Dismantling of the Mock-Up Josef in situ experiment after 10 years of operation – a comprehensive analysis of the bentonite barrier

Abstract The Josef Mock-Up experiment, conducted in the Josef Underground Laboratory, simulated the vertical disposal of a spent nuclear fuel container surrounded by compacted, artificially Na₂CO₃-activated Czech bentonite (“Bentonite 75”) under repository-relevant conditions. This approx. 50% scale model (height 2.2 m, diameter 0.7 m) was managed by the Radioactive Waste Repository Authority and constructed and operated by the Czech Technical University in Prague. After nearly a decade of thermal loading at ~ 90 °C, the system was dismantled in late 2022 and extensively sampled. Over 220 bentonite samples from 18 layers were analysed to assess physical, geochemical, and microbiological changes. The distribution of dry density (between 1350 and 1570 kg·m⁻³) and degree of saturation (Sr; 0.80–1.05) revealed partial homogenisation, with the highest saturation in upper layers due to water ingress. Geochemical/mineralogical analyses showed changes in smectite basal diffraction line (from 12.57 to 11.96–9.98 Å), composition of exchangeable cations (Na⁺ depletion, Ca²⁺/Mg²⁺ enrichment) and composition of water leachates (Na⁺ depletion). The elevated siderite content was analysed near corrosion samples and other metal construction components. These changes were attributed to the combined effects of thermal loading and the interaction of water. Microbiological analysis confirmed near-sterility of the bentonite, with minimal bacterial survival and poor colonisation potential of the input material, likely due to prior Na₂CO₃ activation. The results provided data for updating thermo-hydro-mechanical and geochemical models of bentonite barrier evolution and information on the stability of the bentonite barrier in realistic conditions.