Doctoral Dissertations

Date of Award

12-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Energy Science and Engineering

Major Professor

Hassina Bilheux

Committee Members

S. Babu, H. Choo, P. Liaw, E. Lukosi, S. Zinkle

Abstract

The presence of hydrogen (H) and zirconium hydrides (ZrH1.7) in Zircaloy-4 nuclear fuel cladding has been a significant topic of recent cladding research. Uptake of H into Zircaloy-4 nuclear fuel cladding occurs during normal reactor operation and can lead to material embrittlement and cladding failure. As such, it is important to characterize the H and ZrH1.7 and the associated defects. Six Zircaloy-4 samples (five H-charged and one reference) were prepared such that ZrH1.7 blisters were formed at varying global H concentrations (CH) between 5.8x102 wppm and 1.220x103 wppm. ZrH1.7 blisters were created through the use of a thin nickel (Ni) coating applied prior to H-charging in a Sievert-type apparatus. Non-destructive neutron scattering techniques were then used to characterize H and ZrH1.7 within the Zircaloy-4 samples. Neutron computed tomography (nCT) was used to quantify and spatially resolve CH in three dimensions (3D). Neutron grating interferometry (nGI) was used to identify microdefects within the H-rich regions of each sample. nCT and nGI measurements were collected at the CG-1D imaging beamline at the High Flux Isotope Reactor (HFIR). Neutron diffraction (nD), performed at the VULCAN instrument at the Spallation Neutron Source (SNS), was used to validate the nCT measurements and discern changes in atomic, or d-, spacing due to increasing CH within the ZrH1.7 regions of the H-charged samples. Small-angle neutron scattering (SANS) was done at the GP-SANS instrument at the HFIR to determine the relative quantities of H within the hydride blister regions of each sample. Mean CH values over the ZrH1.7 blister regions of the H-charged samples, calculated from the nCT results, ranged from approximately 4.23x103 wppm to 8.94x103 wppm. The nGI data showed that microdefects were present in the H-rich parts of each sample and that a increase in dark-field image (DFI) contrast caused by those microdefects corresponded to a decrease in neutron transmission in the nCT data. Rietveld refined nD patterns provided weight fractions for the α-Zr and δ-ZrH1.7 phases within the H-charged samples, and subsequent single-peak fitting (SPF) measured changes in d-spacing. The SANS results provided the relative quantities of H within the hydride blisters of the samples and corroborated the nD results.

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