Accelerated Testing of Waste forms using a Novel Pressurized Unsaturated flow (PUF) Method

Abstract

ABSTRACTA new experimental technique has been developed to test waste forms and other proposed engineered-barrier materials under unsaturated conditions. Laboratory experiments using the pressurized unsaturated flow (PUF) apparatus have been performed with a Na-Ca-Al borosilicate glass being studied for immobilization of low-activity tank wastes and a reference borosilicate glass for immobilization of high-level wastes (SRL-202). A complex coupling between glass corrosion, secondary phase precipitation, and unsaturated flow behavior was observed. Precipitation of a family of Na-Ca-Al zeolites was also found to cause an acceleration in the reaction rate of the low-activity waste glass. This same effect was observed after approximately 1 year in high solid-to-liquid ratio batch tests but after only 12 days using the PUF method at the same temperature. The onset of secondary phase precipitation was tracked by monitoring changes in volumetric water content and by inline chemical analysis. Finally, elemental release of the major glass components and solution pH were found to differ under unsaturated flow-through conditions as compared with saturated, batch tests. These findings, while preliminary, have important implications for understanding and modeling glass corrosion behavior under unsaturated conditions.