Microstructure evolution and weathering reactions of Synroc samples crystallized from CaZrTi 2 O 7 melts: TEM/AEM invest
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ZrTiO 4 phase --
Zr-rich zirconolite
-4
Zr-poor zirconolite
--
rutile/perovskite. This sequence is induced by a fractional crystallization process, in which Zr-rich mineral phases tend to crystallize first, resulting in continuous depletion of Zr in melt. Consistent with this melt compositional evolution, Zr content in the zirconolite decreases from the area next to ZrTiO 4 phase to the area next to rutile or perovskite. High-resolution TEM images show that there are no glassy phases at the grain boundary between zirconolite and perovskite. The fractional crystallization-induced textural heterogeneity may have a significant impact on the incorporation of radionuclides into crystalline phases and the resistance of radioniclides to leaching processes. Exsolution lamellae and multiple twinning result from the phase transition from tetragonal zirconia to monoclinic zirconia may decrease durability of the Synroc. Fast cooling of melt may produce more zirconolite phase and relatively uniform texture. In general, however, a Synroc prepared by a through-melt method is less uniform in texture than that prepared by a through-sol-gel method. The reaction path calculation for the alteration of U-bearing zirconolite in an oxidizing fluid shows that zirconolite is first altered into a perovskite-like phase (CaZrO,), followed by rutile (TiO 2 ),
and U"' -bearing phases [Ca(U0J 2Si 60,5'5H20].
of
soddyite
[(UO2) 2SiO 4 2H 20]
and
haiweeite
INTRODUCTION Development of highly durable waste forms is the key to permanent disposal of HLW including surplus weapons-usable plutonium in geologic repositories. Synroc is a durable titanate ceramic waste form with zirconolite (CaZrTi 2O), pyrochlore (Ca(U,Pu)Ti.0 7), perovskite (CaTiO 3), hollandite as major crystalline phases and has been shown to be particularly promising for immobilizing various high level wastes [1-12]. The concept of
Synroc was originally proposed by Ringwood et al. in Australia and the first Synroc fabrication technology was developed by Dosh et al. (1982) [1] at Sandia National Laboratories. During the last two decades, Synroc has been subjected to extensive studies [1-14]. Synroc immobilizes radionuclides by incorporating them into appropriate mineral structures and forming solid solutions. With large polyhedra (with coordination numbers ranging from 7 to 12) in mineral structures, Synroc is able to accommodate a wide range of radionuclides (e.g., actinides, Pu, U, Ba, Sr, Cs, Rb, Tc, etc.) as well as neutron poisons (e.g., Gd) [10]. U- and Pu-loaded Synroc is generally dominated by phases of pyrochlore and zirconolite, a derivative structure of pyrochlore [1, 2, 3, 7, 8, 15]. The pyrochlore phase can incorporate more Pu than the zirconolite phase [3]. Various Synroc formulations (e.g., Synroc-C, Synroc-D, Synroc-E, Synroc-F etc.) have been developed for specific HLWs [I,
47 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
15, 16, 17]. Synroc has been shown to be much more chemically durable and radiationresistant than borosilicate
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