Enabling 3D atomic-scale imaging of hydrogen to investigate hydrogen embrittlement of zirconium alloy fuel cladding in fission reactors

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Zirconium alloys have a critical role in nuclear reactors, providing a hermetic barrier in the fuel rod assembly, which is designed to withstand the high neutron flux and high temperatures of a nuclear reactor environment, for the full operational life of the fuel rod. This ensures that the nuclear fuel contained in the rod is safely isolated from the remaining parts of the reactor. However, despite its many advantages, the life of the fuel rod assembly is often limited by the corrosion behaviour of the zirconium rod. Hydrides can form within the rod, altering the dimensions and stress states of the rod, making it prone to crack. Thus to prevent failure of the rods, these are removed from the reactor well in advance of their useful life. As little as 6% of the energy available in the nuclear reaction can be extracted in the reactor, before the fuel must be removed and disposed of. This leads to higher costs for energy, and critically, higher waste volumes in the reactor output. This waste is in addition to the stockpiled waste within the UK, and is difficult to store and dispose of - again compounding loss. As such, we seek to better understand the interaction of these materials, to minimise waste and to meet the legally mandated carbon dioxide goals for the UK. Understanding the chemistry of these materials, and their interaction with hydrogen is therefore critical - this project seeks to identify how hydrogen interacts with these Zirconium alloys, and to examine the behaviour of mechanical deformation and different chemistries of the material on the hydrogen distribution. This project will develop highly novel methodologies to adapt Atom Probe Tomography (APT), in conjunction with isotopic analysis, to identify how hydrogen is distributed at the atomic-scale within a zirconium sample, and correlate this to the material behaviours that limit the safe operating lifetime of cladding material. The new insights provided will help inform the development of new alloying methods to provide longer-life fuel rods, and inform how zirconium alloys can be designed to optimise their operational lifetimes within a reactor environment. This capability represents an entirely new approach to the study of hydrogen within zirconium alloys. Previously, APT has been used by many researchers to investigate the oxide and suboxide regions of zirconium alloys, to better understand the corrosion effects of alloying elements within Zirconium. However, this project will undertake new research to provide direct experimental evidence at the atomic scale, to understand how hydrogen interacts with zirconium, and how hydrogen cracking effects can be minimised. The project will provide direct experimental data on Zr materials (such as Zr-4) that have been exposed to hydrogen atmospheres, and provide quantitative data on how hydrogen is interacting with the chemistry and microstructure of the host material. It will identify the location and concentrations of hydrogen within these materials to understand how hydrogen, which originates within the water region of the reactor, transports into the material and degrades them. The project falls within the EPSRC Energy research area. It will be undertaken in collaboration with industrial partners at National Nuclear Laboratory and Rolls Royce.