A numerical investigation into the long-term behaviors of Fukushima-derived ¹³⁷Cs in the ocean

The Fukushima nuclear accident in 2011 released large amounts of radionuclides, including ¹³⁷Cs, into the Pacific Ocean. A quasi-global ocean radioactive transport model with horizontal grid spacing of 0.5°×0.5° and 21 vertical layers was thereafter established to study the long-term transport of the Fukushima-derived ¹³⁷Cs in the ocean. The simulation shows that the plume of ¹³⁷Cs would be rapidly transported eastward alongside the Kuroshio Current and its extensions. Contaminated waters with concentrations lower than 2 Bq/m³ would reach the west coast of North America 4 or 5 years after the accident. The ¹³⁷Cs tends to be carried, despite its very low concentration, into the Indian and South Pacific Oceans by 2016 via various branches of ocean currents. Meanwhile, the ¹³⁷Cs concentrations in the western part of the North Pacific Ocean decrease rapidly with time. Up to now the highly contaminated waters have remained in the upper 400 m, showing no evidence of significant penetration to deeper layers.