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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.
Takai, Shizuka; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji
IEEE Transactions on Nuclear Science, 69(7), p.1789 - 1798, 2022/07
Times Cited Count：0 Percentile：0.01(Engineering, Electrical & Electronic)To reduce a large amount of contaminated concrete rubble stored in the Fukushima Daiichi Nuclear Power Station site, recycling low-radioactivity rubble within the site is a possible remedy. To promote recycling while ensuring safety, not only the average radioactivity but also the radioactivity distribution of concrete rubble should be efficiently evaluated because the details of rubble contamination caused by the accident remain unclear and likely include hotspots. However, evaluating inhomogeneous contamination of thick and/or dense materials is difficult using previous measurement systems, such as clearance monitors. This study experimentally confirmed the potential applicability of image reconstruction algorithms for radioactivity distribution evaluation in concrete rubble filled in a chamber. Radiation was measured using plastic scintillation fiber around the chamber (50 
50 40 cm
). Localized hotspots were simulated using standard sources of 
Cs, which is one of the main nuclides of contaminated rubble. The radioactivity distribution was calculated for 100 or 50 voxels (voxel size: (10 cm) or 10 10 20 cm) constituting the chamber. For 100 voxels, inner hotspots were undetected, whereas, for 50 voxels, both inner and surface hotspots were reconstructible. The distribution evaluated using the maximum likelihood expectation maximization algorithm was the most accurate; the average radioactivity was estimated within 70% accuracy in all seven cases.
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.
Namekawa, Masakazu; Fukahori, Tokio
JAEA-Data/Code 2012-014, 206 Pages, 2012/08
JAEA-Data-Code-2012-014.pdf:1.82MB
JAEA-Data-Code-2012-014-appendix.zip:0.17MB
Several numerical data (mass, level energy, spin and parity, abundance, mass excess, beta-decay energy, and half-life) of nuclides were collected with each element into the "Tables of Nuclear Data (JENDL/TND-2012)." These tables are related to the "Chart of the Nuclides" published by Japanese Nuclear Data Committee and Nuclear Data Center of Japan Atomic Energy Agency (JAEA), and data of the nuclides synthesized in the chart. Moreover, new determinations are adopted instead of the values in ENSDF for half-lives of ground state of Se-79 and Sn-126.
Namekawa, Masakazu; Katakura, Junichi
JAEA-Data/Code 2010-017, 822 Pages, 2010/09
JAEA-Data-Code-2010-017(errata).pdf:1.0MBJAEA-Data-Code-2010-017(errata2).pdf:2.24MBJAEA-Data-Code-2010-017-Part1-01.pdf:23.46MBJAEA-Data-Code-2010-017-Part1-02.pdf:25.79MBJAEA-Data-Code-2010-017-Part2-01.pdf:36.26MBJAEA-Data-Code-2010-017-Part2-02.pdf:30.64MB
Neutron cross sections of 406 nuclides in JENDL-4.0 are presented in figures and tables. In the tables, shown are cross sections at 0.0253 eV and 14 MeV, Maxwellian average cross sections (kT = 0.0253 eV), resonance integrals and fission spectrum average cross sections. In the figures, shown are three kinds of figures of cross section curves at 300 K.
Wakasa, Arimitsu; Namekawa, Masakazu; Urano, Hajime; Hayashi, Nobuhiko; Ide, Shunsuke; Ozeki, Takahisa
no journal, ,
no abstracts in English
Shimada, Taro; Namekawa, Masakazu*; Takai, Shizuka; Takeda, Seiji
no journal, ,
It is necessary to confirm the radioactive concentration of recycling contaminated rubbles which is less than the reference concentration for restricted use in the site of Fukushima Daiichi NPS. In order to develop measurement and evaluation methods in the combination of plastic scintillation fiber and inverse analysis for confirmation survey in the 1F site, we studied the measurement methods and geometrical system. We carried out the radiation measurement for simulated recycling rubbles using sealed radiation sources and analyzed gamma radiation fluxes on the PSF. The measurement result was almost agreed with the analytic result by means of correction of position resolution and detecting efficiency for the 
-ray energy of the PSF. It suggested that it was possible to estimate the radioactive concentration of recycling rubbles.
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; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji
no journal, ,
The distribution of surface soil contamination on the site of nuclear facilities after confirmation of the decommissioning may be changed by rainfall. In this study, the radionuclides migration model is developed to evaluate the radiation dose to the public including radionuclides distribution change. The result of radionuclides migration estimation on virtual site condition suggests that radionuclides which is migrated by surface water and soil migration gather in depression area. It is also shown that radionuclides migration on surface is more significant path to the ocean than radionuclides migration in groundwater. Based on these findings, it is suggested that the radiation dose considering change of radionuclides distribution may be larger than that from initial radionuclides distribution.
Miwa, Kazuji; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji
no journal, ,
In the public dose assessment in decommissioning process, remaining radionuclide in surface soil is the main source. The radionuclide in the surface soil migrates on the ground surface with a surface runoff and soil erosion caused by rain. Therefore, we have developed a dose estimation code system in considering radionuclide migration by surface runoff and soil erosion. In the radionuclide migration, the vertical radionuclide profile and the radionuclide concentration in each soil particle size are reflected. Using this dose estimation code system, Cs-137 (Cs) migration during 1 year was assessed in virtual site which has horizontal uniform Cs concentration in surface soil. Cs accumulation occurs in a depression area by radionuclide migration, and raise the air dose rate in the depression area. This result corresponds to the trend of high air dose rate in a depression area observed after Fukushima Daiichi Nuclear Plant accident.
Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji
no journal, ,
Detector responses in the measurement of composite material change according to combination of measuring time, density deference, accumulated condition and radiation source position. It may impact clearance decision unlike measurement of target objects made from a single material such as metal scraps. And also, it is necessary to confirm that radionuclides such as Zn-65, Sb-125 activated in the covered cables are able to be detected in measurement system. In order to evaluate possibility of clearance measurement by the conventional method of measurement and evaluation, we constructed simulation models of measurement systems using plastic scintillation and portable Ge detectors, and simulation of detector response was carried out by PHITS code. Calculation results indicated that measurement of clearance levels of target nuclides is possible in measurement system of covered cables bundled with about 100kg. On the other hand, clearance judgement may be difficult in the case of measurements of control panels which have large housings without segmentation by using the portable Ge detector at once. Multiple measuring with changing position of the detector is required for the case.
