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A series of accidents at the Fukushima Daiichi Nuclear Power Station (F1NPS), following the 11 March 2011 earthquake and tsunami, resulted in the release of radioactive substances into the ocean. In particular, the release of large amounts of radioactive caesium has damaged the fishing industry, leading to voluntary restrictions on fishing and shipping. Oceanic dispersion simulations based on estimates of the pathways and fluxes of radioactive materials provide useful information for assessing the environmental impacts and formulating measures to mitigate the effects of the accident. For the direct release rate from the F1NPS site, an estimation method was developed using the results from nearby monitoring, and the seawater exchange rate was estimated in target volume using a numerical simulation. However, the influence of volume on the seawater exchange rate was not considered. Appropriate volumes must be considered when estimating the effects of future accidents. In addition, the directional coastal transport was underestimated in the simulations of the F1NPS accident because of the low resolution. To estimate the pathways and fluxes of radioactive material to the ocean and understand the distribution of the concentration of radioactive material based on ocean dispersion simulations, a study was conducted using a higher-resolution model. The horizontal resolution of the conventional ocean dispersion model Regional Ocean Modelling System (ROMS), was increased from 1 km to 200 m. The optimal settings of the seawater exchange rate were investigated, and the radioactive caesium transport process in the coastal direction was more accurately reflected. We found that the conventional volume for determining the seawater exchange rate, including the locations of release sources and observation points, is optimal. The validity of this finding was confirmed using experimental equations from previous oceanic tracer release experiments. To estimate future release rates, it an appropriate volume must be defined, for example, depending on the distance between the locations of the release sources and the observation point. In addition, improvements in the accuracy of the simulation of the coastal transport process were observed owing to the higher resolution, which increased reproducibility. However, with a horizontal resolution of 200 m, problems with repeatability near the harbours arose. A higher resolution, achieved using nesting or other methods, would be desirable to deal with releases smaller than those in the F1NPS accident. • A higher resolution model was developed for the assessment of the Fukushima Daiichi Nuclear Power Station accident. • A method for estimating direct leakage rates using the higher resolution model was proposed for future severe accident. • The higher resolution allowed improved transport processes of 137Cs along the coast. [ABSTRACT FROM AUTHOR]

Abstract: 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 68kBqL−1 (6.8×104 BqL−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±0.7PBq ((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 10BqL−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. [Copyright &y& Elsevier]

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. We used the Regional Ocean Model System (ROMS) to simulate the 137Cs activity in the oceanic area off Fukushima, with the sources of radioactivity being direct release, atmospheric deposition, river discharge, and inflow across the domain boundary. The direct release rate of 137Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured 137Cs activities adjacent to the 1F NPP. River discharge rates of 137Cs were estimated by multiplying simulated river flow rates by the dissolved 137Cs activities, which were estimated by an empirical function. Inflow of 137Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of 137Cs activity was large, the simulated results were compared with the annual averaged observed 137Cs activity distribution. Normalized annual averaged 137Cs activity distributions in the regional ocean were similar for each year from 2013 to 2016. This result suggests that the annual averaged distribution is predictable. Simulated 137Cs activity attributable to direct release was in good agreement with measurement data from the coastal zone adjacent to the 1F NPP. Comparison of the simulated results with measured activity in the offshore area indicated that the simulation slightly underestimated the activity attributable to inflow across the domain boundary. This result suggests that recirculation of subducted 137Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic 137Cs activity was small compared to the effect of directly released 137Cs. • The direct release rate of dissolved 137Cs was estimated by the end of 2016. • The river discharge rate of dissolved 137Cs was estimated from 2013 to 2016. • The annual averaged 137Cs activity distribution patterns were similar in each year from 2013 to 2016. • Simulated 137Cs activities attributable to direct release were in good agreement with measured activities. • The effect of river discharge of 137Cs was not detected on oceanic 137Cs distributions. [ABSTRACT FROM AUTHOR]