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Organic matter and containment of uranium and fissiogenic isotopes at the Oklo natural reactors

Nature volume 354, pages 472–475 (1991)Cite this article

Abstract

SOME of the Precambrian natural fission reactors at Oklo in Gabon contain abundant organic matter1,2, part of which was liquefied at the time of criticality and subsequently converted to a graphitic solid3,4. The liquid organic matter helps to reduce U(VI) to U(IV) from aqueous solutions, resulting in the precipitation of uraninite5. It is known that in the prevailing reactor environments, precipitated uraninite grains incorporated fission products. We report here observations which show that these uraninite crystals were held immobile within the resolidified, graphitic bitumen. Unlike water-soluble (humic) organic matter, the graphitic bituminous organics at Oklo thus enhanced radionu-clide containment. Uraninite encased in solid graphitic matter in the organic-rich reactor zones lost virtually no fissiogenic lan-thanide isotopes. The first major episode of uranium and lead migration was caused by the intrusion of a swarm of adjacent dolerite dykes about 1,100 Myr after the reactors went critical. Our results from Oklo imply that the use of organic, hydrophobic solids such as graphitic bitumen as a means of immobilizing radionuclides in pretreated nuclear waste warrants further investigation.

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Authors and Affiliations

  1. Laboratory of Organic Geochemistry, Department of Geosciences, The University of Arizona, Tucson, Arizona, 85721, USA

    Bartholomew Nagy & Mark J. Rigali

  2. Centre National de la Recherche Scientifique, Centre de Géochimie de la Surface, 1 Rue Blessig, 67084, Strasbourg Cedex, France

    F. Gauthier-Lafaye

  3. Centre d'Etudes Nucléaires de Grenoble, 38041, Grenoble Cedex, France

    P. Holliger

  4. Geology Department, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario, M5S 2C6, Canada

    D. W. Davis

  5. Department of Geology, Mount Allison University, Sackville, New Brunswick, EOA 3CO, Canada

    David J. Mossman

  6. US Geological Survey, Federal Center, MS 973, Denver, Colorado, 80225, USA

    Joel S. Leventhal

  7. Department of Geology, The Queen's University of Belfast, Belfast, BT7 1NN, Northern Ireland

    John Parnell

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  1. Bartholomew Nagy
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  2. F. Gauthier-Lafaye
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  4. D. W. Davis
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  7. Mark J. Rigali
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  8. John Parnell
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Nagy, B., Gauthier-Lafaye, F., Holliger, P. et al. Organic matter and containment of uranium and fissiogenic isotopes at the Oklo natural reactors. Nature 354, 472–475 (1991). https://doi.org/10.1038/354472a0

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