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Shimada, Taro; Sasagawa, Tsuyoshi; Miwa, Kazuji; Takai, Shizuka; Takeda, Seiji
Proceedings of International Conference on Environmental Remediation and Radioactive Waste Management (ICEM2023) (Internet), 7 Pages, 2023/10
Nuclear regulatory inspection should be performed on the basis of the risk information during the decommissioning phase of the nuclear power plant. However, it is difficult because the methodology for quantitatively assessing the radiation exposure risk during decommissioning activities has not been established. Therefore, a decommissioning risk assessment code, DecAssess-R, has been developed based on the decommissioning safety assessment code, DecAssess, which creates event trees from initiating events and evaluates the radiation risk resulting from public exposure dose for each accident sequence. The assessment took into account that mobile radioactive inventories that can be easily dispersed in the work area, such as radioactive dust accumulated in HEPA filters attached to a contamination control enclosure, will fluctuate with the progress of the decommissioning work. Initiating events were selected based on the investigation of accidents and malfunctions during dismantling, disassembly, and component replacement activities around the world, and event trees were created from the initiating events to indicate the progress scenario. The frequencies of occurrence were determined with reference to general industry data in addition to the above accidents and malfunctions, and the probabilities of event progression were determined with reference to failure data during the operation phase. The exposure risks during dismantling of components in the reference BWR were evaluated. As a result, the public exposure dose was maximum in case of fire during dismantling of reactor internals and fire spread to combustibles and filters, including radioactivity temporarily stored in the work area. The exposure risk was also maximum because the probability of occurrence of this accident sequence was greater than that of other scenarios.
Sawaguchi, Takuma; Takai, Shizuka; Sasagawa, Tsuyoshi; Uchikoshi, Emiko*; Shima, Yosuke*; Takeda, Seiji
MRS Advances (Internet), 8(6), p.243 - 249, 2023/06
In the intermediate depth disposal of relatively high-level radioactive waste, a method to confirm whether the borehole for monitoring is properly sealed should be developed in advance. In this study, groundwater flow analyses were performed for the hydrogeological structures with backfilled boreholes, assuming sedimentary rock area, to understand what backfill design conditions could prevent significant water pathways in the borehole, and to identify the confirmation points of borehole sealing. The results indicated the conditions to prevent water pathways in the borehole and BDZ (Borehole Disturbed Zone), such as designing the permeability of bentonite material less than or equal to that of the host rock, and grouting BDZ.
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; Mukai, Masayuki; Sawaguchi, Takuma
JAEA-Data/Code 2021-012, 122 Pages, 2022/01
JAEA-Data-Code-2021-012.pdf:3.87MB
Reducing public dose is required when radioactive wastes such as high-level and from reactor core internals etc. are disposed of by means of multi barrier system consist of engineered and natural barriers. In these barriers, engineered barrier is expected to bring out confinement function of waste's radionuclides in the barrier. Materials used as the engineered barriers are altered and performances of the barrier materials are degraded in course of time. To estimate properly the degraded performances, analytical evaluation of long-term change of the engineered barrier state is important. Change state of the engineered barrier is given by mass-transport and geochemical-reaction inside the barrier materials and these phenomena are interrelated, it is necessary to calculate the state by means of coupled analysis procedure. We have developed a coupled mass-transport and geochemical-reaction calculation code (MC- BUFFER) to evaluate alteration of engineered barrier specially targeted for water permeability of bentonite buffer material as one of most important performances to engineered barrier. This report describes functions expected for the engineered barrier, influence parameters for the functions, implementation models in MC-BUFFER, structure and functions of MC-BUFFER, input file format and output examples, execution method of MC-BUFFER, and sample run with MC-BUFFER.
Sasagawa, Tsuyoshi
no journal, ,
no abstracts in English
Sasagawa, Tsuyoshi; Kijima, Tatsuya*; Sawaguchi, Takuma; Iida, Yoshihisa
no journal, ,
no abstracts in English
Kijima, Tatsuya*; Sasagawa, Tsuyoshi; Sawaguchi, Takuma; Iida, Yoshihisa
no journal, ,
no abstracts in English
Sawaguchi, Takuma; Abe, Takeyasu; Sasagawa, Tsuyoshi; Murakami, Hiroaki; Takeuchi, Ryuji; Iida, Yoshihisa; Takeda, Seiji
no journal, ,
no abstracts in English
Sasagawa, Tsuyoshi; Shimada, Taro; Takeda, Seiji
no journal, ,
IAEA standard document (GSR part6) requires appropriate dose control and minimization of radioactive waste for decommissioning of nuclear facilities. It is necessary to develop a method to optimize the amount of radioactive waste and the radiation dose. Therefore, we developed a code to evaluate the optimum conditions of dismantling work by calculating the amount of radioactive waste in radioactive level and the radiation dose according to the conditions of the decommissioning process and analyzing the cost-benefit of both results. Furthermore, based on the information of decommissioning of JPDR, the sensitivity analysis was performed for the case of decontamination and dismantling of three kinds of tanks and associated piping, focusing on the size of cut pieces and the type of container. As a result, different trends were observed for the minimum number of containers of the radioactive waste in radioactive level and the minimum dose to workers. The cost-benefit analysis was performed to derive the optimum conditions of the type of container and the size of cut pieces with cost as an index, and the optimization method by the cost-benefit analysis based on the results of the amount of radioactive waste generated in radioactive level and the radiation dose was shown to be applicable.
Sawaguchi, Takuma; Murakami, Hiroaki; Takeuchi, Ryuji; Takai, Shizuka; Sasagawa, Tsuyoshi; Takeda, Seiji
no journal, ,
In the intermediate depth disposal of radioactive waste, the monitoring boreholes are backfilled and sealed at the end of the monitoring period to prevent potential migration pathways for radioactive materials. However, the method to confirm whether the borehole is properly sealed has not been established, and there is a lack of scientific knowledge to determine the validity of borehole sealing. The aim of this study is to develop the confirmation method. Therefore, based on previous studies, experimental and analytical considerations were performed to identify the confirmation points of borehole sealing. In the former, laboratory experiments were performed to understand the swelling behavior and permeability of bentonite block in the borehole. These results indicated that the initial water content of the bentonite block could affect the internal structure after swelling. In the latter, groundwater flow analyses were performed for the hydrogeological structures with backfilled boreholes to understand the effect of backfill conditions on the borehole sealing. It was shown that the backfill conditions to prevent the formation of water pathways in the borehole were the grouting BDZ (Borehole Disturbed Zone), etc.
Shimada, Taro; Sasagawa, Tsuyoshi; Miwa, Kazuji; Takai, Shizuka; Takeda, Seiji
no journal, ,
It is necessary to develop the methodology for risk assessment during decommissioning so as to select the components or tasks based on the risk-informed at the nuclear regulation inspection. The characteristic is that the radioactive contamination that may be released in the event of an accident fluctuates spatially and temporally as the dismantling work progresses. We have started to construct a methodology for the risk assessment in decommissioning phase. And an outline of risk assessment based on the characteristics of the dismantling work was created. The risk is defined as the product of exposure dose and frequency in this study. As a result of investigating cases of decommissioning and similar trouble events during periodic inspection work, many cases related to fire during cutting work at dismantling were found. Based on the results, quality engineering methods such as FMEA (Failure Mode and Effects Analysis) were used to select the initiating events due to human error. In addition, event trees were created assuming that the events progress due to the failure of mitigation measures for those initiating events.
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.
