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Explanation of the so-called Oklo phenomenon can only be found by associating neutron physics and geological observations. The framework of Reactor Zone #9 (RZ9) from the Oklo uranium deposit in Gabon is discussed. A realistic model of RZ9 is proposed and studied with present-day numerical tools. Anchor points for this model are set by means of a selection of relevant parameters acting on geological conditions and neutron-transport physics, including ore homogeneity, initial reactor thickness, reflector effect and initial neutron poison content. Based on Monte Carlo simulations, the present modeling enables discussion of start-up conditions of the RZ9 reactor at optimal overlap of two opposing long-term processes: geochemical enrichment of the uranium content of the ore and its progressive impoverishment in 235U. On the basis of the present work, the inception and operation of Oklo natural reactors can be discussed from the largest to smallest initial core size, whereas before, only the largest ones had found some explanation.