Vol.4 No.4 2012

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Research paper : Safety assessment of high-level nuclear waste disposal in Japan from the standpoint of geology (T. Yamamoto )−203−Synthesiology - English edition Vol.4 No.4 (2012) disposal is a method that assumes human intervention for management, and the LLR waste other than the one mentioned above is buried in relatively shallow ground.While the forms of radioactive waste disposal differ by the waste type, all must maintain the radiation exposure of the general public at certain value or less, and must keep it as low as reasonably achievable[2]. Considering this point as basic security, the final disposal site must be constructed from several layers of barriers consisting of the artificial barrier that includes the waste material itself and the natural barrier or the surrounding geological structures[3]. The artificial barrier plays the role of containing the radionuclides as long as possible and mitigating the release into the natural barrier. The natural barrier is expected not only to isolate the radionuclides from the biosphere, but also to provide a stable external environment to maintain the constant environment for the artificial barrier over a long period. These barriers are expected to function complementarily. The waste material itself and the multiple barriers around it are generally called the disposal system. In the case of the geological disposal, the time period that must be considered for the safety assessment of the closed disposal system must be over several hundred thousand years. The major issues are how to ensure such long-term security and what kind of standard should be set.The geological disposal business of HLR waste in Japan is based on the “Specified Radioactive Waste Final Disposal Act” established in June 2000. This law states that the site selection is done in three stages as shown below.(1)Selection of the preliminary investigation area (PIA): A survey of literature and other source materials (literature search) is done to select the PIA from the area that is reviewed by literature search.(2)Selection of detailed investigation area (DIA): Land surface investigations including outcrop survey, boring, trench excavation, or physical exploration are conducted at the PIA, and the DIA is selected among the PIA candidates.(3)Selection of the final disposal facility construction site: In addition to the detailed survey on the land, the actual facility is constructed underground, the physical and chemical properties of the strata are conducted, and the construction site for the final disposal facility is selected among the DIA candidates.The Nuclear Waste Management Organization of Japan (NUMO) was established in October 2000 to conduct the disposal work based on the Specified Radioactive Waste Final Disposal Act. This organization has been soliciting disposal site candidates to the cities, towns, and villages throughout Japan since December 2002, but no local government has applied as of October 2011. 3 Extraction of the geological issues for the geological disposal in Japan based on FEP (feature, event, and process)One of the major interests in the geological disposal in Japan is the point, “Is it possible to conduct sufficiently safe geological disposal in Japan where earthquakes strike frequently?” For the security of geological disposal, of course, it is necessary that the geological environment of Fig. 1 Relationship between the issues affecting the site-selection survey and the post-closure safety on the geological disposal of high-level radioactive waste Ensuring the post-closure safetySafety assessments using various scenariosRejection of unsuited site for the geological disposal- Groundwater system- Baseline- Mineral resourcesEnvironments factors- Erosion, sedimentation and sea-level change- Seismicity- Volcanic and magmatic activity- Crustal fluids- Mud volcanoes- Mass movementLong-term external factorsInvestigation/assessment items for geological environmentsEnvironments requirements by NSC, JapanThe Specified Radioactive Waste Final Disposal ActSite requirementsGeological & climatic processes in Japan Islands(NISA)Environments factors: Disposal system domainExternal factorsInternational FEPs(OECD/NEA)Issues affecting the post-closure safety

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