Insights into the interfacial speciation of Ni in the corrosion layer of high burnup Zircaloy-2 cladding: A combined XRD, XAS, and LFDFT study

This work reports atomic-scale experimental and modeling studies of the zirconium oxide microstructure formed at the metal-oxide (M/O) interface, and the crystallography as well as the redox chemistry of alloying element nickel in the corrosion layer found on a serviced boiling water reactor Zircaloy-2 fuel rod cladding. Complying with radioprotection aspects a small-sized sample with only limited radioactivity, taken from a very high burnup (~ 79 MWd/kgU) cladding tube material, was prepared for the investigation using synchrotron-based X-ray microprobe techniques such as X-ray fluorescence (μXRF), X-ray diffraction (μXRD) and X-ray absorption spectroscopy (μXAS). The patterns of Ni K-edge XAS of solute Ni, probed at several location within the corrosion layer, have been recorded and analyzed. Close to the metal-oxide interface both the allotropic forms of zirconium oxide, monoclinic and tetragonal phases, have been identified by XRD analyses. XAS results of the barrier layer at the metal-oxide interface reveal that the oxides contain Ni atoms as divalent cations located in the proximity of oxygen vacancies. Near the M/O interface regions, all oxidized nickel atoms have a Ni2+ homogeneous distribution and no trace or even evidence of any metallic Ni is found in the oxide matrix. The experimental results, also supported by first-principles ligand field DFT calculations, are interpreted in term of energetics, stability, chemical and structural specificity of divalent Ni ions in zirconium oxide microstructure. The present investigation on the basis of a combined experimental and theoretical approach shows that the oxidized nickel ions being situated in the neighborhood of oxygen defects is influential on the hydrogen evolution reaction including the hydrogen intake behavior accompanied with the oxidation reaction in irradiated Zircaloy-2.