Mathematics – Logic
Scientific paper
Nov 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.4479s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, Issue 21, pp.4479-4494
Mathematics
Logic
8
Scientific paper
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 3 O 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 2 O-H 2 -O 2 and H 2 O-H 2 ± CH 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. The presence of H 2 - and O 2 -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 minimum 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 minimum 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.
Pagel Maurice
Savary Véronique
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