engWeber, JulianeBosbach, DirkRoth, GeorgFundamental insights into the radium uptake into barite by atom probe tomography and electron microscopyinfo:eu-repo/classification/ddc/620radiumbaritenuclear waste managementsolid solutionatom probe tomographyRecently, the BaxRa1-xSO4 solid solution has been investigated with regard to its applicability to the long-term safety of spent nuclear fuel (SNF) disposal. As 226Ra originates from the U decay chain, its concentration in SNF builds up with time. In some scenarios for the direct disposal of SNF taken from the Swedish license application for a final SNF repository, 226Ra dominates the dose after 100,000 years. Currently, the solubility of 226Ra is considered to be controlled by the formation of RaSO4 in the Swedish license application as the BaxRa1 xSO4 solid solution characteristics were not sufficiently investigated at the point of submission. The BaxRa1 xSO4 solid solution could be considered as solubility con-trolling phase for Ra if the uptake mechanism of Ra into barite was understood in more detail.Barite can occur as a primary phase in the surrounding of the future repository or as a secondary phase within nuclear waste due to the different positions of Ba and Ra within SNF. In the case of SNF corrosion, Ba would come in contact with water first. Sulfate-containing water would lead to barite precipitation. Therefore, a system is most likely where pre-existing barite is in equilibrium with an aqueous solution into which Ra then enters. Recent studies comprising long-term batch recrystallization experiments propose a kinetically influenced uptake of Ra into barite that equilibrates into a thermodynamically controlled situation within 800 days. This thesis provides the first detailed four-dimensional characterization of the Ra uptake into barite by combining three-dimensional sample characterization with the temporal evolution. To understand the mechanism of Ra uptake into barite, two types of barites (SL and AL barite) obtained from batch recrystallization experiments of previous studies were characterized prior to, during and after the Ra uptake. A combination of different state-of-the-art high-resolution microscopy techniques was used to answer the questions regarding (1) the internal micro-structure of the initial barite (2) the role of this internal microstructure during the Ra uptake and (3) the changes in the Ra distribution within the barite. This study comprises the first characterization of barite by atom probe tomography (APT). By combining APT and transmission electron microscopy (TEM) methods, pores covering the size range from a few nanometers to a few micrometers were identified in the SL barite. The pores were organized in layers parallel to the outer crystal faces. High resolution chemical analysis indicated that the pores contain a solution of water and sodium chloride. By focused ion beam (FIB) tomography, it was revealed that open macropores of several micrometers size are present as well within the SL barite. These partially connected macropores are distributed within the complete barite particles. Therefore, the macropores provide a direct pathway for Ra-containing aqueous fluid to enter the SL barite particles by diffusion within the aqueous solution. In addition, pores were also identified in the AL barite by TEM characterization. The entrapment of solution during mineral precipitation is known for several min-erals at high supersaturation. As barite only precipitates at high supersaturation, nano-scale fluid inclusions as well as macropores probably were entrapped dur-ing the particle growth by precipitation. A microstructure similar to the one of the barite type used in this study was previously reported for other barites. In Ra-free reference experiment, no microstructural changes were noted over recrystallization times of up to 898 days. In prior studies, three different stages of Ra uptake were described based on macroscopic results. Ra-containing barite samples from all three stages were characterized to understand the role of the internal barite microstructure. At the beginning, the nano-scale fluid inclusions disappeared, probably due to coalescing to new macropores. This was part of an Ostwald ripening process driven by the minimization of the free energy in the complete system. Furthermore, chemical analyses by Scanning TEM energy-dispersive X-ray spec-troscopy (STEM-EDX) were conducted and a characteristic evolution of the Ra distribution within the solids with time was observed. After an intermediate, inhomogeneous Ra distribution within the barite, a homogeneous Ra distribution at the scale of STEM-EDX was detected. In conclusion, the Ra uptake into barite takes place in several steps: first, the Ra- diffuses within the aqueous phase into the open macropores where it is incorporated into the barite via a dissolution-reprecitation process. The complex internal microstructure of the barite enlarges the barite surfaces and the uptake process takes place from the inside of the barite particles. The recrystallization of barite in the presence of Ra is completely different from pure barite recrystallization as the uptake of Ra causes fundamental changes within the Ra microstructure. The presence of Ra is a thermodynamic driving force which triggers the complete barite microstructure to rebuild despite the low concentration of Ra. A possible reason for these significantly different effects could be the radiation damage that is caused by the alpha-decay of 226Ra.Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Schriften des Forschungszentrums Jülich. Reihe Energie & Umwelt 367, IX, 143 Seiten : Illustrationen, Diagramme (2017). doi:10.18154/RWTH-2017-02498 = Dissertation, RWTH Aachen University, 2017info:eu-repo/semantics/doctoralThesisinfo:eu-repo/semantics/publishedVersionForschungszentrum Jülich GmbH, Zentralbibliothek2017info:eu-repo/semantics/openAccessDEhttps://publications.rwth-aachen.de/record/685760https://publications.rwth-aachen.de/search?p=id:%22RWTH-2017-02498%22StudentsStudent Financial Aid ProvidersTeachersResearchersinfo:eu-repo/semantics/altIdentifier/issn/1866-1793info:eu-repo/semantics/altIdentifier/doi/10.18154/RWTH-2017-02498info:eu-repo/semantics/altIdentifier/isbn/978-3-95806-220-7