Jonna Partezana

Microstructure and Growth Mechanism of Oxide Layers Formed on Zr Alloys Studied with Micro-Beam Synchrotron Radiation

The structures of oxides formed in water and lithiated water on three Zr-based alloys with varied corrosion behavior were studied with micro-beam synchrotron radiation and optical microscopy. Micro-beam synchrotron radiation (0.2 µm spot) has a unique combination of high elemental sensitivity (ppm level) and fine spatial resolution that allowed the determination of various oxide characteristics such as phase content, texture, grain size, and composition as a function of distance from the oxide-metal interface. Micro-beam X-ray fluorescence shows that the oxides formed in lithiated water have increased levels of Fe absorbed from the autoclave environment indicating greater oxide porosity in these oxides. The phase content, texture, and grain size of oxides were studied in detail using synchrotron radiation micro-beam diffraction for samples corroded in water and lithiated water. A remarkable periodicity was observed in the oxide structures using various techniques including X-ray peak intensities for both monoclinic and tetragonal zirconia, texture, and optical microscopy. The periods were similar to the transition period and were less visible in the oxides that behaved worse in lithiated water. These results are discussed in terms of models of oxide growth and of the differences between alloys.

Nanocrystalline diamond protects Zr cladding surface against oxygen and hydrogen uptake: Nuclear fuel durability enhancement

In this work, we demonstrate and describe an effective method of protecting zirconium fuel cladding against oxygen and hydrogen uptake at both accident and working temperatures in water-cooled nuclear reactor environments. Zr alloy samples were coated with nanocrystalline diamond (NCD) layers of different thicknesses, grown in a microwave plasma chemical vapor deposition apparatus. In addition to showing that such an NCD layer prevents the Zr alloy from directly interacting with water, we show that carbon released from the NCD film enters the underlying Zr material and changes its properties, such that uptake of oxygen and hydrogen is significantly decreased. After 100–170 days of exposure to hot water at 360 °C, the oxidation of the NCD-coated Zr plates was typically decreased by 40%. Protective NCD layers may prolong the lifetime of nuclear cladding and consequently enhance nuclear fuel burnup. NCD may also serve as a passive element for nuclear safety. NCD-coated ZIRLO claddings have been selected as a candidate for Accident Tolerant Fuel in commercially operated reactors in 2020.

Understanding Corrosion and Hydrogen Pickup of Zirconium Fuel Cladding Alloys: The Role of Oxide Microstructure, Porosity, Suboxides, and Second-Phase Particles

We have used a range of advanced microscopy techniques to study the microstructure, the nanoscale chemistry and the porosity in a range of zirconium alloys at different stages of oxidation. Samples from both autoclave and in-reactor conditions were available to compare, including ZIRLO, Zr-1.0Nb and Zr-2.5Nb samples with different heat-treatments. (Scanning) Transmission Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD) and automated crystal orientation mapping with TEM 2,3 were used to study the grain structure and phase distribution. Significant differences in grain morphology were observed between samples oxidised in the autoclave and in-reactor samples, with shorter, less well-aligned monoclinic grains and more tetragonal grains seen in the neutron irradiated samples. A combination of Energy Dispersion X-ray (EDX) mapping in STEM and Atom Probe Tomography (APT) analysis of SPPs can reveal the main and the minor element distributions respectively. Neutron irradiation seems to have little effect on promoting fast oxidation or dissolution of β-Nb precipitates, but encourages dissolution of Fe from Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of Nb in β-Nb SPPs in the oxide reveal the fully oxidised Nb state in the SPPs deep into the oxide, but Nb in the crystalline SPPs near the metaloxide interface. EELS analysis and automated crystal orientation mapping with TEM have also revealed Widmanstatten-type suboxide layers in some samples with the hexagonal ZrO structure predicted by ab initio modelling. The combined thickness of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the estimated instantaneous oxidation rate, suggesting that the presence of this oxygen rich zone is part of the protective oxide that is rate limiting in the key in the transport processes involved in oxidation. Porosity in the oxide has a major influence on the overall rate of oxidation, and there is much more porosity in the rapidly oxidising annealed Zr-1.0Nb alloy than found in either the recrystallised alloy or the similar alloy exposed to neutron irradiation.