GOLDSCHMIDT CONFERENCE TOULOUSE 1998

number of natural fission reactors. These are found within a uranium mineralisation situated between two sedimentary formations in the Franceville series. The reactors were first discovered in the Oklo mine in 1972. The fission reactions have subsequently been dated to 1980+0.05 Ma (Ruffenach, 1978), and were in operation for several hundred thousand years (Hagemann, 1975). The reactors have been investigated intensively since their discovery, partly due to the excellent opportunity ostudy the behaviour of radioelements in a natural environment asa natural analogue for a high-level radioactive waste repository. A recent review of the studies is given by Gauthier-Lafaye et al. (1996)