Abstract
Abstract. Radionuclide impact of the Fukushima Dai-ichi nuclear power plant accident on the distribution of radionuclides in seawater of the NW Pacific Ocean is compared with global fallout from atmospheric tests of nuclear weapons. Surface and water column samples collected during the Ka'imikai-o-Kanaloa (KOK) international expedition carried out in June 2011 were analyzed for 134Cs, 137Cs, 129I and 3H. The 137Cs, 129I and 3H levels in surface seawater offshore Fukushima varied between 0.002–3.5 Bq L−1, 0.01–0.8 μBq L−1, and 0.05–0.15 Bq L−1, respectively. At the sampling site about 40 km from the coast, where all three radionuclides were analyzed, the Fukushima impact on the levels of these three radionuclides represents an increase above the global fallout background by factors of about 1000, 50 and 3, respectively. The water column data indicate that the transport of Fukushima-derived radionuclides downward to the depth of 300 m has already occurred. The observed 137Cs levels in surface waters and in the water column are compared with predictions obtained from the ocean general circulation model, which indicates that the Kuroshio Current acts as a southern boundary for the transport of the radionuclides, which have been transported from the Fukushima coast eastward in the NW Pacific Ocean. The 137Cs inventory in the water column is estimated to be about 2.2 PBq, what can be regarded as a lower limit of the direct liquid discharges into the sea as the seawater sampling was carried out only in the area from 34 to 37° N, and from 142 to 147° E. About 4.6 GBq of 129I was deposited in the NW Pacific Ocean, and 2.4–7 GBq of 129I was directly discharged as liquid wastes into the sea offshore Fukushima. The total amount of 3H released and deposited over the NW Pacific Ocean was estimated to be 0.1–0.5 PBq. These estimations depend, however, on the evaluation of the total 137Cs activities released as liquid wastes directly into the sea, which should improve when more data are available. Due to a suitable residence time in the ocean, Fukushima-derived radionuclides will provide useful tracers for isotope oceanography studies on the transport of water masses during the next decades in the NW Pacific Ocean.
Highlights
The largest amount of radionuclides released into the atmosphere up to now (Table 1), representing the main source of anthropogenic radionuclides in the world ocean, has been from atmospheric tests of nuclear weapons carried out mainly in 1950s and early 1960s (UNSCEAR, 2008)
Large quantities of radioactive materials were released into the atmosphere and coastal waters of the NW Pacific Ocean following a nuclear accident at the Fukushima Dai-ichi nuclear power plant (NPP), which increased 137Cs concentrations in coastal seawater off Fukushima up to eight orders of magnitude above the global fallout background (TEPCO, 2011; MEXT, 2011)
FFigi.g.2.2.A comparison of 137Cs profiles as obtained from the IAEA’97 expedition with the results presented in this paper, which are based on analysis of seawater samples collected during the KOK expedition (Fukushima impact) in the NW Pacific Ocean
Summary
The largest amount of radionuclides released into the atmosphere up to now (Table 1), representing the main source of anthropogenic radionuclides in the world ocean, has been from atmospheric tests of nuclear weapons (global fallout) carried out mainly in 1950s and early 1960s (UNSCEAR, 2008). The Fukushima accident happened due to the failure of the cooling system of the NPP after the Tohoku earthquake, and subsequent unexpectedly high tsunami waves on 11 March 2011. It was classified by the government of Japan on the INES scale (International Nuclear and Radiological Event Scale; IAEA, 2011) at the maximum level of 7, similar to the Chernobyl accident, which happened in 1986 in the former Soviet Union (present Ukraine) (IAEA, 2003). The atmospheric radionuclide releases during the Fukushima accident were estimated to be the highest for 131I (153–160 PBq) and 137Cs (13–15 PBq) (Chino et al, 2012). The contribution of 134Cs was similar to 137Cs as the 134Cs / 137Cs activity ratio was close to one (Masson et al, 2011)
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