Alteration of Microstructure of West Valley Glass by Heat Treatment
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ALTERATION OF MICROSTRUCTURE OF WEST VALLEY GLASS BY HEAT TREATMENT A. C. BUECHELE, X. FENG, H. GU, AND I. L. PEGG Vitreous State Laboratory, The Catholic University of America, Washington, D.C. 20064 Samples of West Valley reference glass WVCM-59 were subjected to isothermal heat treatment according to a systematic schedule of time-temperature combinations. Phases crystallizing during heat treatment were analyzed and quantified using an SEM equipped with an energy dispersive x-ray detector and image processing and analyzing capabilities. Only small amounts of noble metals (e.g. Rh, Pd) and RuO2 were present in the as-melted glass, but these frequently serve as nucleation sites during heat treatment. Irongroup spinels containing Fe, Ni, Cr, and Mn in variable proportions were the most common phases observed, appearing in quantities up to 3.5 vol% in heat treated glass. The formation of a thoria-ceria phase occurred at temperatures of 900°C and below. Acmites formed at temperatures of 800°C and below. A lithium phosphate phase with a particularly interesting morphology developed after prolonged heat treatment at temperatures between 600-750°C. The same phase appeared in a range of glass compositions. However, canister cooling curves preclude such a phase in production glass. Furthermore, no such phase was found in actual canister-cooled glass. INTRODUCTION Glasses suitable for high-level nuclear waste vitrification must meet a number of requirements including processability, durability, and phase stability. In this paper we report findings from a study of the phase stability of WVCM59 which is a candidate West Valley reference glass that was designed at Catholic University. Excessive secondary phase formation could adversely affect glass performance from two perspectives. First is formation of such phases in the melter which may affect processability, particularly if there is a tendency towards sedimentation, since this could lead to accumulation over the course of the melt campaign. Second is phase stability during the cooling cycle which the glass experiences after pouring into the canister. If significant phase separation occurs, properties such as chemical durability may depart from the values expected for the homogeneous waste form. It is useful, therefore, to determine the extent of alteration which can occur under both nominal conditions and overstressed conditions to ensure that data are available to assess any worst-case scenarios which may arise. Our data indicate that WVCM59 is phase-stable over the expected range of melter temperatures and residence times. Similarly, only minimal secondary phase formation, consisting predominantly of spinels, is expected in canister-cooled glass. Prolonged heat treatment at temperatures between 600 and 7500C, yields a lithium phosphate phase with a particularly interesting morphology which we discuss in some detail. Based on measured canister cooling curves [1,2], it is, however, unlikely that this phase would occur in any potential canister cooling scenarios. Mat. Res.
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