Immobilization and leaching mechanisms of radwaste in cement-based matrices

Studies of Cs immobilization continue. On account of its high solubility, it has proved to be difficult to immobilize and high leach rates have been reported from cement-based forms. High specific surface area silicas have been shown to possess significant sorption and retention for Cs in the alkaline cement environment. The mechanism and amount of sorption have been assessed for some representative, commercially-available silicas and siliceous materials. The behaviour of zeolites in cement matrices has been examined. Zeolites containing Cs undergo two types of reaction with cement matrices; ion exchange of Cs in the zeolite with other ions furnished by the cement and pozzolanic reactions, leading to physical consumption of the zeolite. Relatively simple techniques have been developed whereby the separate contributions of the two reactions can be distinguished. Ion exchange is more rapid at 20 deg C than the pozzolanic reaction, but the latter is markedly accelerated at higher temperatures. Diffusion experiments on Cs in cement have also begun. The immobilization of some standard reference waste forms in cement matrices has begun. It is found that the hydration demands of the cement result in the removal of water from salt-rich solutions containing nitrates and sulphates. (author)