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As conclusion of this question it seems that under a dense atmosphere it would be a good idea to place the main city in mountains. Just there is the inconvenient issue that top of mountains lack flat surfaces, suitable for big cities. So the idea is to create proper flat surfaces by blowing up mountain tops on a mountain range to create a long flat surface near some river valley. (hydro power, water, transport).

Feasibility study:

  • when warhead size increases it derives higher and higher share of its energy from fusion, thus it becomes relatively cleaner and cheaper (per megaton)
  • In Sedan test it was fine to walk unprotected at the bottom of the crater in less than a year
  • Krakatau volcano explosion, equivalent of 200 Mt caused a year without summer, but otherwise Earth climate was fine

So what would be the result for nearby landscape? To be precise, I especially mean:

1) If placing a nuclear warhead (In 50 Mt range) to cut the mountain top, what would be the shape of "crater"? (Would the cut be roughly flat? Or maybe concave like in normal crater? Or maybe convex, as the sides of the mountain offered less resistance to the explosion?)

2) For landscaping purposes what would happen to ejecta? (Would it mostly fall in to nearby valley, thus creating some dam? Would it overwhelmingly be spread all over the planet, thus leaving local valley practically unchanged?)

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  • $\begingroup$ "Top of mountains lack flat surfaces": Tibet? Altiplano? $\endgroup$ – AlexP Sep 7 at 12:12
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    $\begingroup$ I think you would be better off using precision meteor strikes rather than nukes to flatten your mountain tops. Not only would you then be free of the radiation issues but the material that the meteors are made of could contribute to the surrounding environment. For example, if the meteors were water ice, they would convert the valleys below the now flat mountain into lakes. $\endgroup$ – Henry Taylor Sep 7 at 14:24
  • $\begingroup$ Why do you think big cities are necessary and/or desirable? (And define "big".) Why do you think mountains are unsuitable for cities? Granted, they're much nicer without them (IMHO, anyway), but there are a number of Earthly cities & large towns built on mountainous or hilly places. $\endgroup$ – jamesqf Sep 7 at 18:08
  • $\begingroup$ @AlexP Yes, when researching it, that were exactly the 2 locations that I found. So as high altitude was not only criteria, it would means that it would be unlikely to be lucky enough to get such area in the right place. $\endgroup$ – Shadow1024 Sep 8 at 9:03
  • $\begingroup$ @jamesqf Big: an urbanised area with a few million people, with space to grow further. Why? Studies show that population density increases productivity, example: newyorkfed.org/medialibrary/media/research/staff_reports/… $\endgroup$ – Shadow1024 Sep 8 at 9:09
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when warhead size increases it derives higher and higher share of its energy from fusion, thus it becomes relatively cleaner and cheaper (per megaton)

Although this claim has popped up before, it isn't strictly true. Whilst there is a lower size limit for a fission device, it isn't necessarily possible to scale up a fission-fusion device to arbitrary sizes without adding additional fissile materials (eg. the U238 tamper or plutonium "sparkplug" in a Teller-Ulam device).

More importantly though, small clean bomb designs not only exist, but have in fact been used. Nuclear weapons were used for peaceful purposes as part of civil engineering works, with varying levels of success and fallout. The three bombs used as part of the Pechora-Kama Canal works were apparently the cleanest ever detonated, with 98% of their yield being from fusion reactions, and they were only 15kt each.

With regards to much larger bombs, note that a substantial proportion of the yield of a Teller-Ulam device can come from the U-238 radiation case and tamper, which is fissioned by the fast neutrons that come out of the D-T reactions in the fusion part of the explosion. In a classic 2-stage design, as much as 50% of the yield of the bomb comes from U-238 fission, and that is gonna yield a non-trivial amount of dirty fallout. The Tsar Bomba test used a lead tamper to reduce yield and fallout, which apparently halved its yield. Still, 50Mt isn't bad...

Additionally, as 80% of D-T and 40% of D-D fusion energy is released in the form of high energy neutrons (and as D-T is the more energetic reaction it is preferred in a weapon, and the neutrons it emits are particularly high energy) you will always get a significant amount of neutron activation in surrounding matter. The amount of radioactive fallout generated by this process, and how long it will last depends very much on what the local landscape is made of. "Safe to walk on after a year" is not the same as "safe to live in" or perhaps more importantly for your use-case, "safe for children to play in and eat dirt because of course they eat a stupid amount of that stuff no matter how hard you try to stop them". The Taiga devices in the Pechora-Kama project produced a significant amount of Cobalt-60 through neutron activation from the fusion neutron flux, much of it from natural ferrous metals present in the soil. With a half-life of >5 years, you may have to wait a while for it to cool down.

Now, bear in mind that the Teller-Ulam design's biggest benefit is the fact that it can be miniaturised to fit in a MIRV warhead. A device intended for landscaping can be arbitrarily large, delicate and complex. You may find that you're able to use other, cleaner ways of producing a big fusion reaction that aren't suitable for weapons. Pure D-D fusion might be possible, and devices with a minimum of metal components (or only ones which don't transmute into dangerous isotopes under neutron bombardment) would be desirable.

Krakatau volcano explosion, equivalent of 200 Mt caused a year without summer, but otherwise Earth climate was fine

Volcanic explosions are not really comparable to nuclear explosions... temperatures are much lower, and energy is released over a longer period of time. I suspect that volcanic explosions will be vastly more effective at moving dirt than nuclear ones. This also makes them much more effective at causing serious climatic changes than nuclear weapons.

Also, don't dismiss the year without a summer. It caused massive worldwide upset causing significant agricultural issues, famine, social problems and polictical upheaval. It wasn't just a bit chilly between spring and autumn.

If placing a nuclear warhead (In 50 Mt range) to cut the mountain top, what would be the shape of "crater"? (Would the cut be roughly flat? Or maybe concave like in normal crater? Or maybe convex, as the sides of the mountain offered less resistance to the explosion?)

You may as well just use a bunch of little warheads scattered around in strategic locations, especially given the existence of small clean bombs. If they're well buried you may find that most of the fallout is contained anyway, and smaller warheads can produce much neater results. Getting a flat mountain top with one big mountain-smasher will be problematic, but a bunch of little ones will produce a post-blast environment that's a bit closer to what you want.

2) For landscaping purposes what would happen to ejecta? (Would it mostly fall in to nearby valley, thus creating some dam? Would it overwhelmingly be spread all over the planet, thus leaving local valley practically unchanged?)

You are in almost complete control over this, especially if you use lots of little warheads. Lots will fall back into the craters or voids left by the bombs exploding. With small enough bombs buried deep enough, you may get very little ejecta at all... lots of big landslides and cliff collapses and so on, but you want the energy to smash the mountain, not fling boulders halfway to the next country.

A bomb powerful enough to spread ejecta all over the world is not suitable for landscaping, and your neighbours (if they exist) will be very unhappy with you.

To follow on from L. Dutch's observation, "I am not that sure that the layer of heavily cracked rocks would make a good substrate to build a city upon" (which is very true), you'll have to spend some time for the debris to settle, and you're gonna want to squish it down and help bind it togther. Getting your heavy construction plant up to the top of a mountain is going to be a slightly larger logistical challenge than merely blowing it up in the first place...

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    $\begingroup$ This is great. Rather than post something else I want to add on the possibility of a trinitite finish - fusing your final layer to glass with a long slow heat. realclearscience.com/blog/2016/04/… $\endgroup$ – Willk Sep 7 at 18:27
  • $\begingroup$ Thanks, I did not not know this part that clean(ish) fusion bombs can be actually quite small. I was more thinking of blowing up the top and building on glassed remains, I'd have to think about the alternative approach of containing explosion underground and building on rubbles. BTW: While technically correct concerning Krakatau, in this case if someone has high enough tech, to be able to run such project, then the issue of reduced agriculture output in that year is barely a nuisance. $\endgroup$ – Shadow1024 Sep 8 at 9:23
  • $\begingroup$ @Shadow1024 in order to get glass, I think you need some suitable surface material... trinite glass for example formed because the test was done in a suitably sandy desert region. A pointy mountain is unlikely to have much sand to produce glass from (it'll wash off or blow away too easily). Other materials will produce other forms of post-nuke substances... "kharitonchiki" at the Soviet test site in semipalatinsk has been described as "vesiculated slag" rather than glass, for example. $\endgroup$ – Starfish Prime Sep 9 at 19:19
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The fireball of a nuclear explosion develops roughly along a spherical pattern, before it starts interacting with the shockwave bouncing back from surfaces it hit.

When the nuclear explosion happens underground, it produces a bubble of vaporized rock, surrounded by cracked rock. Once the vaporized rock cools down the pressure cannot sustain the load and therefore the bubble collapses.

This leads to a shape like the craters that we can observe in all the underground nuclear test grounds around the world.

If the explosion is not contained because the hole is too shallow we get a plume of dust and debris, rising in the atmosphere and being transported by the winds.

For building purposes I am not that sure that the layer of heavily cracked rocks would make a good substrate to build a city upon.

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