Modeling of Spent Fuel Oxidation at Low Temperature
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0985-NN05-04
Modeling of Spent Fuel Oxidation at Low Temperature Arnaud Poulesquen1, Lionel Desgranges2, and Cécile Ferry1 1 Department of Physico Chemistry, CEA, Gif sur Yvette, 91191, France 2 Department of spent fuel studies, CEA, St Paul lez Durance, 13108, France
ABSTRACT During dry storage, the oxidation of the spent fuel in case of cladding and container failure (accidental scenario) could be detrimental for further handling of the spent fuel rod and for the safety of the facilities. Depending on whether the uranium dioxide is under the form of powder or pellet, irradiated or unirradiated, the weight gain curves do not present the same shape. To account for these different behaviours, two models have been developed. Firstly, the oxidation of unirradiated powders has been modelled based on the coexistence, during the oxidation, of two intermediate products, U4O9 and U3O7. The comparison between the calculation and the literature data is good in terms of weight gain curves and chemical diffusion coefficient of oxygen within the two phases. Secondly, the oxidation of spent fuel fragments is approached by a convolution procedure between a grain oxidation model and an empirical parameter which represents the linear oxidation speed of grain boundary or an average distance able to cover the entire spent fuel fragment. This procedure of calculation allows in one hand to account for the incubation period noticed on unirradiated pellets or spent fuel and in another hand to link the empirical parameter to physical as porosity, cracks or linear power, or operational parameters such as fission gas release (FGR) respectively. A comparison of this new modelling with experimental data will be proposed. INTRODUCTION In dry air storage conditions, the main accidental scenario considered is an airtightness defect in the fuel cladding and container involving the exposition of the fuel surface to the air. Because of the residual power provided by the radioactive decay of the fission products, the spent fuel keeps a temperature significantly higher than the ambient. Under these conditions, the rapid transformation of UO2 to U3O8 implies a relative increase of volume of approximately 36%. The resultant swelling is likely to increase the fuel exposure by splitting the cladding and forming U3O8 powder. The release of radionuclides from the waste package should be therefore emphasised due to this increase of the specific surface area. Depending on whether the uranium dioxide is under the form of powder or sintered pellet, irradiated (spent) fuel or unirradiated fuel, the weight gain curves do not present the same shape. For unirradiated powders, a parabolic shape is noticed at the beginning of oxidation whereas a sigmoidal shape is observed for sintered unirradiated pellet with an incubation period depending on the oxidation temperature [1, 2]. Recently, oxidation experiments in air at 200°C have been performed on high burn-up UOX and MOX and shown a sigmoidal shape instead of parabolic one as reported in literature [3].
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