Effect of zirconium-ion irradiation on properties of secondary phase particles in zirconium-oxide film

To estimate the effect of displacement damage by neutron irradiation of an oxide film of zirconium alloy, a Zircaloy-2 sample was corroded in high temperature and high pressure water at 561 K for 1000 h to form an oxide film. It was then irradiated with 3-MeV Zr2+ ions up to 1.3 × 1020 ions/m2 at 573 K. Subsequently, the crystalline properties of the oxide film and nature of secondary phase particles (SPPs) in the oxide film were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and secondary-ion mass spectrometry (SIMS). From the results off the XRD measurement, monoclinic-ZrO2 was found to be predominant at operation temperature of a light-water reactor, and slightly tetragonal ZrO2 was also confirmed. The diffraction peaks of the monoclinic ZrO2 were slightly clearer in the case of the irradiated sample than in the case of the unirradiated sample. Specifically, ratio of tetragonal ZrO2(011) diffraction to monoclinic ZrO2(-111) diffraction was increased by irradaiation. In addition, half width of monoclinic ZrO2(-111) diffraction was decreased with irradiation dose. This result suggests that the crystallinity of the oxide film was macroscopically improved by the ion irradiation. The diffracted wave corresponding to the (011) plane of the tetragonal ZrO2 was also revealed after irradiation. It may suggest that tetragonal ZrO2 is stabilized by accumulation of irradiation defects. However, the crystal structure difference between irradiated and unirradiated is small. Detailed TEM observation of the oxide film confirmed transition from crystalline to amorphous phase in the SPPs near the metal-oxide interface. Furthermore, the diffraction pattern taken from the SPPs near the surface of the oxide film could not be distinguished from that of ZrO2. That is, it was concluded that those near the surface of the oxide film were assimilated into ZrO2. Their iron concentration was less than that near the metal/oxide interface. Following the TEM observation, SEM observation was performed in the vicinity of the area where the TEM image was obtained. According to the SEM observation, the number of SPPs in the ion-irradiated oxide film tended to be lower than that in the unirradiated oxide film. This tendency was more significant near the surface than near the metal-oxide interface. It is therefore concluded that ZrO2 in the oxide film is not easily irradiated; however, the SPPs are affected by the irradia