Retention capacity of lanthanides, thorium and uranium in calcitic fracture mineralisation of granitic rocks

The safety assessment of a repository system for radioactive waste in crystalline rock depends to a large extent on the the fracture system and its properties. Calcite is the most common fracture mineralisation at many crystalline sites in terms of radionuclide retention potential. The aim of the planned work is to investigate calcite fracture mineralisation in crystalline rocks of the Odenwald Crystalline Complex with regard to their ability to retain radionuclides. The investigations will focus on radionuclide retention by incorporation, i.e. substitution of radionuclides in the calcite crystal lattice. Since it is not possible to study radionuclide retention in natural calcite fracture mineralisation, natural analogues are used for these studies. Due to similar chemical behaviour, the lanthanides (REE) can be considered as analogues for the trivalent actinides, and Th and U as analogues for the tetravalent actinides. By (1) analysing the concentrations of REE, Th and U in calcite and in the hydrothermal fluids or deep (palaeo-) groundwater from which the calcites were precipitated, and (2) investigating the physico-chemical formation conditions (e.g. temperature, pressure, pH) by fluid inclusion analyses, it is possible to estimate to what extent and under what conditions the formation of calcite can contribute to the binding of radionuclides in fracture mineralisation.