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This paper deals with the irradiation creep behavior of Zr-base alloys, both under normal operating and power transient conditions. The first part is devoted to the creep experiments, which were conducted in the OSIRIS or SILOE metallurgical test reactors using argon internally pressurized cladding tubes to simulate the normal operating conditions in PWRs. The studied materials are the following Zr-base alloys: stress-relieved (SRA) and fully recrystallized (RXA) low tin Zircaloy-4, the RXA Zr-SnFeV M4 alloy and the FRAMATOME-ANP Zr-l%NbO (M5™). The effects of the alloy composition and metallurgical condition, temperature (280 to 380°C), stress level (0 to 120 MPa), and fast neutron flux (1 to 2 × 1018 n/m2s, E > 1 MeV) on the irradiation creep kinetics are discussed. These irradiation creep results confirm the lower creep rate of RXA Zy-4, M4, and M5™ with respect to SRA Zy-4. In the second part, this paper relates to the clad behavior under pellet-cladding interaction conditions. To this end, a specific irradiation device operated at a nominal temperature of 350°C is used to strain a cladding tube sample at high stress level (about 400 MPa by internal pressure) and under neutron flux (2 × 1018 n/m2s, E > 1 MeV). The diameter elongation is measured online versus time. The results are compared to those obtained during out-of-pile creep tests on irradiated specimens using similar temperature and stress conditions. This experiment allows discussion of the contribution of irradiation-induced creep with respect to the thermal creep at high stress levels relevant to PCI conditions.

Thin tensile specimens of alloy 718 were oxidized under synthetic air, at 1000°C for different durations, then heat treated according to an aeronautical heat treatment (720°C-8h, 620°C-8h) before being tested on a tensile machine at room temperature. Mechanical tests were performed on sufficiently thin specimens (thickness less than 0.3 mm and a gage length equal to 200 mm) in order to highlight the influence of the damaged zone upon the global tensile behavior. It was shown that the mechanical behavior remained unaffected in terms of the flow law but a significant drop in flow stress was seen in the case of oxidized specimens. This drop was at first attributed to a reduction of the loaded section, but this reduction should then be 30 to 40% greater than the intergranular oxidation area measured by SEM or SIMS. To explain this discrepancy, the hypothesis of oxygen diffusion ahead of the intergranular oxidation front was proposed.In order to check this hypothesis, a new method based on SIMS analysis was used. Elemental concentration profiles together with imaging were obtained following a specifically designed method that allowed the sputtering from the metal to the oxide. This analysis showed that Al 2 O 3 forms at the intergranular oxidation front and that no measurable dissolved oxygen is found in the alloy ahead of this front, thus invalidating the proposed hypothesis. Tensile tests on a specimen treated under vacuum at 1000°C suggest that high-temperature heat treatment is responsible for half of the drop in mechanical properties. Finally, comparing the tensile curves of thin specimens of alloy 718 heat treated at high temperature to a standard curve is an accurate method to quantify the damages on this alloy due to the treatment.

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