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

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean by two major pathways: direct release from the accident site and atmospheric deposition. A 1 yr, regional-scale simulation of Cs-137 activity in the ocean offshore of Fukushima was carried out, the sources of radioactivity being direct release, atmospheric deposition, and the inflow of Cs-137 deposited into the ocean by atmospheric deposition outside the domain of the model. Direct releases of Cs-137 were estimated for 1 yr after the accident by comparing simulated results and measured activities adjacent to the accident site. The contributions of each source were estimated by analysis of I-131/Cs-137 and Cs-134/Cs-137 activity ratios and comparisons between simulated results and measured activities of Cs-137. The estimated total amounts of directly released I-131, Cs-134, and Cs-137 were 11.1 +/- 2.2 PBq, 3.5 +/- 0.7 PBq, and 3.6 +/- 0.7 PBq, respectively. Simulated Cs-137 activities attributable to direct release were in good agreement with measured Cs-137 activities not only adjacent to the accident site, but also in a wide area in the model domain, therefore this implies that the estimated direct release rate was reasonable. Employment of improved nudging data by JCOPE2 improved both the offshore transport result and the reproducibility of Cs-137 activities 30 km offshore. On the other hand, simulated Cs-137 activities attributable to atmospheric deposition were low compared to measured activities. The rate of atmospheric deposition into the ocean was underestimated because of a lack of measurements of deposition into the ocean when atmospheric deposition rates were being estimated. Simulated Cs-137 activities attributable to the inflow of Cs-137 deposited into the ocean outside the domain of the model were in good agreement with measured activities in the open ocean within the model domain after June 2012. The consideration of inflow is important to simulate the Cs-137 activity in this model region in the later period of the simulation. The contribution of inflow increased with time and was dominant (more than 99 %) by the end of February 2012. The activity of directly released Cs-137, however, decreased exponentially with time and was detectable only in the coastal zone by the end of February 2012.