No abstract

Thermomechanical processing of Zircaloy-4 cladding plays a major role in determining its deformation behavior. Crystallographic texture and the related anisotropy in mechanical properties of Zircaloy-4 have been shown to be affected by different processing paths. In this program, the deformation behavior of four Zircaloy-4 cladding types was evaluated in laboratory and in-reactor studies under typical pressurized-water reactor (PWR) conditions. In particular, the creep behavior, stress-free growth, and mechanical property changes of these materials were examined. The irradiation program consisted of four specimen cluster assemblies, each containing 16 rods, irradiated in guide tube locations of fuel assemblies in the Oconee-2 reactor. Twelve of the 16 rods in each cluster were prepressurized so that nominal inreactor compressive hoop stresses of 69.0, 86.2, and 103.4 MPa (10, 12.5, and 15 ksi) were maintained. The remaining four rods were open to the coolant and served as stress-free irradiation growth specimens. One specimen cluster was examined after each of four reactor cycles. Local fast neutron fluences ranged up to 1.3 × 1022 n/cm2 (E > 0.1 MeV), while irradiation temperatures ranged from 569 to 591 K (565 to 605°F). Post-irradiation tension and burst tests were conducted at 616 K (650°F) to determine changes in the mechanical properties as a function of fluence. The ex-reactor thermal creep behavior was examined at 672 K (750°F) in a series of multiaxial creep tests where the hoop/axial stress ratio was held constant at several different values. These data were used to construct a partial creep locus at a constant value of the dissipative work function. Results indicate that the in-reactor axial creep and stress-free growth are highly dependent on the cladding fabrication schedules. Most of the axial strain exhibited by the cold-worked, stress-relieved cladding types S1, V1, and V2, consists of irradiation growth, while in the recrystallized type S2, axial creep predominates. In-reactor diametral creep strain exhibited an initial transient regime at fast neutron fluences below 3 to 5 × 1021n/cm2 (E > 0.1 MeV) with a creep rate decreasing to a steady state at higher fluences. In contrast to axial creep behavior, the cladding with the highest level of retained cold work (S1) exhibited the highest diametral creepdown while recrystallized type S2 showed the least creepdown of the four cladding types. The hoop/axial strain rate ratio, εθ/εz, remained essentially constant over the fluence and stress ranges investigated, but its magnitude was significantly different between the S1 and S2 cladding types. A comparison between the in-reactor deformation and ex-reactor thermal creep behavior shows that the relative ranking of the materials in creep resistance is consistent in both environments. An analysis of the anisotropy of in-reactor deformation and a comparison with the thermal creep anisotropy are also provided. Post-irradiation axial tension and burst tests at 616 K (650°F) indicated that the most cold-worked material, S1, maintained the highest strength. In axial tension, it also retained the highest uniform and total elongations. In the burst tests, the more recrystallized cladding types. S2 and V2, exhibited the highest total circumferential elongations. Irradiation-induced changes in mechanical properties were significantly greater in the recrystallized cladding material.

The influence of tin content on the unirradiated mechanical properties of zirconium alloys within the Zircaloy-4 specification has been analyzed. A cold-worked, stress-relieved (CWSR) commercial cladding was produced using similar thermo-mechanical processing encompassing tin contents ranging from 1.22 to 1.54%. Thermal creep testing, tension testing, contractile strain ratio (CSR) testing, and texture analysis were preformed over the range of tin contents. Creep testing was performed at 399°C (750°F), over a series of stress ratios and stress levels. The creep strength of Zircaloy-4 was strongly dependent on the tin content over the allowable composition range. The effect of tin was dependent on the stress state, with the 0:1 stress ratio (hoop stress:axial stress ratio) and 1:1 stress ratio showing greater effects than 2:1 stress ratio. The steady state strain rates increased by factors of 6, 5, and 2 for the 0:1, 1:1, and 2:1 stress states, respectively, as the tin content decreased toward the lower tin content limit of 1.20%. A generalized multiaxjal thermal creep equation was used to model the influence of tin content and to separate the anisotropy and stress dependence. The stress dependence and thermal activation energy were insensitive to alloy content; however, the anisotropy parameters measured under low strain rate conditions moved closer to isotropic conditions as the tin content decreased. It was concluded that the change in the creep anisotropy was due to a change in the predominant slip system from a predominantly basal slip at high tin contents to a mixed basal-prism slip at low tin contents.