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

Abstract The Mock-Up Josef experiment, conducted in the Josef Underground Laboratory, simulated the vertical disposal of a spent nuclear fuel container surrounded by compacted Czech Ca-Mg 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 a 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 (mainly between 1350–1570 kg/m³) and degree of saturation (Sr; 0.80–1.05) revealed partial homogenization, with highest saturation in upper layers due to water ingress. Geochemical analyses showed smectite dehydration (basal peak shift to d001 ~ 9.98), alteration of exchangeable cation composition (Na⁺ depletion, Ca²⁺/Mg²⁺ enrichment), and increased siderite content near corrosion samples. These changes were attributed to combined effects of thermal loading, water interaction, and contact with metal components.Microbiological analysis confirmed near-sterility of the bentonite, with minimal bacterial survival and poor colonization potential of the input material, likely due to prior natrification.The results provided data for updating thermo-hydro-mechanical and geochemical models of bentonite barrier evolution.