Modélisation mécanique des contraintes induites par l'oxydation en air du Zircaloy-4

Titre de la revue : Materials Science Forum Volume : 595-598 N° : PART 1 Pagination : 419-427 Date de publication : 01/10/2008

In the frame of its research work on nuclear fuel safety, the French "Institut de
Radioprotection et de Sûreté Nucléaire" (IRSN) has highlighted the importance of cladding tube
oxidation on its thermomechanical behavior. The occurrence of radial cracking and spallation has
been observed as the main mechanisms for the zirconia layer degradation during transient
experiments. A study of these two mechanisms has been jointly launched by IRSN and Areva-NP.
Thus laboratory air oxidations of fully recrystallized or stress-relieved low-tin Zircaloy-4 cladding
tubes have been performed. Representative oxide layer thicknesses varying from 10 to 100 0m have
been obtained. SEM micrographs of the obtained oxidised samples show that short circumferential
cracks are periodically distributed in the oxide thickness. For specimens with oxide film thickness
greater than 30 0m, radial cracks are initiated from the outer surface of the oxide layer and
propagated radially. Veins characterised by the lack of circumferentially orientated crack are
evidenced. All these phenomena are mainly linked to high compressive stress levels in the zirconia
layer.
A model describing the stress evolution in the oxide and in the cladding has been developed. This
model takes into account the influence of elasticity, cladding creep, oxide growth and thermal
expansion. Deflection tests data [15] are used to calibrate the oxide growth modelling. The model
enables the evaluation of strain or stress profile in the oxide layer and in the base metal. Numerical
results are in good agreement with a large set of axial and circumferential strains measurements.
Further a better understanding of cracking mechanisms is achieved considering the good agreement
between experimental and numerical analysis.