RÉSUMÉABSTRACT Polonium 210, an important naturai reference for radiological protection. Natural polonium 210 which has a relatively short decay period (138 days) and is uranium decay serie daughter should retain more radiological attention because it induces chronic doses higher than that of traces of caesium 137 and plutonium isotopes (238,239,240) in the environment. In the earth's cmst, uranium is present together with radium, radon and its daughter products, at a concentration of about 40,000 Bq t-'. As a consequence of radon 222 exhalation from soils within a metric layer and of lead 210 (radon daughter) falldown, there is an enrichmeut of polonium 210 radioactivity in top layer soi1 relatively to radium concentration, within a factor varying from 2.7 to 8.8 according to environmenial measurements. Human exposure to polonium 210 by ingestion is not negligible. The induced exposure by ingestion of natural polonium 210 is 30 to 70 times higher than that of caesjum from Chernobyl falldown at Helsinki in 1986. Moreover, the latter decreases in relation with the 30 years' decay period while that from radon daughters remains. Comparison with plutonium shows a 500 to 2, OOO times stronger health effect by ingestion for the same number of deposited becquerels on soil. "Hot spots" may occur in the environment: polonium 210 is particularly concentrated by marine fauna, as shown by several measurement programs over the area of north-east Atlantic.' COG'EMA, 2 rue Paul DaUrie6 BP 4, 78/41 VJlizy Cedex, I.'ron
Abstract. To refine assessment of radionuclides concentrations in me environment in the very near field at the La Hague site, COGEMA has decided to develop a dedicated 2D dispersion model. This simplified model aims to provide accurate assessment for the main meteorological situations while assuring a computer answer in a limited time (objective of 20 to 30 minutes on a basic computer for a line mesh). The model has been developed by the University of Paris VI (Numerical Analysis Laboratory) which has provided the software. In order to test and calibrate this research tool, a comparison program with the 3D MERCURE software on academic cases has been launched. Together, a comparison between field measurements of krypton 85, picked up at four monitoring points around the site, and model outputs, has been planned. At the same time, the creation of a user-friendly interface is to be performed to enrich the software, and to facilitate sets of runs for various assumptions (involving model complexity, mesh refinement, meteorological episodes complexity) according to the needs. MOTIVATION AND MODEL ORIENTATIONNumerous models have been worked out to assess pollutant dispersion in the environment. The first models were based on the analytical resolution of a Gaussian dispersion equation, (from a few kilometres to about 20 kilometres). These simplified models are well adapted to flat terrains (Doury, Pasquill for examples).More accurate and complex 3D numerical models have been recently developed to fit better with pollutant concentrations in complex canyon situations. These 3D numerical models have also been developed for regional climate simulation including wind field calculations in different meteorological situations. These codes are usually time consuming, depending on the refinement of the mesh and of the meteorological conditions. By 1999, in the aim of improving the radionuclides concentration assessment in the near field of the La Hague reprocessing site, COGEMA initiated a cooperation with the University of Paris VI, Numerical Analysis Laboratory, to start developing a new model. Considering existing models, the goal was to find an intermediary between Gaussian models and 3D codes, i.e. a compromise between accuracy, speed, robustness and simplicity of use.To gain in computation time, it has appeared acceptable to select a 2D model, given the site characteristics. The dispersion is evaluated in a vertical plane by the solver of the software, supplemented with a Gaussian lateral loss of material, in order to have finally a real transcription of a 3D dispersion. Landscape and buildings are modelled, in order to take into account turbulence due to ground roughness and eddies behind buildings. Moreover, thermal effects are included, through Boussinesq hypothesis (buoyancy of the atmosphere). All these modules can be activated separately.The objective would be to provide results in 20 to 30 minutes on a standard PC, in order to facilitate sets of runs for various assumptions.This 2D model software, called HAG2D, is p...
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