The evolution of backfill resistance and permeability in the compaction of crushed salt
Country / Region: Germany
Begin of project: January 1, 1993
End of project: April 30, 2024
Status of project: November 3, 2023
Crushed salt is considered as the best suitable backfill material in safely encapsulating radioactive waste within a potential salt repository. To the advantage of its chemical and mechanical long-term durability, crushed salt is of the same type of material as the geological repository itself: salt.
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Next to pure crushed salt, we also tested bentonite as a promising additive. Fig. 1 shows both raw materials: lose crushed salt and powder bentonite.
A complete encapsulation of radioactive waste is reached when these lose materials have been compacted by the ductile convergence of the repository walls. Our studies suggest that fully compacted crushed salt behaves mechanically similar to the surrounding host rock. It also maintains the drastic low permeability found in natural salt rocks. As the density of the backfill material increases during compaction, there is also an increase in its resistance to the converging rock mass, which, in turn, slows down the compaction process. This resistance is called backfill resistance. The sketch in Fig. 2 illustrates the setting of a backfilled disposal drift.
Fig. 3: Compacted sample
Source: BGR
The backfill resistance depends on several parameters such as temperature, grain distribution, moisture content, and the speed as well as the force at which the drift walls converge around the backfill . An artificially compacted crushed salt sample is shown in Fig. 3.
Our laboratory investigations mimic the compaction of crushed salt by converging tunnel walls. Herein, we use oedometric and triaxial compaction tests of about one year duration to provide a firm basis for long-term modelling of the crushed salt’s behaviour. Since 2018, two of our tests methods were accredited by the Deutsche Akkreditierungsstelle GmbH (DAkkS).
Next to backfill resistance and permeability measurements, there is also the essential need to understand the underlying microstructural deformation processes of the compaction. Therefore, we investigate many thin sections, electron micrographs and computer tomography recordings for deformation indicators.
BGR’s crushed salt research is carried out in close co-operation with partners. The “KOMPASS 2” joint project with a duration of three years was therefore set up for this purpose on 1 July 2021. Partners are: BGEtec, Institut für Gebirgsmechanik Leipzig (Geomechanical Institute), Clausthal University of Technology, the Gesellschaft für Reaktorsicherheit (Reactor Safety Corporation), as well as Sandia Laboratories (USA), and COVRA (NL).
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Project contributions:
Literature:
KRÖHN, K.-P., STÜHRENBERG, D., JOBMANN, M., HEEMANN, U., CZAIKOWSKI, O., WIECZOREK, K., MÜLLER, C., ZHANG, C.-L., MOOG, H., SCHIRMER, S., FRIEDENBERG, L. (2017): Mechanical and hydraulic behaviour of compacting crushed salt backfill at low porosities - Project REPOPERM - Phase 2. FKZ 02 E 10740 (BMWi), Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, GRS-450, Köln, ISBN 978-3-946607-32-8.
KRÖHN, K.-P., ZHANG, C.L., CZAIKOWSKI, O., STÜHRENBERG, D., HEEMANN, U.: (2015): The compaction behavior of salt backfill as a TNM-process. The Mechanical Behavior of Salt VIII –, In: Roberts/Mellegard/Hansen (eds.), Proceedings of 8th conference on the mechanical behavior of salt, SaltMech VIII, South Dakota, USA, Mai 2015: p. 49– 59. CRC Press, Taylor & Francis Group
HANSEN, F., POPP, T., WIECZOREK, K., STÜHRENBERG, D.: (2015): Salt reconsolidation applied to repository seals. The Mechanical Behavior of Salt VIII –, In: Roberts/Mellegard/Hansen (eds.), Proceedings of 8th conference on the mechanical behavior of salt, SaltMech VIII, South Dakota, USA, Mai 2015: p. 179– 189. CRC Press, Taylor & Francis Group
HANSEN, F., POPP, T., WIECZOREK, K., STÜHRENBERG, D.: (2014): Salt Reconsolidation – Principles and Applications. SAND2014-4502P, Sandia National Laboratories, USA.
STÜHRENBERG, D. (2013): Compaction of Crushed Salt in Consideration of the Moisture Content. In: Proceedings of the 4th US-German Salt Repository Workshop, Berlin Germany, Sept. 2013, Sandia Report, SAND2013-10592P.
STÜHRENBERG, D., SCHULZE, O. (2012): Porosity and permeability of crushed and damaged salt during compaction. Mechanical Behavior of Salt VII –, In: Bérest/Ghoreychi/Hadi-Hassen/Tijani (eds.), Proceedings of 7th conference on the mechanical behavior of salt, Paris, France, April 2012: p. 275–282. CRC Press, Taylor & Francis Group.
KRÖHN, K.-P., ZHANG, C.L., WOLF, J., STÜHRENBERG, D., JOBMANN, M., VON BORSTEL, L. & LERCH, C. (2012): The compaction behaviour of salt backfill at low porosities. Mechanical Behavior of Salt VII –, In: Bérest/Ghoreychi/Hadi-Hassen/Tijani (eds.), Proceedings of 7th conference on the mechanical behavior of salt, Paris, France, April 2012: p. 161–168. CRC Press, Taylor & Francis Group.
KRÖHN, K.-P., STÜHRENBERG, D., HERKLOTZ, M., HEEMANN, U., LERCH, C., XIE, M. (2009): Restporosität und -permeabilität von kompaktierendem Salzgrus-Versatz; Projekt REPOPERM - Phase 1. Bericht GRS Braunschweig, 2009.
ISBN-Nr.: 978-3-939355-29-8
HUERTAS, F., VILLAR, M.V., GARCIA-SINERIZ, J.L., SELLIN, P., STÜHRENBERG, D., ALHEID, H.-J. (2007): Thermo-hydro-mechanical (geochemical) processes in the engineered barrier system. – EU-Projekt NF-PRO, Final Synthesis Report RTD Component 3, Deliverable D 3.6.2.
STÜHRENBERG, D. (2007): Long-term laboratory investigation on backfill. – The Mechanical Behavior of Salt – Understanding of the THMC Processes in Salt, Proceedings of “Saltmech6” Hannover, Germany, Mai 2007: p. 223–229.
STÜHRENBERG, D. (2004): Compaction and Permeability Behaviour of Crushed Salt and Mixtures of Crushed Salt and Bentonite. Conference Proceedings of DisTec2004, International Conference on Radioactive Waste Disposal, p. 511 – 515, Berlin, April 2004.
