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Dispersion of radionuclides in the sea
The Fukushima nuclear plant has rejected 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 few 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 are not able to prescribe in our model realistic scenarios as we do not know how much radionuclides have been rejected, when they have been rejected and how they behave once they reach the sea. That is why we do not claim that our simulations are able to provide a quantification of radioactivity in the sea. We present movies that only intend to give scenarios of dispersion. Only the orders of magnitude of the dilution have to be considered. We use the generic name of Tracer to avoid misunderstanding.
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 this last case, we have assumed that the deposition takes place in a circle of 200km of radius with a concentration maximum at the centre of the circle positioned at the nuclear plant and decreasing with distance. For each source, we consider two cases: one corresponds to dissolved elements, the other one to particles that fall into the sea with a velocity of 5meters 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.