4

A monazite sand placer deposit rich in thorium dioxide is slowly crushed into a sedimentary rock by plate tectonics. Water in it acts as a neutron moderator which ensures the ore formation doesn't blow itself apart before it forms. For some reason, the water vanishes, so the neutron spectrum hardens — i.e. neutrons can be more energetic because there's no moderator to slow them. Spontaneous fission by the 232Th that makes up all the thorium in the formation can now breed fairly large quantities of 233U; previously, the water slowed the neutrons down into thermal neutrons, but now they can be 1+ MeV fast neutrons capable of fissioning 232Th. The 233U — which, unlike 232Th, can sustain a nuclear chain reaction — absorbs spontaneous fission neutrons from the 232Th, starting a runaway chain reaction among itself and eventually destroying the entire ore formation in a gigantic (~750 gigatons/~3.138E21 joules) nuclear explosion.

This is comparable in energy to a supervolcanic eruption (for reference, a Yellowstone eruption would be in the ~875-gigaton/3.661E12 joule range). Due to the extremely low purity of its fissile material, it's highly inefficient — perhaps 1 in 1 million parts of the 233U fission, as opposed to the 1 in 100 to 1 in 10 of early human-made fission explosives) — but its sheer size makes up for that lack of efficiency, with nearly 38,300 metric tons of 233U managing to fission before the ore formation blows itself apart.

The 1.7-kiloton Plumbob Rainier was totally contained only when detonated 899 feet underground. I feel confident an explosion 8+ orders of magnitude more energetic wouldn't fail to punch through 9 times more ground and therefore that radioactive fission products from this will enter the atmosphere. However, I have no idea of the relationship between nuclear detonation magnitude and resultant fission product radioactivity produced, nor the relationship between impact energy and the mass of dust kicked into the atmosphere — a significant consideration, given the amount of energy involved here.

With this in mind, the question is: how bad is this going to be for the environment/biosphere?

There are many factors to consider:

  • the initial shockwaves — expressed both as atmospheric overpressure and via earthquake
  • debris rainout
  • ash and dust injected into the atmosphere; if you can, provide a quantity in tons so I can compare it with the Chicxulub impact
  • radionuclides injected into the atmosphere; if you can, provide a quantity in becquerels/curies/etc. so I can compare it with Chernobyl

Assume this is detonated on Earth. Where on Earth? I'm not sure it matters, this thing is going to be a problem for all of the Earth. If it does matter, assume it pops off right in the middle of Kansas City.

Bonus: how quickly afterwards would it be possible to safely colonize the multi-tens-of-kilometers-wide crater I assume would be left over? The radiation would die down within the year — the Sedan crater proved that much — but what about the heat?

KEY_ABRADE
  • 12,810
  • 2
  • 31
  • 92
  • The mass for a nuke should go supercritical in a very small amount of time, and also even while subcritical, the main deposit will heat up, effectively turning itself into a geyser base, especially when close to surface and not as dense to instantly go supercritical. So the result of that nuke will be far smaller even if initially pressurized. – Vesper Jun 11 '23 at 05:55
  • @Vesper If the main deposit heats up, even while subcritical, it'll get rid of the water that's acting as a neutron moderator, which will result in more and more fission. The ore formation heating up isn't, as far as I'm aware, going to affect its fission properties. Yes, a nuke should go supercritical very quickly, but don't think of this as a successful, human-made nuclear bomb. Think of it as a nuclear fizzle on a giant scale. And, regardless, even if this is somehow physically impossible, I'm more interested in the ecological effects if it were, rather than whether it is or not. – KEY_ABRADE Jun 11 '23 at 06:00
  • You claim that size of the deposit trumps low concentration of U-233 in it. I say a no, as all other materials can also capture neutrons produced by U-233's fission, leading to less than 1.0 effective fission coefficient, water or not. I say it won't blow up but instead generate extra heat, as both alpha and SF of components release energy, so eventually the rock below that deposit would melt and the deposit get dissolved in the mantle. – Vesper Jun 11 '23 at 06:05
  • https://worldbuilding.stackexchange.com/questions/143606/a-list-of-worldbuilding-resources has an extremely comprehensive book called 'The Effects of Nuclear Weapons' by the US DOD, I suggest perusing it. It's halfway down the page in the military category. –  Jun 11 '23 at 06:06
  • 1
    @Vesper Then maybe the percentage of thorium dioxide is higher, so that more U-233 can be made and there are fewer non-fissionable atoms to get in the way of the reaction. Or maybe the increased heat provides the activation energy for a chemical reaction that gets rid of the non-thorium/uranium elements. It's not about whether the nuke is realistic, it's about what its effects are. – KEY_ABRADE Jun 11 '23 at 06:08
  • P252-253 has information on calculating subsurface blast crater size and P409 has a little on the radioactive dust generated. I'm sure there's a lot more in there. –  Jun 11 '23 at 06:11
  • @AncientGiantPottedPlant The "SHALLOW UNDERGROUND EXPLOSION PHENOMENA" (in terms of what it essentially is) and "DEEP UNDERGROUND EXPLOSION PHENOMENA" (in terms of depth) sections are semi-relevant here, but this is 6-8 orders of magnitude larger than any nuclear detonations outlined in those sections. Some extrapolation can be made from those, but in terms of energy, this is more like an impact event or supervolcanic eruption than like a human-made nuclear bomb. When your main cloud touches the edge of space and the crater reaches into the mantle, some of that book probably doesn't apply. – KEY_ABRADE Jun 11 '23 at 06:21
  • @AncientGiantPottedPlant The curves on pages 255/256 get me a scaled burst depth of 17.84 feet, a apparent crater radius of something like 95 feet, an actual crater radius of ~43,676 feet, an apparent crater depth of perhaps 50 feet, and an actual crater depth of ~22,987 feet. The data used for these curves likely doesn't include any on how nukes affect rock ~23k feet underground, unless it's data from the Kola Superdeep Borehole, which I doubt the Soviets were sharing. The fallout information's more useful, but focuses on dosage, not becquerels/quantity of radioactivity, as far as I can tell. – KEY_ABRADE Jun 11 '23 at 06:33
  • @AncientGiantPottedPlant Note that the dry hard rock (what I think is the best option here) curves for apparent crater radius/depth trail off to zero long before they reach a 200 foot depth of burst. When plugged into the actual crater radius/depth equations, they get zeroes. However, I don't think a 750-gigaton explosion only 2.5 kilometers underground wouldn't leave a crater, and the curves being for 1-kiloton detonations backs this up: their data's for explosions nearly 9 orders of magnitude less than what I'm referring to. As such, none of the math I did in the previous equation applies. – KEY_ABRADE Jun 11 '23 at 07:14
  • @KEY_ABRADE I mentally transmuted your GT to MT originally. Any source you could possibly get, even if you can get them, will surely then be purely modelling and not based on anything empirical, given that the Tsar Bomba was 50 MT. I suspect you may be in the 'no one knows, make it up' zone, but let's hope someone really knowledgable proves me wrong. Maybe forgetting about nukes and looking up the effects of asteroids, etc, might be a better bet. –  Jun 11 '23 at 07:16
  • https://www.youtube.com/watch?v=Y3vPbvngero Suggests that a few GT was considered a Doomsday device. Teller thought a 10 GT device would incinerate all of one medium sized country, e.g. France or Germany. So rule of thumbing hard, multiply by 75, then divide by whatever factor it being subsurface makes. –  Jun 11 '23 at 07:20
  • @AncientGiantPottedPlant My current plan without better information is to extrapolate the diameter/depth of the Chicxulub, Barringer, and Sedan craters based on their estimated yields. This gets me rough estimates on crater size — where I got the "crater to the mantle" thing — as well as mass ejected. The nuclear and seismic parts are issues; the Atomic Rockets "Boom Table" claims this is less than the 2011 Japanese earthquake. As for the video: yes, I've seen Sundial, Backyard, GNOMON all of those — impressive, but there are different types of doomsday and this is a more thoroughly doomy one. – KEY_ABRADE Jun 11 '23 at 07:24
  • VTC:Needs More Focus, here's why: Most of the coments are focusing on your backstory, not your question. This means that all the cool detail is actually a distraction from your question. This has the effect of your question needing more focus. However, there's also the problem of some of the details you do have. The United States is ~4,600 km wide. You mention a crater "multi tens-of-km" wide. The *diameter of the earth* is only 12,742 km wide. You're talking about a crater bigger by multiples than... (*Continued*) – JBH Jun 11 '23 at 18:49
  • ... an entire hemisphere of the Earth. That makes answering your question 100% trivial: the event is 100% terminal for all life and the entire ecology of the earth. You probably broke the planet into pieces. I'm sure that's not your goal. However, there are differences of opinion about the effects of, per your example, a Yellowstone eruption. The USGS think's it's non-fatal, the NYT thinks it is. In other words, ... (*Continued*) – JBH Jun 11 '23 at 18:52
  • ... there's no consensus about what the ecological effects of an explosion like this would be other than from a temporary perspective. Basically, it can be as damaging as you, the author, wants it to be (due to narrative necessity) and nobody has the credibility to really gainsay that you're wrong. Knowing all this... and knowing that how a super explosion would effect planetary ecology violates the [help/dont-ask] book rule... what's the specific problem you need help with? – JBH Jun 11 '23 at 18:55
  • 3
    @JBH The comments split between "this seems unrealistic" (to which I said this is about the ecological effects, not its possibility) and a reference to a source (which I discussed with AncientGiantPottedPlant, that source can't accurately describe an explosion this large/deep). Moreover, a crater 30, 40 kilometers across isn't larger than a hemisphere of the Earth. As for narrative necessity: you could argue that for anything; such arguments make up the bones of soft sci-fi. I'd still rather have a rough guess for fallout/ash quantities/etc. than leave this 100% ungrounded in reality. – KEY_ABRADE Jun 11 '23 at 19:18
  • 2
    The question is well defined, with a unique answer (albeit it appears an unknown one so far). VTC because of an objection to a defensible premise is unfair, it's a 'pedant's veto'. –  Jun 11 '23 at 22:48
  • My sincere apologies! I not only read "multi-tens-of-kilometers" I even wrote it... and still my brain kept saying, "multi-tens-of-thousands-of-kilometers." Avoid getting old as long as you can. You still have the problem of providing too much back story that's distracting people, thus the question needs more focus. And you're still dealing with the fact that short of analyzing meteor impacts, there's no science we can draw on to conclusively answer your question short of narrative necessity. We can explain what will happen, but explaining how it will effect the ecology is opinion. – JBH Jun 12 '23 at 03:43

0 Answers0