Remaining porosity and permeability of compacted crushed rock salt backfill in a HLW repository. Final report

The safe containment of radioactive waste is to be ensured by the geotechnical barriers in combination with the containment-providing rock zone (CRZ). The latter is a key element of the recently developed concept of demonstrating the integrity of the geologic barrier (Krone et al., 2013). As stipulated in the safety requirements of the regulating body the CRZ has to have strong barrier properties, and evidence needs to be provided that it retains its integrity throughout the reference period (BMU, 2010). The underground openings excavated in the rock salt will close over time due to the creep properties of the rock salt. This process causes deformations in the surrounding rock salt, which leads to a change in stress state in the virgin rock and may impair the integrity of the containment-providing rock zone. In order to limit the effects of these processes, all underground openings will be backfilled with crushed salt. Immediately after backfilling, the crushed salt will have an initial porosity of approx. 35%, which - over time - will be reduced to very low values due to the creep properties of the rock salt. The supporting pressure that builds up in the crushed salt with increasing compaction slows down the creeping of the salt. Major influencing factors are the temperature (with higher temperatures accelerating the salt creeping) and the moisture of the salt, which - due to the related decrease in the resistance of the crushed salt - facilitates its compaction. The phenomenology of these processes and dependencies is understood to a wide extent. This project investigated the duration until compaction is completed and when and under what circumstances the crushed salt will have the sealing properties necessary to ensure safe containment. Thermo-hydro-mechanical (THM) processes play a crucial role in determining whether solutions which might enter the mine could reach the radioactive waste. This includes changes in material behaviour due to a partial or complete filling of the pore spaces with solution. In this context, the porosity range of < 3% has special significance. The relevant processes influencing the hydraulic parameters of the crushed salt backfill are still not described in sufficient detail. For the compaction of dry crushed salt, various theoretical constitutive approaches have been developed (Spiers et al., 1989, Hein, 1991, Zhang et al., 1993, Heemann, 2004) and verified within the scope of the BAMBUS project (Bechthold, 2004) where the porosity range 10%<φ<35% has been calibrated. The contribution of DBE TECHNOLOGY GmbH to the project consisted mainly of microstructural investigations and by applying discrete element computer codes to evaluate their applicability to simulate compaction processes. This work is described in this report.