Clays at the Natural Nuclear Reactor at Bangombé, Gabon: Migration of Actinides

Abstract

Clay minerals were examined from the natural nuclear reactor at Bangombé. The clay mineralogy of the reactor facies is: illite, kaolinite and minor chlorite (FB formation) ; illite with chlorite and kaolinite in hydrothermal veins (argile de pile) ; illite and chlorite (reactor core); and i) chlorite and kaolinite ± illite ± smectite or ii) illite in the underlying FA formation. Illite crystallinity values are in general highest in rocks that experienced the highest temperatures during reactor operation. Chlorite chemistry varies with respect to position around the reactor: both di-tri and trioctahedral chlorites are present in hydrothermal veins within the argile de pile, dioctahedral chlorites are present in the FA formation within cm of the reactor core, trioctahedral chlorites are present within a meter of the reactor. Chemical data for the argile de pile and the FB formation suggest that formation of the former unit occurred as a result of thermal metamorphism of the FB pelites, as opposed to large-scale dissolution of the sandstones of the FA formation. Autoradiography is used to study the sorptive characteristics of the clays for actinides. The results show that actinide sorption varies in the order: chlorite > illite = kaolinite.

Heterogeneous reduction of uranyl by micas: Crystal chemical and solution controls

“…Eberly et al (1995) noted a close spatial relationship between uraninite and Fe-bearing chlorite in hydrothermal veins associated with the natural nuclear reactor at Bangombe, Republic of Gabon. Based on the textural relationship between uraninite and chlorite, Eberly et al (1996) suggested that chlorite heterogeneously reduced aqueous uranyl to U IV in uraninite. Wersin et al (1994) and Liger et al (1999) demonstrated that pyrite and Fe II adsorbed to ␣-Fe 2 O 3 can heterogeneously reduce U VI to U IV .…”

Special cases of natural analogues: The Gabon and Cigar Lake U ore deposits

“…Along with U, chlorite has also the potential to sorb Pu. This was demonstrated by an experiment in which thin sections from the Bangomb6 deposit remained in contact with a solution containing 10-6M 239pu (Eberly et al 1996). After exposure, the distribution of Pu by autoradiography indicated that adsorption of Pu was much more effective onto chlorite than illite and kaolinite.…”

“…4). Studies of the clay halo at the Oklo and Bangomb~ natural reactors have shown that chlorites are efficient actinide scavengers (Bros et al 1993;Eberly et al 1995Eberly et al , 1996GauthierLafaye et al 1996; and revealed the sorption of significant amounts of U (100-1000ppm) onto Fe-Mg chlorite . Along with U, chlorite has also the potential to sorb Pu.…”

Study of radionuclide leaching from the residues of K Basin sludge dissolution

“…It has also been shown that certain clay minerals have an affinity for actinides (Eberly 1996). found in K basin sludges, and because the KE basin concrete walls were produced from local aggregates, and because soil and sand have blown into the basins over the years, we cannot rule out these possibilities as being present in general sludges, and perhaps a portion of them might survive the acid-dissolution to become part of the residue.…”