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

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.