Predicting the Influence of Hydrogen on Zirconium

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Authors
Aiyeru, Segun
Keyword
Zirconium alloys , Zirconium hydride , Finite Element Modelling , Stress concentration , Hydrogen diffusion , Hysteresis , Precipitation models , CANDU Pressure Tube
Abstract
The corrosion process that ensues when the alloys of zirconium used as structural components and fuel claddings in nuclear reactors react with the heavy water used as coolant in the CANDU® reactors results in the production of hydrogen or deuterium. Some of this hydrogen enters the alloy and causes a gradual increase in the total hydrogen concentration from the initial average hydrogen concentration during fabrication. In addition to the gradients in temperature and concentration, the presence of stress raisers, such as cracks and blunt flaws influences the distribution of hydrogen as the concentration of mobile hydrogen in solution are preferentially increased at regions of localised high tensile stress. Hydride precipitates at these flaws when the solubility limit of hydrogen in the alloy is exceeded. A change of phase accompanies the precipitation of the brittle hydrides resulting in change in the mechanical properties of the alloy as well as additional strain due to volumetric expansion. With the aid of Finite Element Analysis, this thesis describes models of the diffusion of hydrogen in response to gradients in stress, temperature, and concentration; the nucleation of hydride; and the growth of hydride. The commercial finite element code, ABAQUS® combined with subroutine codes written in FORTRAN are developed to fully couple the diffusion – displacement analysis. The fully – coupled hydride precipitation model which is not readily available in any commercial finite element package is robust, reliable, and readily parallelisable allowing for large models to be run while incorporating other complexities such as eigen strain, thermal strain, and creep.
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