Jean Desquines

Modelling of Corrosion Induced Stresses during Zircaloy-4 Oxidation in Air

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.

Influence of Outer Zirconia Transient Cracking and Spalling on Thermomechanical Behaviour of High Burnup Fuel Rod Submitted to RIA

The French “Institut de Radioprotection et de Sîreté Nucléaire” (IRSN) conducted the REP-Na tests in the CABRI reactor within the framework of its research program on nuclear fuel safety. These tests were devoted to the study of Reactivity Initiated Accident (RIA). Cracking and spalling of the fuel rod zirconia layer were observed after several REP-Na tests. Sometimes, an ovalisation of the rod after the RIA transient is also observed. Metallographic examinations showed that the outer and inner zirconia cracks are regularly spaced and that the crack density is linked to the clad plastic hoop strain. An analogy with brittle thin film layering a ductile substrate submitted to a tensile test is made and helps to understand this specific phenomenon. A numerical simulation evaluates the thermo-mechanical behaviour of the rod, including the zirconia influence during a RIA. This work make it possible both to identify the spalling process and to clarify the preferential spalling along the less corroded azimuths for several tests. The influence of the transient spalling on the boiling crisis occurrence in PWR condition is finally addressed.

Coupled Experimental/Numerical Approach to Determine the Creep Behavior of Zr-4 Cladding Under LOCA Condition

The thermo-mechanical behavior of Zircaloy-4 fuel rods under Loss-Of-Coolant Accident (LOCA) conditions is investigated. A custom experimental setup is dedicated to the high-temperature creep ballooning study of 90mm long cladding samples. Creep tests were performed under an inert environment (argon), for temperatures from 750 to 850°C and internal pressures ranging from 1 to 5 MPa. As the high-temperature creep of metals is strongly influenced by the temperature, the setup allow for a heterogeneous thermal distribution along the specimen. A unique test provides a rich database about the steady-state creep of the alloy. A first campaign is dedicated to bare Stress Relieved Annealed Zr-4.