¹³⁷Cs off Fukushima Dai-ichi, Japan. Model based estimates of dilution and fate

In the aftermath of an earthquake and tsunami on 11 March 2011 radioactive ¹³⁷Cs was discharged from a damaged nuclear power plant to the sea off Fukushima Daiichi, Japan. Here we explore its dilution and fate with a state-of-the-art global ocean general circulation model, which is eddy-resolving in the region of interest. We find apparent consistency between our simulated circulation, estimates of ¹³⁷Cs discharged ranging from 0.94 p Bq (Japanese Government, 2011) to 3.5±0.7 p Bq (Tsumune et al., 2012), and measurements by Japanese authorities and the power plant operator. In contrast, our simulations are apparently inconsistent with the high 27±15 p Bq discharge estimate of Bailly du Bois et al. (2012). Expressed in terms of a diffusivity we diagnose, from our simulations, an initial dilution on the shelf of 60 to 100 m²s^-1. The cross-shelf diffusivity is at 500±300 m²s^-1 significantly higher and variable in time as indicated by its uncertainty. Expressed as an effective residence time of surface water on the shelf, the latter estimate transfers to 43±16 days. As regards the fate of ¹³⁷Cs, our simulations suggest that activities up to 4 m Bql^-1 prevail in the Kuroshio-Oyashio Interfrontal Zone one year after the accident. This allows for low but detectable 0.1 to 0.3 m Bql^-1 entering the North Pacific Intermediate Water before the ¹³⁷Cs signal is flushed away. The latter estimates concern the direct release to the sea only.