Comparison between modelling and measurement of marine dispersion, environmental half-time and 137Cs inventories after the Fukushima Daiichi accident

Pascal Bailly Du Bois (1) , Pierre Garreau (2) , Philippe Laguionie (3) , Irène Korsakissok

1 IRSN - Institut de Radioprotection et de Sûreté Nucléaire
2 DYNECO - Dynamiques des Écosystèmes Côtiers
3 GR - Géosciences Rennes

Contamination of the marine environment following the accident at the Fukushima Daiichi nuclear power plant (FDNPP) represents the most important influx of artificial radioactivity released into the sea ever recorded. The evaluation , in near real time, of the total amount of radionuclide released at sea and of the residence time in coastal waters were ones of challenges for nuclear authorities during this event. In the framework of a crisis situation, a numerical hydrodynam-ical model has been built and used 'as is'. The concomitant use of this numerical model and in situ data allows the comparison of the simulated and measured environmental half-times. A tuning of the wind drag coefficient has been nevertheless necessary to reproduce the evolution of measured inventories of 137 Cs and 134 Cs between April and June 2011. After tuning, the relative mean absolute error between measured and simulated concentrations for the 849 measurements in the dataset is 69 %, while the relative bias indicates a model underestimation of 4 %. These results confirm the estimates of the source term, i.e. 27 PBq (12-41 PBq) for direct releases and 3 PBq for atmospheric deposition onto the sea. The parameters applied here to simulate atmospheric deposition onto the sea are within the correct order of magnitude for reproducing seawater concentrations. Quantitative inventories of tracers which integrate dispersion and transport processes are useful to test model reliability. It exhausts the model sensibility to meteorological forcing, which remains difficult to appraise to reproduce mid-to long-term transport.

Keywords: radionuclide; Pacific Ocean Ocean Dynamics; coastal modelling; Fukushima accident; Caesium 137; environmental half-time

Domains: Sciences of the Universe [physics]; Earth Sciences; Oceanography; Environmental Sciences; Biodiversity and Ecology

Pascal Bailly Du Bois, Pierre Garreau, Philippe Laguionie, Irène Korsakissok. Comparison between modelling and measurement of marine dispersion, environmental half-time and 137Cs inventories after the Fukushima Daiichi accident. Ocean Dynamics, 2014, 64 (3), pp.361-383. ⟨10.1007/s10236-013-0682-5⟩. ⟨hal-02433219⟩