THE OXIDATION AND HYDRIDING OF ZIRCALOY FUEL CLADDING IN HIGH TEMPERATURE AQUEOUS SOLUTIONS
Open Access
- Author:
- Chen, Yingzi
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 23, 2006
- Committee Members:
- Digby D Macdonald, Committee Chair/Co-Chair
Mirna Urquidi Macdonald, Committee Chair/Co-Chair
Long Qing Chen, Committee Member
Earle Richard Ryba, Committee Member
W Murray Small, Committee Member - Keywords:
- oxidation/hydriding mechanism
Zircaloy-4
zirconium
passive film
point defect model - Abstract:
- Nearly 90% of today¡¯s fission reactors use Zr based fuel cladding materials. The Boiling Water Reactors (BWRs) and Pressurized Water Reactors (PWRs) are the two most common water-cooled nuclear reactors. Corrosion is the principal threat to the failure of the fuel in these reactors, resulting in the release of fission products to the coolant and hence to the establishment of radiation fields in out-of-core regions of the coolant circuit (e.g., steam generators in PWRs and turbines in BWRs). As is well known, corrosion is an electrochemical phenomenon; however, electrochemical effects are often neglected in corrosion studies on zirconium and its alloys, because of the difficulty in performing well-defined experiments under the appropriate conditions (high temperatures and pressures). In-situ studies have been carried out to examine the electrochemistry of passive zirconium under simulated BWR and PWR coolant conditions by using a controlled hydrodynamic, high temperature/high pressure test cell. The oxidation/hydriding mechanisms are elucidated by measuring the current, impedance, and capacitance of passive zirconium as a function of formation potential. The data are interpreted in terms of a modified point defect model (PDM) that recognize the existence of a passive film comprising a thick oxide outer layer over a thin barrier layer. From the composition of the zirconium passive film and thermodynamic analysis, it is postulated that a hydride barrier layer forms under PWR coolant conditions whereas an oxide barrier layer forms under BWR primary coolant conditions. Transients in current density and the thickness of the passive film formed on zirconium, when stepping the potential in either the positive or negative directions, have confirmed that the rate law afforded by the PDM adequately describes the growth and thinning of the passive film at high temperatures. The experimental results demonstrate that the kinetics of either oxygen or hydrogen vacancy generation at the metal/film interface control the rate of film growth when the potential is displaced in the positive direction, whereas the kinetics of dissolution of the barrier layer at the barrier layer/solution interface control the rate of passive film thinning when the potential is stepped in the negative direction. Both the growth rate and dissolution rate of the passive films formed in the hydrogenated solution are lower than that formed in the dehydrogenated solution. In addition, the effects of second phase particles (SPPs) on the electrochemistry of passive zirconium in the hydrogenated, high temperature aqueous solutions are examined by using different heat-treated Zircaloy-4 samples; i.e., as-received, belta-quenched, and alfa-annealed. Optical microscopy with polarized light and scanning electron microscopy (SEM) were used to characterize the microstructure and determine the size and density of the SPPs. The average size of the second phase particles in the Zircaloy-4 samples was in the sequence of belta-quenched < alfa-annealed < as-received, with the reverse sequence being observed in the areal density. The size of the SPPs is found to be comparable to the passive film thickness. Electrochemical studies show that the size and density of the second phase particles are the determining factors of the electrochemical properties of the passive films. Thus, the belta-quenched sample has the highest corrosion resistance, while the alfa-annealed sample has the lowest. It is also evident that the second phase particles play an important role in determining the electronic character of a passive film, as demonstrated by Mott-Schottky analysis. The second phase particles may cause short circuits in the electrical path across the passive film, which would explain the effect of the size and the density of the SPPs, and hence heat treatment, on the corrosion properties of passive Zircaloy-4.