NATURAL FISSION REACTORS IN THE FRANCEVILLE BASIN, GABON - A REVIEW OF THE CONDITIONS AND RESULTS OF A CRITICAL EVENT IN A GEOLOGIC SYSTEM

Natural nuclear fission reactors are only known in two uranium deposit s in the world, the Oklo and Bangombe deposits of the Franceville basi n: Gabon. Since 1982, five new reactor zones have been discovered in t hese deposits and studied since 1989 in a cooperative European program . New geological, mineralogical, and geochemical studies have been car ried out in order to understand the behavior of the actinides and fiss ion products which have been stored in a geological environment for mo re than 2.0 Ga years. The FrancevilIe basin and the uranium deposits r emained geologically stable over a long period, of time. Therefore, th e sites of Oklo and Bangombe are well preserved. For the reactors, two main periods of actinide and radionuclides migration have been observ ed: during the criticality, under P-T conditions of 300 bars and 400-5 00 degrees C, respectively, and during a distention event which affect ed the Franceville basin 800 to 900 Ma ago and which was responsible f or the intrusion of dolerite dikes close to the reactors. New isotopic analyses on uranium dioxides, clays, and phosphates allow us to deter mine their respective importance for the retention of fission products . The UO2 matrix appears to be efficient at retaining most actinides a nd fission products such as REEs, Y, and Zr but not the volatile fissi on products (Cd, Cs, Xe, and Kr) nor Rb, Sr, and Ba. Some fissiogenic elements such as Mo, Tc: Ru, Rh, Pd, and Te could have formed metallic and oxide inclusion in the UO2 matrix which are similar to those obse rved in artificial spent fuel. Clays and phosphate minerals also appea r to have played a role in the retention of fissiogenic REEs and also of Pu.