Modelling iron-bentonite interactions

The presence of both iron canisters and bentonitic clay in some engineered barrier system (EBS) designs for the geological disposal of high-level radioactive waste (HLW) creates the potential for chemical interactions which may impact upon the long-term performance of the clay as a barrier to radionuclide migration. Natural systems evidence suggests that the sequence of alteration of clay by Fe-rich fluids may proceed via an Ostwald step sequence. The computer code QPAC has been modified to incorporate processes of nucleation, growth, precursor cannibalisation, and Ostwald ripening to address the issues of the slow growth of bentonite alteration products. This, together with inclusion of processes of iron corrosion and diffusion, has enabled investigation of a representative model of the alteration of bentonite in a typical EBS environment. Simulations with fixed mineral surface areas show that berthierine dominates the solid product assemblage, with siderite replacing it at simulation times greater than 10,000 years. Simulations with time-dependent mineral surface areas show a sequence of solid alteration products, described by: magnetite → cronstedtite → berthierine → chlorite. Using plausible estimates of mineral-fluid interfacial free energies, chlorite growth is not achieved until 5000 years of simulation time. The results of this modelling work suggest that greater effort should be placed upon providing key data for iron silicates (e.g. kinetic data, solubilities, and mineral-fluid interfacial free energies), through a dedicated programme of laboratory experimental and natural analogue research.