Alteration products of uraninite from the Colorado Plateau

Abstract

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, during corrosion under oxidizing conditions. The long-term alteration products and processes of uraninite may provide insight into the corrosion of the UO2in spent nuclear fuel. Uraninite, schoepite, calciouranoite, uranophane, fourmarierite, a Fe-rich uranyl phase, and coffinite were identified. The primary uraninites and alteration phases generally have low trace element contents, except for coffinite from Caribou Mine, Colorado that has Y2O3as high as 0.88 wt%. The highest Zr, Ti, Th and REE values of the uraninite are ThO20.17, Y2O30.28, La2O30.03, Ce2O30.10, Pr2O30.02, Nd2O30.14, Sm2O30.12, Eu2O30.04, Gd2O30.08, ZrO20.93, and TiO20.54 wt%, which are in general lower than the corresponding components in secondary uranyl phases (the highest values are ThO20.21, Y2O30.88, La2O30.05, Ce2O30.15, Pr2O30.05, Nd2O30.18, Sm2O30.14, Eu2O30.11, Gd2O30.08, ZrO22.11, and TiO22.74 wt%), suggesting that trace elements preferentially enter the 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 ZrO2decrease, and SiO2, TiO2, CaO and P2O5increase. Alteration causes loss of U, Pb and Zr and incorporation of Si, Ti, Ca and P into uranyl phases. The Ca-rich calciouranoite from Grants, New Mexico, indicates an interaction between the limestone 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 the secondary phases.