Initial spread of sup137/supCs from the Fukushima Dai-ichi Nuclear Power Plant over the Japan continental shelf: a study using a high-resolution, global-coastal nested ocean model

摘要

The 11 March 2011 tsunami triggered by the M9 and M7.9 earthquakesoff the Tōhoku coast destroyedfacilities at the Fukushima Dai-ichi Nuclear Power Plant (FNPP) leading to asignificant long-term flow of the radionuclide 137Cs into coastalwaters. A high-resolution, global-coastal nested ocean model was firstconstructed to simulate the 11 March tsunami and coastal inundation. Based onthe model's success in reproducing the observed tsunami and coastalinundation, model experiments were then conducted with differing gridresolution to assess the initial spread of 137Cs over the eastern shelfof Japan. The 137Cs was tracked as a conservative tracer (withoutradioactive decay) in the three-dimensional model flow field over the periodof 26 March–31 August 2011. The results clearly show that for the same137Cs discharge, the model-predicted spreading of 137Cs wassensitive not only to model resolution but also the FNPP seawall structure. Acoarse-resolution (∼2 km) model simulation led to an overestimationof lateral diffusion and thus faster dispersion of 137Cs from the coastto the deep ocean, while advective processes played a more significant rolewhen the model resolution at and around the FNPP was refined to ∼5 m.By resolving the pathways from the leaking source to the southern andnorthern discharge canals, the high-resolution model better predicted the137Cs spreading in the inner shelf where in situ measurements were madeat 30 km off the coast. The overestimation of 137Cs concentration nearthe coast is thought to be due to the omission of sedimentation andbiogeochemical processes as well as uncertainties in the amount of 137Csleaking from the source in the model. As a result, a biogeochemical moduleshould be included in the model for more realistic simulations of the fateand spreading of 137Cs in the ocean.