Modelling Transport of 14 C-Labelled Natural Organic Matter (NOM) in Boom Clay
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Modelling Transport of 14C-Labelled Natural Organic Matter (NOM) in Boom Clay Alice Ionescu1, Norbert Maes2, and Dirk Mallants2 1 Agency for Radwaste Management (ANDRAD), Str. Campului 1, Mioveni 115400, Romania 2 Belgian Nuclear Research Centre (SCK•CEN), Boeretang 200, 2400 Mol, Belgium ABSTRACT In Belgium, the Boom Clay formation is considered to be the reference formation for HLW disposal R&D. Assessments to date have shown that the host clay layer is a very efficient barrier for the containment of the disposed radionuclides. Due to absence of significant water movement), diffusion - the dominant transport mechanism, combined with generally high retardation of radionuclides, leads to extremely slow radionuclide migration. However, trivalent lanthanides and actinides form easily complexes with the fulvic and humic acids which occur in Boom Clay and in its interstitial water. Colloidal transport may possibly result in enhanced radionuclide mobility, therefore the mechanisms of colloidal transport must be better understood. Numerical modeling of colloidal facilitated radionuclide transport is regarded an important means for evaluating its importance for long-term safety. The paper presents results from modeling experimental data obtained in the framework of the EC TRANCOM-II project, and addresses the migration behavior of relevant radionuclides in a reducing clay environment, with special emphasis on the role of the Natural Organic Matter (NOM) [1]. Percolation type experiments, using stable 14C-labelled NOM, have been interpreted by means of the numerical code HYDRUS-1D [2]. Tracer solution collected at regular intervals was used for inverse modeling with the HYDRUS-1D numerical code to identify the most likely migration processes and the associated parameters. Typical colloid transport submodels tested included kinetically controlled attachment/detachment and kinetically controlled straining and liberation. INTRODUCTION Normally, in reducing clay environments, low values of mobile radionuclide concentrations in the clay pore-water are due mainly to solubility limitation. However, the presence of NOM may enhance the solubility due to complexation/colloid formation and/or may influence the sorption behaviour of radionuclides. Transport processes characteristics of Boom Clay were investigated by means of laboratory and in-situ experiments, taking account of NOM. In this paper, a set of laboratory migration experiments was carried out to clarify the possible role of mobile NOM as radionuclide carrier. For this purpose, radionuclide sources (241Am) were prepared with concentrations as close as possible to their expected equilibrium concentration under in-situ Boom Clay conditions and in contact with 14C-labeled BC Organic Matter (14COM), so-called "double-labeled" migration experiments (details can be found in [1,3]). This paper focuses on the migration of the 14COM. Breakthrough data of the 14C labeled OM from these percolation experiments were used to inversely estimate meaningful values for migration parameters ca
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