A new criterion for the degradation of a defective spent fuel rod under dry storage conditions based on nuclear ceramic
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1124-Q02-06
A new criterion for the degradation of a defective spent fuel rod under dry storage conditions based on nuclear ceramic cracking L. Desgranges1, F. Charollais1, I. Felines1, C. Ferry2, J. Radwan2 1 2
CEA, DEN, DEC F-13108 Saint-Paul lez Durance, France CEA, DEN, DPC F-91191 Gif sur Yvette, France
ABSTRACT Experimental results using environmental SEM on intentionally defected fuel particles showed that oxidation induced cracking could lead to the degradation of HTR coated particles. The interpretation proposed for the swelling resulting from cracking can be extended to irradiated nuclear fuels. That is why a new criterion was proposed to defined safe handling of defective fuel in dry storage condition. This criterion defines the time needed to create an oxidized layer thickness leading to significant cracking.
INTRODUCTION An accident scenario for nuclear spent fuel dry storage consists in cask and fuel rod simultaneous failures that will put nuclear ceramic, mostly made of uranium dioxide, in contact with air. As the temperature expected during the first 100 years of dry storage lies in the range 100-300°C, the nuclear ceramic will be oxidised, leading to the transformation of UO2 into U3O8. Experimental simulation of this accidental scenario showed evidence that the swelling of the ceramic induced by oxidation could lead to the formation of cracks in the cladding and to the ruin of the spent fuel rod. Up to now the swelling of UO2 was assigned to the formation of U3O8, which has a molar volume 36% higher than the one of UO2. That is why previous criteria for safe behaviour of defected fuel rod in contact with air took into account U3O8 molar fraction. The fraction of U3O8 was deduced from analysis of weight gain curves measured during the isothermal oxidation of UO2 un-irradiated powders [1]. The oxidation of UO2 powders is usually partitioned in two stages: the first one is associated to a pseudo parabolic weight gain curve while the second one is associated to a sigmoid weight gain curve. The pseudo-parabolic curve, attributed to the formation of U4O9 and U3O7 on the surface of UO2 powders, indicates a diffusion-controlled mechanism [2], for the modelling of which a finite difference algorithm [3] was recently developed. The sigmoid curve is generally interpreted as the oxidation of U3O7 into U3O8 with a nucleation and growth mechanism [4]. In the case of an un-irradiated fuel pellet, the oxidation process is also characterised by the formation of cracks. Bae showed that two types of cracking are observed [5]. First macro-cracks, associated to intermediate oxide (U4O9, U3O7) formation, occurred at the grain boundaries. It is followed by micro-cracking corresponding to U3O8 formation starting at cracked surfaces, which only enhances spallation. Used nuclear fuel is made of pellets and oxidation induced cracking also has to be taken into account in the safety assessment of an accident scenario in a dry storage facility. Because
macrocracking occurs before U3O8 formation as shown by Bae and confirm
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