Diffusivities of cesium and strontium in concretes and in mixtures of sodium bentonite and crushed rock

Diffusivities of Cs and Sr in concretes and in mixtures of sodium bentonite and crushed rock were measured. The order of magnitude of the apparent diffusivities (D/sub a/) for Cs and Sr in concretes was 10-14-10-13 m2/s and in the mixtures of bentonite and crushed rock 10-13-10-11 m2/s. The calculated effective diffusivities (D/sub e/) in the mixtures of bentonite and crushed rock seemed in some cases to exceed the corresponding values in water, which suggests that the pore diffusion/sorption model cannot explain the transport phenomena in all cases. The experiments where the sorption factors were varied in the diffusion samples suggest that the nuclides diffuse also while being sorbed. A combined pore diffusion-surface diffusion model has been used to explain the transport and the corresponding diffusivities have been evaluated. This paper will give the measured diffusion and sorption data and discuss the models used to describe diffusion in porous sorbing agents. 7 refs., 5 figs., 3 tabs