Abstract
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.
Highlights
The results clearly show that for the same 137Cs discharge, the model-predicted spreading of 137Cs was sensitive to model resolution and the Fukushima Dai-ichi Nuclear Power Plant (FNPP) seawall structure
In our high-resolution model case, which direction and how much 137Cs flowed to the 1F-N and 1F-S sites outside the breakwaters depended on whether or not the model was capable of resolving the local water flushing processes around FNPP
Our results indicate that an underestimation of 137Cs concentration over the shelf predicted by Global-FVCOM was mainly due to the overestimation of 137Cs spreading in the coastal region
Summary
The results clearly show that for the same 137Cs discharge, the model-predicted spreading of 137Cs was sensitive to model resolution and the FNPP seawall structure. Nuclear Power Plant (FNPP) was seriously damaged, resulting in the leaking of large amounts of artificial radionuclides, mainly 131I (t1/2 = 8.02 days), 134Cs (t1/2 = 2.065 yr) and 137Cs (t1/2 = 30.17 yr), from several reactor units into the coastal ocean (Ohnishi, 2012).
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