Interactions of U(VI) with cement alteration products in highly saline solutions

The retention of radionuclides in a repository system for cemented low and intermediate level waste (LLW / ILW) strongly depends on the interaction of the cement products with potentially contacting solutions. The present study focuses on interaction of LLW simulates with MgCl2-rich brines and the consecutive U(VI) release, which are relevant to final disposal of LLW / ILW in German rock salt formations. Results of corrosion experiments with full-scale LLW simulates, conducted for more than 20 years, and of laboratory experiments with cement products are combined with thermodynamic simulations. Aqueous U concentrations in the studied systems are solubility controlled and significantly lower than those reported in literature for dilute cementitious systems. In corroded LLW simulate samples U(VI) rich aggregates with a size of > 20 μm have diuranate like structure according to Raman micro-spectroscopy. Thermodynamic simulations are in agreement with experimentally determined pH and Ca, Mg, Na, K, Cl- and SO42- concentrations. Measured U concentrations match both calculated solubilities of diuranate and uranophane. (authors)