Cesium diffusion in mortars from different cements used in radioactive waste repositories

Cement-based materials are widely used for stabilizing and conditioning radioactive waste in low- and intermediate-level repositories. In this study, the adequacy of four different types of mortars, obtained from four different commercial cements, CEM I, II and IV (A and B), to act as barrier to Cs-137 migration was analysed. Diffusion experiments using the in-diffusion (ID) method with constant tracer concentration in the reservoirs were carried out, at increasing experimental times (150, 380 and 1500 days approximately). In the experiments of less duration, and especially in CEM II and CEM I, two different pathways for diffusion were identified (fast and slow), leading to a double porosity system. However, the "fast" contribution to Cs diffusion, in all cases, could be neglected at larger experimental time, indicating a rearrangement of the pore structure that becomes more fight and homogeneous over time. Thus, if the experiments last enough time, only one diffusion coefficient properly describe Cs transport in these systems. Apparent diffusion coefficients, D-a, obtained for Cs, range between 6.0.10(-13) and 1.0.10(-14) m(2)/s. Mortars produced with CEM IV (A and B) are the most efficient barrier for cesium transport, amongst those analysed in this study. Mortars with CEM II, which contain blast furnace slag, present the highest diffusion coefficient.