Evidence of fast neutron operating in Oklo.

This article focuses on the nuclear and geochemical characterization of natural fission reactor zones (RZs) at the Oklo uranium deposit in Gabon, formed about 2 billion years ago. It compiles and compares key nuclear parameters—such as neutron fluence, duration of operation, restitution factor of uranium-235 (^235U), and fission contributions from uranium-235 (^235U), uranium-238 (^238U), and plutonium-239 (^239Pu)—across several RZs, highlighting the unique features of RZ 13. Unlike most RZs where fission was dominated by thermal neutron-induced ^235U fission, RZ 13 shows a significantly higher contribution from fast neutron-induced ^238U fission, suggesting it operated under different conditions, possibly in a drier environment with less neutron thermalization. These findings are based on isotopic analyses of fissiogenic elements such as ruthenium, neodymium, and lutetium, providing insights into the nuclear reactions and geochemical behavior within these ancient natural reactors.