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Research paper−202−Synthesiology - English edition Vol.4 No.4 pp.202-211 (Mar. 2012) without the safety regulation and assessment utilizing this knowledge to the maximum. Figure 1 shows the outline of the relationship of the investigation and assessment items for geological disposal site investigation and the security after closure of the site. This paper describes the extraction of the geological investigation and assessment items required for the safety assessment of geological disposal, and the methodology for long-term forecast based on the model building for the investigated geological phenomena. 2 What is geological disposal?According to the Japanese law, various radioactive wastes must be disposed in either the category 1 or the category 2 waste disposal facility, depending on the radioactivity level. Category 1 is the so-called geological disposal, and it is a disposal method taken when the wastes must be isolated from the biosphere for a long time, and therefore is buried underground at 300 m or deeper. The wastes that must be disposed in this manner include the high-level radioactive (HLR) waste and certain low level radioactive (LLR) waste (such as the long half-life, low-heat-generating radioactive wastes). The HLR waste is the vitrified HLR waste liquid generated in the processing of the spent fuel. It is characterized as a nuclear waste with a long “lifespan” because it contains radionuclides with a long half-life. This is the reason for selecting the geological disposal method that does not require human management. The time required for the radioactivity of the HLR waste to drop to the level of original uranium ore (high concentration of 1 % grade ore) is about a hundred thousand years after power generation[1]. The category 2 waste 1 IntroductionGeology is a science that studies the series of changes in the earth that occurred in the past, as described in the Principles of Geology by Charles Lyell, written in the early half of the 19th century. The greatest contribution of geology to humankind is the discovery of deep time, and 4.6 billion years of earth history have been unraveled from the records left in the rocks and strata. The stronghold of geology is the understanding of how a region was formed, by looking at the natural phenomenon in a time frame of several thousand to several million years. Geology was traditionally deployed in the exploration of natural resources such as oil and metals, and has recently been applied to mitigate the geological disasters such as earthquakes and volcanic eruptions.Recently, a new issue that must be handled geologically arose due to the changes in social demand. This is the issue of geological disposal where the radioactive waste from nuclear power generation is buried underground, far away from the environment in which people live. To ensure safety of this disposal, the underground geological environment must be stable over a long term of several hundred thousand years. It is impossible to guarantee the safety over such time scale by covering the waste with an artificial structure using engineering methods. Safe disposal is possible when the underground geological environment itself functions adequately as the natural barrier against nuclear waste. Only geological knowledge can provide solutions to the issue of long-term stability assessment required for geological disposal, and geological disposal will not be realized - Methodology of long-term forecast using geological history-Concerning the geological disposal system of high-level nuclear waste, the term subject to safety assessment of the system after closure is considered to exceed several hundred thousand years. We are faced with the major issues of how to guarantee such long-term safety and on what kind of criteria the system should be regulated. Because the Japanese islands lie in the mobile belt where earthquakes and volcanic activities often occur, a variety of geological issues required for the safety assessment have to be taken into consideration. In this paper, issues are extracted from a series of facts or scenario where incidents occur one after another provoked by one incident, and one such example is given of analysis results of the eruption history of volcanic activities. A methodology for long-term forecast addressing the causes of geological phenomena is also presented.Safety assessment of high-level nuclear waste disposal in Japan from the standpoint of geology Keywords : High-level nuclear waste, geological disposal, seismicity, volcanism, long-term forecasting [Translation from Synthesiology, Vol.4, No.4, p.200-208 (2011)]Takahiro YamamotoGeological Survey of Japan, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan E-mail : Original manuscript received March 31, 2010, Revisions received September 27, 2011, Accepted September 27, 2011

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