Surface Alteration of Nd-Bearing Zirconolite Following Hydrothermal Treatment
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Surface Alteration of Nd-Bearing Zirconolite Following Hydrothermal Treatment Zhaoming Zhang, Eric R. Vance and Mark G. Blackford Materials Division, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia Email: [email protected] ABSTRACT Static dissolution tests on Nd-bearing zirconolite were conducted in deionized water at 150°C for up to 6 months. Surfaces, both before and after aqueous dissolutions, were examined using X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). Individual submicron-sized crystals were observed only on some zirconolite grains after hydrothermal treatment for one week, and were identified as brookite (possibly plus anatase) by Transmission Electron Microscopy (TEM). The number of secondary crystals present on the zirconolite surface was, however, reduced significantly after 6 months of durability testing. The results of electron microscopy studies are consistent with those obtained from XPS in that the average Ca and Ti concentrations of the top surface layer (< 5 nm) decreased slightly with respect to Zr, while the average Al concentration increased after durability testing. INTRODUCTION Zirconolite is the primary actinide-bearing phase in synroc, a titanate-based ceramic designed for the immobilization of high level nuclear waste from the reprocessing of spent nuclear fuel [1]. Zirconolite-rich versions of synroc for the immobilization of waste actinides have also been developed [2]. As synroc is envisaged to be disposed in geological repositories, it may come into contact with ground water. Therefore, its chemical durability in an aqueous environment is an important issue. Previous dissolution studies on synroc [3-7] have indicated that zirconolite is the most resistant synroc phase upon exposure to water. However, there are few publications on the dissolution behavior of the zirconolite phase alone. This is partly due to the difficulties of fabricating single-phase zirconolite. Perovskite is a common minor phase in zirconolite and is the least resistant synroc phase (the Ca dissolution rate from perovskite is orders of magnitude higher than that from zirconolite), so it is imperative that perovskite-free zirconolite samples are used to study the true dissolution behavior of zirconolite. The samples used in previous dissolution studies of synroc and individual synroc phases were in the form of as-cut (rough) surfaces, polished discs, thinned TEM specimens and powder. It is known that sample preparation procedures, such as cutting, polishing, ion-beam thinning or crushing, can lead to surface deformation and stress. For example, synroc discs finished with ¼ µm diamond abrasive have a damaged surface layer of ~2 µm thick [3]. It was observed that the deformation introduced by cutting led to preferential attack during dissolution tests of perovskite [8]. Therefore it is highly desirable to use strain-free samples in dissolution tests. It was shown that mechanical damage caused by polishing was reduced at the BaTiO3 surface by s
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