Quality assurance of closure structures in repositories with ultrasound

For the sealing of repositories for radioactive or chemical waste in former salt mines, sealing structures made of salt concrete are tested for their suitability. For some years now, the Bundesgesellschaft Endlagerung (BGE) and the Bundesanstalt für Materialforschung und -prüfung (BAM) have been gathering experience with ultrasonic technology for underground quality assurance in experimental structures. Both commercial and experimental equipment for echo testing as well as specially developed borehole probes are used. All devices are based on low-frequency s-wave point contact probes (25 kHz - 50 kHz), which are combined in different arrays. In several test series it was possible to track unfilled and partially filled cracks over several meters and to map their position. Technical installations could be detected down to a depth of approx. 8 m in the structure. With the help of imaging techniques adapted from geophysics, it was possible to make visible some features that had previously not been detectable. The results serve both the analysis of the experimental structure and the development of a quality assurance system for future real closure structures.