Operando neutron tomography of the water through-diffusion front in illite under fully and partially water-saturated conditions

The time evolution of the water diffusion front in illite, an analogous clay model for clay-rich formations considered in the context of nuclear waste repositories, was imaged using through-diffusion experiments coupled to neutron tomography. Analysis of neutron tomographies under fully and partially water-saturated conditions revealed the absence of disconnected regions due to partial saturation of the pore space. The mobility of water was decreased in partially saturated sample following a reduction in the overall water content. The influence of the diffusion cell components on the obtained water diffusion profiles was investigated in detail using two-dimensional through-diffusion calculations with the CrunchClay reactive transport code. The modeling results confirmed that the presence of polyetheretherketone grids and rubber O-rings can introduce some biases when determining the effective diffusion coefficients. The position of the circulation canals next to the sample was an additional obstacle to water diffusion. All these effects related to the experimental setup were quantified, allowing an accurate determination of the effective coefficients of water and rock capacity factors. Neglecting these effects using a simple one-dimensional diffusion equation resulted in an underestimation of these values of approximately 20%. A correction method is proposed to overcome these biases for the cell geometry considered in this study. The differences reported in the literature between experiments and calculated diffusive transport parameters of water in partially water-saturated conditions must then be attributed to changes in the physical properties of the sample and water transport mechanisms.