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.

Characterization of the conductivity of metal-oxide interface of zirconium based fuel cladding at low and high burnups

“…As it can be clearly seen from Figure 4 the minimum thickness of oxide is not constant below each electrode over the whole area of Pt, since the oxide interface is undulated. This undulation has been reported many times and is a well known phenomenon [25]- [27] with the area of an average protrusion calculated to be approximately 0.9 μm x 0.9 μm. Hence, the path of the current beneath each of the electrodes cannot be totally known.…”

“…Many authors have suggested that the oxide in fact consists of two different regions, an outer heavily cracked porous layer and an inner dense impermeable layer [6], [21], [25], [29], [30]. It is claimed that coolant can penetrate into these outer cracks and hence only the inner dense layer, predominantly acts as a barrier to the penetration of oxidizing species [6], [21], [25], [29], [30]. For this condition to satisfy, an oxygen vacancy gradient formed across this region, following a parabolic trend could be expected with the highest vacancy concentration being at the metal/oxide interface [31], [32].…”

Mechanism of the α-Zr to hexagonal-ZrO transformation and its impact on the corrosion performance of nuclear Zr alloys

“…In previous microstructural studies, both h-ZrO and ZrO2 grains formed during aqueous corrosion have been reported to be very small (20-100 nm) [36][37][38], and it is challenging to carry out micro-texture analysis on such small grains using normal electron backscatter diffraction (EBSD) due to its limited spatial resolution [39]. XRD has been widely used in the study of texture in bulk m-ZrO2, as discussed above, but the crystallography of the h-ZrO suboxide using XRD has rarely been reported due to weak peak intensity from the thin suboxide, and overlapping diffraction peaks originating from multiphase samples [4,40].…”

In-situ TEM study of irradiation-induced damage mechanisms in monoclinic-ZrO2

“…In this study, the zirconium oxides grown by autoclave corrosion were observed to have a high resistance to irradiation-induced amorphisation up to a fluence of 1.9 × 10 16 ions.cm -2 at 10 12 ions.cm -2 .s -1 , but a gradual evolution of microstructure in the bulk oxide can be seen in long term in-situ irradiation experiments as shown in Figure 3. Strong diffraction contrast can be seen in the pre-irradiation sample, Figure 3 (a), arising from the characteristic nanocrystalline grain structure in a corrosion scale grown under simulated reactor conditions, and the complex SAD patterns are also characteristic of the textured zirconium oxide layer [69,70]. The diffraction contrast in the bright-field micrographs gradually changed under irradiation and we observed fewer but brighter spots in the diffraction patterns.…”