Advanced Techniques for the Characterization of Hydrided Zirconium Alloy

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Allen, Gregory
Materials Science , Nuclear Materials , Zirconium , Hydrides
Zirconium alloy pressure tubes are an important component in CANDU nuclear reactors. During operation these tubes can pick up hydrogen as a result of a corrosion reaction, which can eventually lead to the precipitation of a secondary, brittle zirconium hydride phase. Hydrides tend to first form at flaws (stress concentrations), and when they fracture can initiate a time-controlled crack growth mechanism known as delayed hydride cracking (DHC). Since DHC is a known failure mechanism for pressure tubes, and an ongoing concern in the nuclear industry, more fundamental knowledge is required about the behaviour of hydrides precipitated at flaws. Several approaches were employed in this thesis to better characterize the effects and behaviour of hydrides at such stress concentrations. High energy X-ray diffraction, as well as in-situ SEM testing coupled with digital image correlation, were used to map the strains around stress concentrations where hydrides were present. These studies highlighted important differences in the behaviour of the hydride phase and the surrounding zirconium. To gain greater insight into hydride morphology, neutron tomography was used in an attempt to measure the through-thickness hydride distribution at flaws. A finite element model was also developed and verified against the X-ray strain mapping results. This model provided greater insight into details that could not be obtained directly from the experimental approaches, as well as providing a framework for future modeling to predict the effects of hydride precipitation under different conditions. Taken as a whole, these studies provide important information for improving service guidelines and avoiding conditions that favour embrittlement due to hydride precipitation.
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