Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Abdolvand, Hamidreza

Abstract

In CANada Deterium Uranium (CANDU) nuclear reactors, Zr-2.5Nb alloy pressure tubes separate the hot water and cold moderator. Pressure tubes are susceptible to the diffusion of hydrogen from water and formation of a brittle phase called zirconium hydrides. The diffusion and formation of hydrides are affected by the state of stresses within the tubes. As such, it is of great significance to understand the source of the stresses that develop within the tubes. This thesis focuses on the characterization of the micro and nano scale residual stresses that develop in pure zirconium and Zr-2.5Nb polycrystals. With using three-dimensional synchrotron X-ray diffraction (3D-XRD) technique and crystal plasticity finite element (CPFE) modeling, it is shown in chapter 2 that the state of micro-residual stresses in pure zirconium is affected by grain size and the specimen texture. In chapters 3 and 4, the variation of microstructure and residual stress in a neutron irradiated Zr-2.5Nb CANDU pressure tube specimens are studied and compared to an unirradiated specimen. It is shown that the microstructure, texture, and the grain-scale residual stresses of the pressure tube vary as a function of the axial position along the pressure tube.

Summary for Lay Audience

Nuclear energy provides 16% of the total electricity generated in Canada. In Ontario, about 60% of electricity is generated by the CANada Deterium Uranium (CANDU) nuclear reactors. Pressure tubes are one of the main components of the CANDU reactors. Hot water runs through pressure tubes and removes heat from fuel bundles. CANDU pressure tubes are made of a Zr-2.5Nb alloy. The tubes are susceptible to cracking due to the diffusion of hydrogen from water and formation of a brittle phase called zirconium hydride. It is reported in literature that the formation of this brittle phase is affected by the state of localized stresses that develop in the tube. Therefore, the goal of this research is to better understand how and why these stresses develop. This is done by firstly characterizing the state of stresses in pure zirconium crystals in three dimension using an experimental technique known as 3D-XRD. It is found that grain size and how the crystals sit within grains (orientation) have a significant effect on the state of the stresses. In chapters 3 and 4, specimens were removed from different location of a CANDU pressure tube that was in a reactor for 22 years. Electron diffraction and finite element numerical modeling are used to understand how the state of residual stresses varies along the tube. We report that the front-end of the pressure tubes is relatively more stressed compared to other axial positions that we investigated.

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