Reconstruction of cumulative fission yield curve and geochemical behaviors of fissiogenic nuclides in the Oklo natural reactors.

Hidaka, Hiroshi; Konishi, Takashi; Masuda, Akimasa

doi:10.2343/geochemj.26.227

Cited by 18 publications

(8 citation statements)

References 28 publications

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“…Among seven isotopes, 135 Ba, 137 Ba and 138 Ba are produced by fission. Previous studies showed that long-lived fissiogenic Cs isotopes as well as Rb, Sr and Ba were almost completely removed from the reactors because of their incompatibilities with the UO 2 crystalline structure, and their high solubility and reactivity (Brookins et al, 1975;Hidaka et al, 1992). However, small but significant amounts of these elements remained in the core of RZ10, which is well protected from weathering because of its deep location (Hidaka et al, 1993;Hidaka and Holliger, 1998).…”

Section: Introductionmentioning

confidence: 94%

Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

Hidaka

Gauthier-Lafaye²

2008

Geochimica et Cosmochimica Acta

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Ba isotopic studies of the Oklo and Bangombé natural fission reactors in east Gabon provide information on the geochemical behavior of radioactive Cs ( 135 Cs and 137 Cs) in a geological medium. Large isotopic deviations derived from fissiogenic Ba were found in chemical leachates of the reactor uraninites. The fissiogenic Ba isotopic patterns calculated by subtracting the non-fissiogenic component are classified into three types that show different magnifications of chemical fractionation between Cs and Ba. In addition, the isotopic signatures of fissiogenic 135 Ba, 137 Ba and 138 Ba suggest an early differentiation between Cs and Ba of less than 20 years after the production of fissiogenic Cs and Ba. On the other hand, only small excesses of 135 Ba (e < +1.8) and/or 137 Ba (e < +1.3) were identified in some clay samples, which might have resulted from selective adsorption of 135 Cs and 137 Cs that migrated from the reactors by differentiation.

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Section: Introductionmentioning

confidence: 94%

Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

Hidaka

Gauthier-Lafaye²

2008

Geochimica et Cosmochimica Acta

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“…Most of the fissiogenic REE are well retained in the natural reactors. However, the loss of some amount of light REE (LREE) from the reactors has been recognized from the results of bulk rock analyses Hidaka et al, 1992;Menet et al, 1992). Menet et al (1992) measured Nd isotopes of the Oklo reactor 10 and the surrounding rocks, and found the released fissiogenic Nd along carbonate-filled fissures in the range of 0.5 to 1.0 m. Our current interest is to elucidate when and how the fission products have migrated and finally retained.…”

Section: Introductionmentioning

confidence: 96%

Geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at Bangombé, Gabon

Hidaka

Janeczek

Skomurski

et al. 2005

Geochimica et Cosmochimica Acta

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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 to 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 ϭ 0.00609 to 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 ϭ 0.00725) and small amount of fissiogenic REE components. The proportions of fissiogenic and non-fissiogenic REE components in four samples from the core of BAX03 vary in depth ranging from 30 cm to 130 cm beneath the reactor, which suggests mixing between fissiogenic isotopes from the reactor and non-fissiogenic isotopes from original minerals in the sandstone. Significant chemical fractionation was observed between Ce and the other REE in the secondary minerals, which shows evidence of an oxidizing atmosphere during their formation. Pb-isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of U-Pb decay system due to recent geologic alteration. However, systematic Pb-isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes, Pb and U from the reactor in association with dolerite dyke intrusion ϳ0.798 Ga ago and the formation of the secondary minerals by mixing event between 2.05 Ga-old original minerals and reactor materials due to recent alteration.

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“…Thorium is of low abundance in the surrounding rocks, and its presence in the reactor zones is taken to indicate that uranium and plutonium were mostly retained in the reactor core. Plutonium was also retained by clays in the argile de pile, as is evidenced by enrichments of 235 U, the long-lived daughter of 239 Pu, in some reactor clay samples (Pourcelot and Gauthier-Lafaye, 1999 (Hidaka et al, 1992). These studies suggest that retention of fission products by the reactors was variable.…”

Section: Natural Analogs For the Long-term Behavior Of Radionuclides mentioning

confidence: 79%

Environmental Geochemistry of Radioactive Contamination

Siegel

Bryan

2003

Treatise on Geochemistry

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This report attempts to describe the geochemical foundations of the behavior of radionuclides in the environment. The information is obtained and applied in three interacting spheres of inquiry and analysis: 1) experimental studies and theoretical calculations, 2) field studies of contaminated and natural analog sites and 3) model predictions of radionuclide behavior in remediation and waste disposal. Analyses of the risks from radioactive contamination require estimation of the rates of release and dispersion of the radionuclides through potential exposure pathways. These processes are controlled by solubility, speciation, sorption, and colloidal transport, which are strong functions of the compositions of the groundwater and geomedia as well as the atomic structure of the radionuclides.The chemistry of the fission products is relatively simple compared to the actinides. Because of their relatively short half-lives, fission products account for a large fraction of the radioactivity in nuclear waste for the first several hundred years but do not represent a long-term hazard in the environment. The chemistry of the longer-lived actinides is complex; however, some trends in their behavior can be described. Actinide elements of a given oxidation state have either similar or systematically varying chemical properties due to similarities in ionic size, coordination number, valence, and electron structure. In dilute aqueous systems at neutral to basic pH, the dominant actinide species are hydroxy-and carbonato-complexes, and the solubility-limiting solid phases are commonly oxides, hydroxides or carbonates. In general, actinide sorption will decrease in the presence of ligands that complex with the radionuclide; sorption of the (IV) species of actinides (Np, Pu, U) is generally greater than of the (V) species.The geochemistry of key radionuclides in three different environments is described in this report. These include: 1) low ionic strength reducing waters from crystalline rocks at nuclear waste research 4 sites in Sweden; 2) oxic water from the J-13 well at Yucca Mountain, Nevada, the site of a proposed repository for high level nuclear waste (HLW) in tuffaceous rocks; and 3) reference brines associated with the Waste Isolation Pilot Plant (WIPP). The transport behaviors of radionuclides associated with the Chernobyl reactor accident and the Oklo Natural Reactor are described. These examples span wide temporal and spatial scales and include the rapid geochemical and physical processes important to nuclear reactor accidents or industrial discharges as well as the slower processes important to the geologic disposal of nuclear waste.Application of geochemical information to remediating or assessing the risk posed by radioactive contamination is the final subject of this report. After radioactive source terms have been removed, large volumes of soil and water with low but potentially hazardous levels of contamination may remain. For poorly-sorbing radionuclides, capture of contaminated water and removal of radi...

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Reconstruction of cumulative fission yield curve and geochemical behaviors of fissiogenic nuclides in the Oklo natural reactors.

Cited by 18 publications

References 28 publications

Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

Geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at Bangombé, Gabon

Environmental Geochemistry of Radioactive Contamination

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