Distribution, dynamics, and fate of radiocesium derived from FDNPP accident in the ocean

Abstract

This paper gives an overview of the research results obtained in 10 years after the accident at Fukushima Daiichi Nuclear Power Plant (FDNPP), focusing on the distribution and dynamics of 137Cs, which is one of major accident-derived radionuclides. Immediately after the FDNPP accident, 8–21 PBq of 137Cs was transported to the ocean mainly due to direct discharge to the ocean, and deposition in the ocean via the atmosphere. 137Cs in seawater traveled eastward on the surface of the North Pacific Ocean along the Kuroshio Extension over a period of several years. Some of 137Cs is also recirculated to the western margin of the subtropical Pacific via the intermediate layer. The concentration of radiocesium in marine organisms also increased immediately after the accident, and then decreased over time. The concentration of radiocesium in brackish and demersal fishes decreased at a slower rate than the concentration of radiocesium in surface-dwelling fish. The amount of 137Cs accumulated on the seafloor is only about 1% (0.2PBq) of the amount carried to the ocean, but it remains in the sediments in the coastal area for a long period of time and gradually migrates to the seawater and ecosystems near the seafloor.