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Title:Investigation of the mechanical behavior of rim/blister structure hydride phases in Zircaloy-4 by utilizing high energy synchrotron X-ray diffraction
Author(s):Lin, Jun-Li
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Synchrotron X-ray diffraction
Zirconium hydride
Abstract: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 subcritical crack 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 radioactive ssion gas inside the storage casks. This naturally increases the difficulty of transportation of UNF to a permanent storage site. The root-cause of the DHC is hydride re-orientation, where, under sufficient stress, plate-like hydrides dissolve and reprecipitate with plate normal axis parallel to the stress axis. Hydride reorientation provides a brittle pathway 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 hydride reorientation 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 cask storage was investigated. More specifically, the stress-strain behavior of the zirconium and zirconium hydride phases were studied at 200 C 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 cladding material. 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) is the 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 strain behaviors 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 the samples. 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 of delta (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 only slightly 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.
Issue Date:2015-05-01
Rights Information:Copyright 2015 Jun-Li Lin
Date Available in IDEALS:2015-07-22
Date Deposited:May 2015

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