Low Cycle Fatigue Behaviors of the Zircaloy-4 Cladding under the Cyclic Pressurization

Zirconium alloys which are used as a cladding materials in PWR, BWR, and CANDU suffer some kinds of degradation caused by the in reactor corrosion. They also suffer the cyclic deformation by the various kinds of external parameters such as coolant temperature, pressure, and other external forces. Among these, two kinds of source in the cyclic load can be found in the fuel cladding. High cycle fatigue which applies in the flexural direction caused by the external flows is found at the fuel cladding. Nowadays, possibilities of high cycle fatigue are greatly diminished by virtue of the spacer grid. Still, low cycle fatigue in the radial direction caused by the power oscillation remains in the fuel cladding. Normally, power of the reactor is controlled by the control rod by moving up or down to control the nuclear reaction. When the control rod is driven out, power of the fuel rod increased so that cladding undergoes radial expansion against the external coolant and vice versa. Such will occur frequently when the vendors adopt load following operation, which results in the cyclic changes of radial direction to cause low cycle fatigue. Although no failure regarding the radial fatigue in the fuel cladding has been reported until now, it is essential to pile up the fatigue life database under in terms of the fuel design. The objectives in this study are to construct the internal, cyclic pressurization facility for the fuel cladding and to produce stress-life curve of the Zircaloy-4 cladding under the cyclic pressurization