One-year, regional-scale simulation of 137Cs radioactivity in the ocean following the Fukushima Dai-ichi Nuclear Power Plant accident

[摘要] A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant followingthe Great East Japan Earthquakeand tsunami of 11 March 2011 resulted in the release of radioactive materialsto the ocean by two major pathways: direct release from the accident site andatmospheric deposition. A 1 yr, regional-scale simulation of ¹³⁷Csactivity in the ocean offshore of Fukushima was carried out, the sources ofradioactivity being direct release, atmospheric deposition, and the inflow of¹³⁷Cs deposited into the ocean by atmospheric deposition outside thedomain of the model.Direct releases of ¹³⁷Cs were estimated for 1 yr after the accident bycomparing simulated results and measured activities adjacent to the accidentsite. The contributions of each source were estimated by analysis of¹³¹I/¹³⁷Cs and ¹³⁴Cs/¹³⁷Cs activity ratios andcomparisons between simulated results and measured activities of ¹³⁷Cs.The estimated total amounts of directly released ¹³¹I, ¹³⁷Cs, and¹³⁷Cs were 11.1 ± 2.2 PBq, 3.5 ± 0.7 PBq, and3.6 ± 0.7 PBq, respectively. Simulated ¹³⁷Cs activitiesattributable to direct release were in good agreement with measured¹³⁷Cs activities not only adjacent to the accident site, but also in awide area in the model domain, therefore this implies that the estimateddirect release rate was reasonable. Employment of improved nudging data byJCOPE2 improved both the offshore transport result and the reproducibility of¹³⁷Cs activities 30 km offshore. On the other hand, simulated¹³⁷Cs activities attributable to atmospheric deposition were lowcompared to measured activities. The rate of atmospheric deposition into theocean was underestimated because of a lack of measurements of deposition intothe ocean when atmospheric deposition rates were being estimated. Simulated¹³⁷Cs activities attributable to the inflow of ¹³⁷Cs deposited intothe ocean outside the domain of the model were in good agreement withmeasured activities in the open ocean within the model domain after June2012. The consideration of inflow is important to simulate the ¹³⁷Csactivity in this model region in the later period of the simulation. Thecontribution of inflow increased with time and was dominant (more than99%) by the end of February 2012. The activity of directly released¹³⁷Cs, however, decreased exponentially with time and was detectableonly in the coastal zone by the end of February 2012.

[学科分类] 地球化学与岩石