Radium retention by BaxRa1-xSO4 solid solution formation: an electron microscopy and atom probetomography investigation

The migration of radionuclides in the geosphere is to a large extent controlled by sorption processesonto minerals and colloids. On a molecular level, sorption phenomena involve surface complexation, ion exchange aswell as solid solution formation. The formation of solid solutions leads to the structural incorporation of radionuclides ina host structure. Such solid solutions are ubiquitous in natural systems – most minerals in nature are atomisticmixtures of elements rather than pure compounds because their formation leads to a thermodynamically more stablesituation compared to the formation of pure compounds.In some scenarios describing the evolution of a geological waste repository system for spent nuclear fuel in crystallinerocks 226Ra dominates the radiological impact to the environment associated with the potential release ofradionuclides from the repository in the future. The solubility of Ra in equilibrium with a BaxRa1-xSO4 solid solution ismuch lower than the one calculated with RaSO4 as solubility limiting phase. Due to the expected conditions in therepository near field, a likely scenario will be a release of Ra from the spent fuel matrix into a solution in equilibriumwith pre-existing barite. Batch-type laboratory experiments mimicking this scenario were carried out and indicate theuptake of Ra, leading to a reduction of more than 99% of the initial Ra concentration. The grain size and morphologyof the barite grains are very similar before and after the Ra uptake, although ToF-SIMS analyses indicate that Ra wastaken up into the particle volumes. In order to follow the uptake of Ra into barite and identify the possible pathwaysinto the particle volume, we applied a new approach for the detailed characterization of Ra-barites obtained atdifferent stages of the recrystallization experiments utilizing a combination of electron microscopy and atom probetomography (APT). The preparation of the barite samples was done by adapting focused ion beam milling proceduresto the material.A layered structure caused by size and density variations of nano-scaled pores was observed by electronmicroscopy in Ra-free reference samples. The APT reconstruction showed chemical inhomogeneities of H2O, Na andCl present in layers of similar length scale. In conclusion, both findings indicated that the layered structure consisted ofnano-scaled pores filled with NaCl-bearing fluids, providing a fast pathway for Ra into the barite particles.Subsequently, elemental maps of Ra-barites were obtained with energy-dispersive X-ray spectroscopy (EDX)enabling the analysis of the evolution of the Ra distribution within the solid with time. The maps showed anintermediate heterogeneous Ra distribution which becomes homogeneous at equilibrium state