Mechanistic models for the interaction of Cm(III) and Eu(III) with crystalline rocks
Containment of highly radioactive nuclear waste is one of the great social challenge in the 21st century. The most well-regarded scientific concept is the final storage of high-level radioactive waste in deep geological repositories.
Within the nuclear waste repository, reducing conditions are prevalent, therefore transuranium actinides like Pu, Am and Cm are expected to be in their trivalent oxidation state. These trivalent radionuclides are mobile in aqueous solutions. In the safety case, an unexpected ingress of water into the repository is considered. Therefore, it is necessary to assess how well the host rock will retain those actinides.
Most studies dealing with the retention of trivalent actinides on host rock material focus on the determination of retention coefficients and ideally on their verification on a molecular scale using bulk material, i.e. a homogeneous mineral component in the form of powder. Contrary, natural samples inherently show heterogeneity not only from a chemical perspective but also structurally and topographically. Therefore, it is necessary to perform an upscaling of the investigation technique from the molecular level to larger scales.
In order to perform this upscaling, the microscopic time-resolved laser-induced fluorescence microscopy (µTRLFS) was developed and shows great promise. Trivalent lanthanides such as Cm(III) and its chemical analogue the lanthanide Eu(III) show an inherent luminescence when excited by a source of light, from which its chemical speciation can be derived. TRLFS has been used extensively to study luminescent metals and their chemical environment in the past, especially their interaction with different mineral phases. Upgrading conventional TRLFS by introducing a spatial approach yielded µTRLFS. This newly developed technique allows speciation investigation of two-dimensional areas on the scale of millimeters with a 20 µm resolution.
The aim of this thesis within the iCross-project is to use µTRLFS in combination with other surface investigation techniques such as Raman-microscopy, interferometry and autoradiography on natural samples to draw the connection between surface properties such as mineralogy, topography, mineral grain boundaries, and crystal orientation with Eu(III) and Cm(III) sorption quantity and speciation. This will result in a deeper understanding of sorption properties of trivalent actinides in a more realistic environment.
K. Molodtsov, S. Schymura, J. Rothe, K. Dardenne and M. Schmidt, Scientific Reports, (2019), 9, 6287.