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. Within the project SealWasteSafe, we advance construction materials andmonitoring concepts of sealing structures applied for underground disposal ofnuclear or toxic waste. As these engineered barriers have high demandsconcerning integrity, an innovative alkali-activated material (AAM) isimproved and tested on various laboratory scales. This AAM has low reactionkinetics related to a preferential slow release of the heat of reaction incomparison to alternative salt concretes based on Portland cement or magnesiumoxychloride cements. Hence, crack formation due to thermally induced strain isreduced. After successful laboratory scale analysis (Sturm et al., 2021), theAAM is characterised on a larger scale by manufacturing test specimens(100–300 L). Conventional salt concrete (DBE, 2004) and the newly developed AAM arecompared using two specimen geometries, i.e. cylindrical and cuboid. Acomprehensive multisensor monitoring scheme is developed to compare thesetting process of AAM and salt concrete for these manufactured specimens. Theanalysed parameters include temperature and humidity of the material, acousticemissions, and strain variations. Passive sensor systems based onradiofrequency identification technology (RFID) embedded in the concrete,enable wireless access to temperature and humidity measurements and arecompared to conventional cabled systems. Additionally, fibre-optic sensors(FOS) are embedded to record strain, but also have potential to recordtemperature and moisture conditions. Part of this project aims atdemonstrating the high reliability of sensors and also their resistance tohighly alkaline environments and to water intrusion along cables or at sensorlocations. Further technical improvements were implemented so that firstresults clearly show the scalability of the setting process from previoussmall-scale AAM experiments and particularly the high potential of the newlydeveloped approaches. Furthermore, ultrasonic methods are used for quality assurance to detectobstacles, potential cracks and delamination. On the one hand, both active andpassive ultrasonic measurements complement the results obtained from themultisensor monitoring scheme for the produced specimens. On the other hand,the unique large aperture ultrasonic system (LAUS) provides great depthpenetration (up to nearly 10 m) and can thus be applied at in situsealing structures built as a test site in Morsleben by the Federal Companyfor Radioactive Waste Disposal (Bundesgesellschaft für Endlagerung, BGE)as shown by Effner et al. (2021). An optimised field lay-out identified fromforward modelling studies and advanced imaging techniques applied to themeasured data will further improve the obtained results. To characterise theinside of the test engineered barrier and achieve a proof-of-concept, anultrasonic borehole probe is developed to enable phased arrays that canfurther improve the detection of potential cracks. Modelling results and firstanalysis of semispherical specimens confirmed the reliability of thedirectional response caused by the phased arrays of the newly constructedultrasonic borehole probe. Overall, the project SealWasteSafe improves the construction material,multisensor monitoring concepts and ultrasonics for quality assurance. Thiswill help to develop safe sealing structures for nuclear waste disposal. Theoutcomes are particularly valuable for salt as a host rock but partly alsotransferrable to alternative conditions.

SealWasteSafe: materials technology, monitoring techniques, and quality assurance for safe sealing structures in underground repositories