Behaviour of oceanic 137Cs following the Fukushima Daiichi Nuclear Power Plant accident for four years simulated numerically by a regional ocean model

Abstract:

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean by two major pathways, direct release from the accident site and atmospheric deposition.We reconstructed spatiotemporal variability of ¹³⁷Cs activity in the regional ocean for four years by numerical model, such as a regional scale and the North Pacific scale oceanic dispersion models, an atmospheric transport model, a sediment transport model, a dynamic biological compartment model for marine biota and river runoff model. Direct release rate of ¹³⁷Cs were estimated for four years after the accident by comparing simulated results and observed activities very close to the site. The estimated total amounts of directly release was 3.6±0.7 PBq. Directly release rate of ¹³⁷Cs decreased exponentially with time by the end of December 2012 and then, was almost constant. Decrease rate were quite small after 2013. The daily release rate of ¹³⁷Cs was estimated to be the order of magnitude of 10¹⁰ Bq/day by the end of March 2015. The activity of directly released ¹³⁷Cs was detectable only in the coastal zone after December 2012. Simulated ¹³⁷Cs activities attributable to direct release were in good agreement with observed activities, a result that implies the estimated direct release rate was reasonable. There is no observed data of ¹³⁷Cs activity in the ocean from 11 to 21 March 2011. Observed data of marine biota should reflect the history of ¹³⁷Cs activity in this early period. We reconstructed the history of ¹³⁷Cs activity in this early period by considering atmospheric deposition, river input, rain water runoff from the 1F NPP site. The comparisons between simulated ¹³⁷Cs activity of marine biota by a dynamic biological compartment and observed data also suggest that simulated ¹³⁷Cs activity attributable to atmospheric deposition was underestimated in this early period. The simulated river flux of ¹³⁷Cs to the ocean did not effect on ¹³⁷Cs activity in the ocean even if the parameters in this simulation have uncertainties because of the lack of observed data in rivers in the earlier period.