Samuel Armson

Significant cost benefits through plant simplification can be achieved if a soluble boron-free lithiated primary water chemistry can be demonstrated to be viable for small modular reactor operation. However, the mechanisms of accelerated corrosion behavior of the zirconium alloy clad material under lithiated and boron-free autoclave conditions have yet to be fully identified. Advanced microstructural characterization of selected samples from the testing program, combined with atomistic simulation, have allowed for a significant development in the understanding of the mechanism of lithium-enhanced acceleration under boron-free conditions. Density functional theory has been used to identify the most stable accommodation mechanisms for lithium in tetragonal, monoclinic, and amorphous ZrO2 and its impact upon the defect population at an atomic scale. Atom probe tomography has confirmed that lithium predominantly segregates to oxide grain boundaries under elevated lithium conditions. The combination of modeling and advanced characterization has suggested that lithium-enhanced acceleration is linked to a local grain boundary effect caused by the segregation of lithium that increases the oxygen vacancy concentration within the usually protective barrier layer and leads to accelerated corrosion rates.

Understanding the in-reactor corrosion behavior of zirconium alloys is essential for optimizing the lifetime of fuel assemblies. Recent advances in available experimental methods have enabled the characterization of oxide morphology, crystallography, and chemical heterogeneity with unprecedented detail for both autoclave and reactor formed oxides. Advanced high-resolution techniques have already improved the understanding of zirconium alloy corrosion performance. However, they are carried out on small volumes of material and require preparation of thin samples, which can lead to changes in the phase distribution in the oxide and often show varied results from different regions of a single bulk specimen. The present study utilizes high-spatial-resolution electron backscatter diffraction (EBSD) performed on bulk samples to produce spatially resolved microtexture data from nanograined zirconium oxide over a large area, which has not previously been possible. This advanced method of plan-view oxide texture analysis, alongside targeted focused ion beam cross-section measurements and substrate EBSD analysis, has revealed well-defined regions of monoclinic oxide grains that exhibit different textures depending on the orientation of the substrate grain on which they have formed. The observed variations in oxide texture have significant implications on any conclusions drawn solely from methods that are limited to the characterization of small areas—especially where sampling areas are smaller than the substrate grain size. Two competing mechanisms of oxide grain growth and nucleation are discussed, and detailed EBSD analysis illustrates a correlation between local oxide texture and corrosion rate. This analysis is performed on specimens of autoclave-tested Zircaloy-2 and ZIRLO and highlights differences in oxide texture development between the two alloys, indicating the significance of material composition and thermomechanical processing on corrosion behavior.

Although there exists a correlation between autoclave and in-reactor zirconium alloy performances, consistent oxidation kinetics discrepancies in these two environments have been observed and a fundamental understanding of the oxidation kinetics enhancement under irradiation is still lacking. Recent results obtained at the Advanced Test Reactor by the Naval Nuclear Laboratory show that photon irradiation significantly affects zirconium corrosion kinetics. In reactors, various photon sources are present in the core from ultraviolet (UV) to gamma (γ) rays. This study aims at characterizing the effect of UV and γ rays on the corrosion mechanism of Zircaloy-4. To this end, a state-of-the-art autoclave equipped with sapphire windows and connected to a recirculation loop has been installed. Zircaloy-4 coupons were exposed for 7 days at 260°C with and without recirculation or UV irradiation (or both). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) oxide characterizations show the presence of iron (Fe)-rich oxide deposits on top of the zirconium oxide where the sample has been irradiated by UV. The deposit concentration is larger in the static corrosion case and does not significantly influence the zirconium oxidation kinetics. A mechanism is proposed to explain the nucleation of these deposits and the relationship to Chalk River Unidentified Deposit nucleation is discussed. In another experiment, Zircaloy-4 coupons have been irradiated at the MIT reactor in neutron+gamma, gamma, and unirradiated loop conditions. The in-core specimens were exposed to ~1021 n/m2 fast neutron fluence in 290°C water at 7 MPa. Oxide layers have been characterized by SEM and TEM. The oxide grain size, t-ZrO2 fraction, fiber texture, and m-ZrO2 twin boundaries’ density were characterized. The results indicate that, at low dpa, the neutron + γ irradiated sample has a more protective oxide than the γ-irradiated sample, which has a more protective oxide than the nonirradiated sample.

If advanced manufacturing technologies are to be adopted over conventional manufacturing processes in the nuclear industry — the most regulatory challenging industry — rigorous fundamental studies that develop underpinning knowledge, materials performance data, and predictive capabilities are essential. Herein we have employed the use of electron backscatter diffraction (EBSD) and 3D X-ray computed tomography (XCT) to characterize microstructure evolution and porosity consolidation during the early stages of powder metallurgy hot isostatic pressing (PM-HIP). The data herein highlight the mechanisms through which the powder particle size distribution encourages localized plastic deformation and subsequent microstructural recrystallization of Type 316L stainless steel; the effect of powder particle size distribution on the rate of porosity consolidation is also discussed. Specifically, we have determined the temperature and pressure conditions that are required to initiate dynamic recrystallization during HIP, and explain how this is influenced by the powder particle size distribution.