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Local atomic environment of Ni-bearing precipitates in irradiated Zircaloy-2 claddings investigated by micro-beam X-ray absorpti on spectroscopy
G. Kuri, M. Martin, J. Bertsch, C. N. BorcaPaul 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
17th 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)
~ 10150.2 – 0.7X-ray absorption fine structure
~ 1013~ 0.5Atom probe tomography
~ 1015~ 1.2Small angle neutron scattering
~ 1013~ 1Transmission 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.
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� 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 µµµµm2 beam spot size on the sample and allows radioactive specimens for investigation.
Crystal structures of Laves phase and Zintl phase compou nd
BCT Zr 2(Fe,Ni)
1.0 µm
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Outcome : Number and size distributions of SPP; Lattice parameters from selected area diffraction (SAD) patterns.
0.0130.0091
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 (Å2)
(± 15�
)
RD (nm)
(±0.002)
CNSCSpecimen
identification
Irradiated Specimens
Pristine Specimen
Additional results on Laves phase SPP One cycle sample; Cr K-edge EXAFSCubic type Zr(Fe,Cr)2 Lattice parameter a = 7.087 Å
First fourShells (22 atoms)
0.2620.2760.4540.463
2848
NiZrNi Zr
Ni
NoteR (nm)NNeighbourAbsorber
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 (1s22s22p63s23p64s23d84p), 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 prec ipitates
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.
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