Fig 7. Microstructur EvolutionThe microstructural changes of the zircaloy-4 samples before and after hydriding are illustrated in Figs. 7A-E. Very fine microstructure is shown in Fig. 7A, even grains are not clearly visible using optical microscope. At 3500C hydrogen atmosphere, the microstructure of the hydrided sample consisted of -Zr grains and needle which indicates presence of hydrides (Fig. 7B). The hydride morphologies varies with increase of operating temperature hydriding.As discussed in many literature, the solubility of hydrogen drastically decreases with decreasing temperature in zirconium alloys, where hydrogen is more stable as a solution in -Zr than as a gas[1]. Lattice defects such as vacancies may occure during fabrication process of zircaloy-4 tube,which is increasing hydride nucleation possibilities. Hydride nucleation is not homogeneous in zircaloy. In general, hydride nucleation mainly occurs at grain boundaries, but in some cases, many researcher have observed hydrides within the grains[2].During the increase of temperature, needles become thicker and the hydrides concentration increased. Finally, the combined effects of defect and large concentration, the nucleation and growth of hydrides are facilitated in zircaloy samples. Based on XRD analysis, some of hydrides phase were formed in this experiment.