Oxidation kinetics of high burn-up UOX and MOX fuel : the influence of the grain boundaries
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Oxidation kinetics of high burn-up UOX and MOX fuel : the influence of the grain boundaries Lionel Desgranges1, Gurvan Rousseau1, Marie-Pierre Ferroud-Plattet1, Cécile Ferry2, Hélène Giaccalone1, Isabelle Aubrun1, Pierre Delion1, Jean-Michel Untrau1 1 CEA DEC/SA3C C.E. Cadarache 13108 Saint-Paul lez Durance France 2 CEA DPC CEA Saclay, BP 11, 91191 Gif sur Yvette cedex ABSTRACT In a dry air storage facility, the oxidation of irradiated nuclear fuel is considered as the main risk induced by a leakage that would put the nuclear ceramics in contact with air. The oxidation of UO2 in U3O8 leads indeed to a relative volume swelling of more than 30% which could cause the ruin of the fuel rod. In order to fill the lack of data on high burn-up fuel, some oxidation experiments are currently conducted in CEA with UO2 and MOX fuel. Fragments as well as slices of fuel rod, with a high burn-up, have been oxidized in air at 200°C for more than 4000 hours. The associated weight gain curves are presented together with the characterization of some samples picked up at different oxidation times. It is shown that the weight gain curve of high burn-up UOX is not parabolic-shaped, as predicted in literature, but sigmoid-shaped. Moreover it is also shown that the grain boundary oxidation is not uniform inside a fragment. This leads to a new description of the oxidation mechanism of irradiated fuel.
INTRODUCTION In dry air storage facility, the oxidation of irradiated nuclear fuel is considered as the main risk induced by a leakage that would put the nuclear ceramics in contact with air. The oxidation of UO2 in U3O8 leads indeed to a relative volume swelling of more than 30% which could cause the ruin of the fuel rod. Although a huge number of data are available in literature dealing with the oxidation kinetic of un-irradiated UO2 and low burn-up irradiated UOX, only a few data are available concerning high burn-up UOX and MOX fuel which might be (interim) stored. In order to fill this lack of data, CEA and EdF perform in the framework of the PRECCI project specific researches aimed at describing the oxidation kinetic of used nuclear fuel [1]. These studies include experiments on both unirradiated and irradiated nuclear fuel and model is currently under construction to describe the weight gain curve measured in these experiments. In this paper we shall focus on experimental results obtained with irradiated nuclear fuel. In a previous paper [2] the preliminary results of an oxidation experiment in air at 200°C with UOX and MOX fuel were presented and fitted with a model derived from the literature. Since then the experiment continued and a new one with higher burn-up fuels started. Today both experiments have been accumulating more than 4000 hours of oxidation and evidence significant differences which are discussed in the following.
EXPERIMENT ON IRRADIATED FUEL Two sets of experiment were performed with the same method but with different fuels. In each experiment, several fragments of UO2 and MOX fuel were oxidised simultaneously in th
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