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Uraninite: a 2 Ga Spent Nuclear Fuel from the Natural Fission Reactor at Bangombé in Gabon, West Africa

Published online by Cambridge University Press:  03 September 2012

K. A. Jensen ,
R. C. Ewing  and
F. Gauthier-Lafaye
K. A. Jensen
Affiliation:
Department of Earth Sciences, Aarhus University, DK-8000 Aarhus C, keld@geo.aau.dk
R. C. Ewing
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131
F. Gauthier-Lafaye
Affiliation:
Centre National de la Recherche Scientifique, Centre de Geochemie de la Surface, 67084 Strasbourg Cedex, France
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Abstract

Uraninites from the Bangombé natural fission reactor (RZB) and “normal” uranium-ore occur as fine veins in the sandstone host-rock as well as altered, broken, and slightly displaced grains in an illitic matrix, and in nodules and veins of solid bitumen. Inclusions of galena, (Y,Gd)-rich phosphates, a Pb-oxide and a Ti-oxide? were observed. Uraninites just below RZB were partially altered to a uranyl-sulfate. Three generations of uraninite were identified based on their PbO-contents of 8–11.06 wt%, 6 wt% (the largest population), and a younger generation with 3 wt%. The high Pb-uraninites may be the precursor to the low Pb-uraninites. Diffusional loss of Pb is indicated by the presence of a Pb-oxide at the interface to the uraninites. The behaviour of the metallic fission products, incompatible with the uraninite structure, may mimic the behaviour of Pb in these uraninites. The averaged impurity-content ranges from 4.29 to 6.89 wt%, and consists mainly of SiO2, TiO2, ZrO2, FeO, CaO, Al2O3 and P2O5. The averaged content of Y2O3 and the Ln's is less than 0.78 wt% and there is a scattered positive correlation with P2O5. The content of Y + Ln's is generally highest in the uraninites from RZB. Uraninite hydration and the formation of “uranopelite/zippeite” have caused complete loss of Y and the Ln's. These elements seems also to be partially lost by weak phosphatian coffinitization. The analytical results indicate that Y and the Ln's, which are high yield fission products, may be released from uraninite during alteration in the presence of P.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 465: Symposium II – Scienfific Basis for Nuclear Waste Management XX , 1996 , 1209
Copyright
Copyright © Materials Research Society 1997

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References

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