Microstructural and hydriding studies of zirconium based materials for thermal reactor applications

The Zircaloy-2, Zircaloy-4 and Zr-2.5 Nb alloys, used as fuel cladding and other structural components, such as Calandria Tubes and Pressure Tubes, has shown satisfactory performance in pressurised heavy water reactors (PHWR's). However, there is always demand and scope for the better and improved performance of cladding and structural components which requires modification of process flow sheet and development of newer alloys e.g. PHWR reactors designed for higher operating temperature than the current PHWR operating temperature. There is also thrust for development of cladding material with better corrosion and mechanical properties suitable for higher and extended burn up applications. The mechanical properties, irradiation behaviour performance of the material is primarily influenced by the microstructural features such as constituent phases, their morphology, shape, size and size distribution of the precipitates, nature of defects etc. Therefore, tailoring of microstructure and study of the structure-property relationship is very important. The chemistry of the alloy and thermo-mechanical- treatments are manipulated for obtaining microstructure to obtain the desired performance. In the present study, a series of binary, ternary and quaternary alloys have been prepared. The basic phase transformation mechanisms and the microstructure evolution in these alloys after different heat treatments have been studied using optical scanning and transmission electron microscope. Some of the important results obtained will be presented. Hydriding is most important corrosion problem in the zirconium cladding alloys. The mechanism of hydride formation and their crystallographic and microstructural aspects will be presented in detail for relevant zirconium alloys. Zirconium alloys being crystallographically, asymmetric it shows deformation and annealing texture. Result of the studies on evolution of bulk and micro texture during deformation and hydriding will be highlighted. (author)