Department of Nuclear Engineering and Radiological Sciences

Department of Geological Sciences

The University of Michigan, Ann Arbor, Michigan 48109-2104

Uraninite and associated alteration products from the Colorado Plateau were studied by optical microscopy, electron microprobe analysis (EMPA), scanning electron microscopy (SEM) and backscattered electron (BSE) imaging in order to determine the behavior and fate of trace elements, such as Pb, Ca, Si, Th, Zr, and REE. The long-term alteration products and processes of uraninite may be used as a natural analogue of the corrosion of the UO2 in spent nuclear fuel. Uraninite, schoepite, calciouranoite, uranophane, fourmarierite, an Fe-rich uranyl phase, and a uranyl silicate were identified. The primary uraninites and alteration phases generally have low trace element contents, except for a uranyl silicate phase from Caribou Mine, Colorado that has Y2O3 as high as 0.88 wt %. The highest average Zr, Ti, Th and REE contents of the uraninite are ThO₂ 0.17, Y₂O₃ 0.28, La₂O₃ 0.03, Ce₂O₃ 0.10, Pr₂O₃ 0.02, Nd₂O₃ 0.14, Sm₂O₃ 0.12, Eu₂O₃ 0.04, Gd₂O₃ 0.08, ZrO₂ 0.93, and TiO₂ 0.54 wt %, which are in general lower than the corresponding components in secondary uranyl phases (the highest averages are ThO₂ 0.21, Y₂O₃ 0.88, La₂O₃ 0.05, Ce₂O₃ 0.15, Pr₂O₃ 0.05, Nd₂O₃ 0.18, Sm₂O₃ 0.14, Eu₂O₃ 0.11, Gd₂O₃ 0.08, ZrO₂ 2.11, and TiO₂ 2.74 wt %), indicating that trace elements preferentially enter structures of these secondary uranyl phases (Table 2). A compositional profile of a concentric structure in schoepite shows that, with increasing alteration, UO2, PbO and ZrO2 decrease, and SiO2, TiO2, CaO and P2O5 increase. Alteration causes loss of U, Pb and Zr and incorporation of Si, Ti, Ca and P into uranyl phases. The Ca-rich calciouranoite hosted in limestone from Grants, New Mexico, indicates an interaction between the host rock and uranium minerals. Texturally, concentric structures and micro-fractures are common. Concentric structures are usually composed of both uraninite and uranyl phases, while micro-fractures are common in secondary phases.