Zhonghou Cai

Microstructural Characterization of Oxides Formed on Model Zr Alloys Using Synchrotron Radiation

To understand how alloy chemistry and microstructure impact corrosion performance, oxide layers formed at different stages of corrosion on various model zirconium alloys (Zr-xFe-yCr, Zr-xCu-yMo, for various x, y) and control materials (pure Zr, Zircaloy-4) were examined to determine their structure and the connection of such structure to corrosion kinetics and oxide stability. Microbeam synchrotron radiation diffraction and fluorescence of oxide cross sections were used to determine the oxide phases present, grain size, and orientation relationships as a function of distance from the oxide-metal interface. The results show a wide variation of corrosion behavior among the alloys, in terms of the pretransition corrosion kinetics and in terms of the oxide susceptibility to breakaway corrosion. The alloys that exhibited protective behavior at 500°C also were protective during 360°C corrosion testing. The Zr-0.4Fe-0.2Cr model ternary alloy showed protective behavior and stable oxide growth throughout the test. The results of the examination of the oxide layers with microbeam X-ray diffraction show clear differences in the structure of protective and nonprotective oxides both at the oxide-metal interface and in the bulk of the oxide layer. The nonprotective oxide interfaces show a smooth transition from metal to oxide with metal diffraction peaks disappearing as the monoclinic oxide peaks appear. In contrast, the protective oxides showed a complex structure near the oxide-metal interface, showing peaks from Zr3O suboxide and a highly oriented tetragonal oxide phase with specific orientation relationships with the monoclinic oxide and the base metal. The same interfacial structures are observed through their diffraction signals in protective oxide layers formed during both 360°C and 500°C corrosion testing. These diffraction peaks showed much higher intensities in the samples from 500°C testing. The results for the various model alloys are discussed to help elucidate the role of individual alloying elements in oxide formation and the influence of oxide microstructure on the corrosion mechanism.

Microstructure and Growth Mechanism of Oxide Layers Formed on Zr Alloys Studied with Micro-Beam Synchrotron Radiation

The structures of oxides formed in water and lithiated water on three Zr-based alloys with varied corrosion behavior were studied with micro-beam synchrotron radiation and optical microscopy. Micro-beam synchrotron radiation (0.2 µm spot) has a unique combination of high elemental sensitivity (ppm level) and fine spatial resolution that allowed the determination of various oxide characteristics such as phase content, texture, grain size, and composition as a function of distance from the oxide-metal interface. Micro-beam X-ray fluorescence shows that the oxides formed in lithiated water have increased levels of Fe absorbed from the autoclave environment indicating greater oxide porosity in these oxides. The phase content, texture, and grain size of oxides were studied in detail using synchrotron radiation micro-beam diffraction for samples corroded in water and lithiated water. A remarkable periodicity was observed in the oxide structures using various techniques including X-ray peak intensities for both monoclinic and tetragonal zirconia, texture, and optical microscopy. The periods were similar to the transition period and were less visible in the oxides that behaved worse in lithiated water. These results are discussed in terms of models of oxide growth and of the differences between alloys.

Microbeam X-Ray Absorption Near-Edge Spectroscopy of Alloying Elements in the Oxide Layers of Irradiated Zircaloy-2

Hydrogen pick-up of zirconium-based fuel cladding and structural materials during in-reactor corrosion can degrade fuel component, as the ingress of hydrogen can lead to the formation of brittle hydrides. In the BWR environment, Zircaloy-2 fuel cladding and structural components such as water rods and channels can experience accelerated hydrogen pick-up, while Zircaloy-4 components exposed to similar conditions do not. Because the principal difference between the two alloys is that Zircaloy-2 contains nickel, accelerated hydrogen pick-up has been hypothesized to result from the presence of nickel. However, an understanding of the mechanism by which this acceleration occurs is still lacking. This research investigates the link between hydrogen pick-up and the oxidation behavior of alloying elements when incorporated into the oxide layers formed on zirconium alloys when corroded in-reactor. In this study, synchrotron radiation microbeam Xray absorption near edge spectroscopy (XANES) at the Advanced Photon Source was performed on carefully selected BWR-corroded Zircaloy-2 water rods at assembly-averaged burn-up ranging from 32.8 to 74.6 GWd/MTU to determine the oxidation states of alloying elements, such as iron and nickel, within the oxide layers, as a function of distance from the oxide-metal interface at high burn-up. Samples were chosen for comparison based on having similar oxide thicknesses, 1 Dept. of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA, 16802, United States of America 2 Electric Power Research Institute, Palo Alto, CA, 94304, United States of America 3 Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439, United States of America

Hydride Behavior in Zircaloy‐4 during Thermomechanical Cycling

Hydrogen ingress into zirconium alloy fuel cladding during operation in nuclear reactors can degrade cladding performance, both during operation and under dry storage, due to formation of brittle hydrides. At temperature and under stress, hydrogen redistribution and reorientation can occur, reducing cladding resistance to failure. Thus, it is crucial to understand the kinetics of hydride dissolution and re-orientation under stress and at temperature. High-energy and micro-beam synchrotron diffraction are used to study the kinetics of hydride reorientation and hydride distribution near a crack tip in previously hydrided Zircaloy sheet. Reorientation of hydrides in bulk samples is studied in situ (at temperature and under applied tensile stress). In-situ transmission diffraction data provides unique strain and orientation information on the hydrides. Micro-beam diffraction has been performed on previously cracked compact tension specimens under load. Measurement of the hydride distribution and associated strains can be performed with the micro-beam to determine hydrogen response to an applied strain field.

Using Small X-Ray Beams to Understand Corrosion in Nuclear Fuel Cladding

Uniform oxidation by the primary circuit water and associated may soon limit the service of Zr alloy fuel cladding in Light Water reactors. Understanding the differences in corrosion rate between alloys based on the microstructure of the protective oxide may allow us to design better alloying materials for severe duty cycle applications. The use of synchrotron radiation microbeam at APS allows the study of these oxide layers with an unique combination of the wealth of diffraction and fluorescence information and the level of spatial resolution obtained. We will discuss some of our experimental results and the potentials of these techniques in solving engineering problems.Copyright