Experimental and modelling evidence for hydrogen trapping at a β-Nb second phase particle and Nb-rich nanoclusters in neutron-irradiated low Sn ZIRLO

Zirconium-based alloys used for fuel cladding in nuclear fission reactors are susceptible to hydrogen embrittlement during operation, but we currently lack the necessary mechanistic understanding of how hydrogen behaves in the materials during service to properly address this issue. Imaging the distribution of hydrogen within material microstructures is key to creating or validating models that predict the behaviour and influence of hydrogen on material properties, but is experimentally difficult. Studying hydrogen in zirconium-alloys is further complicated by the fact that the most common routes for preparing specimens for Transmission Electron Microscopy and Atom Probe Tomography (APT) analysis, electropolishing and focused ion beam (FIB) milling, are known to induce hydride formation. This introduces uncertainty as to whether the hydrogen distribution in the analysed specimen is actually representative of the entire sample a priori. Recent work has shown that this effect can be mitigated by performing the final specimen thinning stages at cryogenic temperatures. In this paper we use cryo-FIB to prepare APT specimens of neutron-irradiated low Sn ZIRLO, showing that hydrogen is trapped within a beta-Nb SPP and at Nb-rich nanoclusters formed by exposure to neutron irradiation. We then use density functional theory calculations to explain these experimental observations. These results highlight the importance of including niobium-rich features in models used to predict hydrogen pick-up in zirconium alloys during service and delayed hydride cracking during storage.