Large scale in-situ experiments on sealing constructions in underground disposal facilities for radioactive wastes – Examples of recent BfS- and GRS-activities

Highlights: •We show in-situ constructions for the sealing of drifts in various salt formations. •A direct verification of the long-term functionality is often not possible. •The chemical and physical processes involved needs to be developed. •An overall process understanding is required to extrapolate the evolution. •Lab- and large-scale tests are important to enhance process understanding. -- Abstract: This paper shows examples of in-situ constructions and laboratory tests that have been designed and implemented by BfS and GRS in order to demonstrate the technical feasibility of special constructions for the sealing of drifts and shafts in different salt formations. Since it is often not possible to directly verify the long-term functionality (for times scales envisaged in long-term safety assessments) of such constructions, an overall understanding of the main chemical and physical processes involved needs to be developed. Such an understanding is required in order to extrapolate the evolution of a sealing system with a sufficient degree of reliability. Laboratory tests and large-scale in-situ tests are necessary and integral parts of the procedures for enhancing the process understanding. Based on a safety case, BfS has developed a robust decommissioning concept for the closure of the low- and intermediate-level waste disposal facility Morsleben (ERAM) which also takes into account the retention of radionuclides by combined effects of different sealing components. GRS has contributed to this concept with laboratory tests and modelling of the long-term behaviour of the sealing material "salt concrete" foreseen for the sealing constructions in Morsleben. Salt concrete is based on the cement Portlandite i.e., on CaO. Furthermore GRS has also investigated the MgO based "sorel concrete" employed by BfS for the Asse project. The chemical corrosion path and the subsequent permeability changes for both concretes have been investigated with test procedures developed in the GRS laboratory. GRS has also developed a new Self-Sealing material "Selbst Verheilender Versatz – SVV" (SVV, German for Self Sealing Backfill) and tested in laboratory and in-situ experiments. With this material it is possible to achieve an instantaneous and long-lasting permeability reduction within the complex system consisting of the sealing construction and the Excavation Demaged Zone (EDZ) upon brine intrusion. The presentation demonstrates details of the concepts and highlights results from the BfS in-situ experiments in Morsleben and the preceding GRS laboratory and in-situ tests.