Modelling of matrix diffusion in a tracer test in concrete

A laboratory-scale tracer test has been carried out to improve the characterization of the transport properties of the concrete from the Radioactive Waste Disposal Facility at El Cabril (Spain). High entry pressure was employed in order to perform the experiment in a reasonable time span. Lithium, bromide and deuterium were used as tracers. The conceptual model considered matrix diffusion between a mobile pore domain, where water can flow, and an immobile zone without any advective transport. Three geometries have been compared, considering the immobile zone as slabs, spheres or tubes. Porosity of the mobile zone and characteristic time were estimated by calibrating the model results to the measured breakthrough curves of deuterium and bromide. The calculated values showed that the characteristic time depends on the geometry and similar porosity of the mobile zone was estimated for all geometries. The double-porosity conservative transport model could reproduce the deuterium breakthrough curve. However, the bromide behaviour could not be reproduced even when linear retardation was applied.