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RESEARCH PRODUCT

Determination of Diffusion Parameters by a Computer Code (FDP) Based on Mathematica 6.0

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A computer code named Fitting for Diffusion Parameters (FDP) based on Mathematica 6.0 has been developed for modeling through- and out-diffusion experiments. FDP was used to determine the diffusion coefficients (De ) and the rock capacity factors (α) for tritiated water (HTO) and 22 Na+ and the distribution coefficient (Kd ) of 22 Na+ in Opalinus Clay (OPA). The values for De and α were obtained by fitting the results of experimental data of both transient and steady-state phases to the analytical solution of accumulated activity. The quality of the parameters De and α was tested by using them as input parameters in the equation of flux. Moreover, the diffusion parameters of HTO and 22 Na+ were determined also by out-diffusion experiments. Under ambient condition at pH 7.6, the De value of (1.5 ± 0.1) × 10−11 m2 /s for HTO is lower than that of (1.9 ± 1.1) × 10−11 m2 /s for 22 Na+ , which could be explained by the electrostatic attraction between the negative surface charge of OPA and the sodium cations. For the non-sorbing species HTO, α was 0.15 ± 0.01. For the weakly sorbing species 22 Na+ , α was 0.50 ± 0.02 and Kd equaled (1.5 ± 0.3) × 10−4 m3 /kg. The obtained diffusion parameters for HTO and 22 Na+ in OPA are in good agreement with previous results by Van Loon et al. [1, 2]. FDP developed in this study has been used successfully to determine the parameters De and α for the diffusion of 237 Np(V) in OPA [3].Copyright © 2010 by ASME