Transport of nuclear leakage from Fukushima Nuclear Power Plant in the North Pacific

The massive nuclear leakage into ocean from Fukushima Nuclear Power Plant was observed on March 25^th,2011. The transport of leaked radioactive pollutant from the Fukushima Nuclear Power Plant was simulated using a global version of Miami Isopycnic Coordinate Ocean Model (MICOM). The simulation was performed by assuming constant and continuous leakage for 20 days (scenario 1) and for one year (scenario 2) starting from March 25th,2011 and was integrated for 20 years. Taking into account of the annual and inter-annul variability of the ocean circulation,the model for each scenario was forced with four different time periods of NCEP/NCAR daily atmospheric forcing,namely,repeated 2010,1991-2011,1971-1991 and 1951-1971. There is no remarkable difference of transport pathways (both on surface and subsurface layers),transport timescales and vertical penetration for the nuclear waste among different ensemble members. An ensemble mean for each scenario was obtained in order to reduce the uncertainty in the simulations. The results of the ensembles indicate that the nuclear pollutant for both scenarios transports eastward to eastern Pacific and thereafter follows a southwest pathway towards the western Pacific. It takes about 10 to 15 years to reach the coast of East Asia. The transport pathway is governed by the ocean current system of the North Pacific Subtropical Gyre. The transport at the subsurface (100-400 m depth) is about 5 years faster than that at the surface when the nuclear signal reaches the coast of the East Asia. The time evolution of maximum tracer concentration shows that the surface and subsurface maximum concentrations of leaked nuclear pollutant at the end of the simulation are only 0.1% and 1% of the values at the first year. Most of nuclear pollutant stays in the North Pacific throughout the integration with almost 100% of the released nuclear pollutants remaining in the North Pacific for first 10 years and 86±1.5% at the end of the integration. The penetration depth of the nuclear pollutant is less than 1 000 m with relative high concentration between the surface and 600 m depth. The simulations also suggest that the ocean circulation and mixing instead of the source function dominate the variability of the maximum concentration. It should be emphasized that a realistic source function is required and atmospheric fallout and role of ocean ecology should also be taken into account,in order to get a more reliable assessment of possible impact of the radioactive leakage on the ocean environment.