Microbeam X-ray Absorption Near-Edge Spectroscopic Studies of High-Burnup Zircaloy-2 Oxide Layers

Transcription

1 Microbeam X-ray Absorption Near-Edge Spectroscopic Studies of High-Burnup Zircaloy-2 Oxide Layers A.P. Shivprasad 1, A.T. Motta 1, A. Kucuk 2, S. Yagnik 2, and Z. Cai 3 1 The Pennsylvania State University 2 Electric Power Research Institute 3 Advanced Photon Source, Argonne National Laboratory 1

2 Hydrogen pick-up increases at high burnup in Zircaloy-2 in the BWR environment Hydrogen pick-up fraction (%) Zircaloy-2 Zircaloy-4 Miyashita, et al

3 Incorporation of alloying elements in oxide layer could alter oxide properties Corrosion Zirconium Second-phase precipitate Alloying elements in Solid-solution Zirconium oxide Corrosion potential changes from the oxide/metal to oxide/water interface How are alloying elements oxidized? measure the oxidation state of alloying elements upon incorporation in oxide layer X-ray absorption near-edge spectroscopy (XANES) using microbeam Microbeam synchrotron radiation at Advanced Photon Source (APS) gives ability to use a microfocused X-ray beam to probe the oxide layer at the submicron level 3

4 4 Experiment details

5 A focused set of samples was chosen to highlight the differences that lead to high hydrogen pick-up Material Material 10 Material 13 Material 17 Material 21 Fluence (E > MeV) n/cm 2 n/cm 2 n/cm 2 n/cm Average oxide thickness 23 µm 28 µm 3.3 µm 13 µm 21 H content (wt ppm) HPUF (%) Bundle-average burn-up GWd/MTU GWd/MTU GWd/MTU GWd/MTU Archive sample used to account for heat treatment effects only 5

6 Activated water rods sent from Vallecitos to ORNL and prepared as cross-sectional samples Mo rod Brass tube Epoxy 6

7 7 Activated water rods sent from GNF to ORNL and prepared as cross-sectional samples

8 X-ray absorption spectroscopy (XAS) studies X-ray absorption cross-section near the absorption edge Photoelectric absorption Photon absorption mechanisms XANES J. R. Lamarsh, 2001 J. Als-Nielsen and D. McMorrow,

9 Each chemical state has a signature XANES spectrum Edge position relates to oxidation state Every element with a given electronic structure in a given phase has a signature XANES spectrum require standard for each element in different phases 9

10 Standard materials were used for linear combination fitting Nickel standards Iron standards fcc nickel Nickel in Zry-2 metal bcc iron Iron in Zry-2 metal Metal standards Nickel(II) oxide (NiO) Iron(II) oxide (FeO) Nickel(II,III) oxide (Ni 0.86 O) Iron(II,III) oxide (Fe 3 O 4 ) Nickel ferrite (NiFe 2 O 4 ) Nickel ferrite (NiFe 2 O 4 ) Oxide standards Nickel(III) oxide (Ni 2 O 3 ) Iron(III) oxide (Fe 2 O 3 ) 10

11 Standard materials were used for linear combination fitting Example: XANES spectrum in Zry-2 oxide 0.86 μm from the oxide/metal interface in Material 13 11

12 Zr fluorescence counts Micro-focused X-ray beam was used to probe oxide layer along the oxide growth direction Absorption Coefficient, µ X-position X-ray Energy, E 12 11

13 13 XANES results

14 Standard materials were analyzed to use in fitting Nickel standards fcc nickel Nickel in Zry-2 metal Nickel(II) oxide (NiO) Iron standards bcc iron Iron in Zry-2 metal Iron(II) oxide (FeO) Nickel(II,III) oxide (Ni 0.86 O) Iron(II,III) oxide (Fe 3 O 4 ) Nickel ferrite (NiFe 2 O 4 ) Nickel ferrite (NiFe 2 O 4 ) Nickel(III) oxide (Ni 2 O 3 ) Iron(III) oxide (Fe 2 O 3 ) 14

15 Increased Oxidation Standard materials were analyzed to use in fitting Nickel standards Iron standards Increased Oxidation Increased Oxidation Increased Oxidation 15

16 X-ray absorption near-edge spectra were obtained at each oxide location and plotted against position Rising-edge feature White-line 16

17 X-ray absorption near-edge spectra were obtained at each oxide location and plotted against position Rising-edge feature Material 10 Material 13 Rising-edge feature Rising-edge feature Whiteline Whiteline Rising-edge feature Whiteline Whiteline Material 17 Material 21 17

18 XANES spectra were fit with linear combinations of standards to obtain metallic fraction Metal Oxide 18

19 19 XANES spectra were fit with linear combinations of standards to obtain metallic fraction

20 20 Iron oxidized in a similar manner across all samples

21 21 Iron oxidized in a similar manner across all samples

22 22 Nickel remains metallic for extended distances in high hydrogen pick-up fraction (HPUF) materials

23 Nickel remains metallic for extended distances in high hydrogen pick-up fraction (HPUF) materials High HPUF Low HPUF 23

24 Chemical and physical states of iron in oxide layer bcc Fe was observed in previous examinations of the oxide layer using TEM and XANES D. Pêcheur, et al

25 Chemical and physical states of iron in oxide layer fcc Ni was observed as a fitting standard in the Zry-2 oxide layer near the oxide/metal interface A. Couet, et al

26 Chemical and physical states of iron in oxide layer Edge position of the standards was correlated to their oxidation state. This comparison was then applied to the Ni and Fe in the samples. 26

27 27 Chemical and physical states of nickel in oxide layer

28 28 Chemical and physical states of nickel in oxide layer

29 Impact of nickel on hydrogen pick-up Nickel remains metallic to larger distances within the oxide layers of high HPUF materials than those of low HPUF materials Nickel stays metallic to a significant amount for distances larger than barrier layer thickness Metallic nickel should increase oxide conductivity, which would be expected to reduce hydrogen pick-up Surface reaction of hydrogen in oxide layer may be affected by presence of metallic nickel 29

30 Conclusions Microbeam XANES of Ni and Fe in oxide layers found in Zircaloy-2 water rods irradiated to high burn-up have been performed in highand low-hpuf materials A significant difference observed in Ni oxidation between low- and high-hpuf materials. In high-hpuf materials, Ni remained metallic to a significantly greater extent into the oxide layer In Zircaloy oxidation, zirconium oxidizes first, then iron, then nickel. This is consistent with predictions based on Pourbaix diagrams. Results suggest that metallic nickel beyond the protective oxide layer could be at the root of higher hydrogen pick-up in these alloys 30

31 Future work Diffraction analyses of oxide layer microstructures are currently being analyzed Need to determine whether significantly delayed oxidation of nickel is consistently observed in high-hpuf samples 31

32 Thank you for your attention 32