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Backround

The setting is Kansas City, a few centuries after the apocalypse. The Worshippers Of Uranius, a band of 700 mutants, are the only inhabitants of KC, besides a few human slaves from out west. The Worshippers think that Uranius, a nuclear warhead that wasn’t deployed in the apocalyps, is a god, and they worship it with religious reverence.

The Worshippers remain neutral in the conflicts of the wasteland until year 2557, when the Midwestern Empire started their great conquest of the Great Plains. The Worshippers had only a few rifles and little ammunition, so the fight seemed doomed. But, the mutants remembered that they had Uranius on their side.

And so the ritual began

All 700 mutants gathered together in the church hall directly above the Uranius Silo. Every single one was on their knees, praying that Uranius would rapture them to heaven. As they were praying, two other mutants below, in the silo had just finished their last prayer, and a young novice handed them a metal box, which contained the activation codes for Uranius. The went to the holy bomb, and when the Imperial army approached the city, ready for victo.....KABOOM.

Uranius was a 50 megaton bomb, and the silo that contained it was 150 feet deep underground. My question is, what would be the effects of a bomb exploding that deep under the surface, with that yield?

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closed as off-topic by L.Dutch, Aify, Ash, CaM, Giter Jun 27 '18 at 20:15

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    $\begingroup$ That is a fairly well researched question: en.wikipedia.org/wiki/Underground_nuclear_weapons_testing but it is not wordbuilding. $\endgroup$ – b.Lorenz Jun 27 '18 at 19:23
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    $\begingroup$ generously sprinkling mutants doesn't make a worldbuilding question... $\endgroup$ – L.Dutch Jun 27 '18 at 19:24
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    $\begingroup$ Maybe you should have done some research first then. $\endgroup$ – Aify Jun 27 '18 at 19:26
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    $\begingroup$ @Justin Your contrast between mutants and humans is odd. A human with a mutation is still a human until and unless the changes are sufficient to make them a member of a different species. And calling a being who has some genes that mutated at some point in the past a mutant is a little odd since every single gene that you have has mutated many times since your ancestors were single celled organisms. PS I don't think that there is any record of any 50 megaton warheads on missiles in silos in Kansas. Officially Tsar Bomba was the only 50 megaton device ever built. $\endgroup$ – M. A. Golding Jun 27 '18 at 19:55
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    $\begingroup$ Hmmm... sounds like 3955 A.D. from a certain "Planet of the Apes" movie installment $\endgroup$ – Joe Jun 29 '18 at 17:49
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According https://en.wikipedia.org/wiki/Underground_nuclear_weapons_testing#Effects (thanks to b.Lorenz for the link) a contained underground explosion needs to be buried at a depth (in meters) larger than $100·{yield(in.kilotons)}^{1/3}$. Therefore, a 50 megaton bomb should be buried $100·{50000}^{1/3}=3684m$ deep to be contained, so it exploding in its silo should produce the same effect to its worshippers than if it wasn't underground.

The tiny 150 feet depth might reduce a little bit the effect of the bomb on the incoming army, but they are likely to be doomed, too.

It should be noted that 50 megaton is the yield of the largest bomb ever tested, the Tsar Bomba, which is so big that it couldn't be delivered by an ballistic missile and which is probably too big to have any practical military use. In fact, it was two times larger than the largest bomb ever produced by the United States. To keep your apocalypse setting realistic - as realistic as a war between mutants and humans in Kansas City can be -, I would suggest downsizing your bomb.

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  • $\begingroup$ But thats for a nuke burried underneath earth and rock. A nuclear silo is designed to survive a (near) hit of many types of nuclear weapons. It isnt designed for a nuke going off inside but it'll definitely change what happens. Considering the lowest yield of current American ICBM's is 170Kt and the silo likely protected against a blast like that so it might survive the 50Kt blast inside (but likely not). Question is, why is there only 50Kt in there? A stray tactical warhead ended up there? $\endgroup$ – Demigan Jun 27 '18 at 20:18
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    $\begingroup$ @Demigan no, silos do not provide protection from nuclear attacks. $\endgroup$ – Alexander Jun 27 '18 at 20:20
  • $\begingroup$ @Alexander I found one site saying it could survive a near hit and several that said it couldnt. Are there perhaps different types of silo's? $\endgroup$ – Demigan Jun 27 '18 at 21:32
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    $\begingroup$ @Demigan silos are sturdy, but nothing can withstand a direct hit - and explosion of a nuclear warhead inside a silo is an equivalent of a direct hit. $\endgroup$ – Alexander Jun 27 '18 at 21:51
  • $\begingroup$ Please notice that the yield in the question is 50 megatons, not 50 kilotons. Even if the silo was to support a 170 kt explosion (which I don't even think), a 50 megatons explosion is about 300 times larger. Therefore, the silo is not going to have a noticeable effect. $\endgroup$ – Pere Jun 27 '18 at 23:09
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Take a look at this website which has a huge amount of detailed information on sub-surface nuclear blasts. Plugging in plausible numbers from that, I get a crater about 150 meters in diameter.

The blast damage would probably kill people up to 1-2 km away. (I've seen a website with details on how to compute this for buried bombs, but I can't find it now. If I ever do, I'll come back and edit this.)

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