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Journal Articles

Laboratory experiment on runoff of particles deposited on land surface by rainfall at accidents in the decommissioning stage

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$$_3$$O$$_4$$ 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 Fe$$_3$$O$$_4$$ 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.

Oral presentation

Development of dose evaluation method for confirmation of completion of decommissioning, 2; Evaluation method of radioactivity distribution change for safety assessment

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 ($$^{137}$$Cs) migration during 1 year was assessed in virtual site which has horizontal uniform $$^{137}$$Cs concentration in surface soil. $$^{137}$$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.

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