As a consequence of the magnitude 9.0 earthquake occurred about 130 km off the Pacific coast of Japan’s main island Honshu and subsequent tsunami hitting the east coast of Japan on 11 March 2011 the Fukushima Nuclear Power Plants (NPPs) were severely damaged and radioactive material was released to the atmosphere as well as to the ocean.
In this study we are concerned with long-term oceanic-scale dispersion of Cs 137 released from the Fukushima Daiichi NPP. The circulation model uses POM covering the domain 75ºS-65ºN, 0º-360ºE with horizontal resolution, 0.5º×0.5º and 21 vertical sigma layers. The model topography is interpolated from the global 5′ by 5′ ETOPO5 dataset. The circulation model is driven by monthly climatological (COADS) wind stresses and heat fluxes. The initial temperature and salinity fields are set to the Levitus annually averaged temperature and salinity, and the initial velocities are set to zero. A spin up calculation is performed for 10 years. The formulation proposed by Tsumune et al(2003) is used to simulate the transport of the radionuclide which takes into account the transfer effect to the suspended sediment in a simple way.
The information that helped us to determine the source term of radioactive materials for the numerical experiments was the concentrations of radioactive materials in the ocean reported by Tokyo Electric Power Company (TEPCO). The source term of I 131 and Cs 137 released directly into the ocean from the Fukushima Daiichi NPP was estimated using monitoring data from the web site of TEPCO near the northern and southern discharge channels of the Fukushima Daiichi NPP.
Assuming a total release of 9 PBq of 137 Cs from the Fukushima Daiichi NPP into the marine environment (only including liquid releases on the Pacific Ocean), the simulation is carried out up to 2041.
The results are shown as animation. Calculation shows that Cs 137 in surface waters of the Pacific Ocean will reach the US coast after 4-5 years however, the levels will be low (<2 Bq/m3). The north Pacific and Indian Ocean will be affected by Cs 137 of Fukushima origin after 30 years with concentrations below 0.1 Bq/m3. Although results are not shown, it is noted that Cs 137 concentrations at middle (400 m) and deep (800m) water layers will be increasing to the surface levels in 10 and 30 years, respectively.