Diffusion of HTO, 36Cl− and 125I− in Opalinus Clay samples from Mont Terri: Effect of confining pressure

Diffusion coefficients (T=23±2 °C) and accessible porosities for HTO, 36Cl− and 125I− were measured on Opalinus Clay (OPA) samples from the Mont Terri Underground Rock Laboratory (URL) using the through-diffusion technique. The direction of transport (diffusion) was perpendicular to bedding. Special cells that allowed the application of confining pressure were designed and constructed. The pressures ranged from 1 to 5 MPa, the latter value simulating the overburden at the Mont Terri URL (about 200 m). The test solution used in the experiments was a synthetic version of the Opalinus Clay pore water, which has Na+ and Cl− as the main components (I=0.42 M). The measured values of the effective diffusion coefficients (De) and rock capacity factors (α) are: De=1.2–1.5×10−11 m2 s−1 and α=0.09–0.11 for HTO, De=4.0–5.5×10−12 m2 s−1 and α=0.05 for 36Cl− and De=3.2–4.6×10−12 m2 s−1 and α=0.07–0.10 for 125I−. For non-sorbing tracers (HTO, 36Cl) the rock capacity factor α is equal to the diffusion-accessible porosity . The experimental results showed that pressure only had a small effect on the value of the diffusion coefficients. Increasing the pressure from 1 to 5 MPa resulted in a decrease of the diffusion coefficient of 17% for HTO, 28% for 36Cl− and 30% for 125I−. Moreover, the diffusion coefficients for 36Cl− and 125I− are smaller than for HTO, which is consistent with an effect arising from anion exclusion. The diffusion coefficients of HTO and 125I− measured in this study are in good agreement with recent measurements at three other laboratories performed within the framework of a laboratory comparison exercise. The values of the diffusion-accessible porosities show a larger degree of scatter