22 3. THE PACIFIC OCEAN RADIOLOGICAL SCENARIO 3.1. INTRODUCTION Following the magnitude 9.0 earthquake and resulting tsunami which occurred on 11 March 2011, significant amounts of radioactive material were released into the environment from the Fukushima Daiichi NPP accident [2]. Radionuclides released to the atmosphere were transported eastward by a strong jet stream and reached the coast of North America in 4 days [40]. A portion of these radionuclides were deposited on the Pacific Ocean surface by wet and dry deposition processes. In addition, water used to cool the damaged nuclear reactor leaked into the ocean [41]. Thus, two radionuclides inputs into the Pacific Ocean from the Fukushima Daiichi NPP accident are considered: direct release of contaminated water and deposition of radionuclides on the sea surface from the atmosphere. The general large scale marine circulation in this region of the western Pacific Ocean is dominated by the interaction between the Kuroshio current (western boundary current in the north Pacific) which ?ows along the coast of Japan towards the north and curves to the central Pacific Ocean and the Oyashio current, which is a cold current which ?ows from the north. These two current systems converge in the coastal waters o? Fukushima and such convergence leads to the generation of unsteady eddies in the area. It is also known that the Kuroshio current acts as a barrier [42], which prevented the migration of radionuclides released from Fukushima towards the south beyond the latitude of Tokyo, instead they were transported towards the central Pacific. 3.1.1. Previous modelling studies A significant number of modelling studies on the dispersion of radionuclides released from the Fukushima Daiichi NPP accident into the Pacific Ocean have been published in the scientific literature. The first studies were published soon after the accident, thus the spread of 131I and 137Cs was simulated using the Lagrangian model SEA-GEARN developed at JAEA [43]. Moreover, a Lagrangian code was also used to simulate the dispersion of 137Cs and 134Cs in the world ocean up to 30 years after the accident [44], where annually averaged water circulation was used for this purpose. 137Cs dispersion was simulated using a high resolution (1 km) regional model during the first 3 months after the accident. Another similar study found that radionuclides stay close to the coastline for relatively long times and suggested a role for freshwater discharges from land in o?shore dispersion events [45]. Moreover, an Eulerian dispersion model for 137Cs was used to carry out some sensitivity studies in order to highlight the relevant role of winds in the shelf region [46]. Simulations of 137Cs dispersion over a 10 year period in the Pacific Ocean were made [47], where water circulation of the past 10 years was used. It was found that the initial current field is relevant for 137Cs spreading in the first months after the accident, but this relevance fades in the long term. In addition, it was found by the same authors [47] that traces of 137Cs would reach the coast of North America after about 5–6 years, and that very low concentrations would be nearly homogeneous over the whole Pacific after around 10 years. Simulations indicate a fast mixing over the upper 500 m of the water column [48] where it was also found that the radioactive caesium concentration due to the Fukushima Daiichi NPP accident was e?ciently diluted in the North Pacific 2.5 years after the accident. The mesoscale eddies in the Kuroshio Extension played an important role in diluting the radioactive patch. The 137Cs concentrations in the surface, intermediate, and deep layers reduce to the pre-Fukushima Daiichi NPP accident values over the North Pacific around 2.5 years after the occurrence of the Fukushima Daiichi NPP accident. Similar conclusions were obtained by modelling studies, i.e. the plume that