Solute transport properties of compacted Ca-bentonite used in FEBEX project

The present Spanish concept of a deep geological high level waste repositor y includes an engineered clay barrier around the canister. The clay present s a very high sorption capability for radionuclides and a very small hydrau lic conductivity, so that the migration process of solutes is limited by so rption and diffusion processes. Therefore, diffusion and distribution coeff icients in compacted bentonite (i.e. in "realistic" liquid to solid ratio c onditions) are the main parameters that have to be obtained in order to cha racterise solute transport that could be produced after the canister breakd own. Through-Diffusion (TD) and In-Diffusion (ID) experiments with HTO, Sr, Cs a nd Se were carried out using compacted FEBEX bentonite, which is the refere nce material for the Spanish concept of radioactive waste disposal. Experim ents were interpreted by means of available analytical solutions that allow the estimation of diffusion coefficients and, in some cases, distribution coefficients. Analytical solutions are simple to use, but rely on hypothese s that do not hold in all the experiments. These experiments were interpret ed also using an automatic parameter estimation code that overcomes the lim itations of analytical solutions. Numerical interpretation allows the simul taneous estimation of porosity, diffusion and distribution coefficients, ac counts for the role of porous sinters and time-varying boundary concentrati ons, and can use different types of raw concentration data. (C) 2001 Elsevi er Science B.V. All rights reserved.