Rock salt formations are important candidates for radioactive waste disposal. The disposal concept of rock salt is based on the creation of galleries and boreholes to store the radioactive waste. These will subsequently be backfilled with crushed rock salt, which will converge and compact over time. To ensure safe, long-term storage, eventually this backfill should attain porosities and permeabilities comparable to that of dense rock salt, hence sealing in the waste. However, to accurately predict the timescale at which this sealing will occur physics-based descriptions are needed for modelling the long-term behavior of the backfill.
An empirical description of the compaction creep based on experiments on single grain sizes is already available. However, the backfill in a repository will have a distributed grain size, and the individual contribution of each grain size fraction to the overall compaction creep rate is poorly constrained. In addition, a full mechanistic-based model for compaction of backfill down to a few percent porosity is lacking. Hence, extrapolation to longer timescales is uncertain.
Therefore, we have performed experiments on single and distributed grain size fractions and developed a mechanistic-based model that can predict the compaction creep rates of granular rock salt as a function of its grain size distribution. Depending on whether the material is assumed to be subjected to homogeneous stress or strain-rate (in a manner analogue to the Reuss and Voight bounds for elastic deformation), the compaction creep rate can vary up to a few orders of magnitude.
In addition to the effect of grain size, backfill compaction will also be influenced by humidity. To date, most experiments on crushed backfill are performed under either fully or partially brine saturated (i.e. (semi-)flooded) conditions or lab-air conditions. These experiments show that the presence of brine accelerates compaction creep. Therefore, many proposals consider adding brine to the backfill to accelerate creep. However, when no brine is added it is expected that the presence of humid air still accelerates creep with respect to truly dry conditions. The hygroscopic properties of rock salt make it very likely that in-situ backfill will adsorb water from the surroundings until an equilibrium (relative) humidity of around 75% is reached. At present, no experimental data is obtained under humid conditions. Therefore, we are currently performing creep experiments under conditions with a constant humidity to assess the effect of humidity on compaction creep rates.
