Fukushima radionuclides in the marine environment from coastal region of Japan to the Pacific Ocean through the end of 2016

The authors present total amount of released radiocaesium from FNPP1 accident which satisfy mass balance in all components of the environment. The authors also present consensus values of Fukushima Dai-ichi Nuclear Power Plant (FNPP1) derived ¹³⁷Cs in the environment. Total amount of atmospheric release was 15-20 PBq and direct discharge to the ocean was 3-6 PBq, respectively. 12-15 PBq was deposited in the North Pacific and 3-6 PBq was deposited on land, respectively. Therefore 15-18 PBq was injected as sum of atmospheric deposition and direct discharge in the North Pacific Ocean. The authors also present temporal variation of ¹³⁷Cs in coastal region close to FNPP1 site. Based on radiocaesium activity concentration and ¹³⁴Cs/¹³⁷Cs activity ratios decay corrected to 11 March 2011 in surface water, major source of radiocaesium is still FNPP1 site and contribution from global fallout ¹³⁷Cs increased at Tomioka and Hasaki depending on the distances from FNPP1 site. A part of radiocaesium in the Sub Tropical Mode Water (STMW) in the Pacific Ocean is already obducted from ocean interior and enter into northern part of East China sea and in Japan Sea. (author)