¹³⁷Cs off Fukushima Dai-ichi, Japan – model based estimates of dilution and fate

Abstract. 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 Dai-ichi, 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 m² s⁻¹. The cross-shelf diffusivity is at 500 ± 300 m² s⁻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 mBq l⁻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 Bq l⁻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.