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The objective of this work is to study friction surfacing process variability when depositing multilayered coatings. This is motivated by the need to maintain deposition quality when depositing multiple friction surfacing layers, whether for repair, remanufacturing, or new part creation using this solid-state metal additive manufacturing process. In this study, 10-mm-diameter 304L stainless steel rods were used to create up to five layers of 40-mm-long coatings on 304L substrates using a constant set of processing parameters. In-process measurement of forces (X, Y, Z), flash temperature, flash geometry, layer temperature, and post-process measurement of layer geometry, microhardness, and microstructure are used to characterize changes in the friction surfacing process as more layers are deposited. It was observed that with increasing layers: layer thickness and deposition efficiency decrease; offsetting of the deposition towards the retreating side, and temperature in the deposited layer increase; and flash temperature does not change. Metallurgical analyses of friction-surfaced cross-sections revealed fine grain refinement and transformation of base austenite to strain-induced martensite. It is concluded that the process parameters need to be adjusted even after the second or third layer is deposited, corrections to the tool path are required after a couple of layers, and the measured process forces, as well as deposited layer temperature, may be useful to monitor and control the process and its instabilities.

Light water reactors (LWR) use zirconium-alloy fuel claddings, the tubes that hold the uranium-dioxide fuel pellets. Zr-alloys have very good neutron transparency, but during a loss of coolant accident or beyond design basis accident (BDBA) they can undergo excessive oxidation in reaction with the surrounding steam environment. Relatively thin oxidation-resistant coatings on Zr-alloy fuel cladding tubes can potentially buy coping time in these off-normal scenarios. In this study, cold spraying, solid-state powder-based materials deposition technology has been developed for deposition of oxidation-resistant Cr coatings on Zr-alloy cladding tubes, and the ensuing microstructure and properties of the coatings have been investigated. The coatings when deposited under optimum conditions have very good hydrothermal corrosion resistance as well as oxidation resistance in air and steam environments at temperatures in excess of 1100 °C, while maintaining excellent adhesion to the substrate. These and other results of this study, including mechanical property evaluations, will be presented.

Cold spray deposition is being investigated for mitigation of chloride-induced stress corrosion cracking (CISCC) in dry cask storage systems (DCSS) for spent nuclear fuel. Welded regions of austenitic stainless-steel canisters in DCSS are under tensile stress and susceptible to environmental chloride corrosion, which can potentially lead to the formation of CISCC. The low thermal input and high throughput nature of cold spraying make it a viable repair and mitigation option for managing potential CISCC. Cold spray coatings are under compressive stress and act as a barrier in Cl-rich environments. Characterization data including microstructure, hardness, and corrosion resistance are presented for cold spray coatings on stainless steel substrates.

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.

Since the accident at Fukushima, one major goal of reactor safety research has been the development of accident tolerant technologies that can mitigate or delay fuel degradation during a beyond-des...

A novel fabrication method of oxide dispersion strengthened (ODS) steel cladding tubes for advanced fast reactors has been investigated using the cold spray powder-based materials deposition process. Cold spraying has the potential advantage for rapidly fabricating ODS cladding tubes in comparison with the conventional multi-step extrusion process. A gas atomized spherical 14YWT (Fe-14%Cr, 3%W, 0.4%Ti, 0.2%Y, 0.01%O) powder was sprayed on a rotating cylindrical 6061-T6 aluminum mandrel using nitrogen as the propellant gas. The powder lacked the oxygen content needed to precipitate the nanoclusters in ODS steel, therefore this work was intended to serve as a proof-of-concept study to demonstrate that free-standing steel cladding tubes with prototypical ODS composition could be manufactured using the cold spray process. The spray process produced an approximately 1-mm thick, dense 14YWT deposit on the aluminum-alloy tube. After surface polishing of the 14YWT deposit to obtain desired cladding thickness and surface roughness, the aluminum-alloy mandrel was dissolved in an alkaline medium to leave behind a free-standing ODS tube. The as-fabricated cladding tube was annealed at 1000 °C for 1 h in an argon atmosphere to improve the overall mechanical properties of the cladding. Keywords: Oxide dispersion strengthened steel, 14YWT powder, Cold spray process, ODS tube manufacturing

The cold spray materials deposition process has been investigated for manufacturing oxide dispersion-strengthened (ODS) steel fuel cladding tubes. Gas-atomized 14YWT ODS steel powder was used as the feedstock material. A parametric investigation of the cold spray process involving substrate materials of various hardnesses, gas preheat temperatures, and carrier gas compositions was performed to achieve the highest quality deposit. The high-velocity impact of the powder on the substrate led to dissolution of discrete oxide nanoparticles, which subsequently reprecipitated during postdeposition annealing at high temperatures. The tubes were manufactured by deposition on an Al-alloy mandrel substrate and subsequent chemical dissolution of the substrate. A 204-mm-long and 1-mm-thick ODS steel cladding tube was successfully manufactured. The grain growth and distribution of oxide nanoparticles in ferritic steel matrix were identified at elevated temperatures. Overall, the cold spray process holds considerable promise for rapid, cost-effective manufacturing of ODS steel cladding tubes.

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.

In this research, the cladding substrate, Zircaloy-4 was coated with a 3-μm zirconium silicide (ZrSi2). At 700 °C in air, the ZrSi2 coating prevented thermal oxidation and thereby protected the underlying Zicaloy-4 substrate. In contrast, uncoated Zircaloy-4 surface developed an oxide layer that was a few micrometers thick. To investigate the effects of the coating on boiling characteristics, a pool-boiling experiment with saturated water, and a Leidenfrost experiment at low Weber number were conducted under atmospheric pressure. Thermal oxidation of the ZrSi2-coated surface did not affect critical heat flux or Leidenfrost temperature, but thermal oxidation of the uncoated Zircaloy-4 substrate affected its boiling parameters. A thin ZrSi2 coating can be applied easily to a Zircaloy-4 substrate, and may be considered for use in nuclear reactors.

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.

In a series of quenching experiments with smooth/structure hydrophilic/hydrophobic surfaces, the effects of surface wettability and surface structures on the minimum heat flux (MHF) behavior are observed. For a hydrophilic structure surface, early transition from film boiling to transition boiling at the MHF point occurs at the highest temperature (324 °C) in this experimental study. A smooth hydrophilic surface has 274 °C as the MHF temperature while a hydrophobic smooth surface shows the MHF behavior at lower temperature (208.8 °C). In contrast, there is no observable film destabilization with an observable MHF behavior on a hydrophobic structured surface during the quenching process. The test results are described by considering liquid-solid intermittent contact with an oscillating film interface in this study. For the intermittent contact, an equilibrium condition is proposed to describe the boiling transition with surface characteristics.

The objective of this research is to evaluate low-force friction surfacing as a means for repairing cracks in 304L stainless steel canisters. The motivation for reducing process forces is to allow for portability and miniaturization of the system to perform in-field repairs in confined spaces. 304L austenitic stainless steel rod was deposited over a substrate of the same alloy using a CNC machine tool in position control mode while observing the process forces on the back side of the repair. Friction surfacing was performed at spindle speeds up to 20,000 rpm. A fine grain structure was observed in the plastically deformed material. Axial pressures measured during the process showed a significant reduction when tests were performed at 20,000 rpm as compared to lower speeds. Friction surfacing was performed on simulated cracks followed by helium leak testing to evaluate the ability to create an acceptable repair. Closing of cracks near the interface of the substrate and deposit was observed after microstructural analyses. Acceptable helium leak rates through the repair can be achieved.

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.

Irradiation-induced structural evolution and compositional alterations on nanometer length scales of ZrO2-SiO2 nanocomposite and crystalline ZrSiO4, produced by oxidation of ZrSi2 at 1000 °C and 1400 °C, respectively, have been investigated. Irradiations were performed with 3.9 MeV Si2+ at 305 °C up to damage levels of 60 displacements per atom (dpa), and post-irradiation characterization was conducted by cross-sectional scanning- and transmission electron microscopy (STEM and TEM) in conjunction with energy dispersive spectroscopy (EDS), as well as glancing-angle X-ray diffraction (XRD). The multilayered nanocomposite oxide showed extensive phase mixing so that the originally distinct interfaces between ZrO2 and SiO2 phases were obliterated resulting in the formation of a single amorphous Zr-Si-O phase. Crystalline ZrSiO4 grains were completely amorphized at damage levels of 5–15 dpa, as evidenced by disappearance of grain boundaries and as confirmed by electron diffraction. The irradiation-induced phase transformation for both types of oxide structures has the potential to dramatically enhance corrosion resistance in high temperature aqueous and steam environments.

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 sensitivity of surface oxidation and surface roughness to transient pool boiling heat transfer was investigated by performing water quenching experiments of Zircaloy-4 rodlets under increased pressure or subcooled water. The results demonstrated that quenching behavior was notably affected by the surface oxidation and the surface roughness regardless of environmental conditions (saturated water at 0.5 MPa or ΔTsub = 15 K). The minimum film boiling temperature increased with the thickness of surface oxide (3.6 ± 0.2 to 55.6 ± 2.2 µm). Rough surfaces (Ra = 11.4 ± 2.5 µm) showed a large surface heat flux with vigorous vapor generation even in the early stages of quenching. To explain the augmented quenching heat transfer by the surface modifications, a hypothesis that incorporates instantaneous heat transfer during liquid–solid contacts in the stable film boiling regime was proposed. The theoretical model with assumptions elucidated the bubble dynamics of the modified surfaces qualitatively and predicted minimum film boiling points depending on the degree of surface oxidation, which were in good agreement with experimental results. Both surface properties and parameters affecting liquid–solid contact are dominant factors in determining transient pool boiling heat transfer during water quenching.

This study investigates steam corrosion of bulk ZrSi 2 , pure Si, and zirconium-silicide coatings as well as water quenching behavior of ZrSi 2 coatings to evaluate its feasibility as a potential accident-tolerant fuel cladding coating material in light water nuclear reactor. The ZrSi 2 coating and Zr 2 Si-ZrSi 2 coating were deposited on Zircaloy-4 flats, SiC flats, and cylindrical Zircaloy-4 rodlets using magnetron sputter deposition. Bulk ZrSi 2 and pure Si samples showed weight loss after the corrosion test in pure steam at 400 °C and 10.3 MPa for 72 h. Silicon depletion on the ZrSi 2 surface during the steam test was related to the surface recession observed in the silicon samples. ZrSi 2 coating (∼3.9 μm) pre-oxidized in 700 °C air prevented substrate oxidation but thin porous ZrO 2 formed on the coating. The only condition which achieved complete silicon immobilization in the oxide scale in aqueous environments was the formation of ZrSiO 4 via ZrSi 2 coating oxidation in 1400 °C air. In addition, ZrSi 2 coatings were beneficial in enhancing quenching heat transfer - the minimum film boiling temperature increased by 6–8% in the three different environmental conditions tested. During repeated thermal cycles (water quenching from 700 °C to 85 °C for 20 s) performed as a part of quench tests, no spallation and cracking was observed and the coating prevented oxidation of the underlying Zircaloy-4 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.

Cold spray technology is a promising solid-state, powder-based deposition methods for fabrication of coatings, near-net-shape manufacturing, and component repair. In recent years, this technology has been investigated for many applications in the nuclear energy sector. For example, it has been explored for the deposition of coatings of corrosion and oxidation materials on zirconium-alloys fuel claddings in light water reactors (LWR) for achieving more accident-tolerant fuel (ATF) cladding. Other examples of its use for nuclear energy applications include coatings for enhancements in wear resistance and heat transfer, near-net shape manufacturing of next-generation fuel claddings of oxide dispersion-strengthened (ODS) steels, and for mitigation and repair of potential chloride induced stress corrosion cracking in used fuel storage canister systems. This paper reviews published literature on the application of cold spray process for nuclear energy applications, while pointing to gaps that have yet to be addressed.

Two types of physical vapor deposition Cr coatings were deposited on chemical vapor deposited SiC coupons using conventional direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) to mitigate the corrosion of SiC, an advanced fuel and structural material in light water nuclear reactor cores. Coating microstructure was characterized using scanning electron microscopy and scanning transmission election microscopy. X-ray diffraction analysis was conducted for phase identification and qualitative analysis of residual stresses. The mechanical integrity of the coatings was evaluated using scratch testing, micro-indentation, and nano-indentation tests. The microstructure of the DCMS coating consisted of fine columnar grains with nanoscale intercolumnar channels, while the HiPIMS microstructure was denser and free of any columnar defects. The DCMS coating showed a small tensile residual stress, while the HiPIMS coating was under compressive stress. Both types of coatings showed good mechanical integrity in terms of ductility and adhesion to the substrate. Corrosion performance of the coatings was assessed with a 30-day high temperature water autoclave test. Both types of coatings formed a protective Cr2O3 surface layer 20-30 nm thick during the autoclave test. In the DCMS coating, the columnar defects provided permeation pathways for water penetration resulting in oxide formation in the intercolumnar regions in the interior of the coating. Overall, both types of Cr coatings offer promise for the mitigation of hydrothermal corrosion of SiC in light water reactor operating environments.

The oxidation behavior of bulk ZrSi2 at 700, 1000, and 1200 °C in ambient air has been investigated. Parabolic to cubic oxide layer growth kinetics was confirmed by weight gain measurements and the average oxide layer thickness was 470 nm, 6.7 µm, and 37 µm at 700 °C, 1000 °C, and 1200 °C, respectively, after 5 h oxidation tests. Evolution of compositionally modulated nano/micro structures was confirmed in the oxide layer. At 700 °C, Si diffusion resulted in discontinuous Si-rich oxide phases in amorphous Zr–Si–O matrix. At 1000 °C, complex multilayered structures such as fine and coarse irregular spinodal structures, wavy Si-rich oxide, and Si-rich islands evolved. At 1200 °C, additional nucleation of nanoscale ZrO2 particulate phase was observed. The spinodal structures were confirmed to be crystalline ZrO2 and amorphous SiO2, and the thermodynamic driving force for phase evolution has been explained by extension of liquid miscibility gap in the binary ZrO2–SiO2 phase diagram.

Laser surface treatment of amorphous Ti-Al-C (50:25:25 at.%) coating deposited on Zircaloy-4 substrate by dc magnetron sputtering was parametrically investigated as a function of surface temperature and dwell time. The as-deposited coating was transformed to crystalline Ti 2 AlC without significant phase alteration of Zircaloy-4 substrate under optimized conditions (900 °C for 3 s). Thermal analysis via commercial finite element software was applied to guide the selection of parameters for laser surface treatment. Coating performance was evaluated with wear and scratch tests as well as high temperature steam oxidation tests. Although laser treatment led to the formation of microcracks, it was noted to improve the mechanical integrity of the coatings and the oxidation tests indicated the protective nature of the as-deposited and laser-annealed coatings. The oxide particulate morphology and size formed during the steam oxidation tests varied dramatically between the as-deposited and laser-annealed coatings.

A feasibility study of the cold spray deposition process of 304L stainless steel on 304 stainless steel substrates as a mitigation method for chloride-induced stress corrosion cracking (CISCC) has been investigated under various substrate conditions. The study is aimed at the application of this technology to mitigate CISCC that may potentially occur in or near the fusion welded regions of stainless steel canisters in Dry Cask Storage System (DCSS) for used nuclear fuels. Spherical gas-atomized 304L stainless steel powder, in the size range of 25 μm–44 μm, was used as feedstock powder for the cold spray process. The powder was deposited on 304 substrates with various surface conditions: as-polished, oxidized, cold-rolled, and plates with prototypical CISCC. The effects of cold spray parameters on quality of cold spray coatings were investigated. Thickness, porosity, and phases in the as-deposited materials were evaluated using scanning electron microscopy (SEM) and X-ray diffraction (XRD), and correlated with microhardness and adhesion strength measured via micro-indentation and ASTM C633 pulling test, respectively. XRD analysis of the coatings was also conducted to examine the effects of cold spray condition on residual stress state in the coating. Detailed cross-sectional examination of coating/substrate interfaces was performed with transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). Dense and continuous coatings with good adhesion strength, high hardness, and high degree of compressive stress were produced for the various substrate conditions by adjusting cold spray parameters. The results demonstrate that a cold sprayed stainless steel coating is a viable option to provide a physical barrier against CISCC in fusion weld regions of stainless steel in corrosive chloride salt-bearing environments.

The critical heat flux (CHF) of potential accident tolerant fuel (ATF) cladding materials and concepts, Cr and FeCrAl coatings on zirconium-alloy substrates, was investigated experimentally. A cold spray process, which involves the propulsion of high velocity powder particles on to the substrate surface to form a coating upon impact, was used to create the coatings. Surface parameters such as contact angle, surface roughness, and morphology that are associated with CHF in pool boiling were characterized. Non-hydrodynamic effects of newly developed ATF candidate coating materials on CHF were also evaluated. The effects of thermal properties and coating thickness and expected challenges for evaluation of CHF for new ATF cladding materials development are discussed.

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.

Two FeCrAl alloy coatings with different Cr and Al contents were deposited on Zr-alloy substrates via the cold spray deposition method to study the efficacy of these coatings to improve the accident tolerance of Zr-alloy fuel cladding in light water reactors (LWRs). First, the coatings were tested in a 400 °C steam autoclave for 72 h as an accelerated method to simulate the normal LWR operating conditions. In these tests, both coatings resulted in notable improvements in oxidation resistance compared to the Zr-alloy, with the higher Cr containing coating exhibiting a thinner oxide layer. Both coatings provided good oxidation resistance when tested in 1200 °C ambient air environment; however, the formation of a low melting point eutectic (~928 °C) between Fe and Zr resulted in significant melting associated with inter-diffusion between Fe in the coating and the Zr in the substrate. Therefore, to take advantage of the superior oxidation resistance that FeCrAl coatings can provide, a Mo interlayer was deposited, also by the cold spray process, in-between the protective FeCrAl coating and the Zr-alloy substrate, thus creating a dual cold spray layer accident tolerant coated cladding concept. The wear resistance of the FeCrAl coating was superior to the Zr-alloy substrate based on pin-on-disk wear tests, providing an indication of the benefits of such coatings to reduce grit-to-rod fretting (GTRF) damage. The feedstock powders, microstructure, phases, hardness of coatings, as well as oxide and inter-diffusion layers of oxidized coatings were examined using SEM, XRD, and XPS characterization techniques. Keywords: Zircaloy-4, FeCrAl coating, Cold spray process, High temperature oxidation, Interdiffusion barrier

By regulating the geometrical characteristics of multi-scale structures and by adopting heat treatment for protective layer of nanoparticles (NPs), we improved critical heat flux (CHF), boiling heat transfer (BHT), and mechanical robustness of the modified surface. We fabricated 1-mm and 100-μm post structures and deposited NPs on the structured surface as a nano-scale structured layer and protective layer at the same time, then evaluated the CHF and BHT and material robustness of the modified surfaces. On the structured surfaces without NPs, the surface with compactly-arranged micrometer posts had improved CHF (118%) and BHT (41%). On the surface with structures on which NPs had been deposited, CHF increased significantly (172%) due to capillary pumping by the deposited NP layers. The heat treatment improved robustness of coating layer in comparison to the one of before heat treatment. In particular, low-temperature sintering increased the hardness of the modified surface by 140%. The increased mechanical strength of the NP coating is attributed to reduction in coating porosity during sintering. The combination of micrometer posts structures and sintered NP coating can increase the safety, efficiency and reliability of advanced nuclear fuel cladding.

This work examines bubble dynamics for saturated nucleate boiling at atmospheric pressure for pure water on a 0.62 5mm-diameter zirconium wire with nanoparticle surface coating using a high-speed camera. Pool boiling experiments were carried out on the surface-modified wires fabricated by the electrophoretic deposition process using 60- to 80-nm titanium and 60- to 80-nm titania, respectively. Critical heat flux (CHF) enhancements for pool boiling were observed up to 27% for the titanium coatings and 65% for the titania coatings compared to bare zirconium wires. At 300 kW/m2 surface heat flux, the nucleate site density on the titanium coating and the titania coating was observed to be 47% and 45% less than the bare wire. In terms of bubble departure volume, each coating had 16% and 29% smaller volume than the bare surface, respectively. In addition, the average bubble departure frequency of each coating was 43% and 101% higher than that of the bare case. The nanoparticle surface coatings enhanced the natu...

Pool boiling critical heat flux (CHF) measurements have been performed on stainless steel and zirconium wires in nanofluids consisting of oxide nanoparticles (7 to 250 nm) dispersed in water as wel...