Zircaloy-4 Cladding Deformation During Power Reactor Irradiation

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Abstract
The four primary Zircaloy fuel cladding deformation phenomena—axial elongation, circumferential creep, ovalization, and ridging—have been investigated for fuel irradiated in four modern pressurized water reactors. The axial elongation of fueled and nonfueled rods is examined by a regression fit for dependence on fluence, clad texture, yield stress, applied stress and, for fuel rods, fuel pellet length to diameter ratio. For fueled rods, only fluence and stress are found to be important, although the range of texture data is small. For nonfueled rods, the texture is found to influence elongation. The circumferential creep strain data from nonfueled rods are also analyzed by a regfression analysis to estimate the effects of time, flux, applied stress, temperature, yield stress, and texture. Time and flux have weaker than linear influence on strain, while the stress has a stronger than linear influence. The strain rate is almost athermal. The creep strain clearly increases with preirradiation yield strength. However, the primary difference is between recrystallized and stress-relief-annealed cladding with the degree of retained cold work having less effect. The creep rate clearly decreases with an increase in the angle between the basal poles and the radial direction. The effects of carbon content and grain size are also discussed. Ovalization of fueled and nonfueled rods is observed although analysis is difficult. The sensitivity of post-irradiation ovality to initial ovality and the relatively large uncertainty in initial ovality have precluded effective regression analysis although the general effects of applied stress, yield stress, and initial ovality are given. Clad ridging develops in fueled rods, usually in a time period between one and two reactor cycles and almost always after three cycles. However, the maximum ridge height occurs before the end of two reactor cycles.

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