Mass Transfer & Porous Media (MTPM) - Andra

P/MTPM/33A CROSS METHOD OF A NANOSCALEMODEL AND MACROSCOPIC DATATO ESTIMATE ANION EFFECTIVEDIFFUSION COEFFICIENT IN ARGILLITEN.Diaz (1) , F. Goutelard (1) ,, P.Turq (2 )1. CEA/DEN/DPC/SECR/L3MR, CEN Saclay, Bâtiment 450 ,91191 Gif-sur-Yvette Cedex2. LI2C Université Pierre et Marie Curie – 4,place Jussieu-Boîte 51 75252 Paris Cedex 05The Callovo-Oxfordian argillite of the Paris Basin has been proposed as a potential host rock for deepradioactive waste. Because of very low measured permeability, diffusion is assumed to be the maintransport mechanism for solute migration. As 36 Cl - , 129 I - , and SeO 2- 3 are considered to be the majorcontributors to the calculated dose rate associated with the long term underground disposal of nuclearwaste, this study will focus on the diffusion of anionic species in clayey materials. Such speciesexperimentally exhibits slower diffusion compared to water due to anionic exclusion arising from thenegatively charged clay surfaces.In such materials, the diffusion behaviour of anionic species is known to be related to the diffusionproperties of the clayey matrix. The aim of this study is to determine the effective diffusion coefficient ofchloride relatively to that of water in argillite at the micrometer scale taking into account the interfacialphenomena in clay due to its negatively charged surface.A finite element COMSOL Multiphysics code is used to model the diffusion of a species in a 2-D porousmedia at the nanometric scale. This model correlates the effective diffusion coefficient of anion relativelyto that of water in clay to a unique pore size, the ionic strength and the counter-ion valency of the poresolution [1].Through-diffusion experiments have been carried out on illite clay equilibrated with KClO 4 solution. Theeffective diffusion coefficient of D 2 O and Cl - , Br - , I - have been measured for ionic strengths from 0.001 to0.1 Mol/L. The pore size distribution is obtained by N 2 adsorption allowing measurements of pore sizeslower than 180 nm where influence of electrical double layers is likely to occur. CEC and zeta potentialhave also been determined. All this data have been used to implement an up-scaling method which takesinto account the scattering of pore sizes of the clay. The parametric study performed with the nanometricmodel showed that the pore size is one of the main factors influencing anionic exclusion [1]. Thegeometrical variability of the clay in terms of pore size distribution is converted to a distribution ofeffective diffusion coefficient which is randomly implemented into the clayey matrix (figure 1).In a second step, the up-scaling method is extended to one argillite sample from the Callovo-Oxfordian.Through-diffusion experiments are combined with 2-D mineralogical mapping of a sample of Callovo-Oxfordian argillite obtained by scanning electron microspopy and LIBS microprobe [2]. The three majorminerals are identified: carbonates, tectosilicates and phyllosilicates. Carbonates and tectosilicates aredescribed as a non porous media and effective diffusion coefficients are distributed all over the identifiedclay mineral (figure 1).INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCECLAYS IN NATURAL & ENGINEERED BARRIERSFOR RADIOACTIVE WASTE CONFINEMENTPage 487