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

Diffusion coefficients (T = 23 +/- 2degreesC) and accessible porosities for HTO, Cl-36(-) and I-125(-). were measured on Opalinus Clay (OPA) samples from the Mont Tern 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 (a) are: D-e = 1.2-1.5 x 10(-11) m(2) s(-1) and alpha = 0.09-0.11 for HTO, D-e = 4.0-5.5 x 10(-12) m(2) s(-1) and alpha = 0.05 for Cl-36(-) and D-e = 3.2-4.6 x 10(-12) m(2) s(-1) and alpha = 0.07-0.10 for I-125(-). For non-sorbing tracers (HTO, Cl-36) the rock capacity factor a is equal to the diffusion-accessible porosity epsilon. 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 similar to17% for HTO, similar to28% for Cl-36(-) and similar to30% for I-125(-). Moreover, the diffusion coefficients for Cl-36(-) and I-125(-) are smaller than for HTO, which is consistent with an effect arising from anion exclusion. The diffusion coefficients of HTO and I-125(-) 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. (C) 2002 Elsevier Science B.V. all rights reserved.