For three decades, since 1942 to 1972 mankind thought it beat nature when the first sustainable nuclear chain reaction was created. All this changed in 1972 when French Pierrelatte uranium enrichment facility compared samples from the Oklo Uranium mine in Gabon, on the west coast of Africa. The U isotope concentration dropped form the normal 0.72% to 0.6%. This could have meant that some isotopes made it to the wrong hands and nuclear weapons or dirty bombs could be made. The French Commission for Nuclear Energy (CEA) began to investigate. What they found was no less than dumbfounding!
The CEA ran a series of tests only to find that the two most significant isotopes of the uranium mined at Oklo showed strange results. Finding more questions than answers, the CEA investigated the uranium ore in the Oklo mine and found concentration levels as low as 0.44%. The reduction in the U isotope is the result of the process in a nuclear reactor. Eliminating all the impossible, the only answer that was left, as improbable as it sounded, was that the mine operated as a natural fission reactor that generated self-sustaining nuclear chain reactions about Earth about 2 billion years ago. Further investigations found other natural fission reactors in the area.
Natural Fission Reactors
The first nuclear power plant went online in 1951. In 1956, five years after the first nuclear powered electric plant was erected, a chemist from the University of Arkansas, Paul L. Kuroda theorized that a combination of uranium and water could become a natural fission reactor, in earth’s early history.
While examining the concentration in the Oklo mine, it was discovered that the original uranium core had higher concentration levels of the Uranium-235 – the isotope ideal to fuel a fission reaction. Isotope u-235 naturally decays into thorium and releases a neutron in the process. The released neutron races towards another U-235 atom and a fission process is commenced.
One of the reasons fission reactions are controllable today are because of low concentrations of u-235 – 0.7% on average. In comparison 4.5 billion years ago, concentration levels were much higher and broke down as time went by. Scientists claim that at a concentration level above 3.2%, probability says a reaction will continue all on its own. Therefore, a key factor that made the reaction possible was that at the time, the fissile isotope U235 made up about 3.2% of the natural uranium, which is comparable to the amount used in some of today’s reactors. It is unknown if Kuroda ever imagined he would be able to view his theory in practice, but evidently, he’s a lucky scientist!
The Oklo, Okelobondo mines and Bangombe mines, located 35 k”m from the Oklo mines are the only known places that natural nuclear fission occurred. For almost 2 billion years it generated an average of 100 Kwh until the uranium 235 was exhausted and no more fission activity could take place. The reactor eventually slowed to a stop, leaving only a few traces behind that it ever existed – including the enigma of the “missing uranium” – the enigma that sparked the research!