Geochemical Characteristics of an Ancient Nuclear Reactor ^|^ldquo;Oklo^|^rdquo (original) (raw)
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Isotopic study of natural fission reactors at Oklo and Bangombé, Gabon
Journal of Radioanalytical and Nuclear Chemistry, 1999
The Oklo and Bangomb6 uranium ores in the Republic of Gabon are fossils of natural fission reactors. Many elements in these natural fission reactors show isotopic anomalies derived from fission and neutron capture reactions. Isotopic analyses of uraninites and some other minerals provide useful information on the geochemical behavior of fission products and nuclear chemical characterization of the reactors. Integrated isotopic measurements by whole rock analysis with inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry (TIMS) and by in-situ analysis with secondary ion mass spectrometry (SIMS) make it possible to clarify the migration processes of fissiogenic nuclides over a range of scales from micro meters to meters.
Geological Society of America Bulletin, 2001
Oklo-Okélobondo uranium deposit in southeast Gabon. The Okélobondo reactor zone (RZOKE, ϳ310 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 ϳ55 cm thick and contains Յ90 vol% uraninite (Յ91.24 ؎ 0.91 wt% UO 2 and Յ7.22 ؎ 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 Յ60 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 RZO-KE. Fissiogenic Zr, Ce, Nd, and Th (daughter of 240 Pu and 236 U generated by neutroncapture 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 prod-*Present address:
Environmental Science & Technology, 2004
The processes that affect the mobility of uranium and other radionuclides in the environment have been largely studied at both the laboratory and the field scales. The natural reactors found at the Oklo uranium mine in Gabon constitute a unique investigation setting as spontaneous fission reactions occurred two billion years ago. Oklo uraninites contain a large amount of other radionuclides as a result of the fission process. We have investigated the dissolution behavior of four uraninite samples from Oklo as a function of temperature (25 and 60°C) and bicarbonate concentration (2.7-30 mmol/L). We have also investigated the dissolution behavior of minor components of the uraninites (i.e., Nd, Cs, Mo, Yb, and Sb) in relation to the dissolution of uranium. The results of the reported work are in good agreement with the kinetic rate laws derived from other uranium(IV) dioxide studies. Some of the minor components are found to be congruently released from the uraninite phase, while it is postulated that dissolution from segregated phases might affect the final concentrations of some of the rare earth elements, i.e., Nd and Yb. In addition, we have performed dissolution studies at 60°C with two uraninites representative of different geochemical environments at Oklo, to study the uranium dissolution rates as a function of the temperature. This has allowed derivation of apparent activation energies for the bicarbonate-promoted oxidative dissolution of the Oklo uraninites. The dissolution behavior of the minor components of the uraninites at 60°C was found to closely follow the behavior observed at 25°C. This indicates that similar codissolution mechanisms operate in the temperature range studied. The implications for the mobility of uranium and other radionuclides in natural and anthropogenic environments are discussed.
Geochimica et Cosmochimica Acta, 2005
In order to study geochemical processes for migration and fixation of fissiogenic rare earth elements (REE) in association with uranium dissolution, in-situ isotopic analyses using an ion microprobe were performed on U-and REE-bearing secondary minerals, such as coffinite, françoisite, uraniferous goethite, and uraninite found in a sandstone layer 30~110 cm beneath a natural fission reactor at Bangombé, Gabon. Phosphate minerals such as phosphatian coffinite and françoisite with depleted 235 U ( 235 U/ 238 U=0.00609~0.00638) contained large amount of fissiogenic light REE, while micro-sized uraninite grains in a solid bitumen aggregate have normal U isotopic values ( 235 U/ 238 U=0.00725) and small amount of fissiogenic REE components.
Applied Geochemistry, 2003
Mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the Oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides. To address these issues, in situ isotopic analyses by SHRIMP and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products (exchangeable and non exchangeable) and their association with mineral phases. The distribution patterns of U, REE, Zr and Mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances, owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality. The results emphasize the mobility of Zr at Oklo, this element being often considered as ''immobile'' during water-rock interactions. The main output is the demonstration of the net effects of sorption and coprecipitation processes. Chlorite and to a lesser extent illite were found to have adsorbed significant amounts of U, REE, Zr (and probably Th) and less sorbing elements such as Mo. Coprecipitation of secondary UO 2 and P-rich coffinite within the alteration halo is also an important means of retardation. The concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour. No retention effect was found for Se in the immediate vicinity of the reactor and this element may have moved farther from its source of production. These results have interesting implications for the evaluation of long-term containment of radionuclides. They provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary U-minerals. This study also demonstrates the robustness of these retention processes over extremely long periods of time. #
American Mineralogist, 2003
The oxygen isotopic composition of uranium minerals and their nanostructures are sensitive monitors of fluid events and provide key tools for studying the evolution of the Oklo-Okélobondo and Bangombé uranium deposits. However, the post-criticality evolution of the uraninite is only partially understood because the results from the few studies that have reported oxygen isotopic data present conflicting results . The study of uranium deposits is also complicated by the susceptibility of uraninite to alteration and radiation damage (e.g., and because the effects of these processes are most evident at the nano-and µm scale. Oxygen isotope analysis of uraninite on the micrometer scale has been demonstrated using secondary ion mass spectrometry (SIMS) , but such measurements have not yet been related to specific nanostructures. Here we present high-resolution transmission electron microscopy (HRTEM) analyses and in situ oxygen isotopic data obtained by SIMS of uraninite from natural fission reactors at Oklo-Okélobondo. These results are used to assess the extent of meteoric water interaction with the Oklo-Okélobondo natural fission reactors.
Uraninite recrystallization and Pb loss in the Oklo and Bangombé natural fission reactors, Gabon
Geochimica et Cosmochimica Acta, 2005
The Oklo and Bangombé natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO 2 in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombé, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones.
Occurrence of naturally enriched 235U: Implications for plutonium behaviour in natural environments
Geochimica et Cosmochimica Acta, 1993
It is generally accepted that uranium and most of the fission products, with the exception of the alkalis, slkaline earths and tare gases, remained in the irradiated uranium oxides during the nuclear reactions that took place 2.0 Ga ago in the OkIo uranium deposit (Gabon). New isotope investigations show that clay minerals from argillaceous rocks neighbouring the natural fission reactor 10 at Oklo have depleted 235U with z35U/238U ratios ranging between 0.00560 and the common natural value of 0.00725. One sample, however, is enriched in 23sU with a 235U/238U ratio of 0.007682. Leaching experiments of this sample with dilute 1N HCl revealed that the 23sU enrichment is actually restricted to the insoluble residue (235U/238U = 0.0 105 1 1 ), whereas the leachate remains depleted in 23sU. This unique discovery of very enriched uranium, together with samarium, neodymium, rubidium, and strontium isotopic analyses, indicate that a small amount of plutonium could have been more mobile than uranium in the reactor 10, and it is suggested that plutonium was incorporated in the crystallographic structure of clay minerals such as the &lo&es.
Geochimica et Cosmochimica Acta, 2010
Metallic aggregates with a size of a few tens lm and consisting mainly of Ru, Rh, Pd, Te, Pb, As, Sb, S and Bi were found in the acid residue of SD37-S2/CD uraninite taken from Oklo natural reactor zone (RZ) 13. Quantitative analyses of major elements using an electron probe microanalyzer and in situ isotopic analyses of Zr, Mo, Ru, Pb and U using a sensitive highresolution ion microprobe were performed on the metallic aggregates to determine the geochemical behaviors of fission products and actinides and to ascertain the processes of formation of the aggregates in the RZs. The chemical compositions of the aggregates investigated in this study are significantly different from those reported previously, showing lower Pb content and no correlation between the contents of Pb and S in the individual grains. The 235 U/ 238 U ratios in metallic aggregates vary significantly from 0.00478 to 0.01466, indicating chemical fractionation between U and Pu during the formation of the aggregates. The Pb isotopic data indicate that most of the Pb in the aggregates decayed from 2.05 Ga-old uraninite that existed in the RZ originally and that there was chemical fractionation between U and Pb in some aggregates. The Zr and Mo isotopic ratios, 90 Zr/ 91 Zr and 95 Mo/ 97 Mo, for most of the aggregates had small variations, which can be simply explained by constant separate mixing of fissiogenic and nonfissiogenic components. On the other hand, a large variation in the 99 Ru/ 101 Ru ratio (0.324-1.73) cannot be explained only by a two component mixing theory; thus, chemical fractionation between Tc and Ru during the reactor criticality is suggested. The large variations in the 235 U/ 238 U and 99 Ru/ 101 Ru isotopic ratios suggest that the aggregates formed under various redox conditions owing to the radiolysis of water.