Arnaud Quérel

Hints to discriminate the choice of wet deposition models applied to an accidental radioactive release

In nuclear emergency management, wet deposition modelling is of crucial importance for correctly evaluating soil contamination after an atmospheric release. Wet deposition is generally divided into two main processes: in-cloud scavenging (rainout) and below-cloud scavenging (washout). The large number of schemes proposed in the literature for both processes reflects the uncertainties in our current understanding of these phenomena. There is presently no scientific consensus to discriminate between the two processes. In order to improve our understanding of the magnitude of modelling uncertainties, a comprehensive sensitivity analysis was performed by focusing on representation of wet deposition fluxes. A large number of model configurations involving different deposition schemes and modelling options were evaluated by comparison with available observations of soil contamination. The objective is to establish a priority rank order of wet deposition schemes for soil contamination modelling.

Using the Wasserstein distance to compare fields of pollutants: application to the radionuclide atmospheric dispersion of the Fukushima-Daiichi accident

The verification of simulations against data and the comparison of model simulation of pollutant fields rely on the critical choice of statistical indicators. Most of the scores are based on point-wise, that is, local, value comparison. Such indicators are impacted by the so-called double penalty effect. Typically, a misplaced blob of pollutants will doubly penalise such a score because it is predicted where it should not be and is not predicted where it should be. The effect is acute in plume simulations where the concentrations gradient can be sharp. A non-local metric that would match concentration fields by displacement would avoid such double penalty. Here, we experiment on such a metric known as the Wasserstein distance, which tells how penalising moving the pollutants is. We give a mathematical introduction to this distance and discuss how it should be adapted to handle fields of pollutants. We develop and optimise an open Python code to compute this distance. The metric is applied to the dispersion of cesium-137 of the Fukushima-Daiichi nuclear power plant accident. We discuss of its application in model-to-model comparison but also in the verification of model simulation against a map of observed deposited cesium-137 over Japan. As hoped for, the Wasserstein distance is less penalising, and yet retains some of the key discriminating properties of the root mean square error indicator.

Study of Aerosol Particle Scavenging by Rain, Experiments and Modelling

The understanding and the anticipation of the environmental fallout in case of severe nuclear accidents with radioactive releases is crucial for the environment. In this study we aim to improve our knowledge on the aerosol particles scavenging, in particular the washout by raindrops with a diameter larger than 1 mm.

Study of Aerosol Scavenging by Rain in Case of a Radioactive Contamination of the Atmosphere

The analysis of the radioactive aerosol scavenging by rain after the Chernobyl accident highlights some differences between the modelling studies and the environmental measurements. Part of this gap is due to the uncertainties on the scavenging efficiency of aerosol particles by raindrops, in particular for drops with a diameter larger than one millimeter. The IRSN (Institut de Radioprotection et de Surete Nucleaire) has decided to launch an experimental study to measure with a better accuracy the scavenging efficiency of large raindrops.The scavenging efficiency of aerosol has been determined by measuring precisely the mass of aerosol particles collected by a single drop after its path through an atmosphere loaded with particles.The collection efficiencies for drop diameters of 2 mm and 2.6 mm (previously unknown for atmospheric aerosols) are measured. The impact of these new data on modeling of the washout of the atmosphere by the rain is noticed.Copyright

Semi-Analytical Study of Aerosol Washout by Rain

The accidents of Chernobyl and Fukushima have shown the necessity to better understand all the mechanisms implied in the scavenging of aerosol particles released to the atmosphere during a nuclear accident.Among all the phenomena involved in the deposition of aerosol particles, we focus here on the aerosol particles scavenging by the raindrops below the clouds, also called washout (as opposed to the rainout, which concerns scavenging inside the clouds). The strategy of IRSN to enhance the knowledge and the modelling of any mechanism involved in the washout of aerosol particles by rain spans from environmental studies, to analytical ones.The semi-analytical approach chosen here is halfway between these two modes of reasoning. A companion paper is also submitted to the conference to present the microphysical approach chosen at IRSN.In order to perform this study, aerosol particles were dispersed in the TOSQAN chamber, which is a large cylindrical enclosure (4.8 m height with 1.5 m internal diameter). The aerosol particles once dispersed, synthetic rains of different kinds (from stratiform to convective rains) can be activated. Finally, the instantaneous spectral scavenging coefficients are determined from the spectral decrease of aerosol particles concentration in the chamber as a function of time.In order to be able to produce synthetic rains representative of any tropospheric events, a special generator has been designed; it is based on a vibro-rotative disk. This generator is able to produce monodispersed rains at the top of the TOSQAN chamber with rainfall rates from 7 to 15 mm/h and drops diameters from 0.5 to 2.5 mm injected at velocities close to their terminal one.During these tests, the spectral aerosol concentration is measured in line with the help of a Welas granulometer. This instrument is based on white light scattering.The results of these experiments highlight the influence of “meteorological” conditions inside the chamber on the washout of the chamber atmosphere, especially when the relative humidity is reaching saturation.Copyright