THE NATURE OF UNSTABLE OXIDE GROWTH IN ZIRCONIUM AND ZIRCONIUM ALLOYS

Open Access
Author:
Ensor, Brendan Melvin
Graduate Program:
Nuclear Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 07, 2016
Committee Members:
  • Arthur Thompson Motta, Dissertation Advisor
  • Arthur Thompson Motta, Committee Chair
  • Michael Tonks, Committee Member
  • Douglas Edward Wolfe, Committee Member
  • Allison Michelle Beese, Outside Member
  • Ashley Lucente, Special Member
  • John Seidensticker, Special Member
Keywords:
  • zirconium
  • zirconium alloy
  • fuel cladding
  • corrosion
  • synchrotron radiation
  • X-ray diffraction
  • XANES
  • neutron irradiated
  • Zircaloy-4
  • hydrogen
  • breakaway oxidation
  • strain
  • stress
  • plastic deformation
  • oxide growth
  • X-ray fluorescence
  • Raman spectroscopy
  • tetragonal fraction
  • grain size
Abstract:
Zirconium alloys are commonly used as fuel claddings in nuclear reactors due in part to their superior corrosion resistance. The addition of small concentrations of alloying elements prevents the cladding material from undergoing unstable oxide growth under the operating conditions of a nuclear reactor. 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 this thesis. 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 layers characterized with various techniques. Ten model Zr alloys were fabricated and tested in furnace at 600°C for 40 hours in oxygen and in autoclave at 360°C for up to 70 days to determine the causes of breakaway oxidation in pure Zr (and Zr alloys with small concentrations of alloying elements) and the role that alloying elements play in causing this phenomenon. These alloys were carefully selected and included crystal bar Zr, sponge Zr, and alloys with small concentrations of Sn, Fe, and Cr. After testing, the alloys were characterized using scanning electron microscopy (SEM), Raman spectroscopy, and synchrotron μ-X-ray fluorescence (μXRF) to determine how the structure of the oxide, tetragonal phase content, and alloying element distribution affected the formation of unstable oxide. Heterogeneous distribution of alloying elements was linked to regions of unstable oxide (either nodule-like, grain boundary penetration, or differential 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 of unstable oxide growth in Zr and Zr alloys. One of the primary modes of examination of stable oxide layers formed on Zr alloys was microbeam synchrotron X-ray radiation diffraction and fluorescence, performed at the Advanced Photon Source (APS) at Argonne National Laboratory. This synchrotron X-ray source was used to perform μ-X-ray diffraction (μXRD), μXRF, and 3D Laue spectroscopy. The μXRD technique was used to determine the oxide layer phase content, strain, and grain size as a function of corrosion temperature and oxide thickness. The μXRF technique was used to perform Fe X-ray absorption near-edge spectroscopy iv (XANES) to determine the oxidation state of Fe in the metal as a function of distance from the metal-oxide interface for various corrosion temperatures. The 3D Laue spectroscopy technique was used to determine plastic 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 360°C for up to 2804 days in and were periodically weighed to determine oxide thickness. These samples had different coupon thicknesses that altered the surface-to-volume ratio and led to a higher concentration of hydrogen for a given amount of oxide layer growth. The concentration of hydrogen was measured in archived samples to determine the effect of hydrogen concentration on corrosion rate. It was observed that the corrosion rate of Zircaloy-4 increased 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 of advanced oxide growth were associated with the locations of hydrides in the metal. It was hypothesized that the hydrides hardened the metal ahead of the interface and that the metal was then less able to accommodate oxide growth stresses leading to earlier kinetic transition and mechanical cracking of the oxide. Finally, eleven Zircaloy-4 samples exposed to various temperatures (272-355°C) and neutron flux levels (0-11.48 x 1013 n/cm2/s, E > 1 MeV) were examined using μXRD and μXRF to determine the effect of irradiation on oxide grain size, phase content, and the oxidation of Fe at the APS. With increasing neutron fluence, the grain size of the oxide increased, leading to less tetragonal phase in the oxide away from the metal-oxide interface. At the metal-oxide interface, higher amounts of tetragonal phase were observed with increasing 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 presence of alloying elements in Zr and Zr alloys and to the level of stress in the oxide. These two phenomena appear to be the primary causes leading to regions of advanced oxide growth and careful consideration should be given to them when designing and using future Zr alloys in advanced nuclear reactor concepts.