The Japanese approach to developing clay-based repository concepts – An example of design studies for the assessment of sealing strategies

The H12 repository concept for vitrified high-level radioactive waste was developed based on a multi-barrier system, with the emphasis on robust engineered barrier performance to ensure its feasibility for a wide range of geological conditions typically observed in Japan. The decision to use a volunteer siting process requires maximum flexibility of the repository concept to allow it to be adapted to potential sites and hence a wide range of variants of the basic H12 repository design has been developed. In order to evaluate the feasibility and applicability of different repository options to specific siting environments, NUMO has established a set of “design factors” which classify the aspects that need to be considered when evaluating the pros and cons of different repository options.The buffer in the repository concepts is clay-based. An Na-type bentonite from Japan is used as the reference material for all clay-based repository components (buffer, backfill, clay plug, etc.). The characteristics of this bentonite (thermal, mechanical, chemical, hydraulic) have been examined with consideration of various practical constraints (limitation on the repository footprint, the influence of saline water, the interaction of hyperalkaline leachates, practical working environment, etc.).Clay-based seals, which close off the tunnels after emplacement of the engineered barrier system (EBS), may also be key components for assessment of the performance of the repository. Full analyses considering all engineered barrier components (buffer, backfill, clay plug, concrete lining, tunnel, concrete plug) that may be used in a repository will be an essential future task. As a first step towards this goal, a numerical analysis focusing on hydraulic behaviour at the intersections of the disposal tunnels and the main tunnel is presented to illustrate how the design requirements of clay-based seals can be determined.