Model and parameter validation based on in situ experiments in Boom Clay

The demonstration of the long-term safety of a nuclear waste repository relies heavily on earth science models, integrated in performance assessment (PA). Demonstrating the validity of those models is important to have confidence in the safety assessment outcome. The models are therefore subject to quality assurance procedures of which model validation, qualification and verification are essential elements. However, in the context of PA, model validation is often limited due to extreme timescales and the use of natural barriers that can never be entirely characterised. Nevertheless, it is often possible to demonstrate that the models are valid/qualified to describe the processes at hand. In this presentation, this was demonstrated for radionuclide transport in Boom Clay, which is the dominant barrier in the repository system. Two in-situ migration experiments, CP1 and Tribicarb-3D were installed in the HADES URF in 1988 and 1995 respectively to validate radionuclide migration behaviour in Boom Clay at large scale and long-term: the experiments are still running today. Excellent predictions of the tracer’s breakthrough curves at a few metres from the source have been obtained using the conventional ADR equation to describe solute transport and parameters obtained from (small scale) migration experiments in the laboratory.