Mat. Res. Soc. Symp. Proc. Vol. 663 ' 2001 Materials Research Society Dissolution of Synthetic Brannerite at 90C Y. Zhang, K.P. Hart, B.S. Thomas, Z. Aly, H. Li and M. Carter Materials Division, ANSTO, PMB 1, Menai 2234, Australia Email: yzx@ansto.gov.au ABSTRACT Brannerite, as a minor phase, exists in the pyrochlore-rich titanate ceramic formulations designed for immobilization of surplus weapons Pu. The dissolution of synthetic brannerite was studied at 90C using static tests in pH 4 solution, deionized water and Finnsjn synthetic groundwater. After 140 days the normalized U release rates into a pH 4 solution and deionized water reach similar values, ~10 -2 g m -2 d -1 , and are about 2 orders of magnitude higher than those in Finnsjn synthetic groundwater. The normalized Ti release rate into Finnsjn synthetic groundwater is about an order of magnitude higher than those in pH 4 solution and deionized water. The dissolution of brannerite is incongruent in the pH 4 solution and deionized water (preferential release of U over Ti), and nearly congruent in Finnsjn synthetic groundwater. SEM observations of the samples after 140 days in pH 4 solution and deionized water revealed minor surface alteration, in the form of a thin surface layer, probably TiO 2 , as a result of preferential releases of U in both cases. INTRODUCTION Brannerite (UTi 2 O 6 ), a uranous titanate mineral, occurs naturally in many uranium ore bodies. It also exists as a minor phase in the titanate ceramic formulations designed for immobilization of excess weapons plutonium [1]. Its long-term chemical durability is of significance in the formulation design and is being extensively studied at ANSTO [2]. To evaluate the chemical durability of brannerite as an actinide-bearing phase under oxidative groundwater conditions requires a more detailed understanding of its dissolution mechanism; alteration products and properties, kinetics of uranium release, extent of analogy between synthetic and natural brannerite, and a mathematical description of its dissolution process for long-term prediction. This paper presents the results of static dissolution tests in pH 4 solution, deionized water and Finnsjn synthetic groundwater at 90C. 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).