The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16

doi:10.1016/S0016-7037(97)00261-5

Geochimica et Cosmochimica Acta

Volume 61, Issue 21, November 1997, Pages 4479-4494

https://doi.org/10.1016/S0016-7037(97)00261-5 Get rights and content

Abstract

In an underground nuclear waste repository, the chemical behavior of some stored fission products and actinides depends on the redox conditions during their long-term evolution. In this respect, radiolysis is an important phenomenon which can significantly modify the local redox conditions. The Oklo natural fission zones are good examples where the effect of radiolysis can be deduced from a mineralogical and geochemical study. Zones 10 and 16 were studied because they are located at depth of 270 m in an area devoid of any recent water circulation and not subject to the effect of the lateritic alteration occurring elsewhere in this area.

In zone 10, there is a marked evolution of the UPbFeS mineralogy from the center to the periphery of the reactor zone. In the center, uraninite shows silicification and coffinitisation with the formation of galena and native lead; the PbO content of uraninite can be as much as 20 wt%. In the periphery of the reactor zone, some radiogenic lead is present as minium (Pb₃0₄) and in Pb-bearing calcite. In the surrounding sandstones, hematite is widespread.

In zone 16, the mineral paragenesis is generally comparable with that of zone 10 but with some differences. Galena is the only Pb-bearing mineral associated with uraninite crystals. The PbO content of uraninite is always <7 wt%. In the periphery of the alteration zone, barite partly replaces quartz. In the reactor zone, hematite is sometimes replaced by pyrite.

In an area where the fission zone 10 is in contact with sandstones devoid of organic matter, H₂OH₂O₂ and H₂0H₂ ± CH₄ inclusions were observed in healed microcracks in the detrital quartz grains. Based on microthermometric measurements, the salinity of the aqueous solution ranges from 0.2 to 18 wt% eq. NaCl. Raman analysis of the gas phase indicates that the hydrogen to oxygen ratio differs from an inclusion to the other.

The presence of H₂₋ and O₂-bearing fluid inclusions confirms the existence of water radiolysis in the reactor zone. In zones rich in organic matter, oxygen was preferentially consumed in reactions with organic matter and its radiolytic products. In the absence of organic matter and its radiolytic products, oxidizing conditions prevailed and hematite and minium have formed. Consequently, presence of highly oxidizing conditions has important consequences on uranium and fission products mobility. Water radiolysis is also evidenced during the subsequent 1.97 By history of the reactor.

During the fission reactions, redox conditions deduced from Pb-bearing minerals indicate that the sulfur fugacity was very low in zone 10 as shown by the formation of minium and higher in zone 16 with the formation of barite and pyrite. In this paper we discuss the geochemical consequences of such contrasting redox zones and the resulting redox heterogeneity created by water radiolysis.

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^∗: Present address: Universit'e de Paris-Sud XI, Département des Science de la Terre, Batiment 504, 91405 Orsay Cedex, France.

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ArticeThe effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16

Abstract

Precambrian Res.

Chem. Geol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Org. Geochim.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

J. Nucl. Mater.

Geochim. Cosmochim. Acta

Chem. Geol.

Electron microprobe study of uranium minerals and its application to some Canadian deposits

Caracteres minéralogiques des pechblendes de la province hercynienne d'Europe. Comparaison avec les oxydes d'uranium du Proterozofque de differents gisements d'Amerique du Nord, d'Afrique et d'Australie

The potential of natural analogues in assessing systems for deep disposal of high-level radioactive waste

NAGRA NTB-84-41. Baden, Switzerland. EIR BER NR 545, Würenlingen, and KBS TR84-16

A geological investigation upon the effects of ionizing radiation on hydrocarbons

J. Inst. Petrol.

Radiolysis in Nature: Evidence from the Oklo Natural Reactors (SKBF/KBS No. 83-10)

Determination of water, hydrates, and pH in fluid inclusions by micro-Raman spectrometry

Eur. J. Mineral.

Conclusions tithes de l'etude de la migration des produits de fission

UPb age (2.05 x 09 years) of the Oklo uranium deposit

Solutions, Minerals, and Equilibria

Artice
The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16

UPb age (2.05 x 0⁹ years) of the Oklo uranium deposit