The Role of Calcite in Nuclear Waste Disposal Sites

In the safety assessment of nuclear waste disposal sites we have a look on different geochemical processes in the near and far field of a conceivable disposal site. These processes include sorption and incorporation as possibel mechanisms of radionuclide retention in a worst case scenario of an ingress of groundwater. Various minerals in the host rocks as well as primary and secondary phases in the geotechnical barrier were investigated under these conditions elsewhere. Calcite (CaCO3) can be found in every section of the containment and has a couple of features, which makes it interesting for further investigations. A high retention potential because of high sorption capacity as well as the possibility to incorporate guest ions into the crystal lattice at the Ca-ion position is distinctive for calcite [Schmidt 2008, Marques Fernandes 2008]. The long-lived radionuclides (e.g. Plutonium, Curium) determine the long-term radiotoxicity, which defines the considered timespan of the safety analysis. The trivalent radionuclides have an affinity to calcite because of their chemical properties (ionic charge and radius).
To investigate this affinity we conducted different experiments – coprecipitation and batch studies over various periods of time. We can show, that the incorporation of radionuclides and their homologues is dependent on several parameters: i.e. the grain size and specific surface area of calcite, amount and composition of impurities in the calcite and in the background solution. To investigate the structural incorporation we used site-selective timeresolved laser-induced fluorescence spectroscopy with Europium, which serves as a homologue for the trivalent radionuclides because of its great spectroscopic usability [Binnemans 2015] and its similar chemical behavoir. With this method we can distiguish between sorption of the Europium ion onto the calcite surface and incorporation into the crystal bulk by figuring out the amount of water molecules in the first coordination shell of the Europium ion – if there is no water left, incorporation took place. Furthermore we perform X-ray surface diffraction with two high resolution methods, crystal truncation rod and resonant anomalous X-ray-reflectivity. Our experiments were run in situ, which means we have a thin solution film above the crystall. We can demonstrate the influence of different background electolytes (sodium nitrate and sodium iodate) on calcite, which cause a significant surface destabilisation and hence a modification or prevention of sorption and incorporation mechanism. These results are important to examine sorption and structural incorporation on calcite as a process of radionuclide retention in the near and far field of nuclear waste disposal sites.

References
Binnemans, K. (2015): Interpretation of europium(III) spectra. Coord. Chem. Rev., 295, 1-45.
Schmidt, M. et al. (2008): Charge Compensation in Solid Solutions. Angew. Chem., Int. Ed., 47, 5846-5850
Marques Fernandes, M. et al. (2008): Incorporation of trivalent actinides into calcite: A time resolved laser fluorescence spectroscopy (TRLFS) study. Geochim. Cosmochim. Acta, 72, 464-474.