Hydride Precipitation Crack Propagation In Zircaloy Cladding During A Decreasing Temperature History

Author : Pierre Clement Simon
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (125 download)

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Download or read book Phase Field Modeling and Quantification of Zirconium Hydride Morphology written by Pierre Clement Simon and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In light water nuclear reactors, waterside corrosion of the cladding material leads to the production of hydrogen, a fraction of which is picked up by the zirconium cladding. Once the hydrogen concentration reaches its solid solubility limit in zirconium, it precipitates into brittle hydride particles. These nanoscale hydride particles aggregate into mesoscale hydride clusters. Depending on the material's texture and the thermomechanical treatment imposed on the cladding, these mesoscale hydride clusters exhibit different morphologies. In particular, the principal orientation of the hydride platelets in the cladding tube can be circumferential or radial. Because hydrides are usually more brittle than the zirconium matrix, the morphology of the mesoscale hydride clusters can affect cladding integrity. This is in part because radial hydrides can ease crack propagation through the cladding thickness and because the concentration of hydrides in specific locations driven by temperature, hydrogen concentration, and stress gradients can create local weak points in the cladding. This dissertation work investigates the link between precipitation conditions, hydride morphology, and hydride embrittlement in zirconium cladding material. The first part focuses on understanding which physics and mechanisms govern the formation of specific hydride microstructures. A quantitative phase field model has been developed to predict the hydride morphology observed experimentally and identify which mechanisms are responsible for circumferential and radial hydride precipitation. The model accurately predicts the elongated nanoscale hydride shape and the stacking of hydrides along the basal plane of the hexagonal zirconium matrix. When investigating the role of applied stress on hydride morphology, the model challenges some of the mechanisms proposed in previous studies to explain hydride reorientation. Although hydride reorientation has been hypothesized to be caused by a change in nanoscale hydride shape and orientation, the current model shows that these mechanisms are unlikely. This study focuses on the precipitation of nanoscale hydrides in polycrystalline zirconium to understand the physics and mechanisms responsible for the change in hydride microstructure from circumferential to radial under applied stress. It proposes a new mechanism where the presence of an applied stress promotes hydride precipitation in grains with circumferentially aligned basal poles. Nanoscale hydrides, even though they still grow along the basal plane of the hexagonal matrix, now grow and stack radially, thus leading to radial mesoscale hydrides. This mechanism is consistent with experimental observations performed in other studies. The second part of this dissertation focuses on the link between hydride morphology and hydride embrittlement. Although hydride microstructure can significantly influence Zr alloy nuclear fuel cladding's ductility, quantifying hydride microstructure is challenging and several of the metrics currently being used have significant shortcomings. A new metric has been developed to quantify hydride microstructure in 2D micrographs and relate it to crack propagation. As cladding failure usually results from a hoop stress, this new metric, called the Radial Hydride Continuous Path (RHCP), is based on quantifying the continuity of brittle hydride particles along the radial direction of the cladding tube. Compared to previous metrics, this approach more closely relates to the propensity of a crack to propagate radially through the cladding tube thickness. The RHCP takes into account hydride length, orientation, and connectivity to choose the optimal path for crack propagation through the cladding thickness. The RHCP can therefore be more closely linked to hydride embrittlement of the Zr alloy material, thus creating a relationship between material structure, properties, and performance. The new definition, along with previously proposed metrics such as the Radial Hydride Fraction (RHF), the Hydride Continuity Coefficient (HCC), and the Radial Hydride Continuity Factor (RHCF), have been implemented and automated in MATLAB. These metrics were verified by comparing their predictions of hydride morphology against expected values in simple cases, and the implementation of the new metric was validated by comparing its predictions with manual measurements of hydride microstructure performed on ImageJ. The RHCP was also validated against experimental measurements of fracture behavior and it was shown to correlate with cladding failure better than previous metrics. The information provided by these metrics will help accurately assess cladding integrity during operation, transportation, and storage.