Distribution of oceanic 137Cs from the Fukushima Dai-ichi Nuclear Power Plant simulated numerically by a regional ocean model

Radioactive materials were released to the environment from the Fukushima Dai-ichi Nuclear Power Plant as a result of the reactor accident after the Tohoku earthquake and tsunami of 11 March 2011. The measured Cs-137 concentration in a seawater sample near the Fukushima Dai-ichi Nuclear Power Plant site reached 68 kBq L-1 (6.8 x 10(4) Bq L-1) on 6 April. The two major likely pathways from the accident site to the ocean existed: direct release of high radioactive liquid wastes to the ocean and the deposition of airborne radioactivity to the ocean surface. By analysis of the I-131/Cs-137 activity ratio, we determined that direct release from the site contributed more to the measured Cs-137 concentration than atmospheric deposition did. We then used a regional ocean model to simulate the Cs-137 concentrations resulting from the direct release to the ocean off Fukushima and found that from March 26 to the end of May the total amount of Cs-137 directly released was 3.5 +/- 0.7 PBq ((3.5 +/- 0.7) x 10(15) Bq). The simulated temporal change in Cs-137 concentrations near the Fukushima Daini Nuclear Power Plant site agreed well with observations. Our simulation results showed that (1) the released Cs-137 advected southward along the coast during the simulation period; (2) the eastward-flowing Kuroshio and its extension transported C-137 during May 2011; and (3) Cs-137 concentrations decreased to less than 10 Bq L-1 by the end of May 2011 in the whole simulation domain as a result of oceanic advection and diffusion. We compared the total amount and concentration of Cs-137 released from the Fukushima Dai-ichi reactors to the ocean with the Cs-137 released to the ocean by global fallout. Even though the measured Cs-137 concentration from the Fukushima accident was the highest recorded, the total released amount of Cs-137 was not very large. Therefore, the effect of Cs-137 released from the Fukushima Dai-ichi reactors on concentration in the whole North Pacific was smaller than that of past release events such as global fallout, and the amount of Cs-137 expected to reach other oceanic basins is negligible comparing with the past radioactive input. (C) 2011 Elsevier Ltd. All rights reserved.