Experimental and modelling studies on diffusion of Cs, Ni and Sm in granodiorite, basalt and mudstone

Abstract

Through-diffusion experiments were carried out for Cs, Ni and Sm which can have a valence of I, II and III, respectively, through granodiorite, basalt and mudstone which have been considered candidates as natural barriers at 25°C under ambient conditions. The tracer solution was prepared as a mixture of Cs, Ni and Sm. The experiments were continued for a maximum of 596 days. During the experiments, the pH was monitored. The tortuosities of the rocks were measured using a NaCl tracer and a through-diffusion method. The porosities, pore size distributions, specific surface area and dry densities of the rocks were measured by a water saturation method and a mercury porosimetry. Effective and apparent diffusion coefficients ( ϵDp, Da) were obtained for each element. The ϵDp and Da values ranged from 0.57 to 1.4 × 10 −12 and 1.0 × 10 −11 m 2 s −1 for granodiorite, respectively. Those for mudstone ranged from 0.53 to 4.8 × 10 −13 and from 2.6 to 3.9 × 10 −13 m 2 s −1, respectively, and those for basalt ranged from 0.28 to 1.5 × 10 −13 and from 1.6 to 2.1 × 10 −13 m 2 s −1, respectively. The ϵDp and Da values of Cs were the highest of the three elements for all rocks. Both diffusion coefficients for all elements were in the order: granodiorite > mudstone > basalt. The pore size was found to be relatively large for each rock compared with the ionic radius. The ϵDp values were predicted, based on the formation factors by taking into account the porosity and tortuosity. The predicted values were in relatively good agreement with the measured values with deviations of less than about five times.