Abstract: The buffer material as one of the most important components of engineered barrier system is the last defence between waste container and host rock. This barrier plays an important role in stabilizing the repository excavations under the coupled thermal-hydro-mechanico-chemical (THMC) conditions, and in providing low permeability and diffusivities, and long-term retardation. To guarantee the long-term safety of the engineered barrier, a large-scale mock-up facility, named China-Mock-up is constructed in the laboratory of BRIUG. This facility concentrates on the research on Gaomiaozi-Na-bentonite, which is currently considered as the candidate buffer and backfill material for China's high level waste (HLW) repository. Methods of construction and test for buffer material Mock-Up is established. The current experimental data of the facility are reported and analyzed in the paper. The saturation process of the compacted bentonite is influenced by the competing mechanisms of drying effect induced by the heater and the wetting effect by the water penetration. The stress evolution of the compacted bentonite may be influenced by the thermal expansion induced by high temperature and the swelling pressure generated by bentonite saturation. The evolution of displacement of the heater which is to simulate vertical placement of a container with radioactive waste is validated for the first time. The experiment is evaluated THMC processes taking place in the compacted bentonite-buffer during the early phase of HLW disposal and to provide a reliable database for numerical modeling and further investigations of EBS, and the design of HLW repository.

Key words: high level radioactive waste geological disposal, buffer material, Na-bentonite, coupled THMC, large-scale Mock-Up