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A Ti2AlC MAX phase material was deposited on Zircaloy-4 substrate using the cold spray process, with the goal of enhancing the accident tolerance of zirconium-alloy fuel cladding in light water reactors. The samples were annealed up to 923 K to study the intermetallic phase formation between the coating and Zircaloy-4 substrate. Three interface compounds, ZrAl2, ZrAl, and Zr3Al, were identified and the activation energy of the interfacial intermetallic compound growth was determined. The radiation response of the coating material and interfacial intermetallic compounds after 3.5 MeV Zr2+ ion irradiation was investigated by transmission electron microscopy and nanoindentation. Ion irradiation disrupted the nanolamellar structure of the Ti2AlC coating while amorphization was observed some of the intermetallic compounds. Nanoindentation revealed overall hardening of the Ti2AlC coating and Zr3Al phase, with no significant change observed in the ZrAl2 and ZrAl phases.

The development of a cold spray process for the deposition of chromium (Cr) coatings on zirconium-alloys is presented with the goal of improving the accident tolerance of light water reactor (LWR) fuel cladding tubes. The cold spray parameters and feedstock powders were varied to attain the desired coating properties such as thickness, microstructure, and oxidation resistance, on both Zircaloy-4 flat specimens and Optimized ZIRLO™ cladding tubes. The coated samples were tested at temperatures up to 1300 °C in air to investigate the oxidation performance and inter-diffusion between the Cr coatings and the underlying zirconium-alloy substrate. To simulate the performance of the coatings under normal LWR operating conditions, the coated samples were also tested in a steam autoclave at 400 °C and 10.3 MPa. Microstructures, phases, and hardnesses of the feedstock powders and as-deposited coatings were examined, and oxidation and inter-diffusion profiles were quantified in post-oxidation test samples. Overall, cold sprayed Cr coatings show significant promise for enhancing the accident tolerance of zirconium-alloy fuel cladding in LWRs both in terms of performance and cost-effective manufacturability.

FeCrAl has been proposed as a coating layer to enhance accident tolerance of Zircaloy-4 fuel cladding. In this study, we systematically examined the diffusion kinetics and interface compound formation of the spray-coated FeCrAl and Zircaloy-4 substrate. After vacuum annealing at 923 K, 963 K, and 998 K for various time periods, a mixed Zr2Fe phase and Zr3Fe formed at the interface. The formation energies of these two phases were found to be 65 kJ/mol and 58 kJ/mol for Zr2Fe and Zr3Fe, respectively. Bending tests of cantilevers prepared by a focused ion beam technique showed good interface bonding between the coating layer and the substrate. Nanoindentation and micropillar compression tests in the cross-section of polished samples show different mechanical responses of each phase. The substrate exhibited the lowest hardness and most ductile deformation under compression, while interfacial Zr2Fe and Zr3Fe possessed the highest hardness and brittle failure.

The first in-situ ion irradiation and transmission electron microscopy (TEM) of cold spray deposited Cr coatings was performed at the Intermediate Voltage Electron Microscope (IVEM) facility at Argonne National Laboratory for preliminary investigation into the radiation damage tolerance of these materials for use in accident tolerance fuel cladding. The severely plastically deformed microstructure of the cold spray deposited Cr delayed the onset and growth of radiation induced defects when compared to an annealed coating simulating bulk Cr. Evidence to support this conclusion was based on defect density and defect size measurements of TEM images obtained at different irradiation intervals.

Here, we investigate the coupled thermomechanical fracture mechanisms of coated nuclear fuel claddings at Light Water Reactor (LWR) operating temperatures. These coated claddings are a highly attractive, near-term solution that address the demands for accident-tolerant fuel systems that provide greater oxidation resistance. However, the fracture mechanisms of these coatings, which may create channels for oxidation ingression, must be fully understood prior to implementation. Thus, high-temperature expanding plug experiments were conducted on coated cladding specimens at a temperature of 315 °C, consistent with the operating environment of LWRs. In-situ thermomechanical deformation was measured with stereo digital image correlation during heating and mechanical testing to separately resolve contributions of thermal and mechanical strain. Image correlation, supported by acoustic emissions (AE) detection, was also leveraged to track cracking activity during loading. Coating fracture was found to initiate at total hoop strains of 0.34%. The thermal deformation of the coated claddings was investigated via finite element simulations, revealing a bidirectional stress-state within the coating with axial and circumferential strains of 0.026 and 0.031%. This bidirectional stress-state was attributed with the generation of off-axis fracture pattern within the coating as identified via post-experiment scanning electron microscopy. Thus, this study unveiled critical, coupled thermomechanical mechanisms governing the coating fracture of coated claddings at LWR temperatures.

Annealing of attrition-milled, electrolytically produced chromium powder has been investigated to improve cold spray coating deposition efficiency of pure chromium on Zr-alloy substrate for light water reactors with the goal of enhancing high-temperature oxidation resistance. The annealing heat treatment at 800 °C for 5 h induced microstructural transitions in the powder such as the development of equiaxed grains and strain relaxation, both associated with a measured decrease in nano-hardness. Deposition efficiency of the annealed powder was about three times more than the as-received electrolytic Cr powder. In addition, the utilization of the annealed powder reduced substrate deformation effects. A qualitative explanation of the effects powder microstructure on the cold spray deposition process in terms of the resulting coating microstructures and deposition efficiencies has been introduced. Finally, high-temperature exposure tests indicated that oxidation resistance of these Cr coatings was comparable to that previously reported for those produced using gas-atomized Cr powder. This study suggests that annealing of electrolytic Cr powder is a practical and economically favorable pathway to produce oxidation-resistant cold spray Cr coatings. More generally, use of the widely available electrolytic and mechanically milled powder vastly opens scope of cold spray deposition process, where atomized powders may not be either available or challenging to produce.

The cold spray process has been investigated for the deposition of FeCrAl coatings on optimized ZIRLO™ fuel cladding tubes in order to enhance oxidation resistance at high temperatures (>1000 °C) that would be encountered in loss of coolant accident (LOCA) scenarios. The FeCrAl alloy provides excellent oxidation resistance, however Fe forms a eutectic with Zr at temperatures as low as 928 °C resulting in massive inter-diffusion and associated melting. Therefore, a Mo interlayer coating was developed also using the cold spray process to serve as a diffusion barrier between the Fe-based coating and the Zr-alloy substrate. To achieve a combination of desired properties for the dual-layered coating-substrate, an optimization study of spray parameters such as propellant gas pre-heat temperature, helical spray speed, and the number of spray passes was conducted. This parametric study was aimed at achieving dense coatings with desired thickness and coverage of the substrate, and good adhesion of the Mo interlayer coating with subsequent top FeCrAl coating and the Zr-alloy substrate. The optimal FeCrAl-Mo coating protected the Zr-alloy cladding tubes from oxidation at high temperatures of 1200 °C, while at the same time preventing diffusion-induced eutectic melting in the substrate.

The cold spray coating process has been developed at the University of Wisconsin-Madison for the deposition of oxidation-resistant coatings on zirconium alloy light water reactor fuel cladding with the goal of improving accident tolerance during loss of coolant scenarios. Coatings of metallic (Cr), alloy (FeCrAl), and ceramic (Ti2AlC) materials were successfully deposited on zirconium alloy flats and cladding tube sections by optimizing the powder size, gas preheat temperature, pressure and composition, and other process parameters. The coatings were dense and exhibited excellent adhesion to the substrate. Evaluation of the samples after high-temperature oxidation tests at temperatures up to 1300°C showed that the cold spray coatings significantly mitigate oxidation kinetics because of the formation of thin passive oxide layers on the surface. The results of the study indicate that the cold spray coating process is a viable near-term option for developing accident-tolerant zirconium alloy fuel cladding.

The air oxidation behavior of zirconium-silicide coatings for three stoichiometries, namely, Zr2Si, ZrSi, and ZrSi2, at 700 °C has been investigated. These three coatings were deposited on a zirconium-alloy substrate using a magnetron sputter process at a low temperature. Argon gas pressure was observed to have a profound effect on the coating microstructure, with lower pressures favoring a denser and more protective microstructure. Coatings of ZrSi2 stoichiometry clearly showed superior oxidation resistance presumably due to the formation of a thin protective oxide layer, consisting of nanocrystalline SiO2 and ZrSiO4 in amorphous Zr-Si-O matrix. The thermal stability of the coatings was evaluated by annealing in an argon environment, and this also assisted in eliciting the effects of oxidation-induced inward Si migration. Thicker coatings of ZrSi2 were prepared and evaluated for oxidation resistance at 700 °C for longer exposure times, as well as at 1000 °C and 1200 °C. Once again the thin oxide layer provided for significant oxidation resistance. Pre-oxidizing the samples at 700 °C prior to 1000 °C and 1200 °C oxidation tests substantially reduced the extent of oxidation. Insights into the fundamental mechanisms of the oxidation behavior of zirconium-silicide coatings were obtained using a combination of scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy techniques. One potential application of these coatings is to enhance the oxidation resistance of zirconium-alloy fuel cladding in light water reactors under normal and accident conditions.

Recently, the diffusion coefficient of silver (Ag) in zirconium carbide (ZrC) at 1500 °C was determined to help elucidate the diffusive behavior of radioactive 110m Ag in the ZrC coating of the tri-structural isotropic (TRISO) nuclear fuel particle. This study was conducted using a recently developed diffusion experimental technique, known as the encapsulating source method. To verify the validity of this experimental method, modifications to the original experimental design have been made within the domain of this method. Primarily, the ability to retrieve the ZrC disk made it possible to create depth profiles using secondary ion mass spectrometry (SIMS) for more precise depth profiling of Ag concentration in ZrC. Applying this improved experimental design, a sample was fabricated and diffusion annealed at 1500 °C for 2 h. Finally, silver concentrations on the ZrC disk were measured using SIMS. Consequently, the diffusion coefficient of Ag in ZrC 0.95 at 1500 °C was experimentally determined to be about 1.7 × 10 −17 m 2 /s, remarkably consistent with the previously reported value of 2.8 × 10 −17 m 2 /s, also obtained by means of the encapsulating source method. This proves that the encapsulating source method is a reliable experimental technique to determine the Ag diffusivity in ZrC.

Selective modification (e.g. defect creation and amorphization) of diamond surfaces is of interests for functional diamond-based semiconductors and devices. Bombarding the diamond surface with high energy radiation sources such as electron, proton, and neutrons, however, often result in detrimental defects in deep bulk regions under the diamond surface. In this study, we utilized high energy carbon ions of 3 MeV to bombard the polycrystalline diamond compact (PDC) specimen. The resultant microstructure of PDCs was investigated using micro Raman spectroscopy. The results show that the carbon bombardment successfully created point defects and amorphization in a shallow region of ∼500 nm deep on the diamond surface. The new method has great potential to allow diamond-based semiconductor devices to be used in numerous applications.

Coated nuclear fuel claddings offer a promising, near-term solution to address the demand for next-generation, accident-tolerant fuel systems. It is expected that they will possess superior mechanical properties and greater oxidation resistance compared to current cladding technology, allowing for improved performance during beyond design-basis accident conditions. Here, we present a methodology to determine the failure mechanisms of chromium-coated zirconium-alloy claddings under expected, critical loading conditions. Three-dimensional digital image correlation and acoustic emission techniques were used in situ to monitor spatial strain and crack development of the claddings under two key, de-coupled loading conditions: expanding plug and four-point bending. Critical strain levels, at which cracking initiated, were determined to be 0.4% ehoop and 0.4% exx for expanding plug and four-point bending, respectively. A two-dimensional fracture model was also developed for the expanding plug loading condition based on inputs determined from mechanical testing. It was concluded that coating fracture of expanding plug specimens was axisymmetric across the specimen circumference and occurred rapidly through the thickness of the chromium coating. Subsequent high temperature steam oxidation experiments on tested (cracked) expanding plug specimens showed no signs of oxidation degradation to the underlying zirconium alloy, thereby showcasing the effectiveness of the chromium coating. This comprehensive, multi-scale study is intended to inform future testing of next-generation, coated claddings and identify the resulting failure mechanisms that arise in beyond design basis accident conditions.

Oxidation kinetics and microstructural evolution of cold sprayed Cr coatings on Zircaloy-4 at 1130–1310 °C in flowing steam at atmospheric pressure have been studied. The study is aimed at understanding of the response of Cr coated Zr-alloy under a steam environment and high temperatures pertinent to design basis accidents (DBAs) and beyond design basis accidents (BDBAs) in light water reactors (LWRs). Surface morphology, microstructure, and phases of post-oxidation test samples were characterized using Scanning Electron Microscopy (SEM), x-ray diffraction (XRD), and Scanning Transmission Electron Microscopy (STEM). Growth kinetics of the Cr-oxide scale and interdiffusion layers between the Cr coating and the Zr-alloy substrate were quantified from cross-sectional SEM images. Cross-sectional analysis showed that the Cr coatings offered a 50 times reduction in oxidation rate over bare Zircaloy-4 in a 1310 °C steam environment. Oxidation kinetics at 1130 °C followed parabolic law (i.e., n ∼ 0.5 in power law kinetics) but at 1230 °C and 1310 °C the value of n was suppressed to below 0.5 possibly due to the volatilization of Cr species at the two highest temperatures. Interdiffusion at coating/substrate interface resulted in formation of a brittle Cr2Zr or Zr(Fe, Cr)2 intermetallic compound layer on the order of micrometer in thickness and scattered Cr-rich precipitates were observed well below the interface within the Zr-alloy substrate after cooling. These experimental results could provide data to LWR system simulation codes for better estimation of coping time in the event of accidents.