TEM Examinations of the Metal-Oxide Interface of Zirconium Based Alloys Irradiated in a Pressurized Water Reactor

Abstract

Metal-oxide interfaces of three different materials irradiated in a pressurized water reactor have been analyzed by TEM and AEM. Standard Zircaloy-4, low-tin Zircaloy-4, and Zr-2.5%Nb were used for this study. The microstructure of the material on the two sides of the metal-oxide interface, the geometry of the interface, the distribution of different alloying elements, and the oxygen profile have been examined in each material. Results of the examinations showed that the three materials had different microstructure and oxygen distribution on the two sides of the metal-oxide interface. In particular, the following parameters were noticed: a) the geometry of the interface seems to be of a different nature in the case of Zr-2.5%Nb alloy. Unlike the Zircaloy-4 alloys, which show an undulated interface, this material has a "jigsaw" type interface. This point is discussed, and its role on the oxidation is considered. b) Hydrides are observed and analyzed in the vicinity of the interface in the case of low-tin Zircaloy-4, and it is shown that they can have an influence on the occurrence of cracks in this material. c) The origins of stress are discussed, and it is shown that it can have different sources. The crystal structure of the oxides is mainly monoclinic. A tetragonal oxide is observed at some regions, in particular in the standard Zircaloy-4.

Microprobe analysis of neutron irradiated and autoclaved zirconium niobium claddings using synchrotron-based hard X-ray imaging and spectroscopy

“…However, a distinct XANES oscillation structure occurs in this spectrum compared to that of niobium metal. This observation could be explained by the incorporation of oxygen atoms in the Nb-rich phases as observed in previous studies [11,14].…”

“…They ultimately oxidize to form an amorphous oxide. The same behaviour is observed for irradiated Zr-Nb alloys examined by TEM [11,14]. As the oxidation state of different atoms can be determined on an averaged scale by X-ray absorption spectroscopy (XAS), effort is made to use this technique to further examine the speciation of zirconium and niobium in Zr-Nb alloys.…”

“…The oxide thickness of the segment has been determined by scanning electron microscopy (SEM). The details of the microstructure of this neutron irradiated sample are already provided in [11]. Briefly, the material shows a Nb-rich phase which has precipitated in the form of platelets.…”

“…The structure and composition of oxides formed on different zirconium alloys have been subject of several investigations in the past e.g. [6][7][8][9][10][11][12][13]. One of the requirements of the cladding being its resistance to corrosion, the aim of these examinations is to correlate the oxidation behaviour of the alloys to the structure and composition of the oxide layers formed at the surface of the alloys in different oxidation environments.…”

“…Niobium containing alloys have also shown a similar behaviour. As an example, studies performed on autoclaved materials [10] as well as on irradiated materials [11] have revealed Nb-rich metallic phases in the vicinity of the metal-oxide interface in the oxide layer. The oxidation state of the niobium in autoclaved materials has been studied in detail in Zr-2.5%Nb as well as in Zr-20%Nb alloys by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [10].…”