Initial spread of 137 Cs from the Fukushima Dai-ichi Nuclear Power Plant over the Japan continental shelf: a study using a high-resolution, global-coastal nested ocean model

The 11 March 2011 tsunami triggered by the M9 and M7.9 earthquakes off the Tōhoku coast destroyed facilities at the Fukushima Dai-ichi Nuclear Power Plant (FNPP) leading to a significant long-term flow of the radionuclide 137 Cs into coastal waters. A high-resolution, global-coastal nested ocean model was first constructed to simulate the 11 March tsunami and coastal inundation. Based on the model's success in reproducing the observed tsunami and coastal inundation, model experiments were then conducted with differing grid resolution to assess the initial spread of 137 Cs over the eastern shelf of Japan. The 137 Cs was tracked as a conservative tracer (without radioactive decay) in the three-dimensional model flow field over the period of 26 March–31 August 2011. The results clearly show that for the same 137 Cs discharge, the model-predicted spreading of 137 Cs was sensitive not only to model resolution but also the FNPP seawall structure. A coarse-resolution (∼2 km) model simulation led to an overestimation of lateral diffusion and thus faster dispersion of 137 Cs from the coast to the deep ocean, while advective processes played a more significant role when the model resolution at and around the FNPP was refined to ∼5 m. By resolving the pathways from the leaking source to the southern and northern discharge canals, the high-resolution model better predicted the 137 Cs spreading in the inner shelf where in situ measurements were made at 30 km off the coast. The overestimation of 137 Cs concentration near the coast is thought to be due to the omission of sedimentation and biogeochemical processes as well as uncertainties in the amount of 137 Cs leaking from the source in the model. As a result, a biogeochemical module should be included in the model for more realistic simulations of the fate and spreading of 137 Cs in the ocean.

[1]  Hyoe Takata,et al.  Spatiotemporal distributions of Fukushima-derived radionuclides in surface sediments in the waters off Miyagi, Fukushima, and Ibaraki Prefectures, Japan , 2013 .

[2]  Changsheng Chen,et al.  An Unstructured Grid, Finite-Volume, Three-Dimensional, Primitive Equations Ocean Model: Application to Coastal Ocean and Estuaries , 2003 .

[3]  C. Davis,et al.  Effects of surface forcing on interannual variability of the fall phytoplankton bloom in the Gulf of Maine revealed using a process-oriented model , 2011 .

[4]  C. Böning,et al.  Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima , 2012 .

[5]  K. Yoshida,et al.  134Cs and 137Cs activities in coastal seawater along Northern Sanriku and Tsugaru Strait, northeastern Japan, after Fukushima Dai-ichi Nuclear Power Plant accident. , 2012, Journal of environmental radioactivity.

[6]  Tatsuo Aono,et al.  Dispersion of artificial caesium-134 and -137 in the western North Pacific one month after the Fukushima accident , 2012, GEOCHEMICAL JOURNAL.

[7]  S. Jayne,et al.  Radium-based estimates of cesium isotope transport and total direct ocean discharges from the Fukushima Nuclear Power Plant accident , 2012 .

[8]  Jian Lin,et al.  The March 11, 2011 Tōhoku M9.0 earthquake-induced tsunami and coastal inundation along the Japanese coast: A model assessment , 2014 .

[9]  Irina I. Rypina,et al.  Fukushima-derived radionuclides in the ocean and biota off Japan , 2012, Proceedings of the National Academy of Sciences.

[10]  J. Qi,et al.  An unstructured-grid, finite-volume sea ice model : development, validation, and application , 2011 .

[11]  Y. Ishikawa,et al.  Preliminary Numerical Experiments on Oceanic Dispersion of 131I and 137Cs Discharged into the Ocean because of the Fukushima Daiichi Nuclear Power Plant Disaster , 2011 .

[12]  Caskey,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS I . THE BASIC EXPERIMENT , 1962 .

[13]  Changsheng Chen,et al.  An Unstructured Grid, Finite-Volume Coastal Ocean Model (FVCOM) System , 2006 .

[14]  Michio Aoyama,et al.  Impacts of the Fukushima nuclear power plants on marine radioactivity. , 2011, Environmental science & technology.

[15]  J. Smagorinsky,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS , 1963 .

[16]  Haosheng Huang,et al.  A finite volume numerical approach for coastal ocean circulation studies: Comparisons with finite difference models , 2007 .

[17]  Marc Bocquet,et al.  Assessment of the amount of cesium‐137 released into the Pacific Ocean after the Fukushima accident and analysis of its dispersion in Japanese coastal waters , 2012 .

[18]  Irina I. Rypina,et al.  Short-term dispersal of Fukushima-derived radionuclides off Japan: modeling efforts and model-data intercomparison , 2012 .

[19]  M. Ebihara,et al.  Radioactivity concentrations of 131I, 134Cs and 137Cs in river water in the Greater Tokyo Metropolitan area after the Fukushima Daiichi Nuclear Power Plant Accident , 2012 .

[20]  Takeo Ohnishi,et al.  The Disaster at Japan's Fukushima-Daiichi Nuclear Power Plant after the March 11, 2011 Earthquake and Tsunami, and the Resulting Spread of Radioisotope Contamination1 , 2012, Radiation research.

[21]  Takaki Tsubono,et al.  Distribution of oceanic 137Cs from the Fukushima Dai-ichi Nuclear Power Plant simulated numerically by a regional ocean model. , 2012, Journal of environmental radioactivity.

[22]  Changsheng Chen,et al.  A model‐dye comparison experiment in the tidal mixing front zone on the southern flank of Georges Bank , 2008 .

[23]  Iris Kriest,et al.  137 Cs off Fukushima Dai-ichi, Japan – model based estimates of dilution and fate , 2012 .

[24]  Daisuke Tsumune,et al.  Temporal variation of 134Cs and 137Cs activities in surface water at stations along the coastline near the Fukushima Dai-ichi Nuclear Power Plant accident site, Japan , 2012 .