Monte-Carlo Based Numerical Modeling and Simulation of Criticality Conditions Occurrence in Natural Reactor Zone 9 in Oklo Deposit (Gabon)

The occurrence of the criticality with analogue modelled configurations of the fossil reactor zone n°9 (RZ9) from Oklo uranium deposit in Gabon is studied. The RZ9 and the other reaction zones 7 and 8, of the same deposit sector are characterized by a low thickness compared to other zones 1 to 6 and an important presence of organic matter with a lower U content. This makes the simulations performed in the past unable to explain the occurrence of that phenomenon in such zones. In the present work an extrapolation to 2 billion years ago as an initial state of the reactor is done using the MCNP Monte-Carlo based code. To make it more close to the geological reality experimental conditions and geometry, possible porosity, moderators and minimal Uranium content are considered. The minimal critical configuration is obtained by variation of a set of geometrical and physical parameters around the estimated composition of the reactor area. A set of simulations computing the corresponding effective multiplication factor keff, and reactivity are presented. The effect of organic matter as well as the influence on the criticality is discussed.