Durabilities of Pyrochlore-Rich Titanate Ceramics Designed for Immobilization of Surplus Plutonium
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Durabilities of Pyrochlore-Rich Titanate Ceramics Designed for Immobilization of Surplus Plutonium Y. Zhang, K.P. Hart, M.G. Blackford, B.S. Thomas, Z. Aly, G.R. Lumpkin, M.W. Stewart, P.J. McGlinn and A. Brownscombe Materials Division, ANSTO, PMB 1, Menai, NSW 2234, Australia, Email: [email protected] ABSTRACT The chemical durabilities of two Pu-doped pyrochlore samples were studied by Single-PassFlow-Through (SPFT) tests at 70°C. The dissolution of pyrochlore is incongruent with preferential releases of Ca and Gd over Ti, close to stoichiometric releases of U and Ti, and lower releases of Hf and Pu than Ti. Altered pyrochlore and polymorphs of TiO2 (brookite and probably anatase) have been identified on the surface of the leached sample and the principal secondary phase is an unknown polymorph of TiO2 containing Hf and varying amounts of Gd and Pu. These surface alteration phases are consistent with reported studies of natural samples. The releases of U, Gd, Ca and Ti into solution follow linear kinetics, whereas the releases of Pu and Hf exhibit non-linear behavior. The presence of ~5% PuO2 and trace amounts of glass does not appear to have an effect on the overall durability of the material. Further, the low Pu release rate and the similar kinetics for Pu and Hf releases limit the possibility of nuclear criticality under repository conditions. Overall, this study provides useful information on the lower bounds of durabilities of the materials. INTRODUCTION A pyrochlore-rich titanate ceramic formulation has been chosen by the US Department of Energy for immobilization of surplus plutonium [1]. Previous MCC-1 leaching results indicated that impurities and surface finishes have little effect on the normalized elemental release rates [2]. To evaluate the long-term chemical durability of this ceramic waste form, kinetic information is essential, given the long half-lives of the actinide elements and the importance of understanding their long-term release relative to the neutron absorbers (Gd and Hf). In addition, the laboratory experimental results combined with natural analogue studies on pyrochlore group minerals should be able to provide additional evidence to clarify the dissolution mechanism, which would, in turn, provide confidence and aid in extrapolation of laboratory experimental data over geological time scales. In the present study, kinetics of elemental releases from two pyrochlore-rich titanate ceramics have been examined. Their long-term durabilities, based on kinetic information, as well as comparison with natural analogue studies, are discussed.
EXPERIMENTAL Electron microscopy Scanning electron microscopy (SEM) was carried out with a JEOL 6400 instrument operated at 15 kV, and fitted with a NORAN Voyager IV X-ray microanalysis System (EDX). Calibrations for microanalysis were carried out using a comprehensive set of standards for quantitative analysis [3-4]. Transmission electron microscopy (TEM) was carried out with a JEOL 2010F instrument equipped with a Link EDX detector and an EmiSpec ES Vision
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