It has been pointed out that several factors can control the uniform corrosion of zirconium alloys. Alloy optimization of zirconium based alloys used for nuclear fuel cladding is a key to increasing corrosion resistance and reducing hydrogen pickup. However, a complete mechanistic understanding of the role of alloying elements in the corrosion and hydrogen pickup processes is still lacking. Because very small alloying element differences cause large differences in corrosion and hydrogen pickup, it is natural to examine the behavior of alloying elements in the oxide layer for clues to the origin of the differences between alloys.
In this study, we focus on the evolution of the chemical states of two alloying elements, Fe and Nb, when incorporated into the zirconium alloy oxide layers formed during autoclave testing of various zirconium alloys oxidized in pure water. X-ray Absorption Near-Edge Spectroscopy (XANES) measurements to determine the evolution of their oxidation states is performed using micro-beam synchrotron radiation on cross sectional oxide samples. The XANES experiments were performed at the 2ID-D beamline of the Advanced Photon Source (APS) at Argonne National Laboratory.
XANES is a characterization technique in which a series of X-rays of varying energies are used to probe a small sample area. The energy of the X-rays is varied from low to high and the absorption response of the sample yields information about the oxidation state of the elements inside.
Samples of Zr-Nb alloys provided by Westinghouse and oxidized in an autoclave environment to simulate pressurized-water reactor conditions were examined. Depending on the annealing temperature, Zr-Nb alloys can exhibit different secondary phases, namely β-Zr or β-Nb. Below are some transmission electron microscopy (TEM) images of the different precipitates.
By combining the oxidation state information from XANES and the precipitates composition and distribution through TEM, we can assess how the different precipitates oxidize. From this, we can make estimates about the niobium content in solid solution, and correlate our results predictions from the Coupled-Current Charge-Compensation (C4) model which is able to simulate oxidation kinetics while taking into account the effects of space charge caused by aliovalent ions (e.g. niobium) dissolved in the oxide.
- M. Moorehead, A. Couet, J. Hu, and Z. Cai, “Progressing Zirconium-Alloy Corrosion Models Using Synchrotron XANES,” in Proceedings of the 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors: Volume 2, Portland, OR, 2017, pp. 565-576: Springer International Publishing.
- A. Couet, A. T. Motta, B. de Gabory, and Z. Cai, “Microbeam X-ray Absorption Near-Edge Spectroscopy study of the oxidation of Fe and Nb in zirconium alloy oxide layers,” Journal of Nuclear Materials, vol. 452, no. 11-3, pp. 614-627, 2014.