Search

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 hydrodynamical 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 Cs and 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. [ABSTRACT FROM AUTHOR]

Abstract: Gaussian plume models, which are widely used to model atmospheric dispersion, provide an exact analytical solution for line sources such as roads only when the wind direction is perpendicular to the road. Some approximations have been developed to provide an analytical formula for a line source when the wind direction is not perpendicular to the road; however, such formulas lead to some error and the solution diverges when the wind direction is parallel to the road. A novel approach that reduces the error in the line source formula when the wind direction is not perpendicular to the road is presented here. Furthermore, a combination of analytical and numerical line source solutions is used to better represent cases where the wind direction becomes parallel to the road. The improved model was implemented in the Polyphemus modeling platform and it was successfully evaluated against a reference solution as well as observations obtained near a roadway in eastern France. [Copyright &y& Elsevier]