Filtern
Dokumenttyp
- Beitrag zu einem Tagungsband (2)
- Vortrag (2)
Sprache
- Englisch (4)
Referierte Publikation
- nein (4)
Schlagworte
- Ultrasonic imaging (4) (entfernen)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (2)
Underground sealing structures are necessary to seal radioactive or toxic waste in underground repositories. A special version of MgO concrete is used in the mine in Teutschenthal and applied at a large-scale test with the aim to provide a low permeability barrier. The test structures (up to 10 m long) were created by shotcreting. Besides destructive tests, non-destructive ultrasonic measurements have been evaluated for quality assurance to monitor potential anomalies such as cracks, concrete inhomogeneities, and delamination. We show results of ultrasonic measurements with a multi-transducer system used at the front and from the side of the concrete structure. Images are obtained by synthetic aperture focusing techniques. The boundaries between concreting sections are not visible in the ultrasonic images systematically so that a successful concreting is assumed, which is confirmed by the low permeabilities observed. Several reflectors up to a depth of 1.2 m are identified and interpreted as potential damages in the concrete. Phase analysis of prominent reflectors reveal a negative impedance contrast thus indicating air filled voids, cracks or delamination. Boreholes through some of the identified reflectors are used to verify the results from ultrasonic measurements. Additionally, an experiment with incorporated artificial defects was undertaken to analyse the reliability of the applied methods. Overall, the conducted tests show the potential of ultrasonic measurements to detect critical anomalies. Despite challenges at small-scale structures (cm-order), large-scale anomalies can be identified. Consequently, implementing ultrasonic monitoring during and after the construction of concrete sealing structures has shown its potential as a tool for quality assurance, but needs further development and validation.
Multi-sensory monitoring and ultrasound for quality assurance at underground sealing structures
(2023)
Within the safety concepts of underground disposal sites of nuclear waste, engineered barriers play an important role. As these sealing structures have high demands concerning integrity, we aim at advancing the available construction materials, monitoring and inspection techniques within the project SealWasteSafe.
After successful laboratory analysis on the cm-scale, cylindrical and cuboid specimens on the m-scale are produced from classical salt concrete and a specifically developed alkali-activated material. A comprehensive multi-sensory monitoring scheme is applied to compare the setting process of both materials and to demonstrate the sensors’ resistance to highly alkaline environments. Besides temperature and humidity of the materials, strain variations using fibre optic sensors and acoustic emissions are recorded over periods of at least 28 days, partly for more than 8 months. Passive sensor systems based on radiofrequency identification technology (RFID) embedded in the concrete allow for wireless access to temperature and humidity measurements and are compared to conventional cabled systems. After hardening and removal of the outer casings of the specimens, further non-destructive evaluations using ultrasonic echo and thermographic measurements are conducted. Preliminary results clearly highlight differences between the tested materials, particularly showing lower acoustic emission activity for the newly developed alkali-activated material potentially indicating less phase changes or cracks.
Complementary, ultrasonic methods are improved to be used for quality assurance to detect obstacles, potential cracks and delamination at in-situ sealing structure scale. A unique large aperture ultrasonic system (LAUS) with depth penetration as large as 9 m has already successfully been applied at the test site in Morsleben, Germany, of the federal company for radioactive waste disposal (BGE). Modelling studies help to further optimize the measurement layout. Advanced imaging techniques applied to the modelled and measured data will further improve the obtained images of internal structures. Additionally, an ultrasonic borehole probe is developed and constructed using phased arrays to further enhance the detection of potential cracks. Modelling and preliminary results from laboratory specimens prove the feasibility and potential of the directional response even in heterogeneous material such as concrete. Final investigations under in-situ conditions at the test site of the sealing structure are planned.
Overall, the project SealWasteSafe improves the construction material, multi-sensory monitoring, and ultrasound for quality assurance to allow for the development of safe nuclear sealing structures. Although the techniques are tailored for sealing structures within salt as a host rock, they are transferrable to a wider field of applications and alternative disposal conditions.
Underground sealing structures are necessary to seal radioactive or toxic waste in underground repositories. Particularly developed MgO concrete is used in the mine in Teutschenthal and applied at a large-scale test with the aim to provide concrete with low permeabilities. The test structures (up to 10 m long) were created with the help of a shotcrete procedure. Besides destructive tests, non-destructive ultrasonic measurements are used for quality assurance to monitor potential anomalies such as cracks, concrete inhomogeneities, and delamination. We show results of ultrasonic measurements with a multi-transducer system used at the front site of the concrete structure. Images are obtained by synthetic aperture focusing techniques. Concreting sections are not systematically imaged so that a successful concreting is assumed as also indicated by observed low permeabilities. Several reflectors up to a depth of 1.2 m are identified and interpreted as potential damages in the concrete. Phase analysis of prominent reflectors reveal a negative impedance contrast thus indicating air filled voids, cracks or delamination. Boreholes through some of the identified reflectors are used to verify the results from ultrasonic measurements. Additionally, an experiment with incorporated defects is undertaken to analyse the reliability of the applied methods. Overall, the conducted tests show the great potential of ultrasonic measurements to detect critical anomalies. Despite challenges at small-scale structures (cm-order), large-scale anomalies can be identified. Implementing ultrasonic monitoring during and after the construction of concrete sealing structures is recommended as a tool for quality assurance.
Multi-sensory monitoring and ultrasound for quality assurance at underground sealing structures
(2023)
Within the safety concepts of underground disposal sites of nuclear waste, engineered barriers play an important role. As these sealing structures have high demands concerning integrity, we aim at advancing the available construction materials, monitoring, and inspection techniques within the project SealWasteSafe. A specifically developed alkali-activated material is compared to classical salt concrete. A comprehensive multi-sensory monitoring scheme is used at 150-340 l specimens to monitor setting and hardening of both materials. All sensors are demonstrated to resist the highly alkaline environments. Besides cabled and wireless temperature and humidity of the materials, strain variations using fibre optic sensors and acoustic emissions are recorded over periods of at least 28 days, partly for more than eight months. After hardening of the specimens, further nondestructive evaluations using ultrasonic echo and thermographic measurements are conducted.
Preliminary results proof the suitability of the tested sensors and clearly highlight differences between the tested materials. Particularly, the newly developed alkali-activated material shows lower acoustic emission activity indicating less cracking activity. Additionally, unique ultrasonic methods will enable better images of potential internal objects and cracks at in-situ sealing structures. A largescale ultrasonic system is optimised to reliably detect objects at a depth exceeding 9 m while still obtaining a good resolution. Modelling studies show the potential of further increasing the distance between individual transducer arrays. Additionally, a new ultrasonic borehole probe using phased arrays allowing for beam focussing is constructed and tested. Laboratory measurements at a halfcylindrical concrete specimen coincide well with the previous modelling. In total, the presented safe materials, detailed monitoring approaches and ultrasonic quality assurance methods will help to obtain safe sealing structures within salt as a host rock. The concepts can partly be transferred to sealing structures in alternative host rocks and will also be valuable for non-nuclear waste repositories.