Articles | Volume 10, issue 8
Biogeosciences, 10, 5439–5449, 2013

Special issue: Impacts of the Fukushima nuclear power plant discharges on...

Biogeosciences, 10, 5439–5449, 2013

Research article 14 Aug 2013

Research article | 14 Aug 2013

Initial spread of 137Cs from the Fukushima Dai-ichi Nuclear Power Plant over the Japan continental shelf: a study using a high-resolution, global-coastal nested ocean model

Z. Lai1,2,8, C. Chen2,7, R. Beardsley3, H. Lin2,7, R. Ji4,7, J. Sasaki5, and J. Lin6 Z. Lai et al.
  • 1School of Marine Sciences, Sun Yat-Sen University, Guangzhou, 510275, China
  • 2School for Marine Science and Technology, University of Massachusetts-Dartmouth, New Bedford, MA 02744, USA
  • 3Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
  • 4Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
  • 5Department of Socio-Cultural Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha, Kashiwa 277-8563, Japan
  • 6Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
  • 7International Center for Marine Studies, Shanghai Ocean University, Shanghai, 201306, China
  • 8Key Laboratory of Marine Resources and Coastal Engineering in Guangdong Province, Guangzhou, 510275, China

Abstract. The 11 March 2011 tsunami triggered by the M9 and M7.9 earthquakes off the Tōhoku coast destroyed facilities at the Fukushima Dai-ichi Nuclear Power Plant (FNPP) leading to a significant long-term flow of the radionuclide 137Cs into coastal waters. A high-resolution, global-coastal nested ocean model was first constructed to simulate the 11 March tsunami and coastal inundation. Based on the model's success in reproducing the observed tsunami and coastal inundation, model experiments were then conducted with differing grid resolution to assess the initial spread of 137Cs over the eastern shelf of Japan. The 137Cs was tracked as a conservative tracer (without radioactive decay) in the three-dimensional model flow field over the period of 26 March–31 August 2011. The results clearly show that for the same 137Cs discharge, the model-predicted spreading of 137Cs was sensitive not only to model resolution but also the FNPP seawall structure. A coarse-resolution (∼2 km) model simulation led to an overestimation of lateral diffusion and thus faster dispersion of 137Cs from the coast to the deep ocean, while advective processes played a more significant role when the model resolution at and around the FNPP was refined to ∼5 m. By resolving the pathways from the leaking source to the southern and northern discharge canals, the high-resolution model better predicted the 137Cs spreading in the inner shelf where in situ measurements were made at 30 km off the coast. The overestimation of 137Cs concentration near the coast is thought to be due to the omission of sedimentation and biogeochemical processes as well as uncertainties in the amount of 137Cs leaking from the source in the model. As a result, a biogeochemical module should be included in the model for more realistic simulations of the fate and spreading of 137Cs in the ocean.

Final-revised paper