Hydrogen pick-up in Zr alloys - Muzic 2 (Project 1)

Zirconium alloys were selected for use as nuclear fuel cladding and structural fuel assembly components in nuclear reactors in the early 1950s because of an attractive combination of low thermal neutron absorption cross-section, adequate corrosion resistance in high temperature water and reasonable mechanical properties. However, aqueous corrosion and hydrogen pick-up (HPU) of zirconium alloys have now become the major factors limiting the high fuel burn-up or prolonged fuel campaigns in nuclear plant.

The general waterside corrosion kinetics of zirconium alloys has two distinct regions: the first one is an initial pre-transition period of slow parabolic or cubic corrosion rate and then followed by an abrupt increase named ‘’breakaway’’ to almost linear and cyclic corrosion rate in autoclave. HPU happens when hydrogen ions which are produced by oxidation process diffuse from the water/oxide interface to the metal/oxide interface, forming a solid solution at high temperature and then precipitating as hydrides which can lead to hydrogen embrittlement and delayed hydride cracking.

This project will use advanced electron microscopy methods to explore the structure and microchemistry of new low-Sn test alloys that have shown excellent corrosion resistance in autoclave testing (the developmental alloy X2 in particular).  There is almost nothing known about why these alloys have such good properties, and in particular their lowest HPU fraction and a delayed by a factor of up to 2  oxidation kinetic transition is compared to the current generation of commercial nuclear alloys.

TEM samples is made by in situ lift-out focused ion beam (FIB) which can produce TEM thin foils less than 100 nm thick with a large homogeneous electron transparent area. Bright-field (BF) and dark-field (DF) images will give detailed information on grain size and morphology in both metal and oxide, Zr hydrides , SPPs and micropores. Selected area diffraction (SAD) patterns and chemical analysis (EDX/EELS) will further help analysing phases and grain texture, possible links between transformation of phases and can distinguish between face centred tetragonal γ-ZrH and face centred cubic δ-ZrH~1.5. Fresnel contrast in the defocused condition will be used in this work in order to study nano-porosity in the oxide. 3D FIB slicing will provide a quantitative characterisation of hydrides, cracks and SPPs distribution in oxide layer and oxide/metal interface in different stages of oxidation which will help identifying current hydrogen migration paths in oxide scale.