The Natural Nuclear Reactor at Oklo: a Comparison with Modern Nuclear Reactors

Uranium contains only one naturally occurring isotope, 235U, which will sustain a nuclear chain reaction using normal water to moderate and reflect neutrons. At present, this isotope is present in low abundance (0.72%), requiring enrichment to 3% or greater for effective use in commercial nuclear reactors. Two billion years ago, however, the natural abundance of 235U was approximately 3%. Evidence indicates that a rich uranium deposit in Gabon, West Africa achieved nuclear criticality and operated for tens of thousands of years or longer. Comparing the geometric and nuclear characteristics of the Gabon reactor with those of modern, artificial nuclear reactors supports this possibility. An examination of rare earth elements and 235U abundance in the rocks that comprise the reactor zone confirm that a nuclear reactor did operate at this site about 2 billion year ago (Ga), using surface and ground waters to moderate and reflect fission neutrons in order to sustain the chain reaction. Finally, it is apparent that 239Pu was produced in measurable quantities, suggesting that uranium is not the heaviest naturally occurring element known.