Characteristics of radiocesium contamination of dry riverbeds due to the Fukushima Daiichi Nuclear Power Plant accident assessed by airborne radiation monitoring

福島第一原子力発電所周辺の河川敷における無人ヘリによる測定結果から考察する放射性セシウムの変化過程

浅見 和弘*; 太田垣 貴啓*; 石田 睦司; 眞田 幸尚   

Azami, Kazuhiro*; Otagaki, Takahiro*; Ishida, Mutsushi; Sanada, Yukihisa

The Great East Japan Earthquake that occurred on March 11, 2011 generated a series of massive tsunami waves that caused severe damage to the Fukushima Daiichi Nuclear Power Plant, following which a large amount of radioactive materials was discharged from the power plant into the environment. Given the recently developed technologies for unmanned helicopters and their potential application in airborne radiation monitoring, we developed a radiation monitoring system for aerial use. We then used unmanned helicopters to measure the radiation level in areas with soil contaminated by radioactive cesium emitted from the nuclear power plant to evaluate the ambient dose-rate distribution around the site. We found that in dry riverbeds near the nuclear power plant, the dose rate was higher than that in landside areas. In addition, we carried out soil sampling in dry riverbeds. The results did not contradict the results measured by unmanned helicopter. As a result, a possible scenario is that river sediment and adjacent soil containing radioactive materials were carried from the upstream to the downstream due to an event such as a rise in water caused by a typhoon, etc., and remained in the riverbed after the water drained away. The vertical profile of radioactive materials in the soil and the measurement results for river sediment at river mouth areas corroborated the scenario.