Modelling heat and mass transfer around a natural fission reactor at Oklo

The 3D modelling has been used in quantification of thermal influence of a buried nuclear fission reaction on surrounding rocks. The Francevillian uranium deposit at Oklo (Gabon) is unique because it hosted the only natural fission reactor known to have occurred.

In order to quantify the thermal influence and the effects of water circulation induced by the fission reactor at Oklo, several numerical 2D and 3D models are used to evaluate the temperature field (plan of the study)

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• 2.5D Metric scale modelling of a functioning reactor

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• 3D Metric scale modelling of a functioning reactor

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• 2D Kilometric scale modelling of the Franceville basin

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Results and conclusions

• During the reaction, the temperature at the center of the reactor increases by 50 to 200 C above the regional one, according to the assumed heat production of the reactor and/or the duration of the reactions.

• The thermal perturbation around the reactive zone is local, extending to less than 40m

• The temperatures estimated by the numerical model are in good agreement with those obtained from nuclear fission isotopes and fluid inclusions.

• The filtration velocities are low (a few mm.yr-1) and show that the reactor cannot induce by itself general fluid circulation, and that its influence is very limited in space.

• Compared to other modelling methods (2D or axisymmetric), the 3D approach provides better numerical solutions for heat and mass transfers in complex natural objects.

• The setting up of reactor is very rapid (less than 10 y) .

• An external fluid circulation can cause a significant decrease of temperatures for fluid velocities greater than 1 m/y.