High Uranium Content Research Articles

Development of advanced research reactor fuels using centrifugal atomization technology

Rotating disk centrifugal atomization technology has been utilized to fabricate uranium silicide (U3Si, U3Si2) and U-Mo nuclear fuel powders having high uranium content per unit volume for high performance research reactor fuels. Atomized nuclear fuel powders have characteristics of a spherical shape with narrow size distribution, small specific surface area and high purity. The heat treatment time for the formation of U3Si by a peritectoid reaction was reduced from about 72 hours for comminuted powders to about 6 hours for atomized powders due to more rapid solidification of atomized powder with a finer microstructure. The homogeneity of fuel particles in fuel meats was improved by mixing atomized fuel powders with Al powders using a V-shaped tumbler mixer. The atomized powders with a spherical shape and smooth surface were extruded under lower force than the comminuted powders with angular rough surfaces. The dispersed fuel meat with atomized powders resulted in an increased fuel powder loading density and higher thermal conductivity in the heat flow direction. The thermal swelling of dispersed fuel meat decreased due to the reduced specific surface area of spherical atomized nuclear powder. The atomized U-10wt.%Mo dispersion fuel showed less bubble formation than the comminuted fuel after an irradiation test with 40% burnup.

Partitioning of Actinides from High Level Waste of PUREX Origin Using Octylphenyl-N,N'-diisobutylcarbamoylmethyl Phosphine Oxide (CMPO)-Based Supported Liquid Membrane

The present studies deal with the application of the supported liquid membrane (SLM) technique for partitioning of actinides from high level waste of PUREX origin. The process uses a solution of octylphenyl-N,N'-diisobutylcarbamoylmethyl phosphine oxide (CMPO) in n-dodecane as a carrier with a polytetrafluoroethylene support and a mixture of citric acid, formic acid, and hydrazine hydrate as the receiving phase. The studies involve the investigation of such parameters as carrier concentration in SLM, acidity of the feed, and the feed composition. The studies indicated good transport of actinides like neptunium, americium, and plutonium across the membrane from nitric acid medium. A high concentration of uranium in the feed retards the transport of americium, suggesting the need for prior removal of uranium from the waste. The separation of actinides from uranium-lean simulated samples as well as actual high level waste has been found to be feasible using the above technique.

High-performance, flow-based, sample pre-treatment and introduction procedures for analytical atomic spectrometry

Recent and on-going work in the author's laboratory is described with particular reference to the use of flow injection, continuous flow and HPLC procedures for the development of improved analytical methodology for (a) the determination of trace concentrations of As, Cd, Pb and Se and (b) the determination of various arsenic and selenium compounds. The methods have been applied to the analysis of several different sample matrices, including urine, soil, sediments, waters, plants (garlic, onion, apple leaves), yeast, fruit juices, wine, calcium supplements and marine plankton. The dependence of the LOD of an FI HG method on sample volume is examined and the validity of the proposed rectangular hyperbolic relationship established for a number of different analyses. The use of immobilized tetrahydroborate in conjunction with preconcentration of the analyte on the same anion-exchange resin is described as a possible procedure for improving the LOD. When used in conjunction with ETAAS, an LOD of 0.004 µg l –1 for both As and Se was obtained for a sample volume of 10 ml. The procedure was also used in a method for the determination of inorganic arsensic and methylated arsenic(V) species. Methods for the determination of Pb by HG in the presence of hexacyanoferrate(III) were developed and applied to the analysis of urine, soils, waters and apple leaves. In the case of urine, the interference from the chelating agents used in the treatment of patients with elevated lead was overcome by the addition of Sc. The best LOD of 0.03 µg l –1 was obtained for a procedure in which the lead hydride was trapped on the interior of a flame-heated slotted quartz tube under fuel-lean conditions with subsequent atomization when the flame was made fuel rich (by the injection of a small volume of isobutyl methyl ketone). It has been confirmed that it is possible to determine Cd by a ‘cold vapour’ procedure and it was shown that the nature of the atom cell surface played no part in the atomization process, which appeared to be the spontaneous decomposition of the species evolved from acid solution of cadmium on the addition of sodium tetrahydroborate solution. A modest increase in signal was obtained in the presence of nickel and thiourea. An LOD of 0.02 µg l –1 was obtained for a sample volume of 300 µl and the procedure was used for the analysis of NIST SRM 2711 (Montana Soil) in which the interference from the high lead content was overcome by coprecipitation with barium sulfate. Three examples of procedures using manifold designs incorporating an ‘eight-port’ rotary valve are given to illustrate the versatility of this component: the separation of high concentrations of uranium (5000 mg l –1 ) from trace concentrations (1 µg l –1 ) of Al, Be, Li and Mg for determination by ICP-MS, the automated implementation of the co-immobilization of analyte and tetrahydroborate on an anion-exchange resin and the stopped-flow microwave digestion of human urine for the determination of Se by HG and ETAAS. Improvements in the ion-pair (with trichloroacetate) reversed-phase (C8) HPLC procedure (with ICP-MS detection) for the separation of selenoamino acids (and closely related compounds) were made with a new stationary phase (Waters Symmetryshield RP 8 ) and a small-volume spray chamber. The results of extraction procedures indicated that much of the selenium in yeast and garlic is bound in high molecular mass material. So far only a few of the compounds in the extracts have been identified by retention time matching. A reversed phase (C 18 ), ion-pair (tetrabutylammonium) HPLC procedure for the separation of four arsenic species (arsenite, arsenate, monomethylarsinate and dimethylarsonate), with detection by post-column HG-AAS, has been devised and applied to the extracts of soils spiked with these four species. Low recoveries of arsenite were obtained and microwave energy significantly accelerated the oxidation of arsenite to arsenate.

The influence of municipal waste on uranium mill tailings: A hydrogeochemical study on a mixed tailings site

Mining and milling has taken place in Saxony and Thuringia, in eastern Germany, for over 800 years. Radioactive waste rock was dumped without regard for the environment for centuries. The area is now rather densely populated, and some of the smaller dumps and tailings have been used as municipal or industrial landfills after the uranium mine and mill closed. Ground water contamination has occurred down-gradient of an industrial disposal plant in Dresden (Saxony). To determine the source of the contamination, a multi-parameter interpretation was done using234U,238U,226Ra,222Rn,2H,18O,34S, and13C, as well as conventional hydrochemical parameters. Ground water contamination in the natural aquifer beyond the disposal site was determined to be from the tailings due to its oxidizing, acidic character and high concentrations of sulfate, uranium and heavy metals. In this case, the contamination was not caused by recent seepage water from the dump. Water currently leaching from the tailings has been modified by water draining from the superimposed waste, which has had a positive influence on the tailings. Uranium and heavy metals have been immobilized in place due to the changes in the chemical environment.

Adsorptive Stripping Measurements of Chromium and Uranium at Iridium-Based Mercury Electrodes

Iridium-based mercury electrodes are shown to be very suitable for adsorptive stripping measurements of chromium and uranium in the presence of the DTPA and propyl gallate complexing agents. The well-adhered mercury hemispherical electrode offers remarkable durability to withstand various manipulations expected under field deployment and the “pure” mercury surface essential for efficient adsorptive accumulation of the corresponding metal chelates. An electrochemical “cleaning” step ensures complete removal of the adsorbed metal chelate at the end of each run. The same hemispherical surface is thus used over a prolonged period of over five weeks, performing hundreds of runs with RSDs lower than 10%. Detection limits of 0.4 μg/L uranium and 0.5 μg/L chromium are obtained following a 10 min adsorptive accumulation. The electrode responds rapidly to the “switching” between solutions of low and high concentrations of chromium or uranium. Proper choice of the constant current used for stripping potentiometric m...

Spectrophotometric determination of uranium in process streams of a uranium extraction plant

This paper deals with the development and standardization of procedures for the determination of uranium on a routine basis in various process streams of a uranium extraction plant, covering a wide range of concentrations from 350 g 1 −1 down to 5 mg 1 −1 using only a spectrophotometric technique. The self-absorption of uranyl ion in dilute phosphoric acid and the violet-blue colour of the UO 2 2+-Arsenazo III complex in 4 M HC1 were exploited for high and low concentrations of uranium, respectively. The methods described were applied to samples of varying nature such as aqueous, organics and solids, involve minimal sample preparation and do not require prior separation of uranium from impurities. The interfering impurities in different process streams were also studied. Large quantities of silica as undissolved material poses a serious interference in the case of UNS and UNF. Considerable quantities of iron in UNS, UNF, UNR and UNRC cause interference. Possible remedies in these cases are suggested. Problems with the direct spectrophotometric measurement of organic samples is discussed. The effect of the presence of large quantities of ammonium nitrate and sodium nitrate in WD samples on the determination of uranium is also discussed. The results are compared with those obtained by volumetry and X-ray fluorescence spectrometry for higher concentrations of uranium and by extraction-spectrophotometry (ethyl acetate-thiocyanate method) for lower concentrations. Relative standard deviation of 1% and 5% for high and low concentrations, respectively, were obtained, which are adequate as far as process stream samples are concerned. The compared results are in fair agreement. The problems associated with the determination of uranium in these process streams are discussed. Experimental results for 10 different process streams normally encountered in a uranium extraction plant are tabulated.

Weathering Conditions and Behavior of Fission Products in Oklo Reactors

Two billion years ago, natural fission reactions occured in the uranium ore deposit of Oklo (Gabon), mainly due to the high concentrations of fissile 235U. At that time, high uranium contents (40-60 % U) allowed reaching criticality when oxidised uranium bearing fluids met reduced fluids accompanying the migration of hydrocarbons. Natural fission reactors consist of a core made of uraninite crystals embedded in a clay matrix (mainly chlorites). The thickness of the clay layer range between few centimetres to 2 meters. Fission products are mostly located in the uraninite grains but newly crystallised minerals such as phosphate minerals, clays and metallic aggregates retain some proportions of migrated fissiogenic elements [1].

Radon in mines and dwellings in Kosovo and Metohia

The first systematic indoor radon survey in Kosovo and Metohia, Yugoslavia, was performed with passive radon dosimeters of the J. Stefan Institute utilizing CR-39 detector. Some regions, where houses are built from stone with a relatively high uranium concentration (of the order of 10 −5 g g −1), could be classified as radon prone areas (radon concentration around 800 Bq m −3). With the same dosimeter the radon activity concentration was measured in various levels of the Trepča lead and zinc mines. The average radon concentration was found to be relatively low (200–800 Bq m −3) and practically independent of the depth in the mine.

Determination of Neptunium and Plutonium in the Presence of High Concentrations of Uranium by Ion Chromatography–Inductively Coupled Plasma Mass Spectrometry

Ion chromatography (IC) coupled with ICP-MS was used to separate Np, Pu and U, overcoming the isobaric interferences present when the isotopic composition has to be determined and avoiding the influence of the tail of the 238 U peak on the 237 Np and 239 Pu peaks in samples with a high U content. The optimization of the parameters was carried out with a standard solution containing Np, U and Pu (ratio 1:1:1) at the 100 ng ml -1 concentration level. The precision and accuracy of the method were also evaluated using a sample containing 1 ng ml -1 of Np and Pu and 100 ng ml -1 of U. A 200 µl aliquot of the sample (previously treated with silver oxide in order to obtain the elements in a single oxidation state) was injected and, after passing through the separation column, the effluent was passed directly to the nebulizer. Different mobile phases and oxidizing/reducing agents were tested. The use of a high-capacity cation-exchange column (CS10), silver oxide as oxidizing agent and 40 mm 2,3-diaminopropionic acid monohydronitrate in 0.6 m HNO 3 as eluent in isocratic mode provided good resolution and recovery of the elements studied (96–98%). The precision of the method is 2.7, 1.1 and 1.7% for Np:U, Np:Pu and U:Pu, respectively, for a solution containing Np, Pu and U in a ratio of 1:1:1. Isotope dilution analysis applying IC separation was performed for the determination of Pu. The method of standard additions was used for the determination of Np. The agreement between the IC–ICP-MS results and the calculated values was within 15% for most isotopes.

Probability density plot for fission-track grain-age samples

This paper outlines the statistical basis, construction and interpretation of probability-density (PD) plots as applied to the analysis of mixed distributions of fission-track (FT) grain ages. Such distributions can be viewed as being composed of a mixture of elemental component distributions, with each component characterized by a unique average FT age. The original PD plot of Hurford et al. is shown to have a clear statistical basis, as defined by the Gaussian-kernel method of density estimation. This background is used to develop and justify an improved version of the PD plot. Three modifications are recommended: (1) as originally suggested by Galbraith, the PD plot should be constructed using the transform variable z, which is approximately proportional to the logarithm of FT age. By using z i to represent the FT grain ages τ i , each component distribution within a mixed FTGA sample becomes approximately Gaussian-distributed. This approximation is shown to work well for FT dating of zircon grains, which generally have relatively high uranium content and high track densities, and less so for FT dating of apatite grains; (2) estimation of the PD plot is optimized by setting the width of the Gaussian-kernel equal to α( SE( z ̂ i) , where α is a scaling factor with an optimal value of ∼ 0.6 and SE( z ̂ i) is the standard error of the FT estimate for z for the ith grain. This arrangement ensures the best compromise between resolution and smoothness for the final PD plot; (3) probability density is estimated as a function of z but it is best presented as a function of FT age τ. This objective is accomplished by transforming the z coordinates of the PD plot to τ and plotting τ on a logarithmically-scaled axis, which ensures that the original scaling of the PD plot is preserved. With these modifications, the component distributions in a PD plot will appear as symmetric Gaussian-shaped peaks and the area beneath each peak will be proportional to the relative size of the component. Several examples are given that illustrate the general concepts behind the PD plot and the advantages of the recommended modifications.

Uranium-series disequilibrium, sedimentation, diatom frustules, and paleoclimate change in Lake Baikal

The large volume of water, approximately one-fifth of the total surface fresh water on the planet, contained in Lake Baikal in southeastern Siberia is distinguished by having a relatively high concentration of uranium (ca. 2 n M), and, together with the surface sediments, an unusually high 234U 238U alpha activity ratio of 1.95. About 80% of the input of uranium to the lake, with a 234U 238U ratio of 2.0, comes from the Selenga River. Profiles of uranium, as well as the extent of isotopic disequilibrium in a 9 m sediment core collected on Academic Ridge, generally show high values during interglacial periods corresponding to high diatom frustule numbers (DiFr) and biogenic silica (BSi) data that have been reported elsewhere. During glacial periods (low DiFr and BSi), uranium progeny ( 234U and 230Th) were in secular equilibrium with low concentrations of their parent 238U. Radionuclide distributions were interpreted in terms of a quantitative model allowing for adsorption of riverine inputs of uranium onto two classes of sedimenting particles with differing 238U 232Th ratios and uranium progeny in secular equilibrium. If the 234U 238U activity ratio of adsorbed uranium has remained constant, mean sedimentation rates can be independently estimated as 3.6 ± 0.6 and 3.7 ± 0.9 cm · kyr −1 for the decay of 234U and in-growth of 230Th, respectively. These rates are consistent with a mean rate of 3.76 cm · kyr −1, calculated by optimization of the correspondence between adsorbed 238U and δ 18O in dated oceanic sediments. The adsorbed uranium apparently tracks variable river flow during interglacials and is drastically reduced during periods of glaciation. Evidently, uranium has not been significantly redistributed within Baikal sediments over at least the past 250 kyr and is a unique, biologically non-essential, tracer for climate-sensitive processes, which provide their own internal geochronometers, potentially useful for ages up to 1 Myr BP.

Fabrication of UO 2 pellets by gel pelletization technique without addition of carbon as pore former

High density UO 2 pellets for pressurised heavy water reactor fuel were fabricated by a gel pelletization technique from soft UO 2 microspheres prepared by an internal gelation process. Chemical and heat treatment parameters were determined for obtaining soft UO 2 microspheres with optimum properties required for making defect-free, high density, UO 2 pellets. Investigations reveal that a high uranium concentration in the feed broth during the preparation of UO 3 gel particles, conversion to UO 2 via U 3O 8, and optimized calcination and reduction temperatures, result in UO 2 microspheres suitable for obtaining good quality green compacts. Green pellets were densified by low temperature sintering in a C0 2 atmosphere at 1200°C to obtain high density UO 2+ x pellets and reduced at 800°C to UO 2.00 in Ar + 8%H 2 Resulting pellets were free from open porosity and black berry structure.

Distribution of toxic and radiation components in air particulates

The concentrations of several toxic heavy metals and volatile organic compounds (VOCs) in various types of Hungarian fly-ash fine particulates were investigated by means of instrumental neutron activation analysis, X-ray fluorescence analysis and gas chromatography, coupled with mass spectrometry. Within a power station, particulate samples were taken from the boiler zone (BO), from the electrostatic dust filter chamber (FI) and from the flue-gas at the top of the stack (ST). Enrichment rates of the toxic metals both in FI and ST particulate fractions related to the BO concentrations were calculated to enable the temperature dependence on the adsorption of the toxic components to be studied. In addition, both the total amounts of the VOCs and their partial distributions in accordance with the number of carbon atoms were also studied in fly-ash particulates. From them, 31 organic species were identified and determined. Since Hungarian brown coals have high uranium and thorium contents, the specific radioactivities of the daughter isotopes of both the 232Th and 238U decay series were also measured and are discussed.

Studies of uranium—cerium admixture solutions by EDXRF spectroscopy

AbstractUranium and cerium were studied in nitric acid solutions after presentation to an energy‐dispersive x‐ray fluorescence (EDXRF) spectrometer on filter‐paper fixed on Scotch tape. Ce Lα and U Lα peaks were used as analyte lines after excitation with a tungsten tube target at low potential (20 kV). These non‐conventional excitation conditions were considered to be suitable for low contents of cerium strongly but high contents of uranium weakly in a nuclear fuel. Lower net integrated line intensities observed in an actinide as compared with a lanthanide are discussed along with the scattered, Auger and Coster–Kronig intensities responsible for the production of back‐ground before calibrations with known concentrations. Difficulties in establishing an explicit relationship between these with change in matrix composition are discussed. The known concentrations of the mixed solutions when compared with the results from various computer programs using regression analysis showed that the algorithm developed by Plesch and Thiele for Siemens is better than the others. An accuracy of <2% was achieved. The importance of certified, standard or simulated reference materials is realized for quality analysis of occasional samples.

Analysis of toxic and radioactive components in sedimental type air particulates by INAA and XRF

The chemical composition of brown coal fuels and several sedimental type atmospheric particulate pollutants of industrial origin (e.g. fly-ash, bottom-ash, dust particulates from foundry-halls and casting-halls) was studied in relation to their toxic and their natural radioactive contents. In addition, some agricultural crops (e.g. soya bean, sunflower, rape, barley and luceme) sampled, in the near vicinity within a radius of about 10 km of the industrial centre were also analysed for significant distributions of any toxic components. Because of the high uranium and thorium concentrations in Hungarian coal fuels and, consequently, in various fly-ash particulate pollutants, the natural gamma activities of the samples were also measured and the relevant equilibriu states of the232Th and238U natural decay series were studied.

(n,f)-radiography as a means to investigate plants for their contamination with natural and technogenic uranium

Plants which grow in an area where uranium mining took place have been investigated for their uranium content by applying neutron-fission radiography [(n,f)-radiography]. The uranium was either assimilated by the plant from uranium-bearing soil or deposited at the leaves by uranium-bearing dust particles. Human activities like mining and refinement of uranium ores release technogenic uranium in the environment and increase the concentration of this α-emitting element. The technogenic uranium contributes to radioactive pollution. In addition to the natural amount of uranium in dust, water and the fertile (humus) soil layer, the technogenic uranium may increase the effect of uranium on the human organism. In order to perform efficient radiation protection, one has to obtain knowledge about the sources of radiation. The discovery of open sources which release uranium to the environment is of great importance because of the large biological effect of the α-particles emitted by the uranium. Leaves, roots and soil have been sampled at different locations in the territory of a former uranium ore mine, in the surrounding area and in a forest at the edge of the city of Ronneburg, where a small plot is known for its high natural uranium concentration. The plastic foil “Lavsan” was used as track detector, and the irradiation was performed with a fluence of 10 16 neutrons cm −2. (n,f)-radiography applied to plants yields information about the presence of local α-emitting sources of uranium and their location. The density of the fission fragment tracks enables the estimation of the uranium concentration.

Interlaboratory comoparison of analytical methods for the determination of natural series uranium and thorium isotopes in rock samples

Results are repoerted for238U,234U,232Th, and230Th determinations in 19 rock samples from a uranium mine, performed independently, byb three different laboratories. Uranium and thorium isotopoic activities were determkined by alpha spectrometry, after different pre-concentration and counting sample preparation techniques., Additionally, total concentrations of uranium were determined by fluorimetry and gamma spectreometry. the folloing conclusions could be drawn from this intercomparison test: (1) The results for238U specific activity agreed with the amjority of results within 10%. Lincar correlation coefficients between the three data sets were 0.999. However, for a few samples of much higher uranium concentrations, large deviations were observed, indicating problems of, sample heterogencity. (2) For the234U/238U activity ratio data, a still closer agreement was obtained (5%), as computation of the activity ratios did not, require information on the yield of the used tracer spike (232U). (3) The results for232Th specific activities and230Th/234U activity ratios showed larger deviastions between the three laboratories (typically up to 15%, in some cases still ore). Different Th-isotopes (228Th,234Th and229Th) have been used as yield tracers. The data indicates, however, that the observed deviations are not simply a consequence of a systematic difference in the calibration of the different spikes, but, probably cased by other errors such as incomplete sample dissolution, sample heterogencity, tec. The limitations of alpha spectrometry will be discussed and an application of the developed methods shown.

Fission-track dating of granitic rocks from the Eastern Cordillera of Peru: evidence for Late Jurassic and Cenozoic cooling

Apatite and zircon fission-track ages of three samples of the Huachon granite and one sample of the Quiparacra granite, from the Eastern Cordillera of the Andes of central Peru, have been determined. Zircons from the Huachon granite yielded two groups of ages. An older group, made up of zircons with the lowest uranium content, gave an age of c . 270 Ma. The zircons with high uranium content from this granite gave ages of c . 160 Ma. The zircon ages from the Huachon granite show no correlation with elevation. The age of 270 Ma is interpreted as the age of probable emplacement; the 160 Ma age is evidence of a mild cooling (≥200°C) event that ended in late Mid-Jurassic time. This cooling correlates, in central Peru, with thick conglomerates in the Sub-Andes and strong exhumation in the Eastern Cordillera during Bathonian to Malm times. Zircons from the Quiparaca granite, which is a component of a composite batholith of possible Precambrian to Tertiary age, show ages of c. 30 Ma which are interpreted as the emplacement age. These zircons were totally annealed prior to the cooling in the Oligocene. This required temperatures, >200°C. Apatite fission-track dating of the Huachon granite yielded only Neogene ages which reflect the total annealing of the tracks present in the apatite before exhumation began c. 22 Ma. Apatite ages show a correlation with elevation and indicate two periods of cooling during the Neogene at c. 21 Ma and between 12 Ma and the Present. These young ages suggest approximately 4 to 6 km of erosion in the Eastern Cordillera of central Peru in the past 30 Ma. These amounts are consistent with known geological data and fission-track ages obtained from other parts of the eastern Andes.

Effect of depositional environment and sources of pollution on uranium concentration in sediment, coral, algae and seagrass species from the Gulf of Aqaba (Red Sea)

Uranium concentrations were determined in sediment samples, four hard and two soft corals, one seagrass and four species of algae collected from phosphate-polluted sites in the northern reef area of the Gulf of Aqaba. High uranium concentrations were found in all samples examined from a phosphate-polluted site near a phosphate loading berth compared to the unpolluted ones. Uranium levels, U/Ca ratios, concentration and discrimination factors were also high compared to those reported from other regions of the world. The effects of the exported raw phosphate powder as the main source of pollution and depositional environment on the concentration of uranium in the examined species are discussed.

Petrography and paragenesis of organic matter associated with the natural fission reactors at Oklo, Republic of Gabon: a preliminary report

Sixteen known uranium-rich pockets in the sediment-hosted uranium ore deposits in the Oklo area became nuclear fission reactors 1968 ± 50 Ma ago and operated as such for up to 1 Ma. Nuclear criticality was caused by unique and fortuitous geological environments and events. These included the localized high concentrations of uranium in small pockets in the ore bodies, the fact that the relative abundance of the fissile 235U isotope was five times greater in uranium ∼ 2 Ga ago than present, the presence of water which acted as a moderator in the natural reactors and the absence of neutron poisons, which are elements that can prevent nuclear chain reactions. Organic matter is present in all of the Oklo natural reactors, but it is abundant only in reactors 7 to 16. Organic matter has been studied in natural reactors 7 to 9 and at locations at various distances from these reactors. Bitumen in the Oklo reactors is the predominant organic phase. It is now solid bitumen containing dispersed cryptocrystalline graphite in crystal domains smaller than optical microscopic size. Its immediate precursor was liquid bitumen, generated from syngenetic kerogen in organic-rich sedimentary rocks in the Francevillian Basin of Gabon and in the natural reactors themselves by hydrous reaction mechanisms prior to, during, and after nuclear criticality. A preliminary paragenesis of organic matter at Oklo may be defined through the following sequence of events. (a) Protokerogen and kerogen evolve from abundant cyanobacteria in the tidal and deltaic sediments in the basal FA formation of the Francevillian Series, during sedimentation and burial, respectively. (b) During subsequent subsidence of the basin the first generation of bitumen and petroleum occurred. (c) During uplift the sedimentary strata were fractured, and bitumen and petroleum migrated through fractures. Following this, oxidizing aqueous solutions carrying uranyl ions migrated through the highly fractured rocks, met and were reduced by organic matter and precipitated pitchblende or uraninite at the present sites of the Oklo uranium ore deposits and natural reactors. The natural reactors reached nuclear criticality, they were heated and produced the second generation of bitumen through hydrous reactions from the by now solid first-generation bitumen and remaining kerogen. Finally, hydrothermal effects associated with the dolerite dike swarm intrusion 750 ± 150 Ma ago produced yet another localized generation of liquid bitumen from solid organic matter. Possibly other bitumen generations also occurred during and following this time interval. The organic matter in the natural reactors soon became a solid, and was able to prevent the loss of uraninite grains enclosed in it. These uraninite grains trapped and then immobilized fission products until, and to an extent even after, a major tectonic-igneous event, the intrusion of the dolerite dike swarm.