Modelling of a large-scalein-situmigration experiment with¹⁴C-labelled natural organic matter in Boom Clay

Abstract

For the assessment of the long-term safety of a geological disposal of high-and intermediate-level radioactive waste and/or spent fuel in the Boom Clay, a better understanding of the migration behaviour of Natural Organic Matter (NOM) is needed because it can act as a carrier molecule for radionuclides. Therefore, an in-situ migration experiment with 14 C-labelled NOM was performed to study the NOM migration behaviour on a large scale (m), on the long-term (> 10 a) and in directions parallel and perpendicular to the bedding plane (transport anisotropy). The numerical modelling tool HYDRUS2D/3D was used to interpret the results. The model was built stepwise, testing the influence of advection, (non-)linear equilibrium sorption, colloid attachment/detachment and anisotropy. The up scaling of previously determined parameters from small scale lab tests was also tested. A classic diffusion-advection-retardation description, using parameter values in the range of those obtained in the lab tests, provided already reasonable results. Inclusion of a colloid filtration term in the model significantly improved the simulation. Finally, the model was successfully tested against a second dataset and the anisotropy of the Boom clay was brought into account.

Self-Diffusion of Water and Ions in Clay Barriers

“…As evidenced below, this simplicity is instrumental in identifying the key trends associated with water and ion diffusion in clay materials. The diffusion of more complex or less extensively studied species such as oxyanions (Aertsens et al, 2003;Jansson and Eriksen, 2004;Sato and Miyamoto, 2004), actinides Joseph et al, 2013), lanthanides , trace metals (Lange et al, 2009), dissolved gases (Higashihara et al, 2004(Higashihara et al, , 2005, and organic molecules (Chapman and Parker, 2005;Martens et al, 2010) is not addressed here despite its interest in a range of areas. Diffusion in unsaturated conditionsdan important phenomenon in soils (Calvet, 1973;Nye, 1979) and, potentially, in HLRW repositories in certain conditions (Savoye et al, 2010)dis not discussed.…”

Sorption and diffusion of organic acids through clayrock: Comparison with inorganic anions

“…Many studies have been carried out focusing on the influence of organic ligands on RN solubility, sorption and mobility in cement environments (Stockdale and Bryan, 2013) and in clayrocks (Maes et al, 2011;Poinssot and Geckeis, 2012). Numerous studies are also dedicated to the sorption of small organics on clayey materials (see for example Drouin et al, 2010), but only few data are available on diffusiondriven migration in clayey materials (Martens et al, (2010) studied the migration of 14 Clabelled natural organic matter (NOM) through boom clayrock). Such investigations are, however, crucial in order to assess the actual organic transport through the geosphere.…”

Overview of radionuclide migration experiments in the HADES Underground Research Facility at Mol (Belgium)

“…Starting from laboratoryderived data interpreted by the classical diffusion retardation, extra complexity was introduced to account for the colloidal behaviour. This first attempt, which is not discussed further here, resulted in a reasonable fit of the experimental data (Martens et al, 2010) but further fine tuning is needed.…”