3:45 PM - 4:00 PM
[MAG39-08] Importance of understanding the migration behavior of radionuclides for safety assessment of geological disposal of radioactive wastes
★Invited Papers
Keywords:Geological disposal, Safety assessment, Radionuclide migration
In Japan, it has been decided that high-level radioactive waste, which is produced when reprocessing spent fuel from nuclear power plants, should be disposed of in a stable geological formation deeper than 300m (hereinafter referred to as geological disposal). In geological disposal, radioactive waste is safety “isolated” from the human environment by being buried deep underground, and the leaching and migration of radionuclides from the waste into the groundwater are controlled by the inherent ability of the deep underground geological environment combined with engineering counter-measures. In this way, a multi-barrier system will be constructed to "contain" radionuclides within the repository and its vicinity over relevant timescales. The Nuclear Waste Management Organization of Japan (NUMO), which is the implementer of the geological disposal, will assess the safety of the repository after closure of the repository designed by NUMO at each stage of the project, including the selection of a repository site, licensing of the project, and closure of the repository. Specifically, the release of radionuclides into the environment due to the failure of waste containers after the closure of the repository is assumed, the behavior of the repository after closure and the migration behavior of radionuclides in the repository are described as a scenario, and the migration analysis of radionuclides from the waste to the ground surface is conducted accordingly. The radiological impacts (dose, risk, etc.) on humans will be calculated and compared with reference values (NUMO, 2021).
Deep underground has unique characteristics such as slow groundwater flow and low oxygen content. It is important to understand the complex behavior of the repository and the migration process of radionuclides in the deep underground environment in order to make a realistic assessment. The behavior of the repository includes, for example, an increase in the temperature around the tunnel due to the heat generated by the waste, resaturation of the repository, and chemical alteration of the repository materials, such as cementitious materials, metallic materials, and bentonite, due to their reaction with groundwater and with each other. The radionuclide migration processes, which occurred in the field formed as a result of these processes, include redox reactions, dissolution and precipitation reactions of radionuclides in groundwater containing various ions, and reactions between colloids, organic matter, and microorganisms and radionuclides. In the radionuclide migration analysis, it is important to build up understanding of the elementary processes of each phenomenon in order to clear such complex fields.
NUMO has published a report that integrates the scientific knowledge and technologies accumulated to date to provide a comprehensive explanation of how NUMO proceed with site investigations, and perform the design, construction, operation and closure of safe geological disposal facilities, and secure long-term post-closure safety (NUMO, 2021). Through making this report, issues for improving the reliability of safety assessment have been identified. One example of the issues related to the migration processes is the evaluation of sorption behavior in groundwater containing high concentrations of carbonate ions, which is unique to Japan. Due to the lack of knowledge on sorption behavior of uranium in such groundwater, the sorption distribution coefficient, one of the parameters used in the radionuclide migration analysis, had to be set to an extremely small value to avoid overestimating the retardation effect of rock. NUMO carried out a joint research with universities on the sorption of uranium, and as a result of understanding the sorption behavior, it became available to set the reliable sorption distribution coefficient under the relevant environmental conditions.
Reference
NUMO (2021), The NUMO Pre-siting SDM-based Safety Case, NUMO-TR-21-01.
Deep underground has unique characteristics such as slow groundwater flow and low oxygen content. It is important to understand the complex behavior of the repository and the migration process of radionuclides in the deep underground environment in order to make a realistic assessment. The behavior of the repository includes, for example, an increase in the temperature around the tunnel due to the heat generated by the waste, resaturation of the repository, and chemical alteration of the repository materials, such as cementitious materials, metallic materials, and bentonite, due to their reaction with groundwater and with each other. The radionuclide migration processes, which occurred in the field formed as a result of these processes, include redox reactions, dissolution and precipitation reactions of radionuclides in groundwater containing various ions, and reactions between colloids, organic matter, and microorganisms and radionuclides. In the radionuclide migration analysis, it is important to build up understanding of the elementary processes of each phenomenon in order to clear such complex fields.
NUMO has published a report that integrates the scientific knowledge and technologies accumulated to date to provide a comprehensive explanation of how NUMO proceed with site investigations, and perform the design, construction, operation and closure of safe geological disposal facilities, and secure long-term post-closure safety (NUMO, 2021). Through making this report, issues for improving the reliability of safety assessment have been identified. One example of the issues related to the migration processes is the evaluation of sorption behavior in groundwater containing high concentrations of carbonate ions, which is unique to Japan. Due to the lack of knowledge on sorption behavior of uranium in such groundwater, the sorption distribution coefficient, one of the parameters used in the radionuclide migration analysis, had to be set to an extremely small value to avoid overestimating the retardation effect of rock. NUMO carried out a joint research with universities on the sorption of uranium, and as a result of understanding the sorption behavior, it became available to set the reliable sorption distribution coefficient under the relevant environmental conditions.
Reference
NUMO (2021), The NUMO Pre-siting SDM-based Safety Case, NUMO-TR-21-01.