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OKLO Natural Analogue for Transport Processes in a Geological Repository

Objective

The Uranium ore body in Oklo is a unique subject in the world, as natural fission reactions occurred there two billion years ago. It provides opportunities for the study of natural analogy with deep radioactive waste disposal, specially radionuclide mass transfer processes to the surface. However the peculiarities of this uranium ore deposit imply a thorough assessment of the radionuclides and daughters as well as ancient and recent geochemistry and hydrodynamics.
The uranium ore body in Oklo is a unique subject in the world, as natural fission reactions occurred there two billion years ago. It provides opportunities for the study of natural analogy with deep radioactive waste disposal, especially radionuclide mass transfer processes to the surface. This programme involves several different tasks:
in situ sampling, in close collaboration with the mining company;
study and characterization of the source term;
studies on the geochemical systems ruling the migrations;
Modelling.

The natural fission reactors are characterized by very high uraninium concentrations (from 20 to 60% by weight), closely correlated to uranium-235depletion (down to 0.265% of total uranium). Up to now, 16 numbers have been allocated to reactor zones in the Oklo mine, but some numbers (11, 12, 14) appear to be void. Some more, which have not been reached yet by the works, but are known by exploratory coring, are designated by their borehole number. The uranium to lead ratios have made it possible to confirm 2 important times in the history of the bearings: 1970 Mega-annum (Ma) for the fission reactions (zone 10) and 750 Ma for the major lead mobility episode, probably linked to the emplacement of the dolerite dykes.

The petrographical and mineralogical facies of the deep reactors zones are similar to those of the earlier discovered ones (1 to 9), apart from a few specific features, very much related to the lack of supergene weathering.

The study of the retention of fission products by phyllosilicate minerals has progressed when enriched uranium has been evidenced in the acid leaching residue of a clay sample from the roof of reactor zone 10.

The study of trace elements contents in hydrothermal minerals in the vicinity of the reactor zones has been undertaken, with the aim of using these elements as tracers of the fluids which were circulated in the bearings. The analyses are done mainly by neutron activation.

A diagramatic hydrogeologic al section and preliminary hydrodynamic modelling have also been proposed.
Work programme:

This programme involves several different tasks:

1 In situ sampling, in close collaboration with the mining company (C.O.M.U.F., COmpagnie des Mines d'Uranium de Franceville, Mounana, Gabon). To this part of the programme can be added the collecting of new data on the general setting of the Oklo mining area and deposits.

2 Study and characterization of the source term.

3 Studies on the geochemical systems ruling the migrations can in turn be divided into studies on the ancient migrations, contemporaneous to the rock diagenesis, to the nuclear reactions and to the general geological history of the basin, and on the recent to present migrations (tertiary to present?).

3.A The studies of the ancient migrations will encompass several subjects:

the petrography and elemental chemistry of the deposits.
the retention properties of the clays towards radionuclides.
the thermal history of the deposits in and around the reactor zones.
reconstructing the chemical characteristics of the hydrothermal fluids which left their chemical imprint in tracer mineral when they were circulated through the deposits.

3.B The study of the recent migrations will encompass the two following subjects:

the understanding of the hydrogeological conditions.
the hydrochemistry.

4 Modelling: 1st stage models and final integrated models.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Commissariat à l'Energie Atomique (CEA)
Address
Centre D'études De Fontenay-aux-roses 60-68 Avenue Du Général Leclerc
92265 Fontenay-aux-roses
France

Participants (4)

ASSOCIATION POUR LA RECHERCHE ET LE DEVELOPPEMENT DES METHODES ET PROCESSUS INDUSTRIELS
France
Address
Boulevard Saint-michel 60
75272 Paris
Centre National de la Recherche Scientifique
France
Address
1 Rue Blessig
67084 Strasbourg
Centre National de la Recherche Scientifique (CNRS)
France
Address
3 Rue Du Bois De La Champelle
54501 Vandoeuvre-les-nancy
Compagnie des Mines d' Uranium de Franceville
Gabon
Address

Libreville