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During the last two decades, the Mont Terri rock laboratory has hosted an extensive experimental research campaign focusing on improving our understanding of radionuclide transport within Opalinus Clay. The latest diffusion experiment, the Diffusion and Retention experiment B (DR-B) has been designed based on an entirely different concept compared to all predecessor experiments. With its novel experimental methodology, which uses in-situ X-ray fluorescence (XRF) to monitor the progress of an iodide plume within the Opalinus Clay, this experiment enables large-scale and long-term data acquisition and provides an alternative method for the validation of previously acquired radionuclide transport parameters. After briefly presenting conventional experimental methodologies used for field diffusion experiments and highlighting their limitations, this paper will focus on the pioneer experimental methodology developed for the DR-B experiment and give a preview of the results it has delivered thus far. Image 1 • Novel experimental set-up for the study of iodide ion diffusion in compacted clay. • In-situ downhole quantification of trace metal amounts. • Visualization of the impact of small-scale heterogeneities on transport properties. • Long-term and large-scale iodide diffusion data acquisition. • Merging of mineralogical, chemical and structural elements for data interpretation. [ABSTRACT FROM AUTHOR]