R7T7 nuclear glass alteration in a saline medium: in situ experiments in the WIPP [Waste Isolation Pilot Plant] Project

As part of the Waste Isolation Pilot Plant (WIPP) project involving in situ testing in a saline environment at an underground laboratory in New Mexico, R7T7 glass slices were exposed for 6 months, 1 year and 2 years at 90oC in brine in contact with salt of the Salado Formation. Three types of interface were analyzed in this study: glass/brine, glass/salt and glass/glass. Scanning and transmission electron microscope examinations showed that the alteration film on the glass surface consisted mainly of a magnesium silicate gel. The inner portion of the gel layer contained Zr and Al, while neither element was found in the outer portion: Zr and Al may have migrated from the alteration film. After 2 years, the outer portion of the alteration layer was enriched in Ni and Fe, and fine crystallized phyllosilicate flakes were observed inside the gel. The thickness of the alteration film increased to 10 μm after 1 year, but did not appear to vary significantly between 1 and 2 years (9-15μm). (author)