Zirconium and its alloys have been used extensively in both light and heavy water reactors where neutron irradiation is known to cause microstructural evolution, leading to degradation of mechanical properties and dimensional instabilities. Dimensional instabilities due to irradiation growth are particularly crucial for Zr alloy Excel which is the proposed candidate material for the conceptual CANDU-Super Critical Water Cooled Reactors (SCWR) pressure tube. This study employs the in-situ ion irradiation technique and transmission electron microscopy to investigate the irradiation induced microstructural evolution in Zr and Zr alloy Excel. The current study is presented as a manuscript format dissertation comprised of five manuscript chapters. Chapter 3 reports the formation of irradiation induced prismatic defects directly from cascade collapse in pure Zr at low dose (0.008 dpa) in a temperature range of 300oC-500oC. The morphology and yield of these defects are found to be temperature and dose dependent. In Chapter 4, irradiating Zircaloy-2 under similar conditions to pure Zr, reveals that nucleation rate of small prismatic loops increases, whereas their growth is suppressed which indicates that these defect clusters are not only temperature dependent but also impurity dependent. Chapters 5, 6 and 7 report the irradiation induced microstructural changes at various temperatures up to a dose of 10 dpa, in several microstructures of Zr alloy Excel, achieved by different solution treatments. The major focus of these experimental studies was the formation of <c>-component loops in α-phase; decomposition of β-phase; and irradiation induced microchemical changes. It was found that nucleation and growth of <c>-component loops is strongly dependent on irradiation temperature, parent microstructure, and presence of alloying elements. <c>-component loops nucleate above a threshold incubation dose which decreases with an increase of irradiation temperature. Energy dispersive X-ray spectroscopy (EDS) mapping on irradiated microstructures revealed the formation of small Sn clusters in α-phase which have a significant effect on the morphology of <c>-component loops. Fe plays an important role in the nucleation of <c>-component loops, as it distributes itself during irradiation either from β-phase or from pre-existing secondary phase precipitates in α-phase. Furthermore irradiation induced decomposition of β-phase was observed in the form of ω-phase precipitation and irradiation induced clustering of alloying elements. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2013-12-31 23:50:30.352