ALTERATION OF NATURAL UO2 UNDER OXIDIZING CONDITIONS FROM SHINKOLOBWE, KATANGA, ZAIRE - A NATURAL ANALOG FOR THE CORROSION OF SPENT FUEL

Characterization of the uranium-containing solid alteration phases of spent UO2 fuel is required for the long term predictive modeling of groundwater chemistries in contact with a high-level nuclear waste repository. The alteration of uraninite (UO2+x) at Shinkolobwe, Zaire provides a natural analogue for the corrosion of spent nuclear fuel in contact with oxidizing groundwater. An integrated analysis of uraninite alteration product using optical microscopy, scanning electron microscopy, analytical electron microscopy, X-ray diffraction analysis, and electron microprobe analysis demonstrates the presence of coexisting phases at all analytical scales. The uranyl silicates uranophane (H3O)2Ca(UO2)2(SiO4)2 . 3H2O and cuprosklodowskite (H3O)2Cu(UO2)2(SiO4)2 . 4H2O are ubiquitous in the samples studied and replace the uranyl oxide hydroxides becquerelite Ca(UO2)6O4(OH)6 . 8 H2O, compreignacite K2(UO2)6O4(OH)6 . 8 H2O, vandendriesscheite PbU7O22 . 22H2O, fourmarierite PbU4O13 . 6 H2O, billietite Ba(UO2)6-O4(OH) 6 . 8 H2O, and schoepite UO3 . 2H2O by reaction of the oxide hydrates with silica-rich groundwater. A reduction in grain size is apparent as alteration proceeds.