Modelling of corrosion induced stresses during Zircaloy-4 oxidation in air
Titre du congrès :HTCPM 2008 - 7th International Symposium on High Temperature Corrosion and Protection of Materials
Ville du congrès :Les Embiez
Date du congrès :18/05/2008
In the frame of its research program on nuclear reactor fuel safety, the French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN), in partnership with "Électricité de France" (EDF), has conducted a set of tests dedicated to Reactivity Initiated Accidents (RIA) in the experimental reactor CABRI. The importance of cladding tube oxidation on the thermomechanical behavior has also been revealed. The occurrence of radial cracking and spallation has been observed as the main mechanisms of the oxide degradation. A study of these two mechanisms has been recently launched in IRSN with the support of AREVA NP.
In order to study the cracking mechanisms of the zirconia layer, laboratory autoclave oxidations of fully recrystallized or stress-relieved low-tin Zircaloy 4 cladding tubes have been performed. Several 10cm-length specimens, equipped with two end-caps in order to avoid internal oxidation of the ID samples, have been air-oxidized at 470°C. Oxide layer thicknesses varying from 10 to 90/xm have been obtained.
Oxidation kinetics are consistent with literature data. The results have been combined with existing data to derive a correlation for air-oxidation kinetics applicable for temperatures inclu-ded between 350°C and 500°C. The proposed correlation remains valid for long term oxidation durations (at least 150 days).
SEM micrographs of the obtained oxidized samples show a wavy metal-oxide interface. Moreover, short circumferential cracks are periodically distributed in the oxide thickness. For specimens with oxide scale thickness exceeding about 30 /j,m, radial cracks have been initiated from the free outer surface of the oxide layer and propagated radially. Veins characterised by the lack of circumferentially orientated crack have been 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 expansions. Deflexion tests data have been used to calibrate the oxide growth modelling. The code solves at each time step a system constituted with the mechanical equilibrium and the strain compatibility equations. The code determines strain and stress in the oxide layer and in the base metal. Numerical results are in good agreement with a large set of axial and circumferential strains measurement data.