[JimSEA]

In addition to the central question as to whether or not natural nuclear reactions were possible in the crust, this article includes a good discussion of isotopes, half lives and radioactive decay.

[JimSEA]

The article can also be read at Science 2.0

[WayneS]

The only known natural nuclear reactor in earth's history was found in the Oklo uranium mine of Gabon, Liberia on the continent of Africa.

Nah. It wasn't a natural reactor. It was built by aliens...

[yefragetuwrabrumuy]

“The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction. After cooling of the mineral deposit, the water returned and the reaction started again. These fission reactions were sustained for hundreds of thousands of years, until a chain reaction could no longer be supported.”

http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

[Rockingham]

There is also a theory that enough fissile uranium and other nuclear material is concentrated in the earth’s central core to fuel a natural nuclear reactor, thus producing the mysterious excess heat that helps to power the planet’s tectonic and volcanic processes.