On the Oxidation of Uraninite from Natural Reactor Cores
- PDF / 585,852 Bytes
- 8 Pages / 389.34 x 622.08 pts Page_size
- 40 Downloads / 176 Views
equivalent long-term natural processes. The Oklo-Okelobondo uranium mine and the nearby uranium deposit at Bangombd (SE Gabon, western central Africa), are the only known locations where nuclear criticality occurred in a natural setting without human intervention t 11. These natural reactors provide, therefore, unique evidence in helping to understand processes that might occur in radioactive waste disposal sites. One of the most interesting and important questions is how the core material (uraninite) in shallow natural reactors is preserved from complete oxidative dissolution through geological time (2 Gy). The Bangombd reactor is only at a depth of 12 meters from the surface; a thin (5 cm) layer of uraninite in the reactor core is preserved. The uranium mineralisations are associated with tectonic structures (folds, joints and faults) where sandstone is in contact with black shale and also associated with organic material, which typically occurs as up to cm-size nodules in the sandstone. Petrographic analysis shows that uraninite in the reactor core occurs as up to 0.5 mm-size corroded fragments in a matrix of illite, chlorite and traces of recently formed kaolinite [21. At the edge of the reactor, old "bitumen" (partially graphitised) appears to become important and contains numerous inclusions of uraninite. An undefined Pb-oxide or hydroxide was observed on the edge of the reactor [2, 3]. Chemical analysis of unaltered uraninite shows that the PhO-content averages 7 wt.% [2, 31. Immediately beneath the Bangombd reactor the uraninite has been subjected to alteration, partially forming phosphatian coffmite, lead was completely lost during the coffinitization of uraninite. As a result of oxidation, uranyl phases formed in micro-fractures and
817 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
dissolution voids in uraninite [2]. In many cases, later disturbances have caused all or some of the radiogenic lead to have been lost from the uraninite and to have been redeposited as galena (PbS) [4] or other Pb minerals. The crystal structure of a lead-uranyl mineral, fourmarierite ( Pb(U02)40 3(OH) 4.4H 20 ) has been investigated [5]. The mineral has been found at several uranium deposits, such as Shinkolobwe, Shaba, Congo [6] and Koongarra, Australia [7], but has not been found to date in the Oklo/Okelobondo-Bangomb6 natural reactors. A greater understanding of the formation of uranium minerals as resulting from the weathering of natural reactor core (uraninite) material and the rate of the weathering are useful in evaluating the degree of uranium migration. To this end, we have studied the extent of oxidation of uraninite from natural reactors at different depths. EXPERIMENT The mineral material, sample treatment and the experimental methods are listed in Table 1. The experimental methods used in the present work and their purpose are explained as follows: *XRD(X-Ray Diffraction, 2 0 from 5.000 to 80.000); for mineral identification. The samples (0.5 g) were grinded to powder form (30 pg). whic
Data Loading...
