A systematic strategy for the calibration of crushed salt constitutive models

Crushed salt is aimed to act as long-term barrier in a HLW repository in rock salt. The development of its sealing effect against radionuclide migration is determined by the porosity/permeability evolution governed by crushed salt compaction due to rock salt creep. For long-term safety, the time at which the crushed salt reaches barrier properties is crucial. To give a qualified prognosis for the development of barrier properties, numerical simulations with reliable underlying constitutive models are needed. The models must capture THM-coupled processesas observed in experiments/in-situ measurements. In-situ relevant processes must be determined, analysed and credibly extrapolated outside the investigation ranges.

The systematic calibration and verification of constitutive models against experimental data is fundamental. Within current crushed salt projects (Czaikowski et al., (2020), Friedenberg et al., (2024), Friedenberg et al., (2025)) a long-term strategy with key objectives for main research areas  (numerical analysis, physical modelling, experimental investigation on the microscale and the laboratory scale) has been designed and is currently realized (Figure 1).

Figure 1. The long-term investigation strategy 

A systematic calibration strategy for various crushed salt constitutive models against an experimental base is developed (Czaikowski et al., 2020) and first calibration steps are performed (Friedenberg et al., 2024). The strategy is based on a systematicly performed  long-term compaction tests addressing important factors (except grain size/mineralogy due to the use of KOMPASS reference material) influencing the in-situ compaction behaviour. These tests verify the knowledge of the relation between compaction and influencing factors and reduce the areas where extrapolation is required. However, up to date the developed experimental basis (Figure 1) consists of individual tests only. The proof of repeatability and representativeness is necessary; therefore, several laboratories are involved to verify the sample-to-sample variability and repeatability of the experiments (MEASURES project). Due to the collaboration with the SAVER project, an approach for transferability of laboratory generated results with in-situ measurements is developed.

The generated data are then used for benchmark calculations and for calibration of constitutive models. It is successfully shown that by adding more data (more investigated relationships), the bandwidth of results regarding stress-porosity-relation (and as a consequence the porosity evolution with time) is decreased (Figure 2). This approach leads to decreasing uncertainties in the simulation of crushed salt compaction.

Figure 2. Results of numerical simulations for a backfilled drift.

The project partner GRS, BGE-TEC, IfG and TUC acknowledge the funding received by the German Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection, represented by the Project Management Agency Karlsruhe (FKZ 02 E 12214 A-D).

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

References

Czaikowski, O. et al. (2020). KOMPASS-I final report (GRS-608)

Friedenberg, L. et al. (2024). KOMPASS-II final report (GRS-751)

Friedenberg, L. et al. (2025). Multi-Scale Experimental and Numerical Analysis of Crushed Salt Used as Engineered Backfill in a Rock Salt Repository, WM Symposia Phoenix