Author: Suresh K. Yagnik

Publisher:

Published: 2014

Total Pages: 31

ISBN-13:

Localization of hydride precipitates exacerbates the hydrogen embrittlement effects on the deformation and fracture properties of Zircaloy fuel cladding materials. Thus, at comparable hydrogen concentration levels, localized hydride precipitates are more detrimental from the standpoint of cladding integrity during service. Indeed, the hydride precipitates are often non-homogeneously distributed in fuel assembly components; for example, in irradiated fuel cladding, the hydride rim is formed near the outer oxide-metal interface because of the temperature gradient that exists during operation. With increasing fuel burnup, this hydride rim not only becomes denser but might be accompanied by gradients in local hydrogen and hydride concentrations through the rest of the cladding wall thickness. Whereas the importance of hydride spacing and their orientation, as well as the alloy matrix ligaments interspaced with the distributed hydride has been recognized in the literature, little work has been reported on the effects of hydride precipitate distribution on the mechanical properties of Zircaloy fuel assembly component materials. In this paper, we report on an extensive mechanical test program on low-tin Zircaloy-4 specimens from stress-relieved cladding and recrystallized guide tubes, charged with hydrogen to obtain uniform, rimmed, and layered hydride distributions. The hydrogen concentration (0-1200 ppm) and hydride rim thickness (10-90 ?m) were also varied. The strain rate was kept at 10-4/s to simulate in-service steady-state conditions and the tests were conducted both at room temperature and 300°C. All test specimens were of small-gauge-section, cut-outs from cladding, and guide tubes. The loading configurations included slotted-arc test (SAT) on half-ring-shaped specimens and uniaxial tension test (UTT) on dog-bone-shaped cut-outs. Further, prompted by the finite-element analysis of the gauge-section region, a unique geometry of internal slotted-arc specimens with parallel gauge section (ISATP) was chosen. Detailed stress-strain curves for all tests were measured, and post-test fractography and local hydrogen concentrations within the gauge sections were measured by hot extractions. Comparative data on the measured strengths and elongations for the three types of hydride distributions (i.e., uniform, rimmed, and layered) are presented. Quantification and analyses of these effects have provided a general constitutive stress-strain relationship for assessing margins to cladding or guide tube failures.