Stress Reorientation of Hydrides in Recrystallized Zircaloy-2 Sheet

Abstract

A series of measurements was performed to examine the effects of texture, hydrogen concentration, thermal cycling and morphology of hydrides on stress reorientation of hydrides using unirradiated recrystallized Zircaloy-2 sheet materials. It was found that the threshold stress for reorientation of hydrides is approximately 80 MPa, and the threshold stress is not affected by hydrogen concentration, thermal cycling or hydride morphology before the hydride reorientation test. Over the threshold stress, texture, thermal cycling and morphology of hydrides affect the stress reorientation. The modified Ells' equation was proposed for expressing the effect of the texture of Zircaloy-2.

An elasto-plastic fracture mechanics based model for assessment of hydride embrittlement in zircaloy cladding tubes

“…A similar median and standard deviation values as in Fig. 13a can be found in [11] for a case taken after 10 thermo-mechanical load cycles with 48 hydrides that had undergone partial re-orientation and where the maximum length was 257 lm.…”

“…For hydride re-orientation to take place, the hydrogen needs first to be dissolved and then migrate by diffusion and re-precipitate as hydrides with an orientation that depends on the stress and texture. In [11] only relatively few hydrides were radial even after 10 thermo-mechanical load cycles. There are, however, many examples in the literature where most hydrides are radial after load cycles [14,10,15,30,22], so the Cases 5-7 are not unrealistic.…”

“…To this end we use a set of 24 hydrides located primarily in the circumferential orientation. The micrographs were taken directly from [11] and analysed by an in-house image processing tool that uses the MATLAB image processing tool box. Fig.…”

“…This hydride re-orientation is a function of the temperature and the hoop stress. At 400°C typical threshold values for re-orientation are 60-100 MPa [10,11]. The ratio between the radius and the wall thickness is typically 8 for cladding tubes so the hoop stress ðpR=tÞ is sufficient for hydride re-orientation.…”

“…The internal pressure at room temperature is typically 3-7 MPa [9]. When the spent fuel is transferred from wet to dry storage the temperature increases to 400°C, the hydrides dissolve and the pressure increases (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). The hydrides are then reprecitated as partially radial hydrides as the temperature drops.…”