In situ experimental investigation of neon diffusion in the Boom Clay in HADES URL, Belgium

Over the past decade, the Belgian nuclear research centre (SCK CEN) has gathered extensive laboratory data on diffusion coefficients of dissolved gases in water-saturated Boom Clay (BC). Since gas transport around a geological disposal facility occurs at metre scales, a key question is whether centimetre-scale laboratory values remain valid under in situ conditions. To address this, SCK CEN, EURIDICE, and ONDRAF/NIRAS launched the NEMESIS (NEon diffusion in MEgaS In-Situ) experiment in September 2023 at the HADES Underground Research Laboratory in Belgium. Its purpose is to confirm and refine understanding of gas diffusion in BC at realistic scales. NEMESIS reuses four piezometers from the 1990s MEGAS experiment. Dissolved neon is injected into the clay through a vessel–microtube–filter system, and its migration is monitored at three observation filters. This paper outlines the experimental design, strategy, initial findings, and early interpretations. The setup has proven effective under in situ conditions and provides new insights into the coupling between diffusion and water flow. Preliminary best-fit diffusion coefficients from Phase 1 (in-diffusion) are broadly consistent with laboratory measurements. Phase 2 (through-diffusion), to be conducted over the coming years, will deliver additional data to validate results, refine interpretations, and strengthen the characterisation of gas diffusion in Boom Clay.