Compaction of crushed salt for safe containment – a summary of the KOMPASS projects
Authors: Friedenberg Larissa , Bartol Jeroen , Bean James , Beese Steffen , Bollmann Hendrik , de Bresser Hans J. P. , Coulibaly Jibril , Czaikowski Oliver , Düsterloh Uwe , Eickemeier Ralf , Gartzke Ann-Kathrin , Hangx Suzanne , Laurich Ben , Lerch Christian , Lerche Svetlana , Liu Wenting , Lüdeling Christoph , Mills Melissa M. , Müller-Hoeppe Nina , van Oosterhout Bart , Popp Till , Rabbel Ole , Rahmig Michael , Reedlunn Benjamin , Rölke Christopher , Spiers Christopher , Svensson Kristoff , Thiedau Jan , Zemke Kornelia 
Published: September 2023

Abstract. For the underground disposal of high-level nuclear waste
in rock salt formations, the safety concept includes the backfilling of open
cavities with crushed salt. For the prognosis of the sealing function of the
backfill for the safe containment of the radioactive waste, it is crucial to
have a comprehensive process understanding of the crushed-salt compaction
behavior. The crushed-salt compaction process is influenced by internal
properties (e.g., grain size, mineralogy, and moisture content) and boundary
conditions (e.g., temperature, stress state, and compaction rate) and, therefore,
involves several coupled thermal–hydro–mechanical (THM) processes
(Hansen et al., 2014; Kröhn et al., 2017). With the paradigm shift from the limited release of radionuclides to safe
containment due to the German Repository Site Selection Act passed in 2017, the
importance of crushed salt as geotechnical barrier has increased, with a focus on
the evolution of its hydraulic properties. Based on the knowledge gaps in
the current process understanding, the “Compaction of crushed salt for safe containment” (KOMPASS) projects were initiated to
improve the scientific basis behind using crushed salt for the long-term
isolation of high-level nuclear waste within rock salt repositories. The
efforts to improve the prediction of crushed-salt compaction begun during
the first phase of the KOMPASS projects (Czaikowski et al., 2020) and were
followed up in a second phase ending in June 2023. The primary achievements of
the projects are as follows (Czaikowski et al., 2020; Friedenberg et al., 2022):
specification of the KOMPASS reference material, an easily available and
reproducible synthetic crushed-salt material, for generic investigations; development of pre-compaction methods and successful production of samples
in the short term and under in situ loading conditions; formulation of an extended laboratory program addressing the isolated
investigation of known relevant factors influencing the compaction behavior
of crushed salt (Düsterloh et al., 2022); execution of long-term compaction tests addressing isotropic and deviatoric
load changes, temperature, and compaction state; construction of a backfill body using the KOMPASS reference material in the
Sondershausen mine through collaboration with the SAVER (Entwicklung eines salzgrusbasierten Versatzkonzepts unter der Option Rückholbarkeit) project
(Schaarschmidt and Friedenberg, 2022); advancement of the tools for microstructure investigation methods (Svensson
and Laurich, 2022); generation (first stages) of a microphysical process list combining literature
research with our own findings; benchmarking of long-term compaction test for model development and
optimization of various existing models as well as the development of new
models; application of a virtual demonstrator (2D model representing a backfilled
drift in rock salt) for the visualization of developments and the quantification of
the models (Rabbel, 2022).
In summary, the KOMPASS projects contributed to the reduction of
uncertainties and the strengthening of the safety case for using crushed
salt within rock salt repositories.