[en] Nearly two billions years ago, fission chain reactions set up spontaneously in small portions of a uranium deposit in Gabon, and were sustained for sufficiently long periods for the isotopic compositions of many elements to be deeply modified. These natural reactors were found remarkably preserved. Through neutronic analysis, the operating conditions were reconstituted: during reactions, heat release turned out in convection streams, which dissolved quartz and so eliminated large amounts of silica, densifying uranium and deeply modifying geometry and composition of the rocks; reactors were mainly controlled by temperature which modified water density. 3 figs., 1 tab., 5 refs