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The Natural Nuclear Reactor At Oklo: A Comparison With Modern Nuclear Reactors, Last revised 4/16/05 Page 2 of 9 The absence of 236 U (t 1/2 = 2.342 x l0 7 a) indicates that induced fission stopped at least 10 8 years ago. Dating of the strata in which the reactor is found indicates an age of approximately 1.8 Gy (Cowan, 1976). At that time, 235 U had an abundance of approximately 3%, owing to its shorter half-life relative to 233 U (t 1/2 =7.04 x l0 8 yrs and 4.468 x l0 9 yrs, respectively). This paper discusses briefly the geologic setting of the Gabon reactor and compares its nuclear characteristics with those of modern, manmade nuclear reactors in terms of fuel loading, geometry, neutron flux, power, and uranium enrichment. The Gabon Reactor The Gabon reactor consists of several mineralized zones of uranium minerals in sandstone and conglomerate. The uranium probably originated in nearby igneous deposits, dissolved in oxygenated surface waters, and was deposited at an oxidation-reduction front (similar to the roll- front deposits in Texas). The mobilization, therefore, probably did not commence until there was sufficient free oxygen in the earth's atmosphere to allow oxygenation of surface waters (about 2 Ga). The formation in which the deposits reside was deposited about 1.74 (+0.20) billion years ago (Ga). At this time, 235 U had a relative abundance of approximately 3%. According to Neuilly et al, (1972), "the deposit is stratiform. It is located within the sandstone which forms the basis of the Francevillian (sedimentary basin). The sedimentological characteristics of these sandstones suggest fluviodeltaic deposit conditions. Formed essentially of detrital quartz with some accessory feldspars, they have a cement consisting of secondary silica, phyllite, and organic matter (of asphaltic type). The uraniferous mineralization consists primarily of oxides (uraninite, pitchblende). It occurs in the cement which it may almost totally replace in zones bearing the highest grade uranium ore. It is associated with a few sulfides (pyrite, galena, etc). "The thickness of the mineralized layer ranges from 5 to 8 meters. The layer is locally affected by a tectonic movement whose activity served to uplift the sandstone formations bordering on an elongated depression in the sole. The uraniferous mineralization and any redistributions it may have undergone as a result of groundwater flow, appears almost contemporaneous with the sedimentation of the Francevillian." A total of 16 reactor zones have been found in the primary location, and another reactor zone identified about 20 km away (the Bangomb reactor). Current thinking is that a zone of petroleum formation produced an oxygen-free zone because oxygen in groundwater would have been removed by chemical reactions with the petroleum.,