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CEA (Commissariat à l'Energie Atomique) develops several fission gas models with different levels of description (more or less detailed or “mechanistic” descriptions). After a synthesis of the main phenomena which have to be considered in a fission gas model, the MARGARET model is presented. Up to five cavities populations (bubbles nucleated during irradiation or fabrication pores) are taken into account, in addition to the dissolved intra- and intergranular gases. The HBS (High Burnup Structure) formation is also considered. The modeling leads to an ordinary differential equations system (typically of 20 non linear coupled equations), which is solved by the Gear method. The computation time remains reasonably short, which allows validation on the whole set of measurements available, integral ones, as well as local ones.