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Model validation based on in situ radionuclide migration tests in Boom Clay: status of a large-scale migration experiment, 24 years after injection

Abstract 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.