Abstract
Abstract. 134Cs and 137Cs were released to the North Pacific Ocean by two major likely pathways, direct discharge from the Fukushima NPP1 accident site and atmospheric deposition off Honshu Islands of Japan, east and northeast of the site. High density observations of 134Cs and 137Cs in the surface water were carried out by 17 cruises of cargo ships and several research vessel cruises from March 2011 till March 2012. The main body of radioactive surface plume of which activity exceeded 10 Bq m−3 travelled along 40° N and reached the International Date Line on March 2012, one year after the accident. A distinct feature of the radioactive plume was that it stayed confined along 40° N when the plume reached the International Date Line. A zonal speed of the radioactive plume was estimated to be about 8 cm s−1 which was consistent with zonal speeds derived by Argo floats at the region.
Highlights
Aoyama et al.: Radioactive plume of TEPCO Fukushima NPP1 released 134Cs and 137Cs the FNPP1 accident site, April 2011 to December 2011, was presented and the maximum in radiocaesium activity around 2000 Bq m−3 at Hasaki was observed in June 2011, representing a delay of two months from the corresponding maximum in April 2011 at FNPP1
The reasons for the twomonth delay at Hasaki are not yet clear; clockwise current associated with a warm water eddy, whose center is located at 36.5◦ N, 141.4◦ E off Iwaki between Onahama and Hasaki, in mid-May 2011 might have prevented southward transport of 134Cs and 137Cs released from FNPP1 to Hasaki until the end of May 2011 (Aoyama et al, 2012b)
134Cs and 137Cs were released to the North Pacific Ocean by two pathways, direct discharge from the Fukushima NPP1 accident site and atmospheric deposition off Honshu Islands of Japan, east and northeast of the site
Summary
(Inoue et al, 2011, al., 2012a, 2012) were ba;lsAo oreypaSomroateldeidtaanlEd.,da2i0src1tuh2sas;edB.uesseler et During the first month of the release period, 134Cs and. M. Aoyama et al.: Radioactive plume of TEPCO Fukushima NPP1 released 134Cs and 137Cs the FNPP1 accident site, April 2011 to December 2011, was presented and the maximum in radiocaesium activity around 2000 Bq m−3 at Hasaki was observed in June 2011, representing a delay of two months from the corresponding maximum in April 2011 at FNPP1. Before the FNPP1 accident, distribution and inventory of 137Cs which originated from atmospheric weapons tests had been studied in the Pacific Ocean since the late 1950s and the 137Cs inventory in the North Pacific Ocean was 290 ± 30 PBq in January 1970 based on 10◦ × 10◦ mesh data of the 137Cs deposition (Aoyama et al, 2006). In this paper we present the results of our measurements of 134Cs and 137Cs activities in the surface water in sea area both close to the site and the North Pacific Ocean based on the monitoring data and on our observation, respectively. We discuss the behaviours of the radioactive plume in the North Pacific Ocean through March 2012
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