a wall" (meaning thin (several feet, say) (sorry Mr. Trump)) to keep the water out. The barrier needs to be of sufficient thickness so that even if a earthquake moves a fault by several meters, the barrier is still sufficient to prevent significant seepage at the "crack". That's really not too difficult to do, lets just say 20 meters of clay and 10 meters of salt.

The trick is to keep this ceiling compressed so that cracks don't develop, (salt is self-repairing). You'd have to be careful in digging the tunnels to live in. Caves might not be the answer. You want to keep the barrier under compression and caves, even if the rock ceiling is strong enough to support the material above it, could cause decompression (as well as other types of movement). You could ask a question on the Engineering on a different forum. Anyway, I suspect tunnels rather than big caves would be safer, but it's pretty much a wild-assed guess. For what it's worth.

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  • $\begingroup$ My calculations suggest around 38cm of clay is enough to prevent water getting in. m.wolframalpha.com/input/?i=10%5E-9cm%2Fsecond+*1000+years+in+m&x=0&y=0 - 20m is overkill. This doesn't account for faults, but I think clay is pretty good at sealing cracks up. You also only need around 12mm of lead to block half the gamma radiation (so 60mm would block 97%). All in all, this bunker doesn't really seem that hard to construct. $\endgroup$ – Tim Apr 10 '17 at 0:31
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You'd need to create a bubble of clay, concrete, lead, steel corrugated, and some flexible absorbents around your bunker, then make sure something is built on top of it to keep the soil compressed all of the years you are planning to stay there. The main feature you must include is a way to seal the bunker from the outside COMPLETELY and then unseal it when it is safe. This is most difficult to do if you need to reach the surface for the air or water that you want to purify. If you're rich you might as well build a "mole people" drill apparatus to tunnel to the surface. I'd make it 20-50 ft below ground in case the topsoil gets blown away and to keep from detection. Try to build close to a lake and away from fault lines

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Radiation doesn't really "seep", and unless somebody is being very nasty, there isn't enough of it to require hiding in bunkers for decades.

Radioactive materials can be liquids, solids, or gases. However, the vast majority of radioactive fallout is in the form of a somewhat coarse grit -- that's what makes it fall out. This grit falls near the places the nukes fell -- it's basically in a shape of the mushroom cloud, smeared downwind. There will be lots of it directly downwind of where each bomb fell for several miles, and none at all in places that were not bombed. For the levels of fallout currently predicted from a nuclear war, bunkers would be only a few feet deep, and would not need to be carefully sealed or anything. They also would only need to be occupied for a few weeks, as the flip-side to high radiation levels is that highly radioactive material decays pretty quickly.

It will be important to keep radioactive grit and dust out of the bunkers.

The problem comes up if the enemy uses bombs that are deliberately "salted" to produce extremely high amounts of highly radioactive fallout. In this case, all of what I have described applies, except that the fallout is vastly more radioactive and longer-lived. Still, thousands of years would be quite a stretch. You would still have regions that were too radioactive to enter, and regions that were pretty much OK, or at least habitable.

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