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.
In situ migration experiments using different radiotracers have been performed in the HADES Underground Research Facility (URF), built at a depth of 225 m in the Boom Clay formation below the SCK–CEN nuclear site at Mol (Belgium). Small-scale experiments, mimicking laboratory experiments, were carried out with strongly retarded tracers (strontium, caesium, europium, americium and technetium). Contrary to europium, americium and technetium which are subjected to colloid mediated transport, the transport of strontium and caesium can be described by the classic diffusion retardation formalism. For these last two tracers, the transport parameters derived from the in situ experiments can be compared with the laboratory-derived values. For both tracers, the apparent diffusion coefficients measured in the in situ experiments agree well with the laboratory-derived values.In the large-scale experiments (of the order of metres) performed in the URF, non-retarded or slightly retarded tracers (HTO, iodide and H14CO3–) were used. The migration behaviour of these tracers was predicted based on models applied in performance assessment calculations (classic diffusion retardation) using migration parameter values measured in laboratory experiments. These blind predictions of large-scale experiments agree well in general with the experimental measurements. Fitting the experimental in situ data leads to apparent diffusion coefficients close to those determined by the laboratory experiments. The iodide and H14CO3– data were fitted with a simple analytical expression, and the HTO data were additionally fitted numerically with COMSOL multiphysics, leading to about the same optimal values.
Demonstration of the long-term safety of a nuclear waste repository relies on earth science models, integrated in a performance assessment model chain. These models are subject to quality assurance procedures and principles of which model validation, qualification and verification are essential elements. However, in the context of performance assessment, model validation is often limited owing to extreme timescales and the use of natural barriers that can never be entirely characterized. Nevertheless, it is often possible to demonstrate that the models are valid or qualified to describe the processes at hand. In case of geological disposal in Boom Clay, the host formation is the dominant barrier for radionuclide migration and releases to the biosphere. Therefore, it has to be demonstrated that migration of solutes through Boom Clay at relevant scale is adequately understood. Large-scale and long-term in situ migration experiments, such as the CP1 experiment, form a cornerstone in this confidence-building process. In this experiment, accurate predictions of the tracer's breakthrough curves up to 3 m from the source have been obtained using the conventional advection–dispersion–reaction equation to describe solute transport and parameters obtained from (small-scale) migration experiments in the laboratory.
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