Microscopic and Spectroscopic Characterization of Aluminosilicate Waste Form with Cs/Sr/Ba Loading Using Scanning Electron Microscopy, Transmission Electron Microscopy, and X-Ray Diffraction

Abstract

An aluminosilicate waste form has been proposed for the storage and disposal of cesium and strontium isolated from recycled nuclear fuel. To examine the impact of sintering temperature on the waste form product, thermal analysis (thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)) was used to identify key transition temperature ranges. Samples were produced in each temperature range to examine the impact on phase formation and microstructure. Examination of the synthesized materials by X-ray diffraction (XRD) confirmed the formation of the expected Cs- and Sr-aluminosilicate crystalline phases. However, microscopic characterization by scanning electron microscopy (SEM) revealed a spongelike, glassy morphology with high porosity and no observed crystallinity. This discrepancy was investigated by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), which identified the presence of discrete, submicron, crystalline phases within the bulk amorphous matrix. Elemental analysis by energy-dispersive X-ray (EDX) indicated that the strontium and barium were incorporated into the crystalline phase, while the cesium was incorporated into the amorphous matrix. Further analysis of samples synthesized without barium or strontium allowed for the identification of submicron crystalline phases within the amorphous matrix, identifying the source of the cesium aluminosilicate crystal peaks in the XRD patterns, with elemental analysis showing that the cesium was present in both the crystalline inclusions and the amorphous bulk phase.