Effect of curing time on the fraction of 60.CO and 137.Cs leached from cement matrix: Fid-Bau Portal

Effect of curing time on the fraction of 60.CO and 137.Cs leached from cement matrix

Plecas, I. / Peric, A. / Drljaca, J. / Glodic, S. / Kostadinovic, A.

To assess the safety of disposal of radioactive waste material in concrete, curing conditions and time of leaching radionucleides 60.Co and 137.Cs have been studied. Leaching tests in concrete were carried out in accordance with a method recommended by IAEA. All profiles after 28 days, 60 days and 100 days for the radionucleides exhibit the same characteristics. An enhanced initial period of leaching occurs during the first 25 to 30 days or so, followed by a distinct reduction in the leach rate which is broadly maintained up to the long period of leaching. The leach behaviour of concrete materials can be explained as a combination of two processes; surface wash-off, which is not diffusion controlled, followed by a static diffusion stage. Enhanced initial period of leaching can be explained in terms of a rapid equilibrium being established between spaces present in the surface pores of the Portland cement and ions in solution in the leachate; hence the term wash-off. When 60.Co and 137.Cs has been leached from the surface of the waste form, 60.Co and specially 137.Cs must migrate by longer pathways from the bulk through a depleted surface layer before it can enter the solution. It is this second stage which is controlled by diffusion and which dominates the long-term leaching behaviour of the material. Under these circumstances the effect of increased curing time on the diffusion coefficient becomes apparent. For example, the diffusion coefficient at 100 days curing is almost 2 orders of magnitude lower than that obtained after 28 days curing.