UO2 corrosion in an iron waste package

In order to investigate the interactions between spent nuclear fuel, corroding iron waste packages, and water under conditions likely to be relevant at the proposed repository at Yucca Mountain, six small-scale waste packages were constructed. Each package differed with respect to water input, exposure to the atmosphere and temperature. Two of the packages contained 0.1 g UO2. Simulated Yucca Mountain process water (YMPW) was injected into five of the packages at a rate of 200 μl per day for up to 2 years, at which point the solids were characterized with X-ray powder diffraction, scanning electron microscopy, wet chemistry and electron microprobe analysis. Fe(II) is abundant in the corrosion products that form, and the dominant crystalline product in all cases according to X-ray diffraction is magnetite or the structurally similar maghemite. Minor phases included akaganeite (β-FeOOH) and possibly also hematite (Fe2O3), lepidocrocite (γ-FeOOH) and green rust (Fe(II)1−xFe(III)x(OH)2Yx/n). Under these conditions, UO2 is expected to alter to the uranyl silicate uranophane (Ca[(UO2)SiO3(OH)]2·5H2O). Neither oxidation of the UO2 nor any oxidized (uranyl) solid was observed, suggesting that conditions were sufficiently reducing to kinetically hinder U(IV) oxidation.