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 )−209−Synthesiology - English edition Vol.4 No.4 (2012) March 2010. The geological disposal is essentially on hold. Aside from this course of event, reconsideration must be done for the geological disposal in Japan, as there was a major geological problem in the safety assessment for Yucca Mountain. The Yucca Mountain site was located within the basalt magma monogenetic volcano group that was active in the Quaternary period, and various assessments had to be done for the volcanic activities for the site. The US Department of Energy (DOE) that conducted the project assessed the probability of eruption at the site by calculating the average re-eruption interval from the eruption history of the past monogenetic volcanoes, assuming that a mega-eruption that may form a caldera will not occur[11]. However, the eruptive activities of monogenetic volcano groups are distributed unevenly in both time and space, and do not occur uniformly. Considering the peak of the activity and distribution state, it was inevitable that the probability shown by DOE would be considered an underestimate[12]. There is an arbitrary aspect in which the values may differ according to how the scales of time and space are set in calculating the probability, and it must be said that probabilistic forecast without scientific basis for explaining the uneven distribution of the activity frequency lacks reliability. There are some researches that apply the probabilistic assessment of the volcanic activity to Northeast Japan[13], but these are extrapolations of the distribution of the Quaternary volcanoes directly as a function of probability, and do not consider the changes in spatiotemporal distribution or magmatic eruptive volume as shown in Fig. 4 and 5 of this paper.In evaluating the volcanic activities, it is necessary to understand the conditions of magma generation in the deep underground that is the source of volcanic activities, in accordance to the spatiotemporal changes of the activity region. Although it will not be explained in detail in this paper, the rise in partial fusibility was observed through the changes in the chemical composition of the eruptive materials in Numazawa volcano that newly appeared in the aforementioned Northeast Japan back-arc region, and it is thought that the magma was generated by the reheating of the crust[8]. In the major caldera eruption in the Aizu region in southern Northeast Japan, it was shown that the large-scale melting of the lower crust and the rearrangement in the up-down direction of the crust occurred when a large amount of magma was formed, from the geochemical property of the eruptive material. This corresponds well with the underground temperature structure shown seismologically[9]. In the forecast of the volcanic activities, the assessment that steps into the geological, geophysical, and geochemical basis as well as the magma genesis will be demanded. The probabilistic assessment that simply matches the numbers without theoretical backing not only will be questioned, but may also lead to unnecessary confusion as in the Yucca Mountain project. 5 SummaryAs there is a great amount of radioactive wastes already in Japan as the product of nuclear power generation, and they continue to increase, we face a situation where the wastes must be quickly and effectively disposed. Particularly, in geological disposal, the social consensus for this project is mandatory, aside from the issue of long-term safety assessment of the geological disposal environment. While the business executer is responsible to be accountable for safety, the regulating authorities must have the ability to decide whether the application of the disposal business company is appropriate, and to disclose the information to society to obtain consensus. The Research Core for Deep Geological Environments, AIST handles the supportive research for government regulations, and its purpose is to utilize the research results and to provide technological support in the safety regulations. Particularly, the presentation of the safety thinking at a time scale that surpasses the ordinary daily perception, as in the forecast of several hundred thousand years into the future, is an issue that must be solved by the Research Core for Deep Geological Environments. The geological phenomenon may have extremely major effect on society if it happens, even if it is a rare natural phenomenon (event that occurs at extremely low frequency), and we must learn from the experience of the 2011 Earthquake off the Pacific Coast of Tohoku. This paper described the methodology of forecast and the geological issues in geological disposal. Particularly, in the assessment of the natural phenomena that may affect the disposal system, the reconstruction of the geological structure development history as the just history at the candidate site is important, and it is necessary to conduct the assessment at a time scale with sufficient likelihood to eliminate any “unexpected events”. Recently, the Nuclear Safety Commission of Japan stated that the earthquake and volcanic activities should be assessed as rare natural phenomena in the safety regulation for subsurface disposal, which is a form of category 2 waste disposal facility[14]. However, according to the current thinking of the Nuclear Safety Commission, there is arbitrariness in the way in which rare natural phenomenon is determined, and the ways of excluding the arbitrary event settings such as maintaining the low assessed dosage among the various natural phenomena have not been clarified. Moreover, in the geological disposal, which is a non-managed category 1 waste disposal facility, it is assumed that the problematic natural phenomena are eliminated in selecting the site as stated in the Specified Radioactive Waste Final Disposal Act. There is a great disparity in the fact that the earthquake and volcanic activities that were supposedly eliminated are assessed for safety as rare natural phenomena. I shall repeat that in the safety assessment of geological disposal, the basis must be the accurate geological understanding of the candidate site, and the original purpose is the maximum reduction of uncertainty through such understanding.

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