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Sasagawa, Tsuyoshi; Shimada, Taro; Takeda, Seiji
Proceedings of 31st International Conference Nuclear Energy for New Europe (NENE2022) (USB Flash Drive), 8 Pages, 2022/12
In the risk assessment of the decommissioning phase, the inventory of radioactivity accumulated in filters and other materials changes with the progress of dismantling work under normal conditions, and a method that can evaluate the public exposure dose during an accident in which these changes are taken into account is required. The inventories (the mobile radioactive contaminants) include filters in which radioactive dust dispersed by equipment cutting work has accumulated and combustible waste generated by decontamination work. In this study, we developed a method to evaluate the accumulation of mobile contaminants in filters by calculating the amount of dust transferred into the air during equipment cutting operations using a model that evaluates the volume of the cutting kerf width and the dispersion ratio. Furthermore, the amount of the mobile contaminants that accumulates in local filters and building filters for each equipment was evaluated using this method, taking into account differences in cutting methods (underwater or in air) and work processes, and the equipment and work processes that should be focused on during regulatory inspections were studied preliminarily. It was suggested that some equipment cut in air generate the same amount of the mobile contaminants compared to reactor internals with high radioactivity that are cut in underwater. This indicates that the mobile contaminant is one of the important indicators in nuclear regulatory inspections that influence the selection of inspection targets.
Sasagawa, Tsuyoshi; Shimada, Taro; Takeda, Seiji
no journal, ,
In the risk assessment of the decommissioning phase, the inventory of radioactivity accumulated in filters and other materials changes with the progress of dismantling work under normal conditions, and a method that can evaluate the public exposure dose during an accident in which these changes are taken into account is required. The inventories (the mobile radioactive contaminants) include filters in which radioactive dust scattered by equipment cutting work has accumulated and combustible waste generated by decontamination work. In this study, we developed a method to evaluate the accumulation of mobile contaminants in filters by calculating the amount of dust transferred into the air during equipment cutting operations using a model that evaluates the volume of the cutting kerf width and the scattering rate. Furthermore, the amount of the mobile contaminants that accumulates in local filters and building filters for each equipment was evaluated using this method, taking into account differences in cutting methods (underwater or in air) and work processes, and the equipment and work processes that should be focused on during regulatory inspections were studied preliminarily. It was suggested that some components cut in air generate the same amount of the mobile contaminants compared to core structural materials with high radioactivity that are cut in underwater. This indicates that the mobile contaminant is one of the important indicators in nuclear regulatory inspections that influence the selection of inspection targets.
Shimada, Taro; Miwa, Kazuji; Sasagawa, Tsuyoshi; Takai, Shizuka; Takeda, Seiji
no journal, ,
For risk-informed implementation of nuclear regulatory inspections during the decommissioning phase of nuclear power plants, we are developing DecAssess-R, a code that evaluates changes in exposure risk over time according to the decommissioning process based on exposure dose and occurrence probability by accident sequence for each component to be dismantled. Event trees have been constructed from the initiating events that occurred during the decommissioning phase, and the occurrence frequency and accident progress probability of the events have been developed from past trouble cases. Using the developed risk assessment method, we evaluated the exposure risks due to fire as an example of dismantling of a 1.1 MW BWR in the United States. As a result, it was confirmed that the method can provide risk information according to the decommissioning process, not only the quantity of the residual radioactivity inventory, such as the result that the exposure risk is larger for a fire that occurs during air dismantling of peripheral equipment in the reactor building, which has a smaller radioactivity inventory, than for a fire that occurs during underwater dismantling of in-core structures, which has a larger radioactivity inventory. The results confirmed that the information on the risk according to the decommissioning process can be provided.
Shimada, Taro
no journal, ,
For risk-informed implementation of nuclear regulatory inspections during the decommissioning phase of nuclear power plants, we are developing DecAssess-R, a code that evaluates changes in exposure risk over time according to the decommissioning process based on exposure dose by accident sequence and probability of occurrence for each component to be dismantled. The development of DecAssess-R is underway. An event tree that represents the accident sequence from the originating event was constructed from past troubles in the decommissioning phase, and the frequency of occurrence of the originating event and the event progress probability were developed based on the results of the organization of the number of troubles. Using the risk assessment method we are developing, we evaluated the risk of exposure due to fire as an example of the dismantling of a 1.1 million kW-class BWR in the United States. As a result, it was confirmed that the method can provide risk information according to the decommissioning process, not just the size of the residual radioactivity inventory, such as the fact that the exposure risk is larger for a fire that occurs during air dismantling of peripheral equipment in the reactor building, which has a smaller radioactivity inventory, than for a fire that occurs during underwater dismantling of reactor internals, which has a larger radioactivity inventory. The results of this study confirmed that the information on the risk according to the decommissioning process can be provided.