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I'm aware of the Oklo reactor and other natural nuclear fission reactors, in which geological processes can lead to the formation of a sustained, self-regulating uranium fission reactor.

Is it also possible for geological processes (on Earth or elsewhere) to form a critical mass of radioactive elements, leading to a runaway chain reaction and resulting in a nuclear explosion? I realise that nowadays the proportion of ${}^{235}\mathrm{U}$ in uranium deposits is too small to achieve criticality, but if it was large enough in the past for the Oklo reactor to form then that seems to imply that it was high enough for uncontrolled fission to occur as well.

Is it likely that such an event has happened on Earth in the past (on the surface or in the interior), and if so is there any known geological evidence that suggests an actual instance of it?

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  • $\begingroup$ I'm not sure that "explosion" is the correct term. Oklo was more similar to nuclear reactors and not to nuclear bombs. $\endgroup$ – Gimelist Aug 5 '14 at 9:12
  • $\begingroup$ @michael yes, and the question is whether it's also possible for something more similar to a bomb to occur naturally. Was that not clear from the question? $\endgroup$ – Nathaniel Aug 5 '14 at 10:56
  • $\begingroup$ Oh, missed that. Sorry! $\endgroup$ – Gimelist Aug 5 '14 at 14:10
  • $\begingroup$ Do supernovas count? $\endgroup$ – Spencer Jan 1 at 14:57
  • $\begingroup$ @Spencer I don't think so, because as far as I know the energy in a supernova comes from two things: (1) the loss of gravitational potential when the core collapses, and (2) nuclear fusion caused by compression of the core. (I'm not sure of their relative importance.) For this question I'm specifically interested in explosions caused by runaway fission reactions. If that does happen in supernovas or some other astrophysical context it would be very interesting to know about. $\endgroup$ – Nathaniel Jan 2 at 0:22
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This hypothesis has been studied here as a possible explanation of selenogenesis (formation of the Moon). (Forming the Moon from terrestrial silicate-rich material. R.J. de Meijer, V.F. Anisichkin, W. van Westrenen. 2013.)

The only place cited as suitable for spontaneous criticality is the Core-Mantle Boundary (CMB).

The calculations above show that without additional concentration factors, $\mathrm{U}$, $\mathrm{Th}$ and $\mathrm{Pu}$ concentrations in the CMB are insufficient to reach criticality. Additional concentrations can be achieved by a combination of two processes: growth of the relative concentration of the fissile materials by a transient pressure wave, induced by an impact at the Earth’s surface (Anisichkin, 1997; Voronin and Anisichkin, 2001), and/or the development of compositional heterogeneities (de Meijer and Van Westrenen, 2008).

As shown by Voronin (2011) for fissile material at the CMB, an impact of a 100km- diameter asteroid can create a transient pressure increase of several TPa at the CMB, sufficient to concentrate fissile material from a subcritical to a supercritical condition followed by a nuclear explosion. Regarding the development of compositional heterogeneities, it should be noted that small-scale heterogeneities exist in the core-mantle boundary region even today (e.g., van der Hilst et al., 2007): volumes exhibiting both higher-than-average and lower-than-average wave propagation speeds, with diameters as small as 30 km, are now resolvable. Some studies suggest that the bottom of the mantle is partially molten today, forming a so-called ‘basal magma ocean’ (e.g. Williams and Garnero, 1996; Labrosse et al., 2007; Lee et al., 2010).

For another take on natural nuclear explosions, see this paper (Evidence for a large, natural, paleo-nuclear reactor on Mars. J. E. Brandenburg, Orbital Technologies Corporation. LPI 2011 Proceedings.), citing an anomaly in Mare Acidalium on Mars.

It should be noted that nuclear explosions need both critical mass and very low times of reactivity insertion. A gun-type bomb would be the best analogy for low-yield natural explosions on the surface/in the crust, and it is very unlikely to have a naturally occurring acceleration hurl a uranium-rich boulder against a uranium-rich wall.

To detect natural explosions on the surface, one would need to look for fused silica and anomalous $\mathrm{Xe}$ and $\mathrm{Kr}$ isotope ratios at the margins of our instruments' sensitivities.

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  • $\begingroup$ +1, and will accept after giving others a chance to weigh in. The point about fast insertion is interesting - people like to comment on how unexpected the Oklo reactor was, but actually unregulated fission seems quite difficult. But I wonder whether an explosion could occur through another mechanism. E.g. a uranium-rich rock that suddenly gets wet and becomes critical because of water's neutron-moderating effect, for example. $\endgroup$ – Nathaniel Jun 23 '14 at 12:25
  • $\begingroup$ @Nathaniel - if water is outside the rock, neutrons leaving the surface will not come back to make even delayed criticality possible. IANABD, though (not a bomb designer). $\endgroup$ – Deer Hunter Jun 23 '14 at 12:45
  • $\begingroup$ Maybe if it was a porous rock or a pile of gravel, though. $\endgroup$ – Nathaniel Jun 24 '14 at 1:44
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    $\begingroup$ One might also add that the Oklo reactor was self-limiting through intermittent groundwater availability , and could never have achieved a sustasinable chain reaction, even with a much higher 235:238 U ratio. $\endgroup$ – Gordon Stanger Oct 28 '15 at 1:50
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All the evidences are that nuclear explosions could have happened in the mantle during the geological time. The best example being the Permian End Extinction. You can read my paper in:

https://periodicos.ufsm.br/cienciaenatura/article/view/35821

How is it possible to "calculate" about something that we don't know exactly the composition (earth mantle and other parts of the planet) and to say that it is "impossible"? how many things we have seen in Science that were declared "impossible"?

I leave the conclusions to those willing to read and think about the paper. Less math and more facts. In fact each time that I see another fact of the Solar System, it can be explained by this model.

If anyboy can find something that can be wrong or not good in the paper, I ask to write directly to my e-mail: sergiometh@gmail.com . I will glad answer the e-mail. In fact I need some crticism, because nobody has shown none hitherto.

Best regards,

Professor Sérgio Meth

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – gerrit Jan 4 at 12:14

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