[en] Repository site selection in the Japanese HLW disposal programme is based on a volunteering approach, which places special constraints on the process of developing repository designs. In particular, a high degree of flexibility is needed in order to respond to conditions in the specific geological environments in the communities that may come forward. In order to meet this challenge, NUMO has developed a structured process for tailoring repository concepts to siting environments, which considers many options for each design components and assesses them with regard to a number of design goals, long-term safety, operational safety, engineering feasibility, etc. Various associated design demonstration activities have been carried out by R and D organisations. such as JAEA and RWMC, to support the NUMO structured approach. Such activities are presently focused on engineering feasibility and the long-term safety of a range of repository design options. The preliminary results from such demonstration tests have useful feedback for further design improvements. This suggests that optimisation of repository design can be achieved by world rig systematically through an iterative process of design and assessment followed by re-evaluation and design refinement. (authors)