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東京電力ホールディングス(株)福島第一原子力発電所(1F)の事故が発生し、多量の放射性物質が1F周辺に沈着した。日本原子力研究開発機構では、事故後、放射性物質の動態研究を継続して実施している。本報告書は、令和3年度における、近沿岸海域等における放射性物質の状況調査を実施した成果をまとめたものである。具体的には、福島沿岸域57地点において柱状試料を採取し、海底土への放射性セシウムの蓄積状況を示した。さらに海底地形・海底土分布調査を実施し、地形と堆積物の分布状況を把握した。また、河川前面における海底土表層の放射性セシウム濃度の水平分布を計測した。放射性物質の水産物への影響の基礎情報として、魚類の分布状況を調査した。さらに、無人観測船を使用した採水及び採泥の実証試験を実施した。得られた結果より1F前面海域における海底土に含まれる放射性物質の分布と動態について推定を行った。, An accident occurred at the TEPCO's Fukushima Daiichi Nuclear Power Station (1F) in 2011 and a large amount of radioactive materials were deposited around the 1F. Japan Atomic Energy Agency has continued to conduct research on the dynamics of radioactive materials after the accident. This report summarizes the results of the survey conducted in FY 2021 on the status of marine monitoring survey on radioactive substances. Furthermore, a seabed topography and sediments distribution survey was conducted in the coastal area off the Mano River from the Ohta River to understand the topography and sediment distribution. Furthermore, in order to evaluate the inflow of radioactive Cs from the river, the horizontal distribution of the radioactive Cs concentration on the surface sediment in front of the rivers was measured. As basic information on the effects of radioactive materials on marine products, the distribution status of fish was investigated. In addition, a demonstration test of water sampling and sediment sampling was conducted using an unmanned observation vessel. From these results, we estimated the distribution and its dynamics of radioactive Cs in the sediments in the front area on the 1F.

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., 福島大学環境放射能研究所第8回成果報告会

東京電力ホールディングス(株)福島第一原子力発電所(1F)の事故が発生した後、原子力規制庁からの委託を受け、令和2年度に近沿岸海域等における放射性物質の状況調査を実施した。本報告書は、調査により得られた成果をまとめたものである。海水及び海底土のモニタリング結果から調査地点と頻度の適正化に関する評価手法を提示し、効率的かつ効果的な海域モニタリングの在り方について検討した。また、福島沿岸域70地点において柱状試料を採取し、海底土への放射性セシウムの蓄積状況を示した。さらに放射性セシウムの河川からの流入評価のため、沿岸域にセジメントトラップを設置して沈降物を採取するとともに、河川前面における海底土表層の放射性セシウム濃度の水平分布を計測した。平成25年度から平成30年度までに実施された曳航式放射線モニタリングデータについて、天然の放射性核種を考慮した再解析を実施し、沿岸域の海底土における放射性セシウム分布推定図の精度向上を試みた。また、1F沖合のシルトバンド帯にて、海底地形・海底土分布調査を実施し、地形と堆積物の分布状況を把握した。得られた結果より1F前面海域における海底土に含まれる放射性物質の分布と動態について推定を行った。, After the accident of TEPCO's Fukushima Daiichi Nuclear Power Station (1F), the project of marine monitoring survey on radioactive substances have been conducted by the contract research from the Nuclear Regulatory Agency in FY2020. Results obtain in the project are presented in this report. Based on the monitoring results of radioactive substances of seawater and sediments, we suggested the evaluation method for optimizing the survey points and frequency, and examined the proper monitoring method on marine monitoring. In addition, core samples were collected at 70 points at the coast of Fukushima Prefecture to reveal the accumulation of radioactive Cs in the sediments. Furthermore, in order to evaluate the inflow of radioactive Cs from the river, sediment traps were installed at the coastal area to collect sinking sediment, and the horizontal distribution of the radioactive Cs concentration on the surface sediment in front of the rivers was measured. We carried out the revaluation of the towed radiation monitoring data conducted from 2013 to 2018, taking into account the natural radionuclides, and improved the radioactive Cs distribution map in the coastal sediments. In addition, a seabed topography and sediments distribution survey was conducted in the silt band area off the 1F to understand the topography and sediment distribution. From these results, we estimated the distribution and its dynamics of radioactive Cs in the sediments in the front area on the 1F.