Impacts of Fukushima Daiichi Nuclear Power Plant accident on the Western North Pacific and the China Seas: Evaluation based on field observation of Cs.

After the Fukushima nuclear accident (FNA), Cs activities of seawater in the Western North Pacific (WNP) and China Seas were determined in order to examine whether Cs derived from FNA across the Kuroshio was still existed and if there were any indications of FNA's impact. High Cs activities at 200-500m were observed in the south of Kuroshio during 2015, indicating Cs from FNA could cross the Kuroshio. Surface Cs activities were on average 1.43±0.42Bqm for WNP, 1.11±0.14Bqm for South China Sea (SCS) and 1.10±0.29Bqm for East China Sea (ECS) during 2011-2015. The activities and inventories of Cs were almost identical before and after the FNA, indicating the impacts of FNA on the WNP and China Seas were minor. The effective environmental half-lives of Cs in surface seawater were firstly estimated to be 14.4±1.7yrs for ECS and 16.9±2.1yrs for SCS.