Chemical characterization of solid graphitic carbonaceous matter associated with the Oklo natural fission reactors and uranium ore deposits, Gabon (West Africa)
AuthorRigali, Mark Joseph, 1963-
AdvisorParrish, Judith Totman
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PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractSolid graphitic carbonaceous matter (CM) is closely associated with the Oklo uranium ore deposits and several of the natural fission reactors in Gabon, West Africa. This material facilitated the containment of uranium and numerous fissiogenic isotopes in these natural reactors for a period of nearly 2 Ga. Hence it is the subject of detailed studies because it may be useful as an analogue for carbonaceous materials, e.g. technical bitumens, that are currently being considered for the encapsulation and storage of anthropogenic radioactive waste. Chemical and structural analyses of the uraniferous CM associated with the natural fission reactors indicate that it is a polymer-like solid composed mainly of polycyclic aromatic hydrocarbons (PAHs) that are randomly oriented (turbostratic) and vary in size from several to several tens of A. Short-chained aliphatic hydrocarbons and oxygen-bearing moieties are attached to the PAH sheets and frequently bridge adjacent sheets. The Oklo uraniferous CM exhibits very high free radical concentrations, which can exceed 1021 free radicals/g organic carbon. The organic free radicals are stable PAH moieties located at or very near the surfaces of these organic solids, mainly on the internal surfaces of pores. Despite their presence, these pores could not have served as effective conduits for the transport of radionuclides by aqueous solutions through and out of the CM matrix. This is because of the very small average size of the pores, together with the CM's non-wettability by aqueous solutions. The Oklo CM has endured oxidation-reduction reactions during uranium mineralization, exposure to ionizing radiation, and alteration associated with the radiolysis products of water during and after natural reactor operation, perhaps up until the present time. Still, the CM acted as an effective barrier to radionuclide migration out of the CM-rich natural fission reactors. Properties, including its aromatic composition, resistance to alteration by ionizing radiation, and its non-wettability to aqueous solutions, make the Oklo uraniferous CM an effective barrier to radionuclide migration. These properties should be incorporated into man-made carbonaceous materials currently being considered for use in the storage of radioactive waste.
Degree ProgramGraduate College