The uranium deposits in the basin of Franceville (Gabon) host the
only natural fission reactors known in the world. Unique geologic conditions
favoured a natural fission reaction 2 Ga ago. This was detected by anomalous
isotopic compositions of rare earth elements (fission products) and uranium.
In total 16 reactor zones were found. Most of them are mined out.
A current research project of the European Commission concentrates on the reactor zone of Bangombé, which is only 10-11 m below the surface. This reactor zone has been influenced by surface weathering processes.
The migration of fission products can be traced by the anomalous isotope ratios of REE due to the fission process. The normal and constant ratio of 149Sm/147Sm is 0.92. The isotope ratio of 149Sm/147Sm close to the reactor zone is as low as 0.28 due to the intense neutron capture of 149Sm and subsequent decay. Similar changes in isotopic patterns are detectable on other rare earth elements (REE).
The isotope ratios of Sm and other REE of whole rock and fracture samples surrounding the reactor indicate that fission products migrated only a few centimetres above and mainly below the reactor zone. Organic matter (bitumen, kerogene) seems to act as a trap for fission products. REE-patterns show a less intense weathering with depth in the log profile.