Assessing the long-term safety of a deep geological repository for the disposal of radioactive waste depends on an adequate understanding of the processes governing radionuclide transport. From the early days of the research on geological disposal in clay in Belgium, large-scale, long-term in situ migration experiments were started to test whether our knowledge acquired about small-scale samples can be scaled up in time and space. These experiments use multi-filter piezometers to introduce radiotracers in a ‘source filter’ and monitor their breakthrough in ‘monitoring filters’. The CP1 experiment started in 1988 and used HTO as a tracer, while the Tribicarb-3D started in 1995 and used a cocktail of HTO and H14CO−3 . At the start of these experiments, blind predictions were made based on lab-derived parameters and a simple representation of the hydrological system. Several decades later, these blind predictions still describe the data remarkably well. These tests provide valuable data for upscaling and validating the transport models in Boom Clay and allow us to estimate transport parameters at a larger scale. They provide strong arguments that the radiological safety of a deep geological repository in a clay rock can be guaranteed.