Recent Studies Of The Growth And Breakdown Of Oxide Films On Zirconium And Its Alloys

Author : Brendan Ensor
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (971 download)

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Download or read book The Nature of Unstable Oxide Growth in Zirconium and Zirconium Alloys written by Brendan Ensor and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Zirconium alloys are commonly used as fuel claddings in nuclear reactors due in part to theirsuperior corrosion resistance. The addition of small concentrations of alloying elements prevents thecladding material from undergoing unstable oxide growth under the operating conditions of a nuclearreactor. Unstable oxide growth can also occur due to the presence of hydrides or exposure to neutron flux.The role of alloying elements in avoiding the transition from stable to unstable growth is examined in thisthesis. The goal is to determine the mechanism whereby oxide stabilization occurs.To accomplish this goal, a variety of experiments were performed, and the resulting oxide layerscharacterized with various techniques. Ten model Zr alloys were fabricated and tested in furnace at 600Cfor 40 hours in oxygen and in autoclave at 360C for up to 70 days to determine the causes of breakawayoxidation in pure Zr (and Zr alloys with small concentrations of alloying elements) and the role that alloyingelements play in causing this phenomenon. These alloys were carefully selected and included crystal barZr, sponge Zr, and alloys with small concentrations of Sn, Fe, and Cr. After testing, the alloys werecharacterized using scanning electron microscopy (SEM), Raman spectroscopy, and synchrotron -X-rayfluorescence (XRF) to determine how the structure of the oxide, tetragonal phase content, and alloyingelement distribution affected the formation of unstable oxide. Heterogeneous distribution of alloyingelements was linked to regions of unstable oxide (either nodule-like, grain boundary penetration, ordifferential grain-to-grain growth) and hypothesized to cause breakaway corrosion.The examination of stable oxide layers was then used as a baseline for comparison to cases ofunstable oxide growth in Zr and Zr alloys. One of the primary modes of examination of stable oxide layersformed on Zr alloys was microbeam synchrotron X-ray radiation diffraction and fluorescence, performedat the Advanced Photon Source (APS) at Argonne National Laboratory. This synchrotron X-ray source wasused to perform -X-ray diffraction (XRD), XRF, and 3D Laue spectroscopy. The XRD technique wasused to determine the oxide layer phase content, strain, and grain size as a function of corrosion temperatureand oxide thickness. The XRF technique was used to perform Fe X-ray absorption near-edge spectroscopyiv(XANES) to determine the oxidation state of Fe in the metal as a function of distance from the metal-oxideinterface for various corrosion temperatures. The 3D Laue spectroscopy technique was used to determineplastic deformation and elastic strain in the metal as a function of distance from the metal-oxide interface,corrosion temperature, and oxide thickness for crystal bar Zr and Zircaloy-4.Additionally, Zircaloy-4 samples were corroded in autoclave at 360C for up to 2804 days in andwere periodically weighed to determine oxide thickness. These samples had different coupon thicknessesthat altered the surface-to-volume ratio and led to a higher concentration of hydrogen for a given amountof oxide layer growth. The concentration of hydrogen was measured in archived samples to determine theeffect of hydrogen concentration on corrosion rate. It was observed that the corrosion rate of Zircaloy-4increased with increasing hydrogen concentration above the terminal solid solubility (TSS) of the material(and thus the precipitation of hydrides). More hydrogen caused earlier kinetic transition and areas ofadvanced oxide growth were associated with the locations of hydrides in the metal. It was hypothesizedthat the hydrides hardened the metal ahead of the interface and that the metal was then less able toaccommodate oxide growth stresses leading to earlier kinetic transition and mechanical cracking of theoxide.Finally, eleven Zircaloy-4 samples exposed to various temperatures (272-355C) and neutron fluxlevels (0-11.48 x 1013 n/cm2/s, E > 1 MeV) were examined using XRD and XRF to determine the effectof irradiation on oxide grain size, phase content, and the oxidation of Fe at the APS. With increasing neutronfluence, the grain size of the oxide increased, leading to less tetragonal phase in the oxide away from themetal-oxide interface. At the metal-oxide interface, higher amounts of tetragonal phase were observed withincreasing neutron fluence. This could be caused by the redistribution of Fe from second phase particles(SPPs) into the matrix or due to the hardening of the Zr matrix caused by the exposure to neutrons.The cases of unstable oxide growth examined here were linked to both the distribution and presenceof alloying elements in Zr and Zr alloys and to the level of stress in the oxide. These two phenomena appearto be the primary causes leading to regions of advanced oxide growth and careful consideration should begiven to them when designing and using future Zr alloys in advanced nuclear reactor concepts.