Fukushima Radionuclide Dispersion Simulation in the Ocean using JCOPE

Large amount of radionuclides are discharged into the Pacific Ocean from Fukushima Daiichi Nuclear Power Plant (FDNPP) due to the accident associated with the East Japan Earthquake and related huge tsunamis on March 11th, 2011. Using a data-assimilative high-resolution ocean prediction system around Japan, which is called as JCOPE (Japan Coastal Ocean Predictability Experiment), we are conducting radionuclide dispersion simulations to show how the radionuclide spreads and disperse from the Fukushima coastal region to the open ocean. One to two months predictions of the dispersions are also conducted. Early results obtained have been released five times through Ministry of Education, Culture, Sports, Science and Technology (MEXT) during March to May 2011. Since then, we continue the simulation and prediction of the dispersion calculation.

This page shows some results of the radionuclide dispersion simulation by JCOPE2, which is one version of JCOPE system.

A part of this activity has been conducted under a consignment study from Marine Ecology Research Institute (MERI).

Note: Please see description pages of JCOPE models to understand characteristics of the model systems. JCOPE models are developed, operated, and utilized under several assumptions and limitations, and thereby, the results form the model simulations as well.

●Descriptions of JCOPE2 model

JCOPE2 descriptions can be found in “Description” page.

●Currents, Temperature, and Salinity fields simulated in JCOPE2

Latest analysis and forecast results can be found in “JCOPE2 Analysis/Forecast” page.

Ocean observation data obtained by JAMSTEC and MERI under the MEXT “Sea Area Monitoring Action Plan” are specifically assimilated into the JCOPE2 model to improve hindcast and forecast performances of the current fields off Fukushima.

●JCOPE2 particle tracking simulation

The Japan Coastal Ocean Predictability Experiment 2 (JCOPE2) particle tracking model represents the movement of particles by advection and diffusion due to ocean currents, and it includes the half-life decay of the radioactivity for each radionuclide. The surface current data reproduced by an operational ocean forecast system JCOPE2 are used for the advection. JCOPE2 is a data-assimilative ocean general circulation model with horizontal resolution of 1/12°. The diffusion is calculated by a random-walk method based on a Smagolinsky-type parameterization. Concentration of radionuclide in each grid is diagnosed from the number of particles in the grid and the radioactive intensity specified on each particle. Initial radioactivity of the particles in the source grid, which is the closest grid to the location of the Fukushima Daiichi Nuclear Power Plant, is determined from the field observations by Tokyo Electricity Power Corporation (TEPCO).

●Latest result

The dispersion of Cesium-137 simulated by JCOPE2 particle tracking model from March 21st, 2011 to June 14th, 2012 is shown below as one of the examples.

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The simulation results demonstrate that the radionuclide moves north-south direction near the coast of Fukushima due to influences of local winds and, then, gradually spreads out to the east into the open ocean. There is relatively strong southward surface flow about 50 km offshore from the coast, which brings the radionuclide to the south toward Kuroshio. Once it reaches northern part of Kuroshio, the radionuclide is quickly advected to the east along Kuroshio Extension. Then, the radionuclide slowly spreads and diffuses within the mixed-water region between the Kuroshio and Oyashio systems by complex surface flow fields associated with meandering of Kuroshio/Oyashio, meso-scale eddies, and oceanic frontal structures.

The radionuclides directly discharged into the ocean from FDNPP reaches the date line by the end of August, though the concentration would be calculated as large as 0.1 to 0.5 Bq/L. By the end of January 2012, regions with relatively large radioactivity of 0.1 to 0.5 Bq/L move further to the east beyond 155°E. The Kuroshio, flowing along the southern coast of Honshu Island from the west where the radionuclides contamination is rather weak, can be seen as a white belt in the area east of Honshu Island. The Kuroshio flow seems to play a significant role in diluting radionuclides in this particular region.

After March 2012, regions of simulated radioactivity larger than 0.1 Bq/L (shown by light blue color) are gradually shifting toward east, and a large part of the radioactively contaminated water moves beyond the dateline by June. Unfortunately, however, JCOPE2 simulation is limited in the northwestern Pacific Ocean west of the dateline. Therefore, we cannot show how far the radioactively contaminated water distributes at present using JCOPE2 results. On the other hand, the radioactively contaminated water at the sea surface is almost flowing out from Japanese coastal regions due mostly to sweeping effects of Kuroshio and Oyashio along the coast.

We will continue the dispersion calculation using the particle tracking model, with updated ocean current results from the JCOPE2 system.