[en] A high-level nuclear waste disposal is planned to be constructed at deep underground (approximately 500 m). It comprises a multiple barrier system made of a waste canister, sealing clay, the supporting concrete and plug concrete, and the surrounding host rock. Specifically, concrete contains soluble alkali ions, and their contact with the clay and host rock is a concern. Over a long term, the interaction between the alkali ions and clay increases the degradation of the clay, which loses its function of sealing. To observe the interaction area, the authors develop a micro-scale observing method combining high-resolution X-ray computer tomography and local X-ray diffraction. Simultaneously, a full-scale engineering barrier experiment is performed at the Grimsel test site in Switzerland. The authors apply the observing method to an interface mortar used as a plug and bentonite used as sealings. Accordingly, the visualized boundary area exhibit the porous and fusion of materials. (author)