An Overview of the H12 Performance Assessment in Perspective
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An Overview of the H12 Performance Assessment in Perspective Kaname Miyahara, Hitoshi Makino, Tomoko Kato, Keiichiro Wakasugi, Atsushi Sawada, Yuji Ijiri*, Aki Takasu**, Morimasa Naito*** and Hiroyuki Umeki*** Japan Nuclear Cycle Development Institute (at present, *Taisei Corporation, **Nuclear Safety Research Association, ***Nuclear Waste Management Organization of Japan) ABSTRACT The H12 performance assessment (PA) provided a test for the robustness of a HLW repository system concept based on structured siting and design, taking account of a wide range of potentially suitable Japanese geological environments. The generic nature of the host rock in the H12 assessment means, however, that emphasis is placed very much on strong EBS performance. The assessment included a comprehensive evaluation of uncertainty and potentially detrimental factors, including perturbations due to external events and processes. Despite the considerable uncertainty at the current stage of the Japanese program, a safety case that is adequate for the aims of the assessment can be made by a strategy of employing conservatism where there is uncertainty and stressing the reliability and effectiveness of the performance of the near-field. The aim of this paper is to present the H12 PA in a way which makes the PA process clearer and the implications of the results more meaningful, both to workers within the PA field and to a wider technical audience. INTRODUCTION The second progress report, H12 [1], documented research and development progress in the high-level waste disposal project since publication of the earlier H3 report [2]. Its aim was to provide a scientific and technical basis for the geological disposal of HLW in Japan, and to demonstrate to the scientific and technical community, and also to the general public, that safe disposal can be achieved. The concept of geological disposal in Japan is similar to that considered in other countries, being based on a system of multiple passive barriers consisting of the geological environment (geosphere) and an engineered barrier system (EBS; including vitrified waste, an overpack and buffer material). The disposal system considered is generic, as neither host rock nor siting area have so far been identified. The H12 performance assessment (PA) provided a test for the robustness of a HLW repository system, taking account of a wide range of potentially suitable Japanese geological environments. For PA, it is assumed that the repository is sited in a stable geological environment where no significant natural resources exist, that the repository is located at depth (between 500 and 1000 m) and that a sufficient level of quality control of operational procedures is exercised. Such a repository, based on a structured siting and design process [3], provides the following broad intrinsic safety functions: • Protection of the EBS by the geosphere (from physical and chemical perturbations); • Long-term physical and chemical containment of radionuclides by the EBS; • Retardation of radionuclides by physical
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