A long-term single-borehole diffusion experiment (DI) using tritiated water (HTO) and stable iodide ( 127 I − ) was carried out In the Opalinus Clay of the Mont Terri Underground Rock Laboratory (URL). Diffusion coefficients D L and accessible porosity for HTO, 36 Cl − and 125 I − were also measured on centimetric Opalinus clay samples using the through diffusion technique. The evolution of tritium and iodide concentration in the injection system over time and in situ profiles were interpreted with a 3-D numerical simulation. A detailed analysis of the results pointed out the effect of a disturbed zone around the borehole with higher diffusion coefficients. The best estimate values for HTO and iodide in the undisturbed rock are D L = 5 × 10 −11 m 2 /s and D L = 1.5 × 10 −11 m 2 /s respectively. For the laboratory tests, D L values for HTO are in the range of 5 × 10 −11 m 2 /s to 8.5 × 10 −11 m 2 /s. For 125 I − and 36 Cl − the measured values are D L = 1.4 × 10 −11 and D L = 1.6 × 10 −11 m 2 /s respectively.All HTO results obtained with a through diffusion technique are within the same range as those obtained in the in situ tests. The D L values obtained in diffusion cells with 125 I − and 36 Cl − and the value drawn from the interpretation of stable 127 I − concentration profiles from the in situ tests are very close. In fact, some significant uncertainties could be identified (i.e. a likely chemical retention of iodide on argillites, effect of the disturbed zone).
Abstract.Modeling the kinetics of a precipitation dissolution reaction occurring in a porous medium where diffusion also takes place leads to a system of two parabolic equations and one ordinary differential equation coupled with a stiff reaction term. This system is discretized by a finite volume scheme which is suitable for the approximation of the discontinuous reaction term of unknown sign. Discrete solutions are shown to exist and converge towards a weak solution of the continuous problem. Uniqueness is proved under a Lipschitz condition on the equilibrium gap function. Numerical tests are shown which prove the efficiency of the scheme.Mathematics Subject Classification. 65N12, 76S05, 80A30.
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