The influence of some rare earth elements as neutron absorbers on the inception of Oklo natural nuclear reactors.

This article focuses on modeling the criticality conditions of the natural nuclear fission reactors found in the Oklo region of Gabon, specifically examining the influence of initial neutron absorbers—referred to as initial Poisons (IP)—composed of rare earth elements gadolinium (Gd), samarium (Sm), and neodymium (Nd). Using the MCNP5 code driven by Python scripts, the study constructs isocritical lines representing the infinite multiplication factor as a function of uraninite volume fraction and water-filled porosity in a homogeneous U-rich ore matrix. Results indicate that Gd and Sm significantly affect the criticality inception of Oklo's reaction zones (RZs), with higher IP contents delaying criticality onset, particularly in smaller RZs like RZ9, compared to larger ones such as RZ2 and RZ10. This parametric approach provides insight into the varying operational histories of the fossilized natural reactors by linking their criticality conditions to realistic geological and compositional parameters.