SealWasteSafe: materials technology, monitoring techniques, and quality assurance for safe sealing structures in underground repositories
Authors: Lay Vera , Baensch Franziska , Johann Sergej , Sturm Patrick , Mielentz Frank , Prabhakara Prathik , Hofmann Detlef , Niederleithinger Ernst , Kühne Hans-Carsten 
Published: November 2021

Abstract. Within the project SealWasteSafe, we advance construction materials and
monitoring concepts of sealing structures applied for underground disposal of
nuclear or toxic waste. As these engineered barriers have high demands
concerning integrity, an innovative alkali-activated material (AAM) is
improved and tested on various laboratory scales. This AAM has low reaction
kinetics related to a preferential slow release of the heat of reaction in
comparison to alternative salt concretes based on Portland cement or magnesium
oxychloride cements. Hence, crack formation due to thermally induced strain is
reduced. After successful laboratory scale analysis (Sturm et al., 2021), the
AAM is characterised on a larger scale by manufacturing test specimens
(100–300 L). Conventional salt concrete (DBE, 2004) and the newly developed AAM are
compared using two specimen geometries, i.e. cylindrical and cuboid. A
comprehensive multisensor monitoring scheme is developed to compare the
setting process of AAM and salt concrete for these manufactured specimens. The
analysed parameters include temperature and humidity of the material, acoustic
emissions, and strain variations. Passive sensor systems based on
radiofrequency identification technology (RFID) embedded in the concrete,
enable wireless access to temperature and humidity measurements and are
compared to conventional cabled systems. Additionally, fibre-optic sensors
(FOS) are embedded to record strain, but also have potential to record
temperature and moisture conditions. Part of this project aims at
demonstrating the high reliability of sensors and also their resistance to
highly alkaline environments and to water intrusion along cables or at sensor
locations. Further technical improvements were implemented so that first
results clearly show the scalability of the setting process from previous
small-scale AAM experiments and particularly the high potential of the newly
developed approaches. Furthermore, ultrasonic methods are used for quality assurance to detect
obstacles, potential cracks and delamination. On the one hand, both active and
passive ultrasonic measurements complement the results obtained from the
multisensor monitoring scheme for the produced specimens. On the other hand,
the unique large aperture ultrasonic system (LAUS) provides great depth
penetration (up to nearly 10 m) and can thus be applied at in situ
sealing structures built as a test site in Morsleben by the Federal Company
for Radioactive Waste Disposal (Bundesgesellschaft für Endlagerung, BGE)
as shown by Effner et al. (2021). An optimised field lay-out identified from
forward modelling studies and advanced imaging techniques applied to the
measured data will further improve the obtained results. To characterise the
inside of the test engineered barrier and achieve a proof-of-concept, an
ultrasonic borehole probe is developed to enable phased arrays that can
further improve the detection of potential cracks. Modelling results and first
analysis of semispherical specimens confirmed the reliability of the
directional response caused by the phased arrays of the newly constructed
ultrasonic borehole probe. Overall, the project SealWasteSafe improves the construction material,
multisensor monitoring concepts and ultrasonics for quality assurance. This
will help to develop safe sealing structures for nuclear waste disposal. The
outcomes are particularly valuable for salt as a host rock but partly also
transferrable to alternative conditions.