SIROCCO has performed, at the request of the International Atomic Energy Agency (IAEA), simulations using the 3D SIROCCO ocean circulation model to investigate the dispersion in seawater of radionuclides emitted by the Fukushima nuclear plant.
The model uses a stretched horizontal grid with a variable horizontal resolution, from 600m x 600m at the nearest grid point from Fukushima, to 5km x 5km offshore.
The initial fields (T,S,U,V,SSH) and the lateral open boundary conditions are provided by the Mercator PSY4V1R3 system (one field per day, horizontal resolution 1/12 ° x 1/12 °).
At the sea surface, the ocean model is forced by the meteorological fluxes delivered every 3hours by ECMWF. The tidal forcing at the lateral open boundaries is provided by the T-UGO model, implemented for this purpose by the SIROCCO team on the Japanese Pacific coast.
Variables in Creating Simulations for Dispersion of radionuclides in the sea
The Fukushima nuclear plant has injected in the atmosphere and in the seawater radionuclides at different times and in variable quantities. A model of dispersion needs a good knowledge of the source terms to be able to correctly calculate the dispersion.
Of course, very little information is available to build elaborate scenarios of radionuclides emssion. Besides, the evolution of dispersion in the sea also requires to know very well the behaviour of the radionuclides, for example the fraction which is dissolved in the seawater, the particulate fraction and its associated sedimentation velocity.
Even if we knew all this information, radionuclides can aggregate with marine particles and then their sedimentation velocity can evolve. Finally, the oceanic currents computed by our model are not the reality: they are the result of mathematical equations too simple to fully represent the complexity of nature.
The wind which strongly drives the oceanic currents is also a forecast whose accuracy is not known.
We do not know how much radionuclides have been injected, when they have been injected and how they behave once they reach the sea.
That is why we do not claim that our simulations are able to provide an accurate quantification of radioactivity in the sea.
However, in order to build our scenarios, special attention has been paid to the measurements of Cesium 137 concentration taken several times every day by TEPCO at 30 and 300m in front of the nuclear plant.
Two sources of radionuclides are considered.
One corresponds to a direct emission in the sea in front of the nuclear plant (migration of water contaminated by the reactors), the other one corresponds to fallout of atmospheric particles.
In the first case, we introduce a flux at the grid point corresponding to the nuclear plant. This flux is adjusted to produce a concentration close to the values of Cesium 137 measured (see our page of validation ).
In the last case, we used 1-h outputs of the atmospheric transport model Polyphemus/Polair3D (0.25° horizontal resolution) (pers. comm. Marc Bocquet & Victor Winiarek, Ecole des Ponts ParisTech/CEREA)
For each source (direct release and atmospheric), we consider two cases: one corresponds to dissolved elements, the other one to particles that fall into the sea with a velocity of 10meters per day.
Obviously dispersion in the first case will happen at larger scale than for the second case for which deposition of particles on the sea floor reduces the dispersion. Deposition of particles is cumulated over time and will be mapped later on.
In the coastal zone, wind plays an important role on the dispersion of tracers by inducing alongshore currents directed northward or southward (function of the wind direction). Offshore, the local action of wind on currents is concentrated in the surface layer adding a drift on the right of the wind to the large scale currents.
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