ROLE OF ORGANIC-MATTER IN THE PROTEROZOIC OKLO NATURAL FISSION REACTORS, GABON, AFRICA

Of the sixteen known Oklo and the Bangombe natural fission reactors (h ydrothermally altered clastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals), reactors 1 to 6 at Oklo conta in only traces of organic matter, but the others are rich in organic s ubstances. Reactors 7 to 9 are the subjects of this study. These organ ic-rich reactors may serve as time-tested analogues for anthropogenic nuclear-waste containment strategies. Organic matter helped to concent rate quantities of uranium sufficient to initiate the nuclear chain re actions. Liquid bitumen was generated from organic matter by hydrother mal reactions during nuclear criticality. The bitumen soon became a so lid, consisting of polycyclic aromatic hydrocarbons and an intimate mi xture of cryptocrystalline graphite, which enclosed and immobilized ur aninite and the fission-generated isotopes entrapped in uraninite. Thi s mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1.2 b.y.