[en] The sub-surface disposal facility under planning will accept wastes those concentration levels can be a few order higher than that disposed of in current Rokkasho LLW disposal center. Proposed waste contains the reactor core surrounding parts, the ion exchange resin, reactor core-internals replaced or decommissioned, and some of the waste from reprocessing plant operation and decommissioning. In 2004, Nuclear Safety Commission of Japan (NSC) submitted the report titled by 'Commonly Important Issues for the Safety Regulations of Radioactive Waste Disposal'. NSC recommended the risk-informed approach that combines the likelihood of a given scenario with its impact should be introduced in safety regulations governing radioactive waste disposal. The basic concept for the safety of the sub-surface disposal is under consideration by NSC. Considering key issues of NSC, the basic policy of the safety is proposed in terms of radiation protection both for natural processes and human activities, and requirements for the geology and engineered barriers are discussed. The BAT (Best Available Technique) and the predictable long-term behavior of geologic environment are experimentally introduced both in the design of facility and its safety assessment. The concept of co-operation among the site investigation, the facility design and the safety assessment is also proposed. The methodology of facility design is discussed based on the proposed basic policy of the safety. With in mind the safety for the development, operation and closure of disposal facility, key components of the facility are extracted and safety requirements for the design are clarified. Based on safety requirements, safety functions to be added to the facility are assigned. It is examined how safety functions should be divided to key component. According to results of the examination, safety functions are classified as a) a function should be considered in the design for an independent component or should be synthesized among related components, b) a function should be evaluated through the performance assessment based on its specifications in order to judge the necessity of design adjustment. The function b) has an important role to clarify the feedback route from the result of performance assessment to the facility design. The applicability of the proposed design methodology to the bentonite barrier is examined. As a one possible realization of the risk-informed approach that combines the likelihood of a given scenario with its impact, the methodology to construct the likely scenario is examined. In this scenario, the most possible evolution of disposal system is evaluated based on the standard geologic environment, supposing the satisfactory performance of engineered barriers. The scenario should be comprehensively developed with transparency and traceability. At first, possible states of the facility are categorized based on the FEP list and the T-H-M-C coupling matrix. Considering the evolution of the facility from a certain state to another state, some possible bifurcation points are extracted. The logic tree analysis is applied to the bifurcation point and the most possible evolution concerning the bifurcation point is judged. The likely scenario is developed by the succession of judgments for extracted bifurcation points. The applicability of the methodology is examined to estimate the long-term evolution of the hydraulic conductivity of the bentonite barrier. (author)