Analysis of the Role of Water Saturation Degree in HTO, 36Cl, and 75Se Diffusion in Sedimentary Rock

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The aim of this study was to analyze HTO, 36Cl, and 75Se(IV) diffusion behavior in a sedimentary rock, which was obtained from the site initially selected for the emplacement of centralized temporal disposal of radioactive waste in Spain. Different experimental methodologies were used to analyze radionuclide diffusion in this rock: the through-diffusion (TD) method and the instantaneous planar source (IPS) methods. For the conservative tracers HTO and 36Cl, the effective diffusion coefficients, De, were obtained by the TD method, which was applied to the consolidated material taken at different orientations with respect to the bedding plane (parallel and normal). This revealed a negligible anisotropy of the rock. To analyze the effect of the water saturation degree, the IPS method was used, which was shown to be very suitable for evaluating these effects. For these tests, the samples were crushed, adequately hydrated, and compacted. The apparent diffusion coefficient, Da, was determined for all the tracers at five different water saturation degrees. For each of the tracers investigated, the results indicated that, when the water saturation decreased from 100% to 60%, the Da also decreased by at least one order of magnitude: for HTO, this decrease was from 1.5 × 10−9 to 2.3 × 10−10 m2/s; that for 36Cl was from 4.6 × 10−10 to 2.8 × 10−11 m2/s; and that for 75Se was from 3.6 × 10−11 to 8.3 × 10−13 m2/s. The experimental diffusion profiles of HTO and 36Cl could be satisfactorily fitted considering a unique diffusion coefficient, whereas the profiles of 75Se could not. This behavior is related to the existence of different species of selenium in the system, or to different retention mechanisms.