Impacts of Fukushima Daiichi Nuclear Power Plant accident on the Western North Pacific and the China Seas: Evaluation based on field observation of 137 Cs.

After the Fukushima nuclear accident (FNA), 137 Cs activities of seawater in the Western North Pacific (WNP) and China Seas were determined in order to examine whether 137 Cs derived from FNA across the Kuroshio was still existed and if there were any indications of FNA's impact. High 137 Cs activities at 200-500m were observed in the south of Kuroshio during 2015, indicating 137 Cs from FNA could cross the Kuroshio. Surface 137 Cs activities were on average 1.43±0.42Bqm-3 for WNP, 1.11±0.14Bqm-3 for South China Sea (SCS) and 1.10±0.29Bqm-3 for East China Sea (ECS) during 2011-2015. The activities and inventories of 137 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 137 Cs in surface seawater were firstly estimated to be 14.4±1.7yrs for ECS and 16.9±2.1yrs for SCS.