Understanding the precipitation of brittle hydride phases is crucial in establishing a failure criterion for various zirconium alloy nuclear fuel cladding. Accordingly, it is important to quantify the sensitivity of hydride precipitation to the component microstructure. This experimental investigation focuses on two microstructural characteristics and their role as hydride nucleation sites: The grain size and the alloy chemical composition. Samples of commercially pure zirconium (Zr-702) and Zircaloy-4, each with a wide range of grain sizes, were hydrided to 100 ppm and micrographs of the hydride distribution were optically analyzed for inter-granular and intra-granular precipitate sites. For most grain sizes, it was found that a significantly lower fraction of the precipitated hydrides nucleated at grain boundaries in Zircaloy-4 than in Zr-702, suggesting that a higher SPP content encourages the formation of intra-granular hydrides. Moreover, this effect became more prominent as the grain size increased; large-grain specimens contained a higher fraction of intra-granular hydrides than small-grain specimens of both Zr-702 and Zircaloy-4, highlighting the potency of grain boundaries as nucleation sites and how SPPs can influence the hydride distribution profile.

Hydrogen pick-up in zirconium alloys can lead to their structural failure, which is an important problem in the nuclear industry. This investigation focuses on modelling the accumulation of hydrogen in the vicinity of loaded V-notches in four-point bend Zircaloy-4 specimens. In order to account for the anisotropic diffusivity of hydrogen in hexagonal close-packed α-zirconium, a multiscale methodology is proposed to compute notch-tip hydrogen profiles. This methodology unifies continuum scale stress analysis, using the finite element approach, and atomistic scale stress analysis, using the elastic dipole tensor of point defects. The steady state notch-tip hydrogen profiles are determined for different notch geometries and crystal orientations. It was found that hydrogen enhancement is greater but more localised for sharper notches with a smaller flank angles, which is the expected effect of stress. It was also found that hydrogen enhancement is greater if the notch opening plane coincides with the prism plane as opposed to the basal plane. This anisotropic effect is a consequence of the trigonal symmetry of the hydrogen interstitialcy.

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Crack initiation in zirconium alloys is an important issue for the safety of water-cooled fission reactors. Zirconium hydrides that precipitate in service are potential crack nucleation sites. In this work, the deformation and cracking of zirconium hydrides was studied during room temperature deformation of a Zircaloy-4 tensile sample up to fracture. The sample contained a hydrogen concentration of 100 ± 20 ppm. The main aims of this study were to better understand the mechanisms behind the hydride fracture in a polycrystalline matrix, and to identify at which point in the deformation of the Zr matrix the first hydrides break. Cracks thus nucleated may coalesce and propagate through the hydrided Zr-alloy. Scanning electron microscopy (SEM) images of a number of hydrides, both intergranular and intragranular, were taken at discrete increments of deformation during an interrupted tensile test. The results show that cracks in hydrides tend to always occur normal to the applied load, signalling the importance of the external stress. However, evidence is also provided to support the hypothesis that internal stresses generated by microstructural constraints may lead to the fracture of some intergranular hydrides.

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