Secondary creep behavior of Zr-4 claddings under LOCA conditions
The thermo-mechanical behavior of Zircaloy-4 fuel rods under Loss-Of-Coolant Accident (LOCA) conditions in water cooled reactors is investigated. A custom experimental setup is dedicated to the high-temperature creep ballooning study of 90 mm long cladding samples. Creep tests were performed under an inert environment (argon), for temperatures from 750 to 850 C. degrees and internal pressures ranging from 1 to 5 MPa. During its operating life, Zr-4 cladding is submitted to oxidation and hydriding. These parameters strongly influence the creep behavior of Zirconium alloys at high temperatures. A first campaign on as-received Zr-4 was performed. Creep-rates are computed using 2-Dimensional Digital Image Correlation (2D-DIC) and are correlated to local temperatures measured using Near Infra-Red thermography. As-received Zr-4 behavior laws are determined using Finite Element Model Updating (FEMU). Norton exponents and activation energies are determined for thermal-mechanical conditions of the performed tests. The results show that the effect of applied stress on the Norton exponent is evidenced in the study for both α-phase and (α + β)-phase domains. The determined stress exponent magnitudes are very consistent with literature data depending on the thermomechanical test conditions. The activation energy parameter is very sensitive to the thermo-mechanical conditions and remains valid within a limited temperature and stress ranges to the authors' knowledge. The beginning of the (α + β)-phase domain is clearly identified with the activation energy increase. Finally, steady-state creep is accurately modeled depending on ballooning thermo-mechanical conditions.