# How Did Natural Nuclear Reactors Operate?

**URL:** <https://boards.straightdope.com/t/how-did-natural-nuclear-reactors-operate/454489>\
**Category:** Factual Questions\
**Created:** [June 27, 2008, 7:31pm UTC](https://boards.straightdope.com/t/how-did-natural-nuclear-reactors-operate/454489 "2008-06-27T19:31:36Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\
**Post date:** [June 27, 2008, 7:31pm UTC](https://boards.straightdope.com/t/how-did-natural-nuclear-reactors-operate/454489/1 "2008-06-27T19:31:36Z")

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I read that geologists had found evidence of natural chain reactions ocurring in the earth, just as in a man-made nuclear reactor. How did this happen? As i understand it, you have to have a enough fissionable material close together, to initiate a chain reaction. The release of neurons releases heat, and this cuases the material to expand,(or explode), thus ending the chain reaction. So how would a chain treaction take place in a natural deposit of uranium?

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**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\
**Post date:** [June 27, 2008, 7:35pm UTC](https://boards.straightdope.com/t/how-did-natural-nuclear-reactors-operate/454489/2 "2008-06-27T19:35:00Z")

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[Wikipedia article](http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor).

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**Author:** ![Simplicio](https://avatars.discourse-cdn.com/v4/letter/s/c37758/32.png) [@Simplicio](https://boards.straightdope.com/u/Simplicio)\
**Post date:** [June 27, 2008, 7:37pm UTC](https://boards.straightdope.com/t/how-did-natural-nuclear-reactors-operate/454489/3 "2008-06-27T19:37:50Z")

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It was only possible earlier in the Earth’s history when naturally occuring Uranium had a much higher concentration of the fissable isotope (248?). Water could work into the space between deposits and act as a moderator, and the stuff would fissile away.

Since then, the fissable isotope in Uranium has decayed down to much lower concentrations, so no more natural reactors.

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**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\
**Post date:** [June 27, 2008, 7:40pm UTC](https://boards.straightdope.com/t/how-did-natural-nuclear-reactors-operate/454489/4 "2008-06-27T19:40:07Z")

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I don’t know the specifics, but I believe that it was actually less of a sustained reaction, and more of a cyclic one – groundwater acted as neutron moderator (essentially slowing down emitted neutrons to thermal speeds, allowing them to be captured to induce fission), got heated up, evaporated/boiled away, the reaction stopped/slowed down, the water returned, the reaction started up again.

ETA: Too slow! That’s what I get for fact-checking…

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**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\
**Post date:** [June 27, 2008, 11:53pm UTC](https://boards.straightdope.com/t/how-did-natural-nuclear-reactors-operate/454489/5 "2008-06-27T23:53:14Z")

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[QUOTE=ralph124c]  
As i understand it, you have to have a enough fissionable material close together, to initiate a chain reaction.  
[/QUOTE]

[QUOTE=Simplicio]  
It was only possible earlier in the Earth’s history when naturally occuring Uranium had a much higher concentration of the fissable isotope (248?). Water could work into the space between deposits and act as a moderator, and the stuff would fissile away.

Since then, the fissable isotope in Uranium has decayed down to much lower concentrations, so no more natural reactors.  
[/QUOTE]  
Guys, the term is _fissile_. Fissionable material is capable of being fissioned by impingement by energetic neutrons, but only fissile material can sustain a chain reaction with slow (so-called thermal) neutrons or by spontaneous decay.

Other than that, **Simplicio** has it essentially right.

Stranger