Intergranular residual stress analysis of irradiated Zr-2.5Nb pressure tube material**

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Nuclear energy contributes to more than 50% of power production in Ontario. In the core of the CANada Deuterium Uranium (CANDU) reactors, zirconium and its alloys have been used extensively. This is due to having good mechanical properties, and being transparent against the neutrons generated during fission reactions. At micro-level, zirconium alloys comprise of crystals, also known as grains, with specific atomic arrangement that are oriented in specific direction. Two of the main atomic arrangement observed in zirconium alloys are Hexagonal Close-Packed (HCP) and Body-Centred Cubic (BCC). When zirconium crystals are deformed, their response to the applied load depends on the direction of the load. This is called anisotropy. The crystal structure and orientation are the reasons for such observation.**In CANDU reactors, pressure tubes are the primary pressure boundary between the heavy water coolant and the cold moderator. These tubes are made of a zirconium-niobium alloy called Zr-2.5Nb. The alloy has about 5% BCC and 95% HCP crystals that are formed during manufacturing process. During service, hydrogen atoms from water coolant can penetrate into the crystals of the pressure tube and form hydrides. These hydrides are brittle and reduce the expected life-span of the tube. One of the parameters that affects the rate of hydride formation is the state of the stress in the tube's crystals. **It has been observed that the orientation of the hydrides formed in the two ends of the tube are not consistent. This will significantly affect the fracture toughness of the tubes. It is hypothesized that this is due to microstructural variations and development of different level of residual stresses at the two ends of the tube. In this project, samples will be extracted from the two-ends of neutron irradiated pressure tubes to investigate such variations. Finite element modelling will be used with experiments to understand the level of stress variation along the tube. This project will be in collaboration with the Canadian Nuclear Laboratories. The long term goal of this project is to modify manufacturing routes to increase life-expectancy of the pressure tubes. ********************