ANALYTICAL ELECTRON-MICROSCOPY STUDY OF SURFACE-LAYERS FORMED ON THE FRENCH SON68 NUCLEAR WASTE GLASS DURING VAPOR HYDRATION AT 200-DEGREES-C

Extensive solid-state characterization (AEM/SEM/HRTEM) was completed o n six SON68 (inactive R7T7) waste glasses which were alter ed in the p resence of saturated water vapor (200 degrees C) for 22, 91, 241, 908, 1000, 1013, and 1021 days. The samples were examined by AEM in cross- section (lattice-fringe imaging, micro-diffraction, and quantitative t hin-film EDS analysis). The glass monoliths were invariably covered by a thin altered rind, and the surface layer thickness increased with i ncreasing time of reaction, ranging from 0.5 to 30 mu m in thickness. Six distinctive zones, based on phase chemistry and microstructure, we re distinguished within the well-developed surface layers. Numerous cr ystalline phases such as analcime, gyrolite, tobermorite, apatite, and weeksite were identified on the surfaces of the reacted glasses as pr ecipitates. The majority of the surface layer volume was composed of t wo basic structures that are morphologically and chemically distinct: The A-domain consisted of well-crystallized fibrous smectite aggregate s; and the B-domain consisted of poorly-crystallized regions containin g smectite, possibly montmorillonite, crystallites and a ZrO2-rich amo rphous silica matrix. The retention of the rare-earth elements, Mo, an d Zr mostly occurred within the B-domain; while transition metal eleme nts, such as Zn, Cr, Ni, Mn, and Fe, were retained in the A-domain. Th e element partitioning among A-domains and B-domains and recrystalliza tion of the earlier-formed B-domains into the A-domain smectites were the basic processes which have controlled the chemical and structural evolution of the surface layer. The mechanism of surface layer formati on during vapor hydration are discussed based on these cross-sectional AEM results. (C) 1998 Elsevier Science B.V.