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 137Cs concentration in a seawater sample near the Fukushima Dai-ichi Nuclear Power Plant site reached 68 kBq L−1 (6.8 × 104 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 131I/137Cs activity ratio, we determined that direct release from the site contributed more to the measured 137Cs concentration than atmospheric deposition did.We then used a regional ocean model to simulate the 137Cs 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 137Cs directly released was 3.5 ± 0.7 PBq ((3.5 ± 0.7) × 1015 Bq). The simulated temporal change in 137Cs concentrations near the Fukushima Daini Nuclear Power Plant site agreed well with observations. Our simulation results showed that (1) the released 137Cs advected southward along the coast during the simulation period; (2) the eastward-flowing Kuroshio and its extension transported 137C during May 2011; and (3) 137Cs 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 137Cs released from the Fukushima Dai-ichi reactors to the ocean with the 137Cs released to the ocean by global fallout. Even though the measured 137Cs concentration from the Fukushima accident was the highest recorded, the total released amount of 137Cs was not very large. Therefore, the effect of 137Cs 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 137Cs expected to reach other oceanic basins is negligible comparing with the past radioactive input.