The role of nickel in hydrogen pick-up during in-reactor corrosion of zirconium alloys
Open Access
- Author:
- Shivprasad, Aditya Prahlad
- Graduate Program:
- Nuclear Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 01, 2017
- Committee Members:
- Arthur Thompson Motta, Dissertation Advisor/Co-Advisor
Arthur Thompson Motta, Committee Chair/Co-Chair
Nicholas Robert Brown, Committee Member
Seong Han Kim, Committee Member
Clive A Randall, Outside Member
Igor Jovanovic, Special Member
Aylin Kucuk, Special Member - Keywords:
- zirconium alloys
Zircaloy-2
hydrogen pick-up
XANES
oxidation state
X-ray diffraction
zirconium oxide
precipitate oxidation
in-reactor corrosion
water rods
BWR
nickel
high burn-up - Abstract:
- Hydrogen pick-up of zirconium-based fuel cladding and structural materials during in-reactor corrosion can degrade fuel component performance in existing light water reactors (LWRs) and advanced nuclear reactors, such as the LWR-like supercritical water reactors (SCWRs), as the ingress of corrosion hydrogen can lead to the formation of brittle hydrides. In the boiling water reactor (BWR) environment, Zircaloy-2 fuel cladding and reactor core components, such as water rods and channel boxes, can experience accelerated hydrogen pick-up (higher pickup fraction) at high burnup when exposed for one extra 24-month cycle, while Zircaloy-4 components under similar conditions do not. Because the principal difference between the two alloys is that Zircaloy-2 contains nickel, this accelerated hydrogen pick-up has been hypothesized to result from the presence of nickel and its role in the corrosion process when incorporated into the protective oxide layer. Zircaloy-2 and Zircaloy-4 sister samples were corroded in 360 _C water and an additional set of Zircaloy-2 samples was corroded in 400 _C steam. Total weight gain, assumed to be due mostly to oxygen, and hydrogen content were measured as functions of exposure time. The results indicate that Zircaloy-2 samples absorbed more hydrogen than did Zircaloy-4 samples on the basis of total weight gain (hydrogen pickup fraction), though both exhibited similar corrosion kinetics parameters. Microbeam synchrotron radiation X-ray absorption near-edge spectroscopy (XANES) of selected Zircaloy-2 samples at the Advanced Photon Source (APS) was used to probe the oxidation states of nickel and iron in these materials and understand the evolution of the oxidation states of these alloying elements as functions of distance from the oxide/metal interface. Result showed that a significant fraction of nickel atoms remained metallic upon incorporation in the oxide layer. In contrast, iron atoms oxidized much earlier than did nickel atoms and, in most cases, fully oxidized within several micrometers from the oxide/metal interface. A general hypothesis was made that metallic nickel in contact with the coolant may catalyze the surface reactions involved in the hydrogen pick-up mechanism. To understand accelerated hydrogen pick-up of certain Zircaloy-2 samples at high burn-up, additional XANES examinations were performed on Zircaloy-2 water rods exposed in-reactor to high burn-up in commercial BWRs. The first set of samples was corroded in the Limerick-1 reactor, while the second set was corroded in the Dresden-2 reactor. Within each set of samples, fluences, oxide thicknesses, and sample elevations were similar, but hydrogen pick-up fractions were vastly different. In the first set of samples, oxide thicknesses ranged from 28 - 35 μm, but hydrogen pick-up ranged between 15 and 51%. In the second set of samples, oxide thicknesses ranged between 3.5 μm and 16 μm, but hydrogen pick-up ranged from 28 - 69%. All samples were irradiated to fluences between 9.4 and 13.1 ×1021 n/cm2 for neutron energies above 1 MeV. Results of XANES examinations showed a similar correlation between the delayed oxidation of nickel and higher hydrogen pick-up of Zircaloy-2 at high burn-up. A significant fraction (greater than 30%) of nickel atoms were found to be in the metallic state in the porous oxide layer. It was hypothesized that this metallic nickel is responsible for enhancing hydrogen pick-up by catalyzing the surface reactions that affect the overall hydrogen pick-up reaction. This would allow for easier absorption of hydrogen into the protective oxide layer from the coolant. Ab initio modeling of XANES of selected iron- and nickel-containing compounds was also performed and compared to experimental results to help understand how different populations of alloying elements oxidized upon incorporation into the oxide layer. A concurrent study of the microstructure of oxide layers formed on these same irradiated water rods was performed to understand if there was a characteristic microstructure associated with accelerated hydrogen pick-up. Microbeam X-ray diffraction (XRD) at the APS was performed on water rod samples to study oxide texture, phase content, and grain size. A similar examination was performed on steam-corroded Zircaloy-2 to serve as a comparison. Results showed that the oxide layers formed on these samples consisted primarily of highly-oriented monoclinic phase zirconium oxide with a small fraction of tetragonal phase oxide. Monoclinic phase grains were shown to grow as a function of distance from the oxide/metal interface, while tetragonal phase grains remained a constant size, indicating a tetragonal-to-monoclinic phase transformation above a critical grain size of approximately 10 nm. The tetragonal phase fraction was also calculated and observed to maximize near the oxide/metal interface, coinciding with the appearance of the (002)-tetragonal phase diffraction reflection, which appeared to be highly-oriented and strained, but disappeared away from the oxide/metal interface. Findings were consistent with previous microbeam XRD examinations of oxide layers formed on Zircaloy-4 under autoclave conditions. Transmission XRD examinations were also performed on a selected steam-corroded sample to serve as an additional comparison. The observations presented in this study helped to propose a mechanism for oxidation of different populations of iron and nickel upon incorporation into the Zircaloy-2 oxide layer and the effect on the hydrogen pick-up mechanism.