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The 3D compartment model POSEIDON-R was applied to the Northwestern Pacific and adjacent seas to simulate the transport and fate of 90Sr in the period 1945-2010 and to perform a radiological assessment on the releases of 90Sr due to the Fukushima Dai-ichi nuclear accident for the period 2011-2040. The contamination due to runoff of 90Sr from terrestrial surfaces was taken into account using a generic predictive model. A dynamical food-chain model describes the transfer of 90Sr to phytoplankton, zooplankton, molluscs, crustaceans, piscivorous and non-piscivorous fishes. Results of the simulations were compared with observation data on 90Sr for the period 1955-2010 and the budget of 90Sr activity was estimated. It was found that in the East China Sea and Yellow Sea the riverine influx was 1.5% of the ocean influx and it was important only locally. Calculated concentrations of 90Sr in water, bottom sediment and marine organisms before and after the Fukushima Dai-ichi accident are in good agreement with available experimental measurements. The concentration of 90Sr in seawater would return to the background levels within one year after leakages were stopped. The model predicts that the concentration of 90Sr in fish after the Fukushima Dai-ichi accident shall return to the background concentrations only 2 years later due to the delay of the transfer throughout the food web and specific accumulation of 90Sr. The contribution of 90Sr to the maximal dose rate due to the FDNPP accident was three orders of magnitude less than that due to 137Cs, and thus well below the maximum effective dose limits for the public. [ABSTRACT FROM AUTHOR]