Project information

Oklo Natural Analogue Phase II project: Behaviour of nuclear reaction products in a natural environment

Financing institution: CE-FI4W-CT96-0020

Participating institutions: Geological Survey of Sweden (SKB),CEA, ANDRA., ENRESA

Duration: 1996-1999

Project description

Key words

Natural analogue, uraninite, Oklo, radionuclide, migration, reactive transport

Summary

The Oklo site in Gabon represents a unique geological environment where nuclear reaction products have been naturally introduced and still can be detected in fossil reaction zones. Previous studies have shown good chemical analogies of reaction zone materials to a spent fuel repository. Although the site geology is different from that of a deep repository, Oklo is an ideal location to test predictions relating to reates of movement of various radionuclides, including fission products.

Our group is working to build conceptual models of processes related to radionuclide migration/retention and processse regarding the radionuclide water transport. We use reactive transport modeling to test the feasibility of the conceptual hypothesis.

Results: Okelobondo Uranium Deposit

Salas J. and Ayora C. (2004) Groundwater chemistry of the Okelobondo uraninite-deposit area (Oklo,Gabón): two dimensional reactive transport modelling. J. Contaminant Hydrol. 69: 117-137.

Observation:
There is water dissolving UO2 at repository depths

Problem:

Could this situation be found elsewhere in the Earth Crust?

Geological section including the Okelobondo uraninite deposit, a remain of a former fision activity (2,000 Ma ago). Some deep water samples contain more uranium than the suficial ones.

Why UO2 dissolves at depth?:
Type I water: reductand, acidic to neutral
Type II water: oxidant, more alkaline

Eh-pH values measured in the boreholes. The deeper samples are more oxidant.

Hypothesis:
Type I water: redox controlled by Fe minerals
Type II water: redox controlled by Mn minerals

Estability field of the Fe, Mn y U a 25°C species. UO2 is subsaturated in type II waters

2D conceptual model:
Type I water infiltrate the Fe-silicate rich pelites
Type II water infiltrate through Mn-carbonates and do not dissolve Fe-silicates

Hydrogeologic skecth showing the lithological units nd the groundwater flux

pH values predicted by the 2D reactive transport model.
- lower values in surface and WSW area
- higher values in depth and in the ENE area, due to the dissolution of carbonates

Eh values predicted by the 2D reactive transport model.
- lower values in surface and WSW area due to the dissolution of Fe-silicates (Fe2+-Fe(OH)3)
- higher values in depth and in the ENE area, due to the dissolution of Mn-carbonates (Mn2+-MnOOH)
- very low values in the centre due to UO2 dissolution.

Uranium concentrations predicted by the 2D reactive transport model.
There is a plume of uranium downstream the deposit.

Conclusion:
To account for high Eh-pH values at depts it is required the abundance of Mn-carbonate in the recharge area.
This an unusual feature in theEarth crust.