[M-AG39] Radioisotope migration: New development for dynamics study of radionuclides from 1F NPP accident

[MAG39-P03] Evaluating the transport of surface seawater from 1957 to 2021 using ¹³⁷Cs deposited in the global ocean as a chemical tracer.

*Yayoi Inomata¹, Michio Aoyama² (1.Kanazawa University, Institute of Nature and Environmental Technology, 2.University of Tsukuba, Center for Research in Isotopes and Environmental Dynamics)

Keywords:137Cs, Surface seawater in the Global Ocean, Lonterm variation, Inventory

The spatiotemporal variations in the ¹³⁷Cs activity concentrations in the global ocean surface seawater from 1957 to 2021 using the Historical Artificial radioactivity in Marine environment, Global2021 database in order to understand the transport of surface sweater by using ¹³⁷Cs as a chemical tracer. Approximately 577 ± 173 PBq of ¹³⁷Cs derived from the large-scale nuclear weapons tests was deposited into the global ocean. The ¹³⁷Cs inventory in 1970 decreased to 313 ± 94 PBq, suggesting that 54 % of the ¹³⁷Cs deposited from the large-scale nuclear weapons tests was transported downward into the ocean interior on a decadal scale. Except in the North Atlantic Ocean and marginal seas due to the release from the nuclear reprocessing plant, the 0.5-yr average value of ¹³⁷Cs in each box decreased exponentially in 1970–2010, just before the Fukushima Nuclear Power Plant accident, and the expected reduction in 2010 in each box was 64–99.8 % compared to that in 1970. The longer apparent half residence time in 1970–2010 in the Indo-Pacific border region indicates that the ¹³⁷Cs released from large-scale weapons tests was transported from the Pacific Ocean into the Indian Ocean via the Indonesian archipelago on 40–50 years timescale. Until 2010, 58.8 ± 17.6 PBq of ¹³⁷Cs was transported into the Indian Ocean from the Pacific Ocean in the surface seawater, and 49.1±14.7 PBq was transported into the Atlantic Ocean. In 2011, the ¹³⁷Cs inventory in the global ocean mixed layer was increased to 60 ± 13 PBq in comparison with those in the previous year, and the ¹³⁷Cs from the FNPP1 accident was estimated to be 15.4 ± 4.5 PBq. These estimated values agree well with those of the previous studies.