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Theses

Modélisation numérique et assimilation de données de la dispersion de radionucléides en champ proche et à l'échelle continentale

Abstract : Atmospheric dispersion models are used to forecast consequences of accidental releases of radioactivity. Models are based on either analytical, as in the case of pX 0.1, or numerical, as for example in Polair3D, solutions to the advection-diffusion equation. Main drawbacks affecting the quality of a forecast are a limited resolution of input meteorological fields and an incomplete knowledge of depletion processes. An investigation of how those drawbacks could be overcome thanks to coupling the models to measurements in data assimilation procedure is presented in this thesis. Model-to-data comparisons have been performed in the first place. The measurements employed in a validation procedure come either from passive tracer experiments or from accidental releases of radionuclides. A wind tunnel experiment for a near-field dispersion on a scale model of Bugey nuclear power station and continental scale ETEX-I experiment as well as the accidental releases in Chernobyl and Algeciras have been studied here. Secondly, measurements have been coupled to the models in order to improve an evaluation of consequences of accidental releases and to invert their sources, should model performance turn out to be sufficiently good. The standard variational method has been used for a near-field dispersion problem. An adjoint of pX 0.1 has been constructed with the help of an automatic differentiator, and the wind tunnel measurements have been assimilated. Capacity of finding the true source emission rate has been tackled in the first place. Next, data assimilation has been employed to accommodate the modelled and measured concentration fields. The required result has been achieved by optimising the parameters which drive the spatial distribution of a plume. Benefit from data assimilation is subject to the quantity of information contained in the measurements. Therefore, posterior analysis of the monitoring network has been carried out and the possibilities of size reduction in view of operational applications have been addressed. At continental scale, despite working exclusively with the grid boxes containing nuclear installations, the space spanned by sources is of significantly greater dimension than the one spanned by observations. Hence, the reconstruction of source position and its emission profile is an ill-posed problem. Its regularisation has been tackled with the maximum entropy on the mean principle. New cost functions, which embody spatial and temporal confinement of an accidental source, have been built. A source-measurement link has been provided by the adjoint of Polair3D. The method has been tested with both perfect and noisy synthetic measurements. A series of sensitivity studies have been conducted and the quality of reconstruction assessed with an objective indicator. An algorithm meant for operational usage which allows to reduce the number of suspected sites has also been tested. Finally, the results of inversion of the temporal profile of the Algeciras accidental source have been shown.
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Monika Krysta. Modélisation numérique et assimilation de données de la dispersion de radionucléides en champ proche et à l'échelle continentale. Modélisation et simulation. Université Paris XII Val de Marne, 2006. Français. ⟨tel-00652840⟩

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