Prediction of multi-component radioactive waste migration in clay barrier via coupled solute transport and chemical equilibrium speciation

Risk assessment of radioactive waste sites require an accurate prediction of geochemical interactions during transport. The prediction of the long-term leaching behaviour of radioactive substances is an increasingly important issue as awareness of the potential future pollution risks associated with management of such wastes grows. An adequate understanding of the numerous and complex processes which may act to retard or mobilise radioactive contaminants along the transport pathway form the repository to the biosphere is necessary. Modelling of the physicochemical processes which take place between the radioactive substances and soil is an invaluable tool as it is often not possible to conduct experiments over sufficiently long time scales in order to observe the long term leaching behaviour of wastes. A COupled Solute Transport and CHemical Equilibrium SPeciation (COSTCHESP) model has been developed. The model has the capability for simulating transport of multiple components of radioactive substances, thermodynamically reacting chemical, through the clay barrier systems. It consists of two main modules, a finite difference transport module (COST), and an equilibrium geochemistry module (CHESP) which is modified version of MINTEQA2. This linearizes the coupling between the physical and chemical processes and leads to a simple and efficient model to simulate the simultaneous processes of advective-dispersive transport (advection; diffusion, osmotic and ion restriction effect) and geochemical reactions (complexation, exchange, precipitation, adsorption and desorption) under different temperature and pressure. The model will lead to proper identification of the form of specific ions ( i.e., adsorbed and precipitated on solid, and available in solution). The proposed model has been simulated for the transport of strontium through compacted bentonite in a multi-component solution at two different temperature. (author)