Uranium Fission Reactor in Gabon Constructed Two Billion Years Ago
The following is from page 181 of John Lamarsh's text: Introduction
to Nuclear Engineering(Addison-Wesley Publishing Company -
Reading, MA 1983)
... If a canister holding either a whole fuel assembly or solidified waste should disintegrate, even soon after its emplacement in a repository, there is good reason to believe that the fission products and TRU nuclides would not diffuse far into the environment. Strong support for this contention is furnished by what has become known as the Oklo phenomenon. Oklo is the name of a uranium mine in the African nation of Gabon, where France obtains much of the uranium for her nuclear program. When uranium from this mine was introduced into a French gaseous diffusion plant, it was discovered that the feed uranium was already depleted below the 0.711 w% of ordinary natural uranium. It was as if the uranium had already been used to fuel some unknow reactor.
And so it had. French scientists found traces of fission products and TRU waste at various locations within the mine. These observations were puzzling at first, because it is not possible to make a reactor go critical with natural uranium, except under very special circumstances with a graphite or heavy water moderator, neither of which could reasonably be expected to have ever been present in the vicinity of Oklo. The explanation of the phenomenon is to be found in the fact that the half-life of U235, 7.13E8 years, is considerably shorter than the half-life of U238,4.51E9 years. Since the original formation of the earth, more U235 has therefore decayed than U238. This, in turn, means that the enrichment of natural uranium was greater years ago than it is today. Indeed, it is easy to show (see Prob. 2.37) that about 3 billion years ago this enrichment was in the neighborhood of 3 w%, sufficiently high to form a critical assembly with ordinary water, which is known to have been present near Oklo at that time.
The relevance of the Oklo phenomenon to present-day disposal of radioactive wastes is that neither the fission products (identified by their stable daughters) nor the plutonium migrated from the Oklo site in the billions of years since the reactor was critical. ...