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The Okélobondo natural fission reactor, southeast Gabon: Geology, mineralogy, and retardation of nuclear-reaction products

Keld Alstrup Jensen^*,1 and Rodney C. Ewing^,2

¹ Department of Earth Sciences, University of Aarhus, DK-8000 Århus C, Denmark
² Department of Nuclear Engineering and Radiological Sciences and Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2104, USA

Nuclear fission reactors occur in the Oklo-Okélobondo uranium deposit in southeast Gabon. The Okélobondo reactor zone (RZOKE, 310 m depth) was the last reactor zone to be excavated before mining was terminated in December 1997. RZOKE is situated at the base of a 2.5 m deep and 2.7 m wide reactor synform located between the FA sandstone and hydrothermally altered black shales—argile de pile—of the FB Formation. The reactor synform developed by hydrothermal dissolution of the FA sandstone during criticality at ca. 2 Ga. The core of RZOKE is up to 55 cm thick and contains 90 vol% uraninite (91.24 ± 0.91 wt% UO₂ and 7.22 ± 0.53 wt% PbO) embedded in a matrix of Si-rich illite (6.08 to 7.91 Si per formula unit [p.f.u.]). The argile de pile (typically 60 cm) consists mainly of donbassitic chlorite (7.84 to 8.07 octahedrally coordinated Al p.f.u.) cut by fine illite veinlets. Fe-rich chlorite (0.708 to 1.427 Fe p.f.u.) is abundant at the edges of the reactor zone. Organic matter in both the reactor zone and near-field surroundings played an important role in the enhancement of porosity and stabilization of U during formation and operation of RZOKE. Fissiogenic Zr, Ce, Nd, and Th (daughter of ²⁴⁰Pu and ²³⁶U generated by neutron-capture reactions) are well retained in uraninite and (U,Zr)-silicate. The (U,Zr)-silicate formed during local migration of Zr, Sr, U, Pu, and lanthanide fission products during reactor criticality. Fissiogenic Ru, including ⁹⁹Ru—a daughter of ⁹⁹Tc, was mainly retained as ruthenium arsenide (± Pb, Co, Ni, and S). Although RZOKE generally appears well preserved, partially dissolved galena and uraninite, as well as the presence of accessory anglesite(?) and lead-uranyl sulfate hydroxide hydrate, suggest that recently observed deep, oxidized groundwaters have begun to interact with RZOKE.

Key Words: fission products • nuclear reactor • Oklo • retardation • uraninite • uranium

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