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)
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
- 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.