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As I learned, nuclear fission doesn't occur without the control of a human made nuclear reactor, by hitting a neutron to a fissile isotope. Thus, the fission reaction is considedred as a part of 'ARTIFICIAL REACTIONS' category.

But, I've just noticed in a wikipedia article (Natural nuclear fission reactor), that nuclear chain reactions had occurred on Earth about 2 billion years ago, what proves that fission existed naturally before.

Another additional question I was wondering is: Is there any chance that a natural nuclear chain reaction can re-occur on Earth?

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    $\begingroup$ This question appears to be off-topic because it is about semantics rather than physics. $\endgroup$
    – Danu
    Jul 14, 2014 at 15:53
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    $\begingroup$ Would you consider fire to be natural or artificial? $\endgroup$
    – Emilio Pisanty
    Jul 14, 2014 at 15:55
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    $\begingroup$ Your title mentions fusion, but you completely ignore that part in the question? There are most definitely natural fusion reactors, as every star is one. $\endgroup$
    – Kyle Oman
    Jul 14, 2014 at 16:22
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    $\begingroup$ @YounesseBagachoul Natural fission reaction did occur naturally and it did not destroy anything but a number of uranium nuclei. Why do you think it would destroy "everything"? $\endgroup$
    – mpv
    Jul 14, 2014 at 16:26
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    $\begingroup$ And as we keep telling you, it is meaningless to continue discussing unless you provide precise definitions of 'natural' and 'artificial'. What makes it impossible for a phenomenon to be both natural and artificial? Do you need to make an exception for fire? What about dams? $\endgroup$
    – Emilio Pisanty
    Jul 14, 2014 at 16:42

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There isn't really a difference between "natural" or "artificial" reactions. All reactions are just "things that can happen". Some things only happen in certain circumstances, and those circumstances may be very unlikely to occur without being specifically engineered, but there is no reason in principle why they could not happen naturally.

There is evidence that there have been natural fission reactors on earth and there is no reason in principle why it could not happen again, although I don't think we know of anywhere that looks likely to go critical in the near future and as the earths uranium radioactively decays to other elements it becomes less and less likely.

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For a fission chain reaction to spontaneously happen in uranium there are some requirements to be fulfilled. A) there should be at least 4% U-235 in the mix of uranium isotopes. B) there should be a moderator to decrease the energy of the neutrons which result from the fission of a uranium core.

2 billion years ago the percentage of U-235 was large enough to sustain a chain reaction. Furthermore there was water seeping in the uranium which acted as a moderator. The heat from the fission turned the water into steam and stopped the chain reaction. This went on and off. So there was no continuous chain reaction for a billion years. Since then radioactive decay decreased the percentage of U-235 as it's half life is shorter than that of U-238. so no fission chain reaction will spontaneously occur in the earth crust.

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