Comparison of lab-scale experiments for the determination of non-reactive tracer diffusion coefficients in Boom Clay

The transport parameters of radionuclides, the diffusion coefficient and the retardation factor, in clay-rich host rocks investigated for deep geological disposal, can be determined using various types of laboratory experiments. The present study aimed at comparing three types of laboratory experiments commonly used in the context of radioactive waste for the determination of the transport properties of two non-reactive tracers, HTO and ¹²⁵I, in Boom Clay. Two Boom Clay samples with orientation parallel and perpendicular to the bedding were selected to assess the role of sample heterogeneity and anisotropy. Each sample/orientation was subjected to through-diffusion experiments, pulse injection experiments with pressure differences over the clay core ranging from 5 to 19 bars and a back-to-back experiment. In the case of HTO, the three types of experiments led to a consistent set of transport parameters for each Boom Clay sample. Over the three types of experiment, the average apparent diffusion coefficients and accessible porosity vary at maximum a factor 1.4 and 1.3, respectively for a same sample type/orientation. The porosity accessible to HTO was similar to the porosity calculated from the water content. In the case of ¹²⁵I, the three types of experiments also showed consistency for the determination of the apparent diffusion coefficients with values varying within a factor 1.7 for the same sample type/orientation. However, variation in accessible porosity between the experiments revealed a physical modification of the Boom Clay sample occurring during the pulse injection experiments and affecting ¹²⁵I transport. Transport anisotropy was consistently observed in all the types of experiment and for both HTO and ¹²⁵I with a higher anisotropy observed in the samples richer in clay. Slightly higher total porosity and lower ¹²⁵I accessible porosity were noticed in the clay-rich samples in comparison to silt-rich samples.