Abstract
Vertical profiles of radiocesium (134Cs and 137Cs) were measured in the western subarctic area of the North Pacific in 2017. The highest concentration of 134Cs, which was derived from the accident of the Fukushima Dai-ichi Nuclear Power Plant in 2011, was 0.14 Bq m−3 (or 1.19 Bq m−3 after the decay correction to the accident date). Although the vertical inventory of 134Cs decreased between 2014 and 2017, the inventory in 2017 was larger than that expected. That was probably arose from the return of some portion of the high-concentration water mass along with the anticlockwise subarctic gyre current.
Highlights
A massive earthquake in the North Pacific Ocean off Japan on 11 March 2011 resulted in serious damage to the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) operated by Tokyo Electric Power Company (TEPCO)
Before the FNPP1 accident, radiocesium was released into the North Pacific Ocean by the atmospheric testing of nuclear weapons mainly in the 1950s and 1960s
The radiocesium derived from the FNPP1 accident in 2011 was still observed in the surface mixed layer above about 200 m depth in the western subarctic area in June 2017, about 6 years after the accident
Summary
A massive earthquake in the North Pacific Ocean off Japan on 11 March 2011 resulted in serious damage to the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) operated by Tokyo Electric Power Company (TEPCO). The consequent giant tsunami enlarged the damage because the FNPP1 is located on the North Pacific shoreline of the Japanese island of Honshu. Radiocesium (134Cs and 137Cs) released from the damaged FNPP1 to the atmosphere in March and April 2011 was deposited on land in the eastern area of Honshu island and sea surface in the North Pacific [1, 2]. The radiocesium measurements in the coastal area by TEPCO [10] and the Japanese Government [11] revealed that another major source of radiocesium in the North Pacific was the direct discharge of radioactive water from the FNPP1 between late March and early April. The total (atmospheric deposition and direct discharge) release of 134Cs or 137Cs into the North Pacific Ocean was calculated to be 12–26 PBq approximately
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