Reactor Zone Research Articles

Operating regime of a biphasic oil/aqueous hollow-fibre reactor with immobilized lipase for oil hydrolysis

Lipase from Candida rugosa was immobilized on Cuprophane membrane in a hollow-fibre module for possible application for oil hydrolysis. A simple immobilization technique was applied and 154.8 mg m −2 lipase was immobilized with approximately 40% mass yield. The reactor operating regime was investigated with respect to flow patterns in reactor zones, lipase desorption and hydrolysis rates. Flow pattern studies in the experimental system revealed nearly plug flow with a stagnant region in the reactor lumen and significant flow channeling in the shell zone. Mass transfer limitations were apparent in the lumen zone at flow rates below 15 cm 3 min −1. Higher flow rates up to 50 cm 3 min −1 in the lumen zone and in the range 2.5–26 cm 3 min −1 in the shell side had no apparent influence on hydrolysis rates. At the enzyme load employed in this study, loss of enzyme activity due to desorption had little effect on reactor stability and oil hydrolysis.

Experimental Limit on the Probability of Neutral Pion Emission in Neutron-Induced Fission of235U Nuclei

Results of an experiment on the search for the emission of π0 mesons in neutron-induced fission of 235U nuclei based on the detection in coincidence of two γ-quanta from the π0-meson decay performed with the nuclear reactor developed at the Scientific-Research Nuclear Physics Institute at Tomsk Polytechnic University are described. The experimental setup comprised two Cerenkov spectrometers of total absorption for the detection and measurement of γ-quantum energy placed in two coaxial horizontal channels of the nuclear reactor. Water filters placed in the channels were used to protect the detectors from β-, γ-, and neutron fluxes. To reject the cosmic radiation, scintillation counters of large area, operating in anticoincidence mode, were placed above each spectrometer. The experimentally measured limitation on the probability of neutral pion emission in neutron-induced fission of nuclei was established ≤4.1·10–11 with a 90% confidence level. The energy spectrum of γ-quanta from the active reactor zone was measured in the range 32–65 MeV. The parameters of the spectrum deviated from the results of theoretical calculations and from the experiment performed with the nuclear pulse reactor developed at the Joint Institute for Nuclear Research (Dubna).

Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon)—inferences from U, REE, Zr, Mo and Se isotopes

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 UO2 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.

REE mobility in groundwater proximate to the natural fission reactor at Bangombé (Gabon)

The natural nuclear fission reactor at Bangombé is located at a depth of approximately 12 m and has undergone supergene weathering and chemical exchange with groundwater under moderately oxidizing conditions. This reactor has been studied as an analogue for the migration of radionuclides in a geologic repository. Five water samples were taken from a drill hole in the reactor zone and from drill holes that are situated along the direction of the groundwater flow. Dissolution of phosphates in the weathering profile provided an important mechanism for the mobilization of phosphorous and REE. This phosphorous allowed the formation of secondary minerals, e.g., phosphatian coffinites and Fe-uranyl phosphate hydroxide hydrates. The filtered groundwaters (dissolved phase) obtained directly from the reactor zone had anomalous 143Nd/ 146Nd, 145Nd/ 146Nd and 149Sm/ 147Sm isotope ratios of 0.7235, 0.4933 and 0.843, respectively, confirming mixing between a fissiogenic and normal REE component. In this dissoved phase, a small fraction of the Nd (2.3 at.%) is of fissiogenic origin. The suspended loads (filtered particulates) of the same groundwater from within the reactor show similar isotopic anomalies with a fissiogenic Nd contribution of 3.2 at.%. Similar fissiogenic Nd contributions are observable for groundwater samples from outside the reactor zone (3 m distance). Both dissolved and suspended load samples from the more distant well (25 m) have normal isotopic ratios. Thus, the prevailing physico-chemical conditions within this groundwater system allow the migration of the fissiogenic REE over at least 3 m but less than 25 m. Although REE phases are abundant in the reactor zone, the REE concentrations of the water in contact with the sediments of this zone are very low (<17 ppt). The most important lanthanide- and actinide-bearing phases are uraninite, coffinites and Fe-uranyl phosphate hydroxide hydrate. The presence of all of these phases retards the mobility of the fissiogenic lanthanides and actinides.

Uranium transport around the reactor zone at Bangombé and Okélobondo (Oklo): examples of hydrogeological and geochemical model integration and data evaluation

The sites at Bangombé and Okélobondo (Oklo) in Gabon provide a unique opportunity to study the behaviour of products from natural nuclear reactions in the vicinity of reactor zones which were active around two billion years ago. The Commission of the European Communities initiated the Oklo Natural Analogue Programme. One of the principal aims was to study indications of present time migration of elements from the reactor zones under ambient conditions. The hydrogeological and hydrochemical data from the Oklo sites were modelled in order to better understand the geochemical behaviour of radionuclides in the natural system, by using independent models and by comparing the modelling outcome. Two modelling approaches were used: M3 code (hydrochemical mixing and mass balance model), developed by the Swedish Nuclear Fuel and Waste Management Company (SKB) and HYTEC (reactive transport model) developed by Ecole des Mines de Paris. Two different reactor zones were studied: Bangombé, a shallow site, the reactor being at 11 m depth, and OK84 at Okélobondo, situated at about 450 m depth, more comparable with a real repository location. This allowed the validation of modelling tools in two different sedimentary environments: one shallow, with a more homogeneous layering situated in an area of meteoric alteration, and the other offering the opportunity to study radionuclide migration from the reaction zone over a distance of 450 m through very heterogeneous sedimentary layers. The modeling results indicate that the chemical reactions retarding radionuclide transport are very different at the two sites. At Bangombé, the decomposition of organic material consumes oxygen and at Okélobondo the oxygen is consumed by inorganic reactions resulting, in both cases, in uranium retardation. Both modelling approaches (statistic with M3 code and deterministic with HYTEC code) could describe this situation. The goal of this exercise is to test codes which can help to describe and understand the processes taking place at the sites, validate the models with in situ data, and thus build confidence in the tools used for future site characterization. Ultimately, this allows identifying and selecting processes and parameters that can be used as input into repository performance assessment calculations and modelling exercises.

Effect of Feed-Stream Configuration on Gas-Phase Chlorination Reactor Performance

Chlorination of hydrocarbons is an industrially important process used for the production of commercially viable environmentally friendly chemicals. The highly exothermic nature of these reactions necessitates a thorough study of reactor stability and product feasibility. Here, computational fluid dynamics (CFD) is used to analyze the performance of a coaxial right-cylindrical insulated reactor for different inlet flow configurations. Chlorination reactions involve a large number of radicals and other intermediates, and hence, direct simulations using traditional CFD techniques are difficult because of the stiff nature of the reaction scheme involved. A novel algorithm for reaction computation, in situ adaptive tabulation (ISAT), is used to obtain considerable computational gains. The joint probability density function (JPDF) transport equation for the scalars with closed terms for reaction is solved using a Monte Carlo particle algorithm in tandem with a finite-volume (FV) Reynolds-averaged Navier−Stokes (RANS) method. The particle method handles transport of 15 scalars along with enthalpy and feeds back mean field values of temperature and molecular weight that are used by the FV code to correct the flow for reaction. The scalar scatter plots conditioned on the mixture fraction are used to study the details of the kinetics in different reactor zones. Comparison of premixed and segregated inlets is done to determine reactor stability and product yield. Conclusions are then drawn about fundamental properties of the reactor and broad considerations for reactor design.

Efficient mixing and reactions within microfluidic channels using microbead-supported catalysts.

A strategy for efficiently mixing solutions and carrying out multistep catalytic reactions in microfluidic systems is described. The approach involves immobilizing catalysts on microbeads, placing the beads into well-defined microreactor zones, and then passing reactants through one or more of the reactor zones to yield products. The catalyst-modified beads effectively mix reactants and increase the effective surface area of the channel interior, both of which improve reaction velocities compared to open channels. This approach is demonstrated using two sequential reactions catalyzed by glucose oxidase and horseradish peroxidase. In addition to providing a general route to chemical synthesis within microfluidic systems, this design strategy may also be applicable to modeling reaction pathways within cells and to bio/chemical sensing applications.

A miniaturized cyclic PCR device—modeling and experiments

Abstract With the aid of thermal and fluidic modeling using CFDRC ACE+™, we designed and fabricated the first miniaturized cyclic polymerase chain reaction (PCR) device in low-temperature cofired ceramics. The device comprises of a serpentine channel with different cross-sectional areas in different reactor zones to provide adequate residence time for the melting, annealing, and extension reaction to take place. This is in contrary to the thermal cycling in the batch PCR system. With a flow rate of 15 μl/min, the designed time to complete 30 PCR cycles is less than 40 min, given the total volume of the device 19 μl, provided an internal pump may be implemented to reduce the dead volume. We have demonstrated DNA amplification in this device, using an external peristaltic pump, and the PCR product was used with a DNA bioelectronic sensor chip (Motorola e-Sensors™) for genotyping experiment.

Two Pilot Plant Reactors Designed for the In Situ Bioremediation of Chlorobenzene-contaminated Ground Water: Hydrogeological and Chemical Characteristics and Bacterial Consortia

The SAFIRA in situ pilot plant in Bitterfeld, Saxonia-Anhalt, Germany, currently serves as the test site for eight different in situ approaches to remediate anoxic chlorobenzene (CB)-contaminated ground water. Two reactors, both filled with originallignite-containing aquifer material, are designed for themicrobiological in situ remediation of the ground water bythe indigenous microbial consortia. In this study, thehydrogeological, chemical and microbiological conditions of theinflowing ground water and reactor filling material are presented,in order to establish the scientific basis for the start of thebioremediation process itself. The reactors were put intooperation in June 1999. In the following, inflow CB concentrationsin the ground water varied between 22 and 33 mg L-1; achemical steady state for CB in both reactors was reachedafter 210 till 260 days operation time. The sediments werecolonized by high numbers of aerobic, iron-reducing anddenitrifying bacteria, as determined after 244 and 285 days ofoperation time. Furthermore, aerobic CB-degrading bacteria weredetected in all reactor zones. Comparative sequence analysis of16S rDNA gene clone libraries suggest the dominance of Proteobacteria (Comamonadaceae, Alcaligenaceae, Gallionella group, Acidithiobacillus) and members of theclass of low G+C gram-positive bacteria in the reactorsediments. In the inflowing ground water, sequences withphylogenetic affiliation to sulfate-reducing bacteria andsequences not affiliated with the known phyla of Bacteria, were found.

In Situ Isotopic Analysis of Uraninite Microstructures from the Oklo-Okélobondo Natural Fission Reactors, Gabon

ABSTRACTUranium deposits can provide important information on the long-term performance of radioactive waste forms because uraninite (UO2+X) is similar to the UO2 in spent nuclear fuel. The Oklo-Okélobondo U-deposits, Gabon, serve as natural laboratory where the long-term (hundreds to billions of years) migration of uranium and other radionuclides can be studied over large spatial scales (nm to km). The natural fission reactors associated with the Oklo- Okélobondo U-deposits occur over a range of depths (100 to 400 m) and provide a unique opportunity to study the behavior of uraninite in near surface oxidizing environments versus more reducing conditions at depth. Previously, it has been difficult to constrain the timing of interaction between U-rich minerals and post-depositional fluids. These problems are magnified because uraninite is susceptible to alteration, it continuously self-anneals radiation damage, and because these processes are manifested at the nm to μm scale. Uranium, lead and oxygen isotopes can be used to study fluid-uraninite interaction, provided that the analyses are obtained on the micro-scale. Secondary ionization mass spectrometry (SIMS) permits in situ measurement of isotopic ratios with a spatial resolution on the scale of a few μm. Preliminary U-Pb results show that uraninite from all reactor zones are highly discordant with ages aaproaching the timing of fission chain reactions (1945±50 Ma) and resetting events at 1180±47 Ma and 898±46 Ma. Oxygen isotopic analyses show that uraninite from reactors that occur in near surface environments (δ18O= −14.4‰ to −8.5‰) have reacted more extensively with groundwater of meteoric origin relative to reactors located at greater depths (μ18O= −10.2‰ to −7.3‰). This study emphasizes the importance of using in situ high spatial resolution analysis techniques for natural analogue studies.

Radioelement transport in the Bangombé nuclear reactor zone (Gabon) – Evidence from U and Sm isotopes

ABSTRACTAs a natural analogue study of relevance to the safety margin of radioactive waste repository conditions, mineralogical and isotopic studies were carried out to assess the effects of the alteration of the Bangombé natural nuclear reaction zone. Mobilization and retention of reactor products were identified around the reactor. The high temperature alteration associated with the heat released during nuclear reactions lead to the partial dissolution of uraninite and the sorption of released U mostly by Al-chlorite. This was followed by the precipitation of secondary U-phases such as Zr-bearing uraninite and U-Ti oxides. The recent (&lt;1 Ma) low temperature alteration lead to further dissolution of reactor core uraninite and mobilization of U and fissiogenic REE. Migrations took place laterally through hydraulically conductive fractures within sandstones and vertically through compacted shales, likely by diffusion-type process. Adsorption onto clays and Fe-oxides is the dominant mechanism of retention as inferred by the results of selective extraction experiments. The amounts released appear to be low compared with the quantities initially produced in the core and the transfers may have been limited within several meters around the reactor.

Nitrification in saline wastewater with high ammonia concentration in an activated sludge unit

A nitrifying activated sludge reactor fed with a high salinity medium was operated efficiently at ammonia loading rates between 1 and 4 g NH 4 +–N l −1 d −1. The system became completely inefficient at inlet salt concentrations higher than 525 mM due to the mixed inhibition effect of salts and ammonia. The final product was mainly nitrate although dissolved oxygen limitations caused sporadic ammonia and nitrite accumulations. Specific nitrifying activity decreased due to the saline effect. A set of activity tests showed that in the continuous reactor non-adapted biomass is rather more sensitive than biomass to the saline effect. Physical properties of biomass in the reactor (sludge volumetric index and zone settling velocity) were not affected by the saline concentration, a biomass concentration of 20 gVSS l −1 was achieved.

Induced Radioactivity and Waste Classification of Reactor Zone Components of the Chernobyl Nuclear Power Plant Unit 1 After Final Shutdown

The dismantlement of the reactor core materials and surrounding structural components is a major technical concern for those planning closure and decontamination and decommissioning of the Chernobyl Nuclear Power Plant (NPP). Specific issues include when and how dismantlement should be accomplished and what the radwaste classification of the dismantled system would be at the time it is disassembled. Whereas radiation levels and residual radiological characteristics of the majority of the plant systems are directly measured using standard radiation survey and radiochemical analysis techniques, actual measurements of reactor zone materials are not practical due to high radiation levels and inaccessibility. For these reasons, neutron transport analysis was used to estimate induced radioactivity and radiation levels in the Chernobyl NPP Unit 1 reactor core materials and structures.Analysis results suggest that the optimum period of safe storage is 90 to 100 yr for the Unit 1 reactor. For all of the reactor components except the fuel channel pipes (or pressure tubes), this will provide sufficient decay time to allow unlimited worker access during dismantlement, minimize the need for expensive remote dismantlement, and allow for the dismantled reactor components to be classified as low- or medium-level radioactive waste. The fuel channel pipes will remain classified as high-activity waste requiring remote dismantlement for hundreds of years due to the high concentration of induced 63Ni in the Zircaloy pipes.

Migration of fissiogenic Sm in fractures at the natural fission reactor site of Bangombé and effects of neutron capture on remote samples

The Sm isotopic composition of samples from fractures below the reactor zone (RZ) Bangombé is investigated by thermal ionization mass spectrometry to shed light on neutron capture effects and migration of fission products. A well-defined mixing line from normal to fissiogenic Sm isotopic composition, which reveals that migration of normal and fissiogenic Sm at the reactor site of Bangombé is more significant than that at other RZs, is found.A small shift in the Sm isotopic composition in one fracture, which was 3m below the RZ Bangombé, shows neutron capture effects in situ. The shift indicates a neutron fluence of about 4×1015neutron/cm2. The neutron fluence originates from the fission reaction and is consistent with a macroscopic neutron capture cross-section.

Weathering process at the natural fission reactor of Bangombé

The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found. Most of them are mined out. The reactor zone of Bangombe, is only 10–11 m below the surface. This site has been influenced by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombe during the course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns, the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced by the anomalous isotope ratios of REE because of the fission process. The 149Sm/147Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of 149Sm and subsequent transmutation, whereas 147Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE. Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model.

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

Research Article| January 01, 2001 The Okélobondo natural fission reactor, southeast Gabon: Geology, mineralogy, and retardation of nuclear-reaction products Keld Alstrup Jensen; Keld Alstrup Jensen 1Department of Earth Sciences, University of Aarhus, DK-8000 Århus C, Denmark Search for other works by this author on: GSW Google Scholar Rodney C. Ewing Rodney C. Ewing 2Department of Nuclear Engineering and Radiological Sciences and Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2104, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Keld Alstrup Jensen 1Department of Earth Sciences, University of Aarhus, DK-8000 Århus C, Denmark Rodney C. Ewing 2Department of Nuclear Engineering and Radiological Sciences and Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2104, USA Publisher: Geological Society of America Received: 26 Apr 1999 Revision Received: 29 Nov 1999 Accepted: 22 Feb 2000 First Online: 01 Jun 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 Geological Society of America GSA Bulletin (2001) 113 (1): 32–62. https://doi.org/10.1130/0016-7606(2001)113<0032:TOLNFR>2.0.CO;2 Article history Received: 26 Apr 1999 Revision Received: 29 Nov 1999 Accepted: 22 Feb 2000 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Keld Alstrup Jensen, Rodney C. Ewing; The Okélobondo natural fission reactor, southeast Gabon: Geology, mineralogy, and retardation of nuclear-reaction products. GSA Bulletin 2001;; 113 (1): 32–62. doi: https://doi.org/10.1130/0016-7606(2001)113<0032:TOLNFR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Modifications to the Theory of Coupled Reactors

The mathematical simulation of neutron-physical processes in multipoint models of kinetics is examined. Promising modifications of such models along the lines of enlarging the assortment of weighting functions, description of the couplings between the reactor zones, and taking account of the peculiarities of the neutron kinetics in the general form of mixtures of the fissioning nuclides are formulated. Corrections are introduced into the assumptions of the well-known theory of coupled reactors.

Alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the Oklo–Okélobondo and Bangombé natural nuclear reaction zones (Franceville basin, Gabon)

Large-scale light rare earth element (LREE), uranium, lead and phosphorus migration has been evidenced in the FA Lower Proterozoic sandstones of the Franceville basin (Gabon) hosting Oklo natural nuclear reaction zones (RZ) in relation with extensive accessory mineral alteration by highly saline diagenetic brines (28.7 wt.% NaCl eq. to 30 wt.% CaCl 2 eq.) at about 140°C and 1 kbar. Monazite is the most severely altered accessory mineral in the coarse-grained sandstones of the basal FA formation. Detrital monazite crystals are altered to Th–OH silicate microcrystalline phase with very low concentrations of U and LREE. The Th/La ratio increase from non-altered (Th/La∼0.27) to altered sandstones (Th/La∼1.14) shows that about 76% of the LREE was leached. This corresponds to the leaching of 2.01×10 9 metric tons at the scale of the FA formation in the Franceville basin. Similarly, the Th/U increase from monazite (Th/U=18.6) to the Th-silicate phase (Th/U=88.7) is interpreted as a result of an alteration by oxidizing brines with leaching of U together with LREE and P. It corresponds to the leaching of 9.06×10 6 metric tons of uranium. This amount of uranium largely exceeds the known uranium reserves from the Franceville basin. In zircon crystals, the cores are generally homogeneous, weakly fractured and well preserved as attested by the Archean ages (2867±24 and 2865±51 Ma) obtained by ionic microprobe analysis on zircon of the FA Formation, respectively, from the marginal and central parts of the basin. Their composition corresponds to the pure end-member (Zr,Hf)(SiO 4), poor in Th and U (Th/U∼1). At the contrary, their rims, which present several growth zones with cracks fillings, are enriched in REE, P, Th and U with higher Th/U ratios (5–10). Both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings. For example, in zircon, the Pb of galena crystals (3–23 wt.%) largely exceeds the amount of Pb (maximum 0.1 wt.%) that would have been produced in situ by radioactive decay in this mineral. Nearly all the lead were introduced into altered zones of accessories. Dissolution of accessory minerals occurred at 2000 Ma, producing a porous and distorted crystal structure which has allowed a later incorporation of Pb. Galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions. Altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non-radiogenic Pb isotopic compositions, confirming the external origin of lead. Pb isotopic evolution models indicate a crystallization age sometime after 1000 Ma, both for galena located close to and far from U mineralizations and reactor zones, which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms, between 1000 and 750 Ma, at the scale of the Franceville basin. The present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride-rich fluids during the diagenesis of a sedimentary basin.

Extreme light rare earth element mobilization by diagenetic fluids in the geological environment of the Oklo natural reactor zones, Franceville basin, Gabon

Extreme light rare earth element mobilization by diagenetic fluids in the geological environment of the Oklo natural reactor zones, Franceville basin, Gabon

Galena Crystallization and the Origin of Sulfur in the Oklo and Bangombé Natural Reactors: The Effects of a ca. 900 Ma Thermal Event

ABSTRACTGalena in the ca. 1950 Ma old natural fossil fission reactors in Gabon crystallized sometime between 980 Ma and 750 Ma during a period of regional extension and the intrusion of a dolerite dyke swarm. The S isotopic composition of galena, containing radiogenic Pb from uraninite, gives information about the origin of the S. Results from ion microprobe analyses of galena from the reactor zones indicate that S mainly originates from the surrounding sediment. Galena in a thin, altered dolerite dyke also contains non-magmatic S. The presented data gives no positive evidence for the involvement of magmatic S during the ca. 900 Ma galena crystallisation, however, the possibility cannot be ruled out.