Microscopic evidence of grain boundary moisture during granular salt reconsolidation

Abstract:

Very low permeability is a principal reason salt formations are considered viable hosts for disposal of nuclear waste and spent nuclear fuel. Granular salt is likely to be used as back-fill material and a seal system component. Salt formation pressures will promote reconsolidation of granular salt, eventually resulting in low permeabilities, comparable to native salt. Understanding the consolidation processes, dependent on the stress state, moisture availability and temperature, is important for demonstrating sealing functions and long-term repository performance. As granular salt consolidates, initial void reduction is achieved by brittle processes of grain rearrangement and cataclastic flow. At porosities less than 10%, grain boundary processes and crystal-plastic mechanisms govern further porosity reduction. When present, fluid assists in grain boundary processes and recrystallization. Fluid inclusions are typically found in abundance within bedded salt crystal structure and along grain boundaries, but are rarely observed internal to domal salt grains. We have observed fluid canals and evidence of moisture along grain boundaries in domal salt. In this research, we investigate grain boundary moisture in granular salt that has been reconsolidated under high temperatures to relatively low porosity. Mine-run salt from the Waste Isolation Pilot Plant and Avery Island was used to create cylindrical samples, vented and unvented, which were reconsolidated at 250°C and stresses to 20 MPa. Unvented reconsolidation retains essentially all the grain boundary moisture as found ubiquitously on scanning electron photomicrographs of consolidated samples which revealed an inhomogeneous distribution of canals from residual moisture. This contrasts significantly with the vented samples, which had virtually no grain boundary moisture after consolidation. Microstructural techniques include scanning electron, stereo-dynascopic, and optical microscopy. The observations will be used to support evaluation of a constitutive model for reconsolidating granular salt to be used to predict thermal-mechanical-hydrologic responses of salt repository seal structures and backfilled rooms.