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The radiocesium levels in coastal waters off Fukushima deceased greatly after the accident of the nuclear power accident 1FNPP, whereas depuration in marine fish was different, especially of rockfish. Matstumoto et al., 2015 suggested that excretion of 137Cs was slower than expected;; Ishimaru et al., 2019 demonstrated that stable cesium content in the rockfish was comparatively higher than those of other species; high content of radiocesium in prey organism of rock fish. Howeverthe detailed clarification of the mechanism of its high level and slow depuration was not enough clarified. It was expected to confirm the excretion rate of radiocesium from the rockfish furthermore; determination of the transition state of radiocesium between rockfish and environment; examination of the contribution of radiocesium transfer along food chain focusing on bio-availability. 137 In this study, Cs concentrations in demersal fish including rock fish collected from southern coastal waters of 1FNPP to identify the radiocesium transfer state of equilibrated or transition. Concerning the cesium bio-kinetics in the rockfish was examined the depuration rate of radiocesium by measuring the radioactivity of 137Cs in living fish by Ge detector under experimental condition to identify the biological half-life. With respect to radiocesium food chain transfer, labile fraction of 137Cs in prey organisms and stomach content were examined by collected samples from off Fukushima. Finally, the dynamic biological transfer model was applied to reconstruct the 137Cs level in rockfish using derived parameters, and the theoretically derived mechanism was verified by the observed time-series of 137Cs concentrations in the rockfish. As a result, there was no significant difference among stable cesium content in muscle of three demersal fish including the rockfish. The biological half-lives suggested as 140-220 days by the modelling study (Ishimaru et al., 2019) was revised to be approximately 210 days from the long-term experiment of - spectroscopy of 137Cs in living rockfish measurement. The result demonstrated that the biological half-life of radiocdesium in the rockfish was evaluated as slower than 90 days for teleost species derived from the radio-isotope experiment. The 137Cs levels seawater and of the stomach content and muscle of the rockfish collectedfrom southern area of the 1FNPP were 3-7 mBq/L, 1-2 Bq/kg-wet, 2-7 Bq /kg-wet. These results demonstrated that food chain transfer of 137Cs to the rockfish was still substantial to partly keep the radioactivity higher than the levels of pre-Fukushima, whereas it was not found why the radiocesium content in prey organisms higher than theoretical understanding suing seawater level., 6th International Conference on Environmental Radioactivity

The accident of the Fukushima Dai-ichi nuclear power plant in March 2011 released a large amount of radiocesium into the North Pacific Ocean. Vertical distributions of Fukushima-derived radiocesium were measured at stations along the 149°E meridian in the western North Pacific during the winter of 2012. In the subtropical region, to the south of the Kuroshio Extension, we found a subsurface radiocesium maximum at a depth of about 300 m. It is concluded that atmospheric-deposited radiocesium south of the Kuroshio Extension just after the accident had been transported not only eastward along with surface currents but also southward due to formation/subduction of subtropical mode waters within about 10 months after the accident. The total amount of decay-corrected 134Cs in the mode water was an estimated about 6 PBq corresponding to 10–60% of the total inventory of Fukushima-derived 134Cs in the North Pacific Ocean.

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.

著者人数: 22名, Accepted: 2011-08-01, 資料番号: SA1003198000