NATURAL URANIUM AS A TRACER IN RADIONUCLIDE GEOSPHERE TRANSPORT STUDIES

Uranium (U) is a common element in the earth's crust and its solubility characteristics allow it to be moved by naturally occurring geochemical processes. This study aims to understand U transport in crystalline bedrock, which is vital for assessing potential radionuclide escape from deep nuclear waste repositories. The investigation focused on the mobile fraction of U in water-conducting fractures near a small U-Th deposit, with particular attention to U fractions that come into contact with groundwater. Uranium-series disequilibria were examined to gather spatial and temporal information on U transport activities. A combination of in situ studies and laboratory experiments enhanced experimental methods, enabling insights into U fixation mechanisms. Both reversible and irreversible fixation of U were characterized, revealing that the attachment of U to ferric compounds occurs through mechanisms that secure U more firmly than adsorption. Observations indicated the movement of U isotopes within the rock matrix close to fractures on a centimeter scale. The findings significantly contribute to understanding the geochemical evolution and potential behavior of radionuclides in underground waste repository environments.