Hydrogen pickup of zirconium-based fuel cladding and structural materials during in-reactor corrosion can degrade fuel components because the ingress of hydrogen can lead to the formation of brittle hydrides. In the boiling water reactor (BWR) environment, Zircaloy-2 fuel cladding and structural components such as water rods and channels can experience accelerated hydrogen pickup, whereas Zircaloy-4 components exposed to similar conditions do not. Because the principal difference between the two alloys is that Zircaloy-2 contains nickel, accelerated hydrogen pickup has been hypothesized to result from the presence of nickel. However, an understanding of the mechanism by which this acceleration occurs is still lacking. We investigated the link between hydrogen pickup and the oxidation behavior of alloying elements when incorporated into the oxide layers formed on zirconium alloys when corroded in the reactor. Synchrotron radiation microbeam X-ray absorption near-edge spectroscopy (XANES) at the Advanced Photon Source was performed on carefully selected BWR-corroded Zircaloy-2 water rods at an assembly-averaged burnup ranging from 32.8 to 74.6 GWd/MTU to determine the oxidation states of alloying elements, such as iron and nickel, within the oxide layers as a function of distance from the oxide-metal interface at high burnup. Samples were chosen for comparison based on having similar oxide thicknesses, processing, elevation, reactors, and fluences but different hydrogen pickup fractions. Examinations of the oxide layers formed on these samples showed that (1) the oxidation states of these alloying elements changed with distance from the oxide-metal interface, (2) these elements exhibited delayed oxidation relative to the host zirconium, and (3) nickel in Zircaloy-2 remained metallic in the oxide layer at a longer distance from the oxide-metal interface than iron. An analysis of these results showed an apparent correlation between the delayed oxidation of nickel and higher hydrogen pickup of Zircaloy-2 at high burnup.
The photoelectrochemical investigation was performed by measuring the photoelectrochemical behavior of various alloys such as Zircaloy 2, 304 stainless steel (SS) and Alloy X-750 in 0.01M Na2SO4 at 25C or in high purity water at 300C under intense ultraviolet (UV) illumination. UV was selected because its photon energy (~5 eV) is similar to the energy gap of the electron-hole pairs in zirconium oxide. The data show that the photoexcitation of ZrO2 caused the corrosion potential to shift in the anodic direction and produced anodic photocurrents under the oxidizing water chemistry condition when Zircaloy 2 electrode was galvanically coupled with dissimilar electrodes, such as Alloy X-750, 304 SS, or Pt, causing accelerated corrosion of Zircaloy. It is thus postulated that the photoelectrochemical enhancement of surface reaction kinetics at the ZrO2 surface may be responsible for the radiation-enhanced corrosion on Zircaloy (i.e., shadow corrosion). In addition, it is revealed that no significant change in galvanic current between Zircaloy 2 and other dissimilar electrodes was measured in high temperature water containing only hydrogen that resulted in similar ECPs. These data thus clearly provide an explanation for why the radiation-enhanced corrosion of Zircaloy only occurs in BWRs but not PWRs. Based on galvanic corrosion, impedance, and transmission electron microscopy analysis, it is proposed that the defect structure of equiaxed grain layers may be responsible for the photoelectrochemical response of ZrO2 in high temperature water. The mechanism of shadow corrosion is still highly debated but it appears to be similar to a process of galvanic corrosion in connection with sufficiently conducting Zr oxide structure. The galvanic corrosion data suggests that a Zr coating on the fuel assembly spacer (Alloy X-750 or 304 SS) may mitigate the shadow corrosion in BWRs.
The photoelectrochemical investigation was performed by measuring the photoelectrochemical behavior of various alloys such as Zircaloy 2, 304 stainless steel (SS) and Alloy X-750 in 0.01M Na2SO4 at 25°C or in high purity water at 300°C under intense ultraviolet (UV) illumination. UV was selected because its photon energy (∼5 eV) is similar to the energy gap of the electron-hole pairs in zirconium oxide. The data show that the photoexcitation of ZrO2 caused the corrosion potential to shift in the anodic direction and produced anodic photocurrents under the oxidizing water chemistry condition when Zircaloy 2 electrode was galvanically coupled with dissimilar electrodes, such as Alloy X-750, 304 SS, or Pt, causing accelerated corrosion of Zircaloy. It is thus postulated that the photoelectrochemical enhancement of surface reaction kinetics at the ZrO2 surface may be responsible for the radiation-enhanced corrosion on Zircaloy (i.e., shadow corrosion). In addition, it is revealed that no significant change in galvanic current between Zircaloy 2 and other dissimilar electrodes was measured in high temperature water containing only hydrogen that resulted in similar ECPs. These data thus clearly provide an explanation for why the radiation-enhanced corrosion of Zircaloy only occurs in BWRs but not PWRs. Based on galvanic corrosion, impedance, and transmission electron microscopy analysis, it is proposed that the defect structure of equiaxed grain layers may be responsible for the photoelectrochemical response of ZrO2 in high temperature water. The mechanism of shadow corrosion is still highly debated but it appears to be similar to a process of galvanic corrosion in connection with sufficiently conducting Zr oxide structure. The galvanic corrosion data suggests that a Zr coating on the fuel assembly spacer (Alloy X-750 or 304 SS) may mitigate the shadow corrosion in BWRs.
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