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

Abstract

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 ( D e) and rock capacity factors ( α) are: D e=1.2–1.5×10 −11 m 2 s −1 and α=0.09–0.11 for HTO, D e=4.0–5.5×10 −12 m 2 s −1 and α=0.05 for 36Cl − and D e=3.2–4.6×10 −12 m 2 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 ε. 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.