Micro-focussed XAFS spectroscopy to study Ni-bearing precipitates in the metal of corroded Zircaloy-2

Abstract

The present work concerns an investigation of the local atomic environment of Ni-containing secondary phase precipitates (SPP) present in the metal-part of Zircaloy-2 cladding tubes. An unirradiated Zircaloy-2 and two specimens irradiated in a commercial nuclear power plant are characterized using μ-focussed synchrotron radiation, and by x-ray absorption fine structure (XAFS) spectroscopy. The patterns of Ni K-edge XANES and EXAFS of SPP in unirradiated and irradiated cladding are found different. Considering the fact that Ni-bearing SPP in the unirradiated samples are mainly Zintl phase Zr 2 (Fe, Ni) type, a detailed EX-AFS analysis of near-neighbor Ni atoms has been made. The result of a curve fit for the first two shells shows that about 2 Ni(Fe) and 8 Zr atoms are coordinated at 2.68 and 2.77 Å, respectively, around a central Ni atom in the SPP. XANES data analysis provides total electronic density of states at the Fermi level of unirradiated Zr 2 (Fe, Ni). At the Ni K-edge EXAFS spectra of irradiated SPP, however, only a single scattering peak is observed demonstrating the structural disorder introduced by the neutron irradiation. The coordination number of the Ni neighboring shells is reduced markedly due to the formation of point and extended defects in the damaged SPP lattice. Dissolution of Ni from the SPP is also evident from the data. The results of this study provide a further basis for the description of both crystallographic and electronic structures of intermetallic secondphase precipitates found in Zr-based alloys.

Nano-scale chemical evolution in a proton-and neutron-irradiated Zr alloy

“…Interestingly, vacancy migration in the Zr 2 Ni system was determined to be anisotropic with a preference in the [001] direction [57], which will of course have a profound effect on point defect interactions and larger, extended defects and their evolution, especially in regard to SPP clusters containing both Fe-Cr and Fe-Ni regions. The Fe-Ni system does indeed become defective under neutron irradiation, as Kuri et al have demonstrated by comparing the EXAFS and XANES with and without neutron irradiation in a BWR [62]. The degree of disorder is, however, difficult to discern and although Kuri et al described a lack in postirradiation long-range order, the spectra show signs of local order that may exceed that of an amorphous system.…”