Microbeam X-Ray Absorption Near-Edge Spectroscopy of Alloying Elements in the Oxide Layers of Irradiated Zircaloy-2

Abstract

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.

Understanding the role of Fe, Cr and Ni in Zircaloy-2 with special focus on the role of Ni on hydrogen pickup

“…The interaction of the {Ni Zr : V •• O } cluster with H + ions might explain why Ni has been observed as detrimental to the HPUF of Zr based cladding by offering a mechanism in which Ni 0 is involved in a catalytic process. Given that Ni has been observed recently by XANES analysis to have a zero oxidation state close to the oxide metal interface [29], these results perhaps provide a mechanism for increased HPUF. Since one of the hydrogen atoms is bonded to the Ni and the other bonded to an oxygen, this offers the possibility of that single H or H + species subsequently de-bonds (either thermally or through some other mechanism) and continues its migration into the Zr metal.…”

“…This seems unusual, as it suggests that despite the single substitutional ion exhibiting a 2+ or a 3+ charge state, it is energetically favourable for Ni in the cluster to retain all of its electrons. Yet this was reported by Shivprasad et al [29] following experimental work on samples of Zircaloy-2 with higher HPUF. The significance of this, with respect to the mechanism by which Ni might increase HPUF, is discussed in Section 3.2.…”

“…Similar to Fe, enriched concentrations of Ni are also observed along grain boundaries in Zr metal, which extend into the oxide [28]. Recent XANES work by Shivprasad et al [29] showed the presence of metallic Ni in the non-protective oxide layer of Zircaloy-2 samples exhibiting high HPUF, hinting at a possible role of Ni 0 . Fe, Cr and Ni all have very low solid solubility in α-Zr, thus they tend to form into second phase precipitates (SPPs) in the alloy, and have previously been assumed to influence corrosion and HPUF by methods other than oxide layer doping.…”

Cu and Sb in tetragonal ZrO2 on fuel cladding

“…This has the same electronic structure as metallic Ni 0 . It has been suggested that Ni 0 may make a key contribution towards the negative impact of Ni in increasing the hydrogen pickup in Zircaloy-2 cladding [11,12]. The potential influence of {Cu Zr : V •• O } was therefore investigated though a similar approach implemented in previous work [12].…”

“…Recent work has shown the presence of metallic Ni 0 in tetragonal ZrO 2 and linked it to higher hydrogen pickup in Zircaloy-2 samples [11]. Modelling work based on density functional theory (DFT) has gone further and identified a possible atomic scale mechanism for this increased hydrogen pickup [12].…”