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My question here is if there is any circumstance, however unlikely, that could lead to explosive nuclear fission that was not set off by intelligent creatures. I'm working on some bizarre planets for my heroes to find, and I was thinking of one where nuclear explosions are an environmental hazard. Is that possible?

Say you've got a planet with extremely abundant uranium 235 (or whatever other nuclear fuel you require). Like huge boulders of it in ore strains in the ground and rolling around on the surface.

The geology, atmosphere, oceans, volcanic activity, size and structure of this planet are all flexible. The only constraint is that it must have a solid surface (it isn't a gas giant).

Could earthquakes, volcanoes, or meteor strikes set off the naturally abundant nuclear fuel, making subterranean or surface explosions? Any other unlikely but plausible scenarios for 'all-natural' nuclear explosions?

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    $\begingroup$ That's a difficult scenario. On a planet like that, natural fission will be running very high, making it a molten lava world. $\endgroup$ – Alexander Apr 4 '18 at 18:59
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    $\begingroup$ Also, as has been pointed out in some of the comments, if you have boulders of (near-pure?) U235 rolling around, everything under them has to be denser. Which doesn't lend itself to a) not having the entire planet be undergoing fission, and b) having your heroes ever arrive on the surface of the planet. $\endgroup$ – jdunlop Apr 4 '18 at 23:47
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    $\begingroup$ Not an answer to your question (hence not being in an answer), but you might want to just consider frequent bombardment by small-impactors. Same boom, far greater plausibility. A moderate-sized meteor that strikes the surface or breaks up in the atmosphere will result in the same scale of energy release as a groundburst/airburst nuclear weapon. $\endgroup$ – jdunlop Apr 4 '18 at 23:50
  • $\begingroup$ flexible? I imagine a planet with a solid surface which has an atmosphere consisting of pure hydrogen that is as big as the Sun ;D $\endgroup$ – user6760 Apr 5 '18 at 7:40
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Pretty much no. The reason is that you can't accumulate a critical mass slowly. It must be assembled quickly. How quickly? With pure U 235 (which is easiest, I believe of all isotopes to make go bang) the original Manhattan Project design was to use a big bunch of gunpowder (well, not gunpowder, though it might have worked, but another more convenient chemical explosive) to shoot what amounted to a sub-critical U 235 bullet into a sub-critical U 235 target. (bang) BANG!

This doesn't work with plutonium. If you tried the same thing, you'd get a fizzle because the subcritical chunks are being assembled slowly enough that the two chunks heat up and melt and go Poof! before they impact. (The problem is that when we make P 239 it is invariably contaminated with P 240 which produces enough neutrons spontaneously to set the thing off before it's fully assembled.) See https://en.wikipedia.org/wiki/Gun-type_fission_weapon for more.

It turns out that the natural Oklo uranium deposit in Gabon underwent a natural fission process a couple billion years ago. This was not a bomb, but it's estimated that it produced 100 kW of heat continuously for a few hundred kYears. See https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor for more.

The timescale of natural processes for concentrating uranium is thousands to millions of years. The time scale needed to get a Bang! when the mass goes critical is milliseconds. It's very hard to imagine a plausible scenario which bridges this gap.

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    $\begingroup$ Through the wiki link you gave, I've found the concept of predetonation, in which pieces of subcritical mass have a reaction before being fully squished together into a critical mass, and that predetonation reaction pushes them back apart. So if one of my proposed subcritical U235 boulders rolled down a hill and bumped another one, the reaction would be violent and push them apart, but nothing like a nuke going off. Am I getting that right? These boulders would have to smash together at crazy speeds is what you're saying? $\endgroup$ – Jared K Apr 4 '18 at 19:55
  • $\begingroup$ @Jared K: That's the way I understand it. (It's called a "fizzle" in the nuke business.) I suppose if one of the boulders was at the bottom of a very tall cliff and the other rolled down a slope, went shooting off the edge of the cliff, fell far enough get to up to a high enough speed, and then happened to hit the first lump perfectly, you might get an explosion. The necessary speed is probably between a few hundred and a thousand mph. $\endgroup$ – Mark Olson Apr 4 '18 at 20:06
  • $\begingroup$ Admittedly that does sound just a bit contrived. $\endgroup$ – Mark Olson Apr 4 '18 at 20:16
  • $\begingroup$ Could such a sudden concentration not be created by a shift in tectonic plates during a earthquake? $\endgroup$ – Daniel Apr 5 '18 at 16:55
  • $\begingroup$ It's pretty hard to imagine how. But maybe more to the point, remember that it's really REALLY hard to imagine how just-barely-subcritical masses of pure U-235 could form naturally... $\endgroup$ – Mark Olson Apr 5 '18 at 17:57
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You would need to concentrate a lot of reasonably pure uranium in a very little volume in a very short time, otherwise what you get is either a fizzle or a meltdown (most likely) or a slow, slow cooking like the Oklo reactor.

Pretty much the only way of obtaining the required condition of prompt criticality is smashing together several (at least two) pieces of fissile material to both increase the mass and its density. Temperature changes alone may achieve this (e.g. shifting plutonium from its beta to its alpha phase), but the likeliest result by far is a fizzle. That's one of the reasons why a nuclear weapon requires extreme precision in machining and operation, and preventing a nuclear explosion is relatively straightforward, if dangerous.

Also, if the uranium is present from planet birth, it will sink to the core of the planet and guarantee it'll remain a molten inferno for a very long time. And still no fissionables on the surface.

You need a Heavy Element Heavy Late Bombardment :-).

You could have some weird condition that brings about neutron reflecting elements and gets a "natural" Demon Core flash, but that's really farfetched too.

So, farfetched for farfetched... one even more contrived, but spectacular possibility is this:

  • a double star system, with a primary luminous star of about 7 solar masses and a 2.3 SM neutron star.
  • the system came about after a complicate merger of two smaller neutron stars
  • a planet, possibly once orbiting the primary star, is in orbit around the neutron star (it could orbit the primary too, but the neutron star would need to be farther out).
  • the planet has captured an appreciable quantity of the heavy element accretion plume once orbiting the neutron star.
  • it turns out that the neutron star magnetosphere is weirder than expected, and occasionally gives rise to colossal core eruptions. Instead of gamma ray bursts or X-ray showers, this neutron star actually ejects neutron plumes.
  • These were rarely detected by Earth astronomers because a free neutron plume decays before reaching a distance of a few AUs, so the resulting gamma signal would be very low luminosity.
  • But now, our planet is occasionally bathed in dense streams of neutrons. They both enrich and transmute surface ores, and occasionally detonate some of them. The effect would be immediate and catastrophic, and quite spectacular; you don't need fissile deposits, more common fissionable materials will do.

(One of the reasons this couldn't really work is that at those densities, the "weak" gamma ray burst from neutron decay is sufficient to sterilize the planet twice over. Not to speak of the neutrons themselves, which are no fresh breath of spring. But maybe... an even weaker breakout from a rapidly spinning neutron star...)

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Not exactly.

It won't ever be a boom like our nuclear weapons, more like a massive but relatively much slower release of enormous amount of energy. Even then, it's going to be very rare. It's also much less likely to happen on a planet which is stable enough for people to walk around on. For instance, one hypothesis for the formation of the Moon was a natural nuclear 'explosion' that ejected the material for the Moon from a rapidly spinning mass that was the Earth and Moon as one.

Of course, if you've got this much radioactive material on a planet, you're probably not expecting it to be very hospitable.

What needs to happen is for enough uranium to get together and go supercritical. There are some ways this can occur more easily that needing boulders of uranium rolling around to come in contact and reach critical mass. You can use the presence or absence of neutron moderator or neutron reflectors. For instance a large lump of uranium that would otherwise go into meltdown can be moderated by the presence of water. The water not only keeps things cool, it slows down neutrons, keeping them from slamming into the uranium and knocking off more neutrons. Dry out the water and the whole thing melts down. Alternatively you can add a neutron reflector and reflect neutrons back at the uranium to make it go supercritical (this is what happened with the Demon Core).

Of course, you can just cross the line of scientific accuracy and say that these sub-critical lumps of uranium are under immense pressure when the neutron moderator goes away, causing them to build pressure to the point that they explode violently.

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    $\begingroup$ That paper you reference regarding a nuclear explosion to form the Moon requires so much hand-waving that the authors could do a live-action Red Bull Gives You Wings commercial. Essentially it comes down to a "and then a miracle happens" level. I wonder if they couldn't find a geologist to co-author because they'd be laughing so hard. $\endgroup$ – Keith Morrison Apr 5 '18 at 3:00
  • $\begingroup$ @KeithMorrison I believe the advising professor on that work is a geologist. The paper was also accepted for publication in the journal of Chemical Geology. I found the paper from an article from the NASA Lunar Science Institute. This leads me to believe it's not as ridiculous as you make it out to be, but I'm not a geologist. Can you explain why it's so obviously ridiculous? $\endgroup$ – Samuel Apr 5 '18 at 3:16
  • $\begingroup$ (1) The paper postulates that the explosion was created by a sphere of uranium-thorium enriched rock anywhere from 60 to 210 kilometers in radius at an average density of 5500 kg/cubic meter, triggered by the pressure wave of a 100 km diameter asteroid impacting. $\endgroup$ – Keith Morrison Apr 5 '18 at 4:44
  • $\begingroup$ (2) First, they need a process to concentrate all that enriched material in one spot. Then they need to create a place for it to concentrate that is, at best, highly speculative. Then they need it to have the right shape so that a pressure wave doesn't cause only a portion to fission and the rest to fizzle. $\endgroup$ – Keith Morrison Apr 5 '18 at 4:53
  • $\begingroup$ (3) That's already a lot of things that have to work before this could theoretically happen, but then the killer for me is that somehow this only happens once, with zero evidence that it ever happened again, anywhere else, that anything like it has ever happened. Large impacts blowing stuff off planets, on the other hand, is something that's known to happen, that we have actual evidence of. $\endgroup$ – Keith Morrison Apr 5 '18 at 4:59
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Let's imagine we have few uranium rock with less than critical mass each, but enough mass in total. Put some mineral between them. It is better if this mineral absorb neutrons. Then put this mix under ground at high pressure.

To detonate this 'device' you need to melt said mineral and let surroundings absorb it. Now we have critical mass of uranium under high pressure.

Two problems here: I'm not sure that mineral with such properties exist and not sure that explosion will create enough pressure to overcome pressure of earth crust.

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Lets borrow some ideas from Robert L. Forward's Camelot 30K. Now, as with most of his aliens I see no way they could evolve, but lets go with something much simpler:

We have a spacegoing plant (It originally evolved in atmosphere and managed to make the transition as it's planet of evolution slowly lost it's atmosphere and then was destroyed by passing within the Roche limit of a gas giant.) It's seeds are incredibly tough, long-lived and radiation resistant. When one comes to rest in a sufficient (which would be quite low) gravity field the plant sprouts. There are plants that throw seeds but this plant took the idea much farther. It sent down a big taproot that is made out of an explosive. At the end of it's life cycle the plant dies, the root dries out and eventually detonates. The seeds are cast a considerable distance this way, sometimes even at escape velocity (the plant normally grows on carbonaceous asteroids.)

At this point a variation arose that used a block of metal in the root to direct the blast better. Denser metals work better, a version evolved that used uranium. At this point it encountered a very young asteroid belt containing uranium that has a far higher percent of U-235 than we have.

It still built the uranium blocks in it's taproot, but now it's actually possible for them to go critical. A critical mass does us no good but as it approaches criticality it gets warm, the root dries and detonates. If the blocks are arranged properly they are blown together and you get a nuclear yield. At first it will be small but it throws the seeds much farther than before (and dispersing it's seed is the big issue for this plant), it's a big reproductive help. Better bombs evolve as well as tougher seeds to survive the launch. Interstellar travel becomes likely rather than requiring quite a fluke.

Now, one of these seeds has fallen on the planet your heroes are exploring. You need fresh uranium and a decent amount of it but you don't need the sort of concentrations that others are talking about which would almost certainly cook your planet.

Now, trapped under an atmosphere the plant can't throw it's seeds offworld but they get dispersed.

Now, this plant was adapted for life in space and conservation is it's driving principle, it's life cycle is going to be slow and it's not going to look like a traditional plant. Your heroes will probably mistake it for a mineral formation until the biologists figure it out--and note that messing with a mature one is asking to get yourself nuked.

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    $\begingroup$ Honestly, I do not think your actual answer is that good but thinking about it, if you want the explosions to be an ongoing threat, there must be an ongoing process creating the conditions for the explosions. And much like with the free oxygen in atmosphere, a biological process really is the simplest solution. So you IMHO do have the right basic idea, I just can't bring myself to like the details enough to upvote... sigh. $\endgroup$ – Ville Niemi Apr 6 '18 at 9:02

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