Fission product retention in newly discovered organic-rich natural fission reactors at Oklo and Bangombe, Gabon

Conference · Fri Jan 01 04:00:00 EST 1993 · Transactions of the American Nuclear Society; (United States)

OSTI ID:5854065

Nagy, B; Rigali, M J ^[1]

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1. Univ. of Arizona, Tucson (United States)

The discovery of naturally occurring fission reactors in the rock strata of the Paleoproterozoic Francevillian Basin in the Republic of Gabon in equatorial West Africa led to several programs to define migration and/or retention of uranium and fissiogenic isotopes from/in the natural reactor zones. Although much understanding has been gained, new insight is needed regarding the chemical and physical parameters that control movement and retention of fission products over almost two billion years from/in the natural reactors. Seventeen known natural fission reactors sustained criticality for 0.1 to 1 million years in hydrothermally altered sedimentary rocks 1968 +/- 50 million years ago. These natural nuclear reactors attained criticality because of high concentrations of uranium in small pockets in uranium ores, the lack of neutron poisons, and because at the time they reached criticality, the abundance of [sup 235]U was five times greater than it is today. Water acted as a moderator, and temperature in the natural reactors was between 160 and 360[degrees]C. Both the uranium-rich pockets and the uranium ore bodies in which these pockets are located were formed when aqueous solutions moving through highly fractured zones in the Francevillian sedimentary rocks met organic-rich sediments. This resulted in the reduction of U(VI) in the dissolved uranyl ions to U(IV), causing the precipitation of pitchblende and uraninite. It has been proposed that between 2.2 and 1.9 billion years ago, the earth's atmosphere experienced a remarkable temporary rise in O[sub 2] content; this event may account for the uranium-bearing, oxidizing aqueous solutions in the Francevillian rocks.

• OSTI ID: 5854065
• Report Number(s): CONF-930601--
• Conference Information: Journal Name: Transactions of the American Nuclear Society; (United States) Journal Volume: 68
• Country of Publication: United States
• Language: English

Related Subjects

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12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
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Terrestrial-- Radioactive Materials Monitoring & Transport-- (1990-)
ACTINIDES
AFRICA
AQUEOUS SOLUTIONS
DEVELOPING COUNTRIES
DISPERSIONS
ELEMENTS
ENVIRONMENTAL TRANSPORT
FISSION PRODUCTS
GABON
ISOTOPES
MASS TRANSFER
MATERIALS
METALS
MINERALS
MIXTURES
NATURAL NUCLEAR REACTORS
ORES
OXIDE MINERALS
PITCHBLENDE
RADIOACTIVE MATERIALS
RADIOACTIVE MINERALS
RADIONUCLIDE MIGRATION
ROCKS
SEDIMENTARY ROCKS
SOLUTIONS
URANINITES
URANIUM
URANIUM MINERALS
URANIUM ORES