Mass Transfer & Porous Media (MTPM) - Andra

P/MTPM/37AN EXPERIMENTAL PROTOCOLFOR THE INVESTIGATION OF DIFFUSION OFIONS THROUGH ARGILLACEOUS BARRIERSM.A.M. Kedziorek, A.C.M. BourgEnvironmental HydroGeochemistry (LHGE), Université de Pau et des Pays de l’Adour, Pau,France (monika.kedziorek@univ-pau.fr)INTRODUCTIONRepositories for radioactive wastes will involve a series of barriers to retard the potential dissemination ofradionuclides towards the Hydro- and Bio-spheres. Clays are sought to achieve this objective for both nearfieldand far-field environments because of their low permeability and high adsorption capacity. In theANDRA approach the barriers investigated are compacted MX80 bentonite and Callovo-Oxfordian argilite,the first as added clay and the second as host rock formation.EXPERIMENTALIn order to investigate diffusive radionuclide transport in argillaceous barriers, we developed anexperimental set up which permits either through-diffusion or in-diffusion experiments. Typically, forthrough-diffusion experiments, the sample is set in a cell at the desired dry density, between two porousplates. Water is circulated through the porous plates (Figure1), the concentration of the element of interestis followed as a function of time in the upstream and downstream reservoirs.RESULTSTwo types of solids were investigated: MX80 bentonite and argilite (Callovo-Oxofordian). In a first step,the diffusion studies were performed on untreated raw bentonite, and subsequently to obtain representativeconditions more rapidly (concerning ionic strength and pH) the bentonite samples were pre-equilibrated forthe specific conditions before the experiments. The argilite samples were used, as delivered, under oxygenfreecontrolled atmosphere.Four elements (Cs, Ni, Pb, Se) were investigated for conditions either controlled by the argilite(compaction, pH) or imposed on the bentonite (pH, ionic strength, dry density) (Figure 2).Effective diffusion coefficients (De) were determined and discussed.Upstreamporous plateDownstreamporous plateClay plug (reactivetracers diffusethrough)Upstream solution(NaNO 3 + reactive tracers)Figure 1: Schematic description of the experimental set up.Downstreamsolution (NaNO 3 )INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCECLAYS IN NATURAL & ENGINEERED BARRIERSFOR RADIOACTIVE WASTE CONFINEMENTPage 495