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?
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.
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…
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