This paper describes a new safety concept for the Japanese geological disposal program, which is a development of the conventional multi-barrier system concept. The Japanese government established the ‘Nuclear Waste Management Organization of Japan’ (NUMO) as an implementation body in 2000 based on the “Final disposal act” following the publication of the “H-12 Report”, which confirmed the scientific and engineering feasibility of HLW geological disposal in Japan. Since then, NUMO has undertaken further technical developments aimed at achieving safe and efficient implementation of final disposal. The safety concept developed in the “H-12 Report” provides sufficient safety on the basis of site-generic considerations. However, it is considered to be over-conservative and therefore does not represent the most probable performance of the engineered or natural barriers. Recently, concrete measures have been proposed requiring the safety case to be presented in terms of a realistic assessment of the most probable performance. This approach takes into account the safety functions of both engineered and natural barriers as well as the long-term static geochemical equilibrium. In particular, the evolution of the safety performance of engineered and natural barriers can be efficiently augmented by the realistic long-term geochemical equilibrium.

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