Investigation of the mechanical behavior of rim/blister structure hydride phases in Zircaloy-4 by utilizing high energy synchrotron X-ray diffraction

Synchrotron X-ray diffraction;Zirconium hydride

Lin, Jun-Li

关键词: Synchrotron X-ray diffraction;    Zirconium hydride;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/78567/LIN-THESIS-2015.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Zircaloy-4 is one of the most common fuel cladding materials used in light water reactors due to excellent mechanical properties, corrosion resistance and low thermal neutron absorption cross-section. However, in service absorption of hydrogen by Zircaloy-4 (Zry-4) cladding is potentially problematic. One issue is the delayed hydride cracking (DHC) phenomenon which occurs during storage of used nuclear fuel (UNF). DHC is a time-dependent subcriticalcrack growth mechanism that can be induced by tensile stress hoop stress within the fuel cladding, even when this hoop stress is lower than the yield stress of the cladding materials.DHC eventually leads to the failure of the cladding, potentially resulting in the release of radioactivession gas inside the storage casks. This naturally increases the difficulty oftransportation of UNF to a permanent storage site. The root-cause of the DHC is hydridere-orientation, where, undersufficient stress, plate-like hydrides dissolve and reprecipitate with plate normal axis parallel to the stress axis. Hydride reorientation provides a brittlepathway for crack propagation and may potentially decrease the lifetime of UNF in storage.The concern of DHC have engendered studies of the microstructure and mechanical response of the hydride phase to externally applied tensile stress in order to understand hydridereorientation behavior. In this thesis, the mechanical response of the zirconium hydride phase in Zry-4 material under an applied load at the temperature similar to the dry caskstorage was investigated. More specifically, the stress-strain behavior of the zirconium andzirconium hydride phases were studied at 200C with different hydrogen concentrations.The hydride phase was intentionally created with a rim/blister (a region with high density of hydride) within the sample, similar to the hydride rim/blister observed in UNF claddingmaterial. High energy synchrotron X-ray diffraction at the APS 1-ID was utilized to study the mechanical response of hydride and matrix phase with the rim/blister distribution in a cold worked stress-relieved (CWSR) Zry-4 sheet. The only hydride phase observed in the Zry-4 sheet material for hydrogen concentration from 97 to 977 weight parts per million (wppm) isthe delta phase. The lattice strain evolution of zirconium hydride and the zirconium phase as a function of the applied stress was studied. Two stages of different stress-lattice strainbehaviors were identified. Load partitioning between the zirconium matrix phase and the hydride phase is responsible for this change in behavior. The highest internal stress or the effective von Mises stress was analyzed on the (220) hydride plane. The degree of load transfer from the matrix to the hydride phase increased as hydrogen content increased in thesamples. This load transfer may potentially lead to the early fracture of the brittle hydride phase.The evolution of the full-width at half maximum (FWHM) and the integrated diffraction intensity was also studied. The FWHM ofdelta (220) hydride reflection decreased in the direction parallel to the applied stress axis and increased in the direction perpendicular to the applied stress axis, which is inconsistence with published results of the FWHM signature of reoriented hydride. The integrated diffraction intensity of delta (220) hydride reflection onlyslightly decreased in the direction parallel to the applied stress axis and remained unchanged in the direction perpendicular to the applied stress axis. Therefore, no strongly direct ev-idence of hydride reorientation was observed, which implies that the hydride blister/rim structure is fairly stable under the environment tested in this study.