Could a nuclear explosion occur naturally on a planet with crazy enough (but still possible) conditions?

Question

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?

Answer 1

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.

Answer 2

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...)

Answer 3

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.

Answer 4

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.

Answer 5

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.