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

核能占安大略省电力生产的50%以上。在加拿大的氘铀堆芯中，锆及其合金被广泛使用。这是由于它具有良好的机械性能，并且对裂变反应中产生的中子是透明的。在微观层面上，锆合金由晶体（也称为晶粒）组成，具有特定方向取向的特定原子排列。在锆合金中观察到的两种主要原子排列是六方密排（HCP）和体心立方（BCC）。当锆晶体变形时，它们对外加载荷的响应取决于载荷的方向。这被称为各向异性。晶体结构和取向是造成这种现象的原因。在CANDU反应堆中，压力管是重水冷却剂和冷慢化剂之间的主压力边界。这些管子由一种叫做Zr-2.5Nb的锆铌合金制成。该合金在制造过程中形成了约5%的BCC和95%的HCP晶体。在使用过程中，来自水冷剂的氢原子可以渗透到压力管的晶体中并形成氢化物。这些氢化物易碎，降低了管子的预期寿命。影响氢化物形成速率的参数之一是管中晶体的应力状态。已经观察到，在管的两端形成的氢化物的取向是不一致的。这将显著影响管的断裂韧性。据推测，这是由于微观结构的变化和在管的两端不同水平的残余应力的发展。在本项目中，将从中子辐照压力管的两端提取样品来研究这种变化。有限元模型将与实验一起使用，以了解沿管的应力变化水平。这个项目将与加拿大核实验室合作。该项目的长期目标是修改制造路线，以增加压力管的预期寿命。

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.