Natural reactor

Natural Reactors refer to a handful of Uranium deposits that have been discovered, mostly in Oklo, Gabon. These deposits, first discovered around 1972, were noted to have an unusually low concentration of U-235. A total of fifteen natural reactors have been discovered in three different ore deposits in this mine in Oklo.

French physicist Francis Perrin was the first to declare that these deposits were the result of the natural fission of Uranium, causing much debate as to whether natural processes could spark the fairly complicated process of nuclear fission.

The current natural concentration of the U-235 isotope in naturally occurring Uranium is roughly 0.7 percent, with U-238 comprising the other 99.3 percent. Given the half-life of U-235, one can extrapolate that 1.7 billion years ago, the concentration of U-235 should have been roughly 3% -- enough to start a nuclear chain reaction given the right conditions. The Uranium uncovered in Oklo was discovered to have only half the naturally occurring amount of U-235, indicating some process must have reduced that amount of U-235, presumably fission.

File:Gabon Geology.gif

A diagram of the conditions in Oklo

Perrin claims that the right conditions did indeed exist in the mine in Oklo to carry out nuclear fission 1.7 Gy ago. Water seeped through the cracks in sandstone and acted as a moderator to the fast-moving neutrons emitted by the radioactive Uranium that would have otherwise simply dissipated. Once slowed down, these neutrons were then able to break apart other Uranium nuclei, and a chain reaction was established. Eventually, the heat produced would boil away the moderating water, and the reaction would creep to a halt -- until more water managed to seep down, starting the process anew. This self-moderating fission process could have gone on for a million years.

Sources

• Natural Reactor Fact Sheet
• Ancient African Nuclear Reactors, has a picture of the Uranium in the mine