Daily computer simulations are suggesting that, so far, the hazardous radioactive materials being released into the sea by the Fukushima nuclear plants are still largely restricted to areas near the coast. In the model being run by French researchers, the powerful Kuroshiro current —the Pacific's version of the Gulf Stream—tends to block contaminated seawater from flowing southward toward Tokyo Bay while picking up little contamination itself.
Figuring out where Fukushima radioactivity is going involves all the complexities and uncertainties that plagued early efforts to model the meanderings of oil last summer in the Gulf of Mexico, and then some, says ocean modeler Claude Estournel. She is a member of the SIROCCO group  at the University of Toulouse in France running the SYMPHONIE-NH ocean model under the auspices of the French national research agency CNRS. The International Atomic Energy Agency requested that the group run its model centered on the Fukushima plant on the coast northeast of Tokyo. The resulting daily simulations posted  on the Web are rife with uncertainties, Estournel cautions, so that the group is presenting only "scenarios of dispersion" that provide an "orders of magnitude" idea of the actual amounts of radionuclides in the sea.
Caveats aside, the modeling is strongly confirming oceanographers' intuition. The highest concentrations in the model are still within 5 kilometers or so of shore but have been carried up to 50 kilometers north and south by wind-driven currents. That contamination was released directly into the sea.
Radionuclides first released into the atmosphere only to fall into the sea are 20 to 100 times less concentrated but spread more widely, spanning 600 kilometers along the shore and reaching 150 kilometers offshore. In the model, concentrations are 1000 times lower than near Fukushima in the Kuroshiro as it shears off the southern extent of contamination and heads east into the Pacific. The modeling will guide Japanese authorities as they scramble to sample the expanse of ocean liable to contamination.