Thermodynamic modelling of cement degradation: Impact of redox conditions on radionuclide release

Abstract

Portland cement, used as a solidification matrix and/or as backfill and construction material will determine the aqueous chemistry of a low- and intermediate level radioactive waste repository for very long periods of time. Based on a recently developed cement degradation model, the present work aims to investigate the influence of Eh on the solubility of actinides during the degradation of cement. Uranium is used as a representative for the actinides. Literature data are used to estimate the solubility of uranium bearing solid phases, which were found to be stable in cement-like systems. Log K so values of -41 and of -66 are estimated for a solid called x-phase and for uranophane. In a strongly reducing environment (−450 mV) uraninite is the stable solid phase and uranium solubilities in the range 10 −8 to 10 −10 M are modelled. Under moderately reducing conditions (−300/−200 mV) the solubility limiting phase depends on the pH of the cement pore solution, and at pH∼12 a phase transformation is predicted. Modelled solubilities lie between 10 −7 and 10 −10 M. Uranium speciation in solution and the nature of the solubility limiting phase are very sensitive to both Eh and pH, whereas total dissolved uranium is not. It is concluded that the predictability is fair for strongly reducing conditions, but the uranium-cement system is not very well understood at moderately reducing conditions. The study raises a series of still unresolved questions, which clearly indicate the need of further experimental data.