Modelling Long-Term Corrosion of Cemented Waste Forms in Salt Brines

Since 1979, leaching and corrosion experiments have been performed at the Asse II salt mine using simulated full-scale cemented waste forms. The cement blocks, doped with ¹³⁷Cs and uranium, have been exposed to saturated salt brines. The brines have been sampled regularly and analysed with respect to the mobilized radionuclides, pH, and major solution components. In 2006, four of the cement blocks samples were recovered and solid samples were obtained by drilling vertically into the corroded cement forms. The drill cores as well as abraded drill dust were analysed with respect to chemical and mineralogical compositions, distributions of radionuclides and major waste components, thermogravimetric and mechanical properties. Results of these methods show consistently that cement forms, having an initial W/C = 0.5, were homogeneously corroded in MgCl₂ rich brine. No vertical or radial variations were found with respect to major chemical components and mineral phases. In the cement blocks corroded in NaCl saturated brine, spatial heterogeneities in the distribution of major anions (chloride, nitrate) and the trace cation Cs⁺ show that these cement / NaCl systems have not been completely homogenized within 22 years. Concentration profiles of Cl⁻, $NO3−$ and Cs⁺ were compared with calculated profiles to quantify diffusion coefficients and to get information on the kinetics of relevant processes. Simulations of chemical alteration of the cement / brine system indicate that both MgCl₂-rich and NaCl systems are close to equilibrium with respect to stabilities of secondary mineral phases.