U- and Hf-Bearing Pyrochlore and Zirconolite and their Leached Layers Formed in Acidic Solution: Tem Investigation
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U- AND Hf-BEARING PYROCHLORE AND ZIRCONOLITE AND THEIR LEACHED LAYERS FORMED IN ACIDIC SOLUTION: TEM INVESTIGATION Huifang Xu1, Yifeng Wang1, 2, Pihong Zhao3, William L. Bourcier3, Richard Van Konynenburg3, and Henry F. Shaw3 1. Transmission Electron Microscopy Laboratory, Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131, USA. 2. Sandia National Laboratories, Carlsbad, NM 88220, USA 3. Lawrence Livermore National Laboratory, L-219, Livermore, CA 94550, USA E-mail: [email protected] Abstract Transmission electron microscopy results from a sintered ceramics with stoichiometry of Ca(U0.5Ce0.25Hf0.25)Ti2O7 show the material contains both pyrochlore and zirconolite phases and structural intergrowth of zirconolite lamellae within pyrochlore. (001) plane of zirconolite is parallel to (111) plane of pyrochlore because of their structural similarities. The pyrochlore is relatively rich in U, Ce, and Ca with respect to the coexisting zirconolite. Average compositions for the coexisting pyrochlore and zirconolite at 1350 °C are Ca1.01(Ce3+0.13Ce4+0.19U0.52Hf0.18)(Ti1.95Hf0.05)O7 (with U/(U+Hf) = 0.72) and (Ca0.91Ce0.09)(Ce3+0.08U0.26Hf0.66Ti0.01)Ti2.00O7 (with U/(U+Hf) = 0.28) respectively. A single pyrochlore (Ca(U,Hf)Ti2O7) phase may be synthesized at 1350 °C if the ratio of U/(U+Hf) is greater than 0.72, and a single zirconolite (Ca(Hf,U)Ti2O7) phase may be synthesized at 1350 °C if the ratio of U/(U+Hf) is less than 0.28. An amorphous leached layer that is rich in Ti and Hf forms on the surface after the ceramics has been leached in pH 4 buffered solution. The thickness of the layer ranges from 5 nm to 15 nm. The leached layer functions as a protective layer and therefore reduces the leaching rate. 1. Introduction A durable titanate ceramic waste form (Synroc) with pyrochlore (Ca(U,Pu)Ti2O7) and zirconolite (CaZrTi2O7), a derivative structure of pyrochlore, as major crystalline phases has been shown to be particularly promising for immobilizing various high level wastes containing fissile elements (239Pu and 235U) [1-12]. General stoichiometry for zirconolite and pyrochlore can be expressed as ABTi2O7. Tetravalent actinides generally occupy site B positions with coordonation number of 7 or 8. Thermodynamic study indicates that U- and Pu-pyrochlore phases are stable with respect to rutile, perovskite, and their oxides UO2 and PuO2 [12]. However, Th-pyrochlore is unstable with respect to rutile, perovskite, and its oxide ThO2 [12]. The Synroc ceramic waste form will be thermodynamically stable in aqueous silica-depleted and aqueous carbonate-depleted groundwater environments (such as WIPP site in salt rock formation) [13]. The concept of Synroc was originally proposed by Ringwood et. al. [2] and the first Synroc fabrication technology was developed by Dosch et al. [1]. During the last two decades, Synroc has been subjected to extensive studies [1-14]. Synroc immobilizes radionuclides by incorporating them into appropriate phases and forming solid solutions. With larg
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