Model developments to estimate movements of radioactive cesium with ocean sediment
after the Fukushima Dai-ichi nuclear power plant accident.
Shin-ichi Ito1,2, Kazuhiro Aoki3, Hiroshi Kuroda4, Takashi Setou3, Kazuhiro Takeuchi5, Daisuke Hasegawa1,
Hideki Kaeriyama3, Ambe Daisuke3, Tsuneo Ono3, Shigeho Kakehi1, Hiroshi Yagi6, Kouichi Sugimatsu6 and
Akiyoshi Nakayama6
1
Tohoku National Fisheries Research Institute, FRA, Shiogama, Japan. E-mail: goito@aori.u-tokyo.ac.jp
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan
3
National Research Institute of Fisheries Science, FRA, Yokohama, Japan
4
Hokkaido National Fisheries Research Institut, FRA, Kushiro, Japan
5
Institute of Environmental Informatics, IDEA Consultants Inc., Yokohama, Japan
6
National Research Institute of Fisheries Engineering, FRA, Kamisu, Japan
2
Radiocesium (Cs: Cs134+137) due to the Fukushima Dai-ichi Nuclear Power Plant accident was released into
the ocean by three pathways: 1) direct release of high Cs liquid wastes to the ocean after the accident (mainly
until April 2011), 2) the deposition of airborne Cs to the ocean surface, and 3) riverine Cs discharge originated
by the deposition of airborne Cs on the land. The Cs concentration in seawater has been decreased rapidly, but
the Cs concentration in the ocean sediment is still high in partial area of coastal region. To investigate the
movements of Cs137 in the ocean sediments, an ocean circulation-wave-sediment coupled model has been
developed and Cs137 was incorporated. Both the ocean circulation and wave models were downscaled from the
North Pacific model (one degree) to the coastal model (1/90 degree) through the northwestern Pacific model
(1/10 degree). Observations of bottom boundary layers were also conducted to estimate model parameters and
validate the model simulations. After several case studies, the horizontal distribution of median particle diameter
of ocean sediment and the horizontal transport of suspended substances in the coastal region were reasonably
hindcasted. Two scenarios were considered: 1) Cs absorption by ocean sediments from high Cs waters and 2)
discharge of Cs absorbed particles from the Abukuma River. The simulation results revealed that the riverine
effects is limited and the major distribution of Cs in the ocean sediment was determined by the high Cs water
pathways after the accident and the medial particle diameter of the ocean sediment.