Estimate of Fukushima-derived radiocaesium in the North Pacific Ocean in summer 2012

Y Inomata,M Yamada,Y T Tanaka,E Oka,M Aoyama,D Tsumune,M Kajino,T Tsubono,T Yamagata,H Nagai,Y Kumamoto,T T Sekiyama

doi:10.1007/s10967-018-6249-7

Abstract

Distributions of radiocaesium (134Cs and 137Cs) derived from the Tokyo Electric Power Company (TEPCO) Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in the North Pacific Ocean in the summer of 2012 were investigated. We have estimated the radiocaesium inventory in the surface layer using the optimal interpolation analysis and the subducted amount into the central mode water (CMW) by using vertical profiles of FNPP1-134Cs and mass balance analysis as the first approach. The inventory of the 134Cs in the surface layer in the North Pacific Ocean in August–December 2012 was estimated at 5.1 ± 0.9 PBq on 1 October 2012, which corresponds to 8.6 ± 1.5 PBq when it was decay corrected to the date of the FNPP1 accident, 11 March 2011. It was revealed that 56 ± 10% of the released 134Cs into the North Pacific Ocean, which was estimated at 15.3 ± 2.6 PBq, transported eastward in the surface layer in 2012. The amount of 134Cs subducted in the CMW was estimated to be 2.5 ± 0.9 PBq based on the mass balance among the three domains of the surface layer, subtropical mode water, and CMW.

Highlights

As a result of an extraordinary earthquake and subsequent giant tsunami on 11 March 2011, Tokyo Electronic Power Company Fukushima Dai-ichi Nuclear Power Plant, locate 37.42°N, longitude 141.03°E, received serious damage
It was recorded that extremely high radiocaesium activity concentrations reaching 6.8 × 107 Bq m−3 were observed on 6 April 2011 at the site of the FNPP1 accident [3, 4]
We focused on the distribution of the central North Pacific Ocean in 2012, because the centre of the radioactive contaminated seawater in the surface layer was observed in the central region of the North Pacific Ocean, at approximately 180° in August–December 2012 [21]

Summary

Introduction

As a result of an extraordinary earthquake and subsequent giant tsunami on 11 March 2011, Tokyo Electronic Power Company Fukushima Dai-ichi Nuclear Power Plant (hereafter referred to as FNPP1), locate 37.42°N, longitude 141.03°E, received serious damage. Because of the damaged FNPP1 reactors, large amounts of radiocaesium (137Cs, with a half-life of 30.2 years and 134Cs, with a half-life of 2.06 years) were directly released into the North Pacific Ocean by atmospheric deposition and direct discharge of liquid-contaminated stagnant water from the FNPP1 accident site, mostly in March and April 2011 [1, 2]. The radiocaesium released by atmospheric deposition and direct discharge of liquidcontaminated stagnant water into the North Pacific Ocean. The directly discharged radiocaesium in the liquid-contaminated stagnant water into the North Pacific Ocean was estimated to be 3.5 ± 0.7 PBq using the 131I/137Cs activity ratio, which is a useful tracer to distinguish the release pathways between direct release and atmospheric deposition [2, 3]. It is considered that more than 75% of the radiocaesium released into the atmosphere from the FNPP1 event was deposited into the North Pacific Ocean

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