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Diffusion coefficients (T=23 +/- 2 degrees 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 (alpha) 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 36Cl(-) and D(e)=3.2-4.6 x 10(-12) m(2) s(-1) and alpha=0.07-0.10 for 125I(-). For non-sorbing tracers (HTO, 36Cl) the rock capacity factor alpha 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 approximately 17% for HTO, approximately 28% for 36Cl(-) and approximately 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. |
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