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Local atomic environment of Ni-bearing precipitates in irradiated Zircaloy-2 claddings investigated by micro-beam X-ray absorption spectroscopy G. Kuri, M. Martin, J. Bertsch, C. N. Borca Paul Scherrer Institute, Nuclear Energy and Safety Department, CH 5232 Villigen PSI, Switzerland Reference & Acknowledgment [1] G. Kuri, C. Degueldre, J. Bertsch, S. Abolhassani, Applied Physics A (2010) 625-633. The authors would like to thank Paul Scherrer Institute and Swissnuclear for supporting this work. Introduction 17 th International Symposium on Zirconium in the Nuclear Industry, February 3-7, 2013, Hyderabad, India Results Methods and Specimens Zircaloy-2 cladding (in BWRs) exhibits a higher hydrogen pickup fraction compared to Zircaloy-4 (in PWRs), and the largest micro-structural difference between these two materials is that Zry-2 contains both Ni and (Fe, Cr) bearing secondary phase precipitates (SPP) while Zry-4 is Ni-free. Electron microscopic investigations have shown that accelerated hydrogen pick at higher burnup in Zry-2 can be correlated with irradiation induced simultaneous dissolution of both Fe and Ni, and the crystallinity of the SPP. The concomitant function of Ni, Fe or Cr in transporting hydrogen (if at all) in the metal through SPP is not determined. Additionally, many aspects of local atomic structures and electronic environment of elements in these SPP remain unknown. This work concerns an experimental investigation of the local atomic environment of nickel containing SPP present in the metal part of irradiated Zircaloy-2 cladding tubes. Cubic Zr(Fe,Cr) 2 C15 C14 C36 C16 TEM, SEM/FIB and EPMA X-ray absorption spectroscopy (EXAFS and XANES) ~ 10 15 0.2 – 0.7 X-ray absorption fine structure ~ 10 13 ~ 0.5 Atom probe tomography ~ 10 15 ~ 1.2 Small angle neutron scattering ~ 10 13 ~ 1 Transmission electron microscopy Precipitates density [cm -3 ] Diameter [nm] Experimental methods An unirradiated reference Zircaloy-2 and three irradiated cladding specimens (average burnup of 11, 34 and 66 MWd/kgU) obtained from a Swiss BWR plant are characterized. Using micro-focused synchrotron radiation, and by X-ray absorption fine structure (XAFS) spectroscopy samples have been analyzed. The facility is available at the Paul Scherrer Institute Swiss Light Source (PSI-SLS). The beam line delivers 1 μm 2 beam spot size on the sample and allows radioactive specimens for investigation. Crystal structures of Laves phase and Zintl phase compound BCT Zr 2 (Fe,Ni) 1.0 μm Outcome: Number and size distributions of SPP; Lattice parameters from selected area diffraction (SAD) patterns. 0.013 0.0091 0.0145 0.268 0.277 1.5±0.3 7.5±0.3 Ni-Ni Ni-Zr Un-irradiated Zircaloy-2 Quality Factor (R) 2 (Å 2 ) (± 15 ) RD (nm) (±0.002) CN SC Specimen identification Irradiated Specimens Pristine Specimen Additional results on Laves phase SPP One cycle sample; Cr K-edge EXAFS Cubic type Zr(Fe,Cr) 2 Lattice parameter a = 7.087 Å First four Shells (22 atoms) 0.262 0.276 0.454 0.463 2 8 4 8 Ni Zr Ni Zr Ni Note R (nm) N Neighbour Absorber Crystallographic data Experimentally determined structural parameters Outcome: Next neighbor atomic environment of Ni, Fe and Cr in the SPP and atomic scale structural modifications of second phase particles resulting from irradiation effects. These results need to be correlated with the measured hydrogen content in the samples. The aim is to provide an improved understanding of SPP evolution due to irradiation damage and their influence on accelerated hydrogen pick-up at higher burn-up found in Zircaloy claddings. Summary & Conclusions Successful application of XAFS to analyze SPP in Zircaloy-2. In the irradiated material no significant coherent scattering contributions beyond the Ni first neighbor shell, loss of Ni from the SPP, reduced coordination number in the Ni-Zr pair, and disruption of the average bond distances are found (EXAFS part). Structural parameters of Ni-containing SPP (only in the metal) are determined. These results are essential to analyze SPP in the oxide layer. The electronic structure of the irradiated SPP is also altered. Although Ni d band is normally full (1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 8 4p), band hybridization results in unoccupied density of states of d character above the Fermi level. Information on the electronic states and band mixing are yet to be obtained (from XANES part). Ni sublattice is much more stable than Cr and Fe sublattices. Measurement in the corroded layer in unirradiated as well as irradiated cladding specimens containing oxidized SPP is yet to be done. Hexagonal Zr(Fe,Cr) 2 Comparison of experimental techniques for analyzing precipitates Sample transport PSI-SLS XAS beam line Reference sample 1 cycle 3 cycles 6 cycles SC: scattering configuration; CN: coordination number; RD: radial distance Next step: determination of structural parameters Experimental data of Cr K-edge are shown. Measurements are also performed at the Fe K-edge. Preliminary results show a reduction of the Cr-Cr(Fe) coordination number and bond contraction in the first shell, when compared with the pristine material.
