The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16
- CREGU and G.R. CNRS-CREGU, Vandoeuvre-les-Nancy (France)
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 minimum (Pb{sub 3}O{sub 4}) 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, H{sub 2}O-H{sub 2} {+-} CH{sub 4} 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. 41 refs., 15 figs., 3 tabs.
- OSTI ID:
- 585491
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 61, Issue 21; Other Information: PBD: Nov 1997
- Country of Publication:
- United States
- Language:
- English
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