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Aono, Tatsuo*; Nishikawa, Jun*; Otosaka, Shigeyoshi*; Takata, Hyoe*; Misono, Toshiharu; Nakanishi, Takahiro; Miura, Hikaru*; Kambayashi, Shota*; Fukuda, Miho*; Sakurada, Masanobu*; et al.
no journal, ,
From 2016 to 2020, the Tohoku Marine Ecosystem Research Vessel Shinsei-maru conducted four research cruises on the dynamics and bioavailability of radionuclides around off Fukushima. Seawater, sediments and organisms were collected mainly around off the Fukushima Daiichi Nuclear Power Plant (FDNPS). The purpose of those cruises is to observe the flux of radionuclides released into the ocean by the FDNPS accident and to understand the bioavailability of radionuclides. In order to understand the contaminated levels of the marine environment, the concentration of radioactive cesium (Cs) in seawater and sediments around off Fukushima was investigated. As a result, the concentration range of dissolved Cs-137 in the surface seawater near the FDNPS from 2016 to 2018 was 10-23 mBq/L, but decreased to 7 mBq/L in 2020. In terms of points, the concentrations of dissolved Cs-137 tended to decrease with time. On the other hand, in sediments, the Cs-137 concentrations were higher in the surface layer (0-2 cm) than the deep layer from 2016 to 2017 and tended to decrease with depth, but after 2018, subsurface vertical maximum of Cs-137 concentrations was observed at some stations. Although these concentrations tended to increase in the subsurface layer, the concentrations in the sediment also tended to decrease with time. In this presentation, we report on the concentration variation of Cs-137 and their characteristics in seawater and sediments around off Fukushima.
Kawahara, Rika*; Ochi, Kotaro; Yamaguchi, Katsuhiko*; Torii, Tatsuo*
no journal, ,
In this study, the distribution of radioactive materials was measured in the Hamadori area of Fukushima prefecture using a Compton camera. Compton cameras are lightweight and capable of visualizing radiation distribution, which makes them useful for in-situ measurements. This time, in addition to the Compton camera, several instruments, such as a survey meter, were used to measure radiation and compare them. As a result, the radiation distribution (2D image) in the environment was clarified by the Compton camera. However, the distance to the hot spot cannot be grasped in the 2D image. Therefore, in order to grasp the radiation source in detail, we aim to create a 3D map of radiation source position and its intensity. Since the Compton camera requires the dose rate for each nuclides, we believe that it will lead to the prediction of the dose rate distribution in the future.
Katengeza, E.*; Sanada, Yukihisa; Ochi, Kotaro; Iimoto, Takeshi*
no journal, ,
This study aimed at evaluating the vertical distribution of radiocesium in bottom sediment core samples from 45 ponds sampled in 2015-2019 and clarifying the impact of the vertical distribution on the efficiency of decontamination by stripping the top 10 cm of bottom sediments. The relaxation mass depths and depth of sediment containing 90% of the inventory were larger than on land indicating faster downward migration of radiocesium in ponds. This may have been caused by accumulation from the ponds' catchments where radiocesium enrichment occurred. This indicates that local behavior of radiocesium in reservoirs are more important to decontamination outcomes.