Fundamental Insights into the Radium Uptake into Barite by Atom Probe Tomography and Electron Microscopy

Recently, the Ba$_{x}$Ra$_{1-x}$SO$_{4}$ solid solution has been investigated with regard to its applicability to the long-term safety of spent nuclear fuel (SNF) disposal. As $^{226}$Ra 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 applicationfor a final SNF repository, $^{226}$Ra dominates the dose after 100,000 years. Currently,the solubility of $^{226}$Ra is considered to be controlled by the formation of Ra-SO$_{4}$ in the Swedish license application as the Ba$_{x}$Ra$_{1-x}$SO$_{4}$ solid solution characteristics were not sufficiently investigated at the point of submission. The Ba$_{x}$Ra$_{1-x}$SO$_{4}$ solid solution could be considered as solubility controlling 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. Acombination of different state-of-the-art high-resolution microscopy techniques was used to answer the questions regarding (1) the internal microstructure 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. [...