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 )−211−Synthesiology - English edition Vol.4 No.4 (2012) 3 Overall processes leading to the decision of waste disposal site and the positioning of the researchComment (Akira Ono)I think it is easier for the readers to understand if you show the overall processes leading to the decision and where this research is positioned within the processes, because there are many people involved as disposal sites for radioactive waste are determined.Answer (Takahiro Yamamoto)In accordance to your comment, I added the relationship between this research and the security after closure of the geological disposal repository in Fig. 1. The ways of conducting the geological disposal business or the procedure for selecting the final disposal site are set by the Specified Radioactive Waste Final Disposal Act, and the selection of the disposal site itself is conducted by NUMO, the executive body, but the content of the safety regulation has not been established. The final goal of this research is to establish the index for selecting and deciding the investigation and assessment items for the disposal site selection investigation. For example, as described in this paper, places that may be affected by volcanic activities or places in which new volcanic activities may occur are not appropriate as disposal sites. The theme of this research is the methodology for how to establish the assessment method to determine such matters.4 Responsibility of the current generation to the future generationQuestion (Akira Ono)In this paper it is stated that it is necessary to assess the safety of the radioactive waste disposal at a time range of several hundred thousand to a million years, and that is because this time span is required for the radiation level of the HLR waste material to decrease to a level of naturally existing radioactive substances. After this time range has elapsed, the current humanity will be in the next evolutionary stage, and it is imagined that the current civilizations, cultures, and ethnic groups will be totally gone. I think this paper is based on the thinking that the current humankind should be responsible for the earth environment and biosphere of such a distant future. What does the author think about the adequacy of this thinking?Answer (Takahiro Yamamoto)I think the generation that received the benefit of nuclear power should be responsible for the disposal of the waste. Since the accident at the Fukushima Daiichi Nuclear Power Plant, the public opinion is to move away from nuclear dependency, but the disposal of the existing radioactive wastes that had been produced cannot be avoided in decommissioning the plants. “No nuclear” cannot be achieved only by stopping the power generation, and our generation must be responsible for the final disposal of the nuclear waste whether we continue or discontinue nuclear power.As for the human activities in the future, Japan is engaging in the safety assessment after closure of the geological disposal sites with this point in mind, just as in other countries. For example, it says “mineral resource” in “Investigation/assessment items for geological environment” in Fig. 1. This is because the presence of underground resource that may be mined is set as an exclusion condition, to avoid the disposed materials to come in contact with the human activities in the future.5 Scenario for the assessment of risk occurrence in the futureQuestion (Akira Ono)The risk of disposed radioactive waste to humans at a time in the future may be the product of the radiation of radioactive substances at that time and the possibility that the radioactive substances may affect humans.The radiation level of radioactive substances can be estimated to decrease logarithmically from the nuclear physics data. On the other hand the possibility that the radioactive substances from the waste substances may affect humans is a geological issue. What scenario of geology is envisaged for the risk assessment?Answer (Takahiro Yamamoto)The natural barrier in geological disposal not only attempts isolation by distance. The waste materials including the artificial barrier buried deep underground will eventually lose the containment function due to corrosion. After that, the radioactive substances are expected to undergo reduction in radioactivity in the process of travelling through the natural barrier by underground water, and through delay by absorption by minerals and dilution by underground water, before it arrives to the ecological environment on the surface. The safety assessment after closure is based on such underground water transfer scenario.

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