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Shimada, Taro; Shimada, Asako; Miwa, Kazuji*; Nabekura, Nobuhide*; Sasaki, Toshihisa*; Takai, Shizuka; Takeda, Seiji
JAEA-Research 2024-004, 115 Pages, 2024/06
JAEA-Research-2024-004.pdf:6.02MB
We have studied the confirmation method for the termination of decommissioning of nuclear facilities based on the site release flow presented at the Nuclear Regulation Authority (NRA) study team meeting in 2017, and organized it as a procedure for the site soil. First, the effects of radionuclides released by the Fukushima Daiichi Nuclear Power Station accident are excluded as background radioactivity, and the distribution of radioactivity concentration of facility origin on the site is evaluated using geostatistical method kriging. Then, considering the downstream transport of sediment by surface runoff generated by rainfall that exceeds the infiltration capacity of the ground surface, a series of evaluation procedures are presented to evaluate the exposure dose reflecting future changes from the evaluated radioactivity concentration distribution, and a comparison method with the assumed 0.01 mSv/y as a dose criterion is proposed. Furthermore, an example of the procedure for evaluating the distribution of contamination in the subsurface was also presented for the case where groundwater is affected.
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.
Miwa, Kazuji; Iimoto, Takeshi*
Journal of Radiation Protection and Research, 48(2), p.68 - 76, 2023/06
In the process of discussion on possibility of using radionuclide-contaminated soil and debris generated by radiation disasters, strategy on the proper management of radiation exposure protection while considering the source of the contaminated materials is necessary. We proposed five interpretations of radiation protection to contribute the promotion of discussion on possibility of using a part of low-level-radionuclide-contaminated soil and debris in post-accident rehabilitation. Interpretations I to III are based on the idea of "using a reference level to protect the public in post-accident rehabilitation," whereas IV and V are based on the idea of "using the dose constraint to protect the public in post-accident rehabilitation when the sources are handled in a planned activity."
Shimada, Taro; Namekawa, Masakazu*; Miwa, Kazuji; Takeda, Seiji
Proceedings of Waste Management Symposia 2023 (WM2023) (Internet), 8 Pages, 2023/02
It is supposed that radioactive dust deposited at the land surface will be moved downstream and concentrated at the depression by overland flow at heavy rain after the accidental release of radioactive dusts accumulated at the filters in the decommissioning stage of nuclear facilities. The authors are developing a calculation code to evaluate distribution changes of radioactivity on the surface and public dose considering the conditions such as rainfall, topography and types of cover surface. It is necessary to construct methods for setting parameter values used for the calculations based on the actual situation. Therefore, the parameter values were obtained by the experiments where Fe
O
powder spread on the cover surface such as smooth and aged-asphalt, concrete and bare soil, was eroded by overland flow and raindrops and they were collected at the lower end of the slope at a minute interval. The collected weights of overland flow and FeO powder were measured. Based on the Manning's roughness coefficient for smooth asphalt already known as a fixed value, the erosion velocity coefficient was evaluated. Then Manning's roughness coefficients for other cover surfaces were obtained using the erosion velocity coefficient. Manning's roughness coefficients were slightly smaller than the range of literature values. In addition, elevations for the cover surface were measured by 3D scanner as point cloud data, and the surface roughness were evaluated. The values of surface roughness and Manning's roughness coefficients had a correlation. It indicated a possibility to utilize the surface roughness to set the Manning's roughness coefficient for the evaluation of radioactivity distribution change by heavy rainfall.
Miwa, Kazuji; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji
MRS Advances (Internet), 7(7-8), p.165 - 169, 2022/03
We have developed evaluation method of radiocesium (RCs) migration by surface runoff and soil erosion in considering vertical distribution of RCs in initial contaminated soil and concentration of RCs in different particle size. RCs migration on ground surface during single year has been evaluated in virtual site contaminated uniformly by Cs-137. As a result, RCs has concentrated in the impoundment, and 0.18% of total inventory in the site migrated into the sea. These results suggest that surface migration of RCs effects increasing of external exposure at impoundment and internal exposure from ingestion of marine product.
Miwa, Kazuji; Obata, Hajime*; Suzuki, Takashi
Journal of Nuclear Science and Technology, 57(5), p.537 - 545, 2020/05
Times Cited Count：2 Percentile：15.33(Nuclear Science & Technology)This study investigated the vertical distribution of Iodine-129 (
I) which is mainly produced by European nuclear reprocessing plants in the Chukchi Sea and Bering Sea. I was found to be distributed almost uniformly in fallout level, and an increasing in I concentration levels caused by high I water inflow from the Atlantic Ocean was not observed. Additionally, we revealed the vertical distribution of iodide, one chemical form of iodine, from the Bering Shelf area to the Chukchi Sea for the first time. The increasing tendency of iodide near sea bottom was observed.
Miwa, Kazuji; Takeda, Seiji; Iimoto, Takeshi*
Radiation Protection Dosimetry, 184(3-4), p.372 - 375, 2019/10
Times Cited Count：0 Percentile：0.00(Environmental Sciences)The Ministry of the Environment has indicated the policy of recycling the contaminated soil generated by decontamination activity after the Fukushima accident. By recycling to coastal reclamation which is one of effective recycling application, dissolved radiocesium and absorbed radiocesium on soil particles will flow out to the ocean by construction, therefore evaluating radiocesium transition in ocean considering the both types of radiocesium is important for safety assessment. In this study, the radiocesium outflow during constructing and after constructing is modeled, and radiocesium transition in ocean is evaluated by Sediment model suggested in OECD/NEA. The adaptability of sediment model is confirmed by reproducing evaluation of the coastal area of Fukushima. We incorporate the sediment model to PASCLR2 code system to evaluate the doses from radiocesium in ocean.
Miwa, Kazuji; Terasaka, Yuta; Ochi, Kotaro; Futemma, Akira; Sasaki, Miyuki; Hirouchi, Jun
Nihon Genshiryoku Gakkai-Shi ATOMO
, 61(9), p.687 - 691, 2019/09
This report summarizes the contents of the session of the Health Physics and Environment Science Division, which was held in Atomic Energy Society of Japan 2019 Spring Meeting. In this session, six students and young researchers who engaged in the field of nuclear energy and radiation gave a lecture about health physics and environmental science research through their expertise. After the all presentations end, we took discussion time about the issues and future development in this field with all attendees. In this report, we summarized each lecture outline and discussion contents.
Shimada, Taro; Miwa, Kazuji; Takeda, Seiji
Nihon Genshiryoku Gakkai-Shi ATOMO, 61(7), p.531 - 534, 2019/07
Rubbles less than 5 
Sv/h of surface dose rate, which are stored outdoor in the Fukushima Daiichi NPS (1F) site, will be recycled and applied in a restricted reuse only within 1F site in the future. However, there is no precedent for establishing the reference values such as dose and/or concentration for reuse or recycling under the existing exposure situation. In this study, we suggested a concept for establishing the reference radioactive concentration of recycling material for the restricted use in the 1F site. In addition, based on the concept, we calculated the reference radiocesium concentrations of the recycling material used for paved roads and the bases of concrete building.
Kataoka, Noriaki*; Nakajima, Junya; Miwa, Kazuji; Hirota, Seiko*; Tsubota, Yoichi; Yamada, Ryohei; Fujimichi, Yuki*; Ishikawa, Junya*; Sunaoshi, Masaaki*
Hoken Butsuri (Internet), 54(2), p.140 - 145, 2019/06
It is the written report of the joint meeting of young researchers in the field of radiation protection and radiation biology.
Shimada, Taro; Miwa, Kazuji; Takeda, Seiji
Progress in Nuclear Science and Technology (Internet), 6, p.203 - 207, 2019/01
Rubbles less than 5 Sv/h of surface dose rate, which are stored outdoor in the Fukushima Daiichi NPS (1F) site, will be recycled and applied in a restricted reuse only within 1F site in the future. In this study, we suggested a concept for establishing the reference radioactive concentration of recycling material for the restricted use in the 1F site. Reference radiocesium concentration is calculated so that increased dose rate by restricted reuse does not exceed 1 Sv/h which is the minimum value of dose rate map in the 1F entire site. In order to justify the restricted reuse under the reference concentration calculated, additional occupational dose, dose rate at the site boundary and groundwater concentration at the outlet to the ocean are evaluated and confirmed that the values are below 2 mSv/y, 1 mSv/y and 1 Bq/cm
of 
Cs and 
Cs, respectively. And then calculated the reference radiocesium concentrations of the recycling material used for paved roads and the bases of concrete building.
Miwa, Kazuji; Shimada, Taro; Takeda, Seiji
Progress in Nuclear Science and Technology (Internet), 6, p.166 - 170, 2019/01
In this study, in order to validate the restricted use of recycling material at the reference radiocesium concentration (determined in series report (1)), we evaluated worker annual doses, air dose rate at the site boundary and impact of migrated radiocesium into groundwater. Firstly, we evaluated the additional annual dose for workers, on the assumption that typical workers coming in contact with the source after construction (Road: 1.2 mSv/y, Building: 1.3 mSv/y). Secondly, we evaluated the air dose rates by distance from road and building including recycling material, and investigated the distance for not exceeding 1 mSv/y (including additional dose rate by recycling and background dose rate of 0.6 mSv/y) at the site boundary (Road: 25 m, Building: 1 m). Thirdly, we evaluated the Cs migration in groundwater, and investigated the distance required for satisfying the operation target value (Cs: 1 Bq/L, Cs: 1 Bq/L) at the boundary (coastal line) (Road: 10 m, Building: 10 m).
Miwa, Kazuji; Shimada, Taro; Takai, Shizuka; Nabekura, Nobuhide; Takeda, Seiji
no journal, ,
A part of the debris with relatively lower radioactive concentrations, which have been stored in the site of Fukushima Daiichi Nuclear Power Station (1F), have been planned to be recycled for restricted purpose in the 1F site. In this study, we evaluated reference values of upper radiocesium concentrations which are practicable to recycle materials for some applications such as road. Radiation doses for all workers in the 1F site are controlled. There is no previous reference values estimated for restricted recycling under the condition of controlled radiation doses for workers in the 1F site. Therefore, we evaluated the reference values not to exceed the additional air radiation dose as 1Sv/h with recycling the debris, which is the estimated minimum dose rate in the 1F site regarded as background. As a result, the highest value of radiocesium concentration of recycling materials was evaluated to be 100,000 Bq/kg in the case of use as the sub-base in the concrete paving road.
Shimada, Taro; Miwa, Kazuji; Takeda, Seiji
no journal, ,
Rubbles less than 5 Sv/h of surface dose rate, which are stored outdoor in the Fukushima Daiichi NPS (1F) site, will be recycled and applied in a restricted reuse only within 1F site in the future. However, there is no precedent for establishing the reference values such as dose and/or concentration for reuse or recycling under the existing exposure situation. In this study, we suggested a concept for establishing the reference radioactive concentration of recycling material for the restricted use in the 1F site. In addition, based on the concept, we calculated the reference radiocesium concentrations of the recycling material used for paved roads and the bases of concrete building.
Miwa, Kazuji; Takeda, Seiji; Iimoto, Takeshi*
no journal, ,
The Ministry of the Environment has indicated the policy of recycling the contaminated soil generated by decontamination activity after the Fukushima accident as material. By recycling to coastal reclamation which is one of effective recycling application, dissolved radiocesium and absorbed radiocesium on soil particles will flow out to the ocean by construction. It is suggested by previous studies that the both types of radionuclides affect concentration of radionuclides in marine organism. In safety assessment for the recycling, it is necessary to consider transition of nuclides in both types. Therefore, we developed a model to evaluate the radionuclides transition in the ocean considering the both types of radionuclides which flow out during constructing and operation. The concentration of radiocesium in ocean is estimated with time, and the impact of internal exposure dose by sea food ingestion is evaluated through comparison with other major exposure pathway.
Miwa, Kazuji; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji
no journal, ,
For development of technical foundation to confirm the completion of the decommissioning of nuclear facilities, JAEA is developing the methodology to evaluate the dose in considering radionuclides migration by not only groundwater migration but also surface water migration and surface soil migration. We studied estimating methods for radionuclide migration between the surface water and surface contaminated soil, changes of radionuclide distribution, and direct outflow of radionuclides to the sea. In addition, preliminary analysis of the surface water migration and soil migration based on the topography and rainfall conditions is performed by using a general-purpose water and soil runoff evaluation code. The methodology will be applied to evaluate the dose by using the out put of the surface water migration and soil migration.
Miwa, Kazuji; Iimoto, Takeshi*
no journal, ,
ICRP Publ.103 shows that reference levels are used for radiation protection of public exposure in existing exposure situation. However, there is no specific description about actual operation of reference levels. There is a possibility that a more rational radiation protection concept can be applied depending on the scale of environmental pollution in the existing exposure situation. This study proposes variation of radiation protection concept for public exposure assuming reusing radiation contaminated material which is in existing exposure situation.
Shimada, Taro; Miwa, Kazuji; Takeda, Seiji
no journal, ,
Rubbles less than 5Sv/h of surface dose rate, which are stored in the Fukushima Daiichi NPS (1F) site, will be recycled and applied in a restricted use only within 1F site in the future. In this study, we suggested a concept for establishing the reference radioactive concentration of recycling material for the restricted use in the 1F site, and then calculated the reference radiocesium concentrations of the recycling material used for paved roads and the bases of concrete building. In order to prevent the working time from being decreased, it is necessary for the restricted use to suppress the additional dose to a level of value. Therefore, focusing on the current background air dose rate in the 1F site, we suggested the required condition that air dose rate increased by use of contaminated recycling material instead of non-contaminated did not exceed the variation range of the background dose rate. In addition, to validate the restricted use under the reference concentration, following three items should be confirmed; (1) additional dose for worker using the recycling material does not exceed 2mSv/y, 10% of worker dose limit; (2) the impact to air dose rate evaluated at the 1F site boundary does not exceed the 1mSv/y including direct and skyshine radiations from wastes stored in the site; (3) the impact of migrated radionuclides into groundwater does not exceed the operational target value. The reference radiocesium concentrations for recycling aggregate taken from contaminated concrete rubbles were evaluated for paved road and bases of concrete building.
Sawaguchi, Takuma; Miwa, Kazuji*; Shimada, Taro; Takeda, Seiji
no journal, ,
In the previous dose assessment for the radioactive waste disposal, the dissolved radionuclides leaking from the repository were assumed to flow directly into the living environment (ocean, lake, river, etc.) through natural barriers. However, based on the knowledge after the Fukushima Daiichi Nuclear Power Plant accident, it was considered that radionuclides via groundwater could sorb and desorb with soil near spring water points, and that radiocesium was mainly transferred as sorbed to suspended particles in the living environment. In this study, in order to contribute to the dose assessment for intermediate-depth disposal, we analytically understood the influence on the migration in the living environment with or without consideration of the nuclide sorption on the seabed soil during spring water inflow into a coastal zone. In addition, the effects were also evaluated for the presence or absence of the nuclide sorption/desorption on suspended particles and the particle sedimentation. As a result, the radioactivity concentrations in seabed soil and seawater immediately above the seabed were higher in the estimate that considered the sorption/desorption and the sedimentation than in the estimate that did not. These results indicate that it is important to consider the radionuclide sorption on the seabed soil and the migration of radionuclides sorbed on suspended particles in the estimation of radionuclide migration in the living environment because these phenomena could cause the increase of radionuclide concentrations in the interface layer and the seabed soil and the higher exposure due to benthic fish and shellfish ingestion, etc.
Miwa, Kazuji; Shimada, Taro; Takeda, Seiji
no journal, ,
In this study, in order to validate the restricted use of recycling material at the reference radiocesium concentration (determined in series report (1)), we evaluated worker annual doses, air dose rate at the site boundary and impact of migrated radiocesium into groundwater. Firstly, we evaluated the additional annual dose for workers, on the assumption that typical workers coming in contact with the source after construction (Road: 1.3 mSv/y, Building: 1.3 mSv/y). Secondly, we evaluated the air dose rates by distance from road and building including recycling material, and investigated the distance for not exceeding 1mSv/y (including additional dose rate by recycling and background dose rate of 0.6 mSv/y) at the site boundary (Road: 25 m, Building: 1 m). Thirdly, we evaluated the Cs and Sr migration in groundwater, and investigated the distance required for satisfying the operation target value (Cs-134: 1 Bq/L, Cs-137: 1 Bq/L, Sr-90: 5 Bq/L) at the boundary (coastal line) (Road: 10 m, Building: 10 m).
