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Oxidation of metals in water has been explored to generate sustainable carbon free hydrogen and power. This paper studies the thermal ignition of micro aluminum powders in high-pressure deionized (DI) water for cogeneration of hydrogen, heat, and alumina. Thermal ignition (temperature runaway) is necessary to ensure achieving high reaction temperatures, past the saturation temperature, where alumina formation is thermodynamically favorable. Aluminum powders with nominal particle diameters of 1, 10 and 45 μm with spherical morphology and 2 μm with flake morphology were mixed with DI water to form slurries with equivalence ratio of 1.3 in a 100 mL batch reactor. The reactor was pressure maintained between 500 and 2000 psig using a back-pressure regulator, and heat was injected into the slurry through an electric band heater. It was found that 1 and 2 μm powders ignite in compressed water, but only the 2 μm slurries achieved temperatures above saturation temperature at pressures above 700 psig. On the other hand, 10 and 45 μm powders require saturated steam for ignition at pressures above 1000 and 1200 psig, respectively. Hydrogen volume data indicate that larger powders at higher pressures optimize reaction yield, which is likely due to delayed ignition which preserves the active aluminum content and the particle-water reaction interfaces. Scanning electron microscopy images illustrate that the byproduct particles crack upon ignition at the saturation temperature, which increases the specific surface area by an order of magnitude relative to the aluminum powders. Pore size distribution data indicates the formation of micro, meso and macropores upon the ignition. X-ray powder diffraction data indicate that reaction temperatures above 450 °C promote the formation of alumina (Al 2 O 3) instead of boehmite (ALOOH). • Hydrogen, heat and alumina are cogenerated from reaction of aluminum with water. • Pressure controls the onset of reaction. • Specific surface area of the powder has a significant impact on ignition. • Presence of steam is necessary for ignition of larger powders. • High specific surface area alumina powder is produced. [ABSTRACT FROM AUTHOR]

Aluminum nanoparticles present excellent combustion characteristics, but such advantage is weakened due to particle agglomeration. This work focuses on this crucial behavior of aluminum nanoparticles in propellant and explore its underlying evolution mechanism from an atomic view. Molecular dynamic simulation method is used to simulate the agglomeration behavior under typical temperature of aluminum-based propellant. A two-staged accumulation-aggregation behavior is observed under low temperature due to atom movement driven by surface diffusion and grain boundary diffusion. A third stage of agglomeration dominated by viscous flow mechanism is achieved when the particle temperature is above 1500 K. The transition from aggregation to agglomeration is initiated by the solid-liquid phase transformation of the oxide shell, which happens under temperature lower than the melting point of alumina due to the formation of aluminum suboxide with low melting point. Decreasing the oxide layer thickness and integrity as well as particle size could accelerate the agglomeration and thus weakening the size advantage of aluminum nanoparticles. • Staged agglomeration behavior of ANPs and critical transition point is determined. • Underlying mechanism of agglomeration based on atom diffusion modes is revealed. • The transition is triggered by bi-directional atom diffusion to form Al suboxide. • Influences of oxide layer properties and particle size are explored. [ABSTRACT FROM AUTHOR]

To further our understanding of iron particle combustion, especially in the liquid state, i n s i t u optical measurements are performed for the size evolution and temperature of laser-ignited iron particles in a narrow size range around 45–55 μ m. The particle size evolution is monitored by a high-speed shadowgraphy system, and the particle temperature is probed using a two-color pyrometer synchronized with the shadowgraphy system. Before a particle reaches the peak temperature, its diameter grows linearly with elapsed time. Near the peak temperature, three types of particle size behavior are detected, namely, smooth transition, micro-explosion, and rapid inflation. For particles with smooth transition, the size and temperature reach the maximum values at the same time, and thereafter the size remains constant, while the temperature drops until rapid solidification of the iron oxide droplet. The micro-explosion and rapid inflation clearly indicate towards a quick gas release inside the particle. The ambient oxygen concentration has a strong effect on the growth rate of the particle size before the peak temperature but no influence on the final particle size. The measured relative particle diameter at the peak temperature suggests that iron has been fully oxidized and the degree of oxidation could be higher than FeO. At the onset of solidification, gas release occurs again, resulting in a second inflation or burst of already inflated particles. This suggests that the oxygen dissolved in the liquid iron oxide is at least more than needed to form solid Fe 3 O 4 , and the solidification process is accompanied with gaseous oxygen release via the phase transition: L2 ⟶ Fe 3 O 4 (s) + O 2 (g) at 1855 K, where L2 represents liquid iron oxide with dissolved excess oxygen. [ABSTRACT FROM AUTHOR]

We present a numerical study of the combustion of single iron particles in an O 2 –N 2 atmosphere. By resolving the full boundary layer, mass and heat transfer are accurately modeled, including Stefan flow. Only the conversion of Fe to FeO is taken into account and evaporation is implemented to investigate the formation of nano-sized iron-oxides products. Temperature- and composition-dependent heat capacity and density are used and phase transitions from solid to liquid (and vice-versa) are accounted for by the apparent heat capacity method. The model is validated by comparing the time to maximum temperature (t max) and the maximum temperature (T max) of a 40 and 50 µm particle in an O 2 –N 2 atmosphere with experiments. The current model, which assumes infinitely fast internal transport, can well estimate the maximum particle temperature and the time to reach this maximum temperature, but it does not capture the particle size effect on the maximum temperature. Even though the particle temperature stays below its boiling temperature, a small but non-negligible amount of mass is lost due to evaporation of the particle. Evaporation of the particle and oxidization of the gaseous Fe-containing species in the boundary layer limit the maximum temperature of the particle when increasing the oxygen concentration. By means of a sectional model, the formation and growth of the iron oxide nano-particles is numerically investigated. It is shown that somewhat further downstream of the particle, the volume fraction of nano-particles attains a maximum. An average nano-particle diameter of 40 nm is found at t / t max = 1 in the wake of a 50 µm iron particle burning at 21% oxygen concentration. [ABSTRACT FROM AUTHOR]

Ignition temperatures of metal particles play an essential role in not only the fundamental theories of non-volatile dust flames but also the robust operation of practical metal fuel burners. The present paper introduces a novel approach to accurately measure the ignition temperature of an isolated particle. Micron-sized single particles are injected downwards into a quartz tube heated externally by a premixed flame near the bottom end. During free fall, a particle, if sufficiently small, closely follows the gas-phase temperature that increases gradually from top to bottom. By measuring the ignition position of the particles, the ignition temperature is determined from the gas-phase temperature profile that is quantified a priori. Applying the approach together with high-speed imagining and diffuse backlight-illumination techniques, the ignition temperature of approximately 30–60 μ m iron particles in O 2 /N 2 mixtures are comprehensively measured at oxygen mole fractions of 10%–50%. The experimental results reveal that the measured ignition temperatures is in the range of 1030–1130 K that are independent of the oxygen mole fraction and the particle size. In contrast, the particle size significantly influences the ignition probability. Smaller particles have lower probabilities to ignite. At the oxygen mole fraction of 10%, ignition is only observed for iron particles larger than approximately 45 μ m. For all other cases, ignition is detected for all particle diameters. Possible mechanisms underlying the experimental observations are discussed. Novelty and significance statement A novel approach to measure the ignition temperature of an isolated, micron-sized particle is developed. Compared to existing methods, the new approach provides a convenient way to determine the ignition temperature accurately and examine the size dependence of the ignition characteristics. For the first time, size-resolved ignition temperatures of isolated iron particles are reported. For a fixed particle diameter, the existence of an ignition probability is revealed. The quantitative experimental results have a high potential to be widely used to validate models of iron particle ignition. [ABSTRACT FROM AUTHOR]

Simultaneous, in-situ , optical diagnostics are performed to measure initial diameters and the temporal temperature evolution of iron microparticles burning in hot laminar oxidizing atmospheres with 10–30 vol% O 2. The pre-ignition particle diameter and temperature evolution during combustion are monitored using synchronized high-speed diffuse backlight-illumination and two-color pyrometry techniques, respectively. Average temperature histories are obtained for particles sorted into three size fractions centered at 40, 45, and 50 μ m with a span of 5 μ m. Increasing the oxygen level from 10 to 30 vol%, particles burn faster and reach a higher peak temperature that increases approximately from 2400 to 3200 K. From their temperature trajectories, the peak temperatures of individual particles are extracted and correlated with their initial diameters. It is observed that the maximum particle temperature decreases with the increasing particle diameter, attributed to the enlarged radiative and evaporative heat losses relative to the chemical heat release of the larger particles that burn in the diffusion-limited regime. In addition, the size dependence of the maximum particle temperature enhances considerably when the particle peak temperature increases from approximately 2400 K to around 2800 K, but its further variation is small as the particle peak temperature continues approaching the boiling point of the particles. This observation does not align with previous non-size-resolved measurements that have lower temporal resolutions and wider particle size distributions. Possible reasons for this inconsistency are discussed. A theoretical analysis is performed to quantitatively reveal the role of surface radiation and evaporation in the size dependence of the particle peak temperature. The results suggest that at relatively low particle temperatures the size dependence is determined mainly by radiation and that the effect of evaporation becomes more dominant at higher particle temperatures. Moreover, with increasing particle temperature, radiation strengthens the size dependence of the particle peak temperature. On the contrary, evaporation weakens the size dependence at higher temperatures because of the increasing sensitivity of vapor pressure (evaporative heat loss) to the temperature according to Clausius–Clapeyron relation. Novelty and significance statement This work presents, for the first time, the simultaneous, in-situ measurements of the initial sizes and time-resolved temperatures of micrometer-sized iron particles burning at elevated gas temperatures. Using current size-resolved measurements, particle-temperature evolutions for several particle diameters are statistically obtained with significantly increased precision. This high-fidelity experimental database over a wide range of operating conditions is a very valuable reference for the iron fuel community to validate and improve numerical models of iron particle combustion. Due to the novel design of the experiment, in this work negative correlations between the initial diameters and the maximum temperatures of isolated burning iron particles are observed and theoretically explained. This result does not align with previous non-size-resolved measurements that have lower temporal resolutions and wider particle size distributions. [ABSTRACT FROM AUTHOR]

• Ignition of Mg particles in CO 2 includes melting, evaporation, breaking and ignition phases. • Fractures occur preferentially at weak locations with low compressive strength limits. • Ignition of Mg particles occur almost simultaneously at the moment of Mg vapor injection. • Increasing pressure shortens ignition delay and accelerates gaseous combustion rate of Mg/CO 2. • Ignition and combustion of Mg particles in CO 2 are dominated by a liquid-gas reaction. Combustion studies of individual Mg particles under CO 2 atmosphere are essential for the development of Mg/CO 2 powder rocket engines for in-situ resource utilization (ISRU) technology on Mars. In this work, the ignition and combustion characteristics of individual micron-sized Mg particles prepared by gas-atomization method under CO 2 atmosphere were investigated in detail. It was found that the ignition delay of Mg particles was determined by two factors, i.e., ambient pressure and particle size, and the former had a greater influence on the ignition delay of the particles. As the pressure increased, the ignition delay time of the particles decreased significantly. For particle sizes between 30 μm and 35 μm, the ignition delay times of Mg particles at 0.9 MPa and at 1.6 MPa were only ∼2 % and ∼1 % of that at atmospheric pressure. The ignition delay time of Mg particles increased linearly with the increasing particle size at both atmospheric pressure (0.1 MPa) and high-pressure (1.6 MPa) CO 2. During melting and evaporation, the newly generated non-dense oxides grew unevenly along the surface of Mg particles, leading to inhomogeneity in the mechanical strength of the oxide layer. Fracture occurred preferentially at weak locations with low compressive strength limits, resulting in selectivity in the direction of the Mg vapor injection at the moment of shell breaking. Finally, it is proposed that the ignition and combustion of individual Mg particles should be dominated by a liquid-gas reaction and that pressure and particle size are crucial for the combustion performance of micron-sized Mg particles. Classical understanding of Mg/CO 2 combustion for in-situ resource utilization technology of Mars exploration relies on the gas-phase reaction, however, difficultly explaining the overall ignition and combustion evolution. A novel observation and recognition of traditional Mg/CO 2 combustion reaction through self-built real-time microscope with high-speed cinematography and highly-focused inductive laser ignition, indicating that the oxides of individual Mg particles in CO 2 atmosphere grow non-uniformly along the surface, which leads to the preferential shell breaking of Mg vapors at the weak points with lower mechanical strength and eventually causes the particles to be ignited. Highly temporally and spatially resolved visualized images, spectrum, temperature, and product SEM information firstly demonstrate that the ignition and combustion of individual Mg particles in CO 2 are dominated by a liquid-gas reaction, experiencing the surface and gas-phase combustion and confirming that high pressure significantly improves the ignition performance of Mg particles and promotes gas-phase combustion in CO 2. [ABSTRACT FROM AUTHOR]

Combustion of micron-sized particles of iron, a promising renewable alternative to fossil fuels, was investigated in a drop tube reactor (DTR) under overall lean conditions. A bimodal particle size distribution was observed after combustion: black micron-sized particles consisting of a mixture of iron oxides, with oxidation degrees ranging from 60% to 90%, and reddish fine particles of Fe 2 O 3. The oxidation degree of the products showed no obvious variation with the reactor temperature. The coarse product particles were comparable in size with those of the raw iron particles, independent of temperature. The evaporation of iron represented a mass loss of 1%–2%. The primary nm-sized particles agglomerated to form aggregates of the order of 1 μ m. A simplified model for the combustion was used to analyze the results. It was based on the Particle Equilibrium Composition approach by van Gool et al. (Appl Energy Combust Sci 2023;13:100115) that assumes the oxidation from Fe to FeO to be limited by external diffusion and further oxidation by thermodynamic equilibrium. The model predicted larger oxidation degrees than observed experimentally. The difference was believed to be due mainly to stratification in the drop tube reactor, resulting in local fuel-rich conditions. The small impact of reactor temperature on the final oxidation stage indicated that diffusion limitations, rather than kinetic barriers, were rate controlling. • Lean combustion of μ m -sized iron particles in a drop tube reactor resulted a bimodal product distribution with coarse melted particles and aerosols. • The oxidation degree of the coarse particles was 60%–90%, independent of reactor temperature in the 700-1200 °C range. • The incomplete oxidation to Fe 2 O 3 was attributed to mixing limitations. • The lack of temperature effect on oxidation degree indicated diffusion control. • The evaporation of iron represented a mass loss of 1%–2%. • The primary nm-sized particles agglomerated to form aggregates of the order of 1 μ m. [ABSTRACT FROM AUTHOR]

Significant heat generation will be required for humans and equipment to the lunar night. In this work, we investigate the use of a sustainable in-situ thermite material as a fuel to provide the thermal energy required to keep components in working conditions. Magnesium is used as a reactive metal fuel, with lunar regolith simulant ball milled to sub-micron sizes as a solid oxidizer producing exothermic reactions. Enhanced combustion is achieved by controlling particle size and composition of the thermite mixture. Simulant particles ranging from hundreds of nanometers to tens of microns in diameter are tested, as well as the magnesium fuel compositions of 20% to 40% by weight. Small pellets of 3 mm in diameter and 3 mm in height are ignited by laser in both air and vacuum to quantify the combustion properties in different environments. High speed video, infrared camera and pyrometry techniques are taken to quantify the sample combustion properties. These pellets demonstrate the burning rates between 2.3 and 5.9 mm/s and temperatures ranging from 1100 to 1480 °C in vacuum and in air conditions, respectively. The samples composed of 20% magnesium and 80% regolith simulant release around 400 J/g, and sustain elevated temperatures for 15 s after combustion, making them suitable for in-situ lunar heating. Novel 2D temperature mapping allows greater understanding of the simulant thermite combustion. Based on the results, we discuss the design considerations that would need to be made in the creation of an in-situ metal fuel heating lunar system. • Micro/nanoparticle combustion of magnesium/JSC-1A lunar regolith simulant. • Effects of composition and particle size suggest tunable reaction properties. • Self sustained combustion in vacuum with an energy release of 400 J/g. • Reaction temperatures over 1400 °C and burning rates of 2.3 to 5.9 mm/s. • 2D temperature data for future modeling studies. [ABSTRACT FROM AUTHOR]

This paper proposes a 0D modelling strategy for the combustion of a single iron particle. The primary objective was to accurately represent the evolution of the particle temperature, including key parameters such as the peak temperature T max and associated characteristic burn time τ b , and oxidation dynamics in a wide range of conditions. An optimisation approach, rather than a purely mechanistic model, was chosen to further close the current gap between numerical simulations and experimental observations. The model considers oxidation processes, heat transfer, solid–liquid phase changes and dissociative evaporation. Intra-particle reaction rates are controlled by external O 2 diffusion combined with an optimised O 2 absorption reduction quantity γ , but at the end of the combustion process by a more adequate empirical kinetic rate. A first combustion stage involving the reaction 2 Fe + O 2 → 2 FeO is followed by two successive stages with the respective reactions 6 FeO + O 2 → 2 Fe 3 O 4 and 4 Fe 3 O 4 + O 2 → 6 Fe 2 O 3 . This oxidation strategy is based on the Fe-O phase diagram and experimental observations of oxidation beyond FeO. Mass and enthalpy balances for the particle gave its temperature evolution, which was compared with experimental data and state of the art modelling approaches. The numerical overestimation of T max in environments with elevated O 2 concentration was addressed via the optimised quantity γ , which was modelled as piecewise constant, changing once at a predetermined burn time based on experimental measurements of the burn time τ b. Correlations for the moment of reduction of the quantity γ and for its initial value were introduced, both related to the initial particle diameter and O 2 molar fraction in the gas. Further model refinement is required to enhance the accuracy of simulated cooling rates in high-temperature combustion environments, where experimental data are particularly scarce. In another scenario, characterised by reduced O 2 conditions and hitherto unexplored in the numerical modelling literature, an underestimation of T max was found even assuming the maximum possible oxidation rate. This observation has prompted the authors to question the suitability of the correlations used in existing models to calculate the convection coefficient, which seems to be slightly overestimated. The proposed simple and efficient modelling framework has demonstrated its potential ability to accurately reproduce key combustion characteristics of a burning iron particle in a wide range of conditions. And it will thus serve as a good starting point for the simulation of heterogeneous particle-laden reactive flows. • Iron particle combustion model with time-controlled oxygen absorption reduction. • Optimisation of simulated combustion characteristics using experimental data. • Proposition of correlation models for the estimation of oxygen absorption rate. • Favourable comparison of results with those of state of the art modelling approaches. • Tendency to underestimate peak particle temperature in reduced oxygen conditions. [ABSTRACT FROM AUTHOR]

The present experimental study focuses on the determination of the oxidation progress and internal structures during the combustion of single iron particles using combined in-situ optical measurements and ex-situ material examination of rapidly quenched particles. Narrowly sieved iron particles with a mean diameter of 49 µm are ignited and burn in the hot exhaust of a premixed CH 4 /O 2 /N 2 flat flame with remaining 20 vol% O 2 , provided by a laminar flow reactor with well-defined thermal and flow boundary conditions. During particle combustion, key parameters of oxidation progress are determined optically in a time-resolved manner, such as the time-resolved particle surface temperature and the reaction time relative to the instant of the particle peak temperature. Furthermore, individually burning particles are rapidly quenched at different combustion stages and then extracted from the exhaust gas using an isokinetic extraction probe. The bulk composition of the quenched particles with respect to α -Fe and its three oxides FeO, Fe 3 O 4 , and Fe 2 O 3 is determined using Wide-angle X-ray Scattering and 57Fe Mössbauer Spectroscopy. Combining the information obtained from in-situ and ex-situ measurements, it is shown that iron particles oxidize rapidly to FeO during the initial stage of combustion, followed by a much slower oxidation to Fe 3 O 4 as the particles cool down. The peak particle temperatures are measured during the fast initial oxidation. Finally, particles sampled from representative positions of the oxidation process are analyzed by Energy-Dispersive X-ray Spectroscopy and Focused Ion Beam Scanning Electron Microscopy, revealing different particle morphology and internal structures. For the first time, the clear presence of an oxide shell and iron-core structure in quenched particles suggests that liquid iron and liquid iron oxide are layered during liquid phase combustion, if the particle remains within the miscible gap. • Iron particle peak temperature is near the stoichiometric ratio of Fe and O. • Iron particles combust in a core shell structure initially. • Micron sized iron particle oxidation history plotted on Fe O phase diagram. • Particle sampling with well characterized sampling probe. [ABSTRACT FROM AUTHOR]

• Metal alloy microparticles directly derived from the waste stream of additive manufacturing. • Experiments were conducted on micro-sized AlSi10Mg particles via two unique approaches. • Reactivity was investigated in gaseous oxygen and using a solid oxidizer (CuO nanoparticles of 500 nm) in thermal analysis. • Combustion tests were performed in ambient conditions as well as at low pressure (100 Pa) Metal powders including aluminum, iron and metal alloys have been considered as a fuel for energy generation without releasing carbon dioxide and being recyclable. While additive manufacturing processes desire feedstock microparticles with a specific size range, a significant amount of metallic powders is left in its waste stream. In this work to investigate the potential applications of these alloy powders, they are mixed with solid metal oxide particles for combustion tests in ambient and low-pressure environments. The thermochemical properties and exothermic characteristics are measured on the mixtures of micro-sized AlSi10Mg particles with a median diameter of 13 µm and CuO nanoparticles of 500 nm. Experimental results indicate, compared to Al micron particles with a similar average diameter, the reaction between AlSi10Mg powder and oxygen shows a comparable onset temperature (around 1000⁰C), a similar activation energy (316.9 kJ/mol), but a much higher energy release (11.5 kJ/g vs. 4.2 kJ/g). When mixed with CuO nanoparticles, AlSi10Mg/CuO and Al/CuO mixtures exhibit a similar reaction kinetics which is characterized by the diffusion of gaseous oxygen produced from the thermal decomposition of CuO. In the ambient combustion tests, the AlSi10Mg/CuO powder shows a 20 % shorter ignition delay, 3 times faster burning rate, and up to 2 times higher impulse than these of the Al/CuO powder. When a low air pressure (about 100 Pa) is applied, the AlSi10Mg/CuO powder is successfully ignited and combusted, while the micron-Al/CuO powder cannot be ignited. We conclude the micro-sized Al-Si-Mg alloy particles recycled from the advanced manufacturing processes can be a good candidate for metal fuel. [ABSTRACT FROM AUTHOR]

The ignition temperature is closely linked to the reaction front speed in iron dust flames, a crucial target quantity when trying to predict the characteristics of such flames. To this end, the ignition of iron microparticles is analyzed in this paper by means of single particle simulations which are compared with recent ignition experiments by Ning et al. Different particle sizes and oxygen concentrations in the gas phase are investigated, since iron particles in a dust flame are polydisperse and experience vastly different oxygen atmospheres. Several modeling approaches have been proposed for the reaction kinetics governing the ignition process and in this work, two of the most widely used models are considered: one describing the kinetics as a surface reaction and the other assuming the diffusion of iron cations through the solid oxide layer as the rate-limiting mechanism. The onset of the particle oxidation can be characterized by the solid-phase oxidation time (SOT) during which inert heating and thermal runaway of the iron microparticles occur. Therefore, the SOT allows for the assessment of the predictive capabilities of the models regarding ignition. With this background, it is the objective of this work to gain insights into the rate-limiting mechanisms controlling the ignition and to assess, whether the state-of-the-art models require further calibration. Uncertainty assessment is performed for the SOT predictions reflecting experimental uncertainties for particle sizes. It is found that the current models mostly underpredict the SOT which is a strong indication that the limiting factors for all cases of iron oxidation are still not sufficiently well understood. The experimental measurements indicate that the SOT depends on the oxygen concentration of the surrounding gas. Possible rate-limiting mechanisms are discussed and new kinetics parameters are proposed from model calibration with the experimentally measured SOT. The ignition temperature of the calibrated model is compared with measured ignition temperatures from various experimental studies in the literature and reasonable agreement is found. • Kinetic-limited iron oxidation is influenced by O 2 concentration in ambience. • State-of-the-art 0D models underpredict solid oxidation times (SOT) from experiments. • First Order Surface Kinetics model qualitatively captures trend of the SOT. • Solid Oxide Layer Diffusion model requires extension to capture influence of O 2. • New kinetics parameters are proposed for First Order Surface Kinetics model. [ABSTRACT FROM AUTHOR]

• In situ resource utilization of carbon dioxide. • Detailed analysis of multilayer oxidation shell structures provided. • Moving micron-scale single-particle combustion was observed and measured. • Temperature data enhances burning time measurement and mode classification. An experimental system integrating micro-sized single particle generation, ignition, and observation was proposed and constructed for investigating the combustion characteristics of magnesium single particles in carbon dioxide. Defining the onset of combustion using the boiling point temperature allows for precise measurement of combustion time without artificial interference. Results revealed four modes of combustion for magnesium particles (Vapor-phase combustion, surface reaction, Micro explosion and flash-out mode) in carbon dioxide. Vapor-phase combustion is diffusion-controlled (with a D 0 1.76 scaling, where D 0 is the initial particle diameter). Additionally, the micro-explosion mode primarily occurs in particles with diameters exceeding 70 μm, and the combustion time is rather insensitive to the particle size (Typically 0.5 ∼ 2.2 ms in this study). The characteristic temperatures of the particles were determined using two-color pyrometry, indicating that the particle combustion temperature decreases with increasing particle size. Magnesium particles typically reach temperatures approximately 800 K higher than normal vapor-phase combustion temperatures before experiencing micro-explosions. Analysis of combustion products shows that the oxide shell formed by the combustion of magnesium particles has a three-layer structure. Both the outer and inner layers are mixed layers of magnesium oxide and solid carbon produced during combustion reactions, and the middle layer is the initial oxidation layer of the particles. [ABSTRACT FROM AUTHOR]

Combining laser ignition tests with quenched particle cross-sectional analyses, this study explores the formation of bubbles and microexplosions in burning boron agglomerates, and discusses the plausible mechanisms behind the microexplosion (expansion–rupture) phenomenon. High-resolution images reveal that these phenomena are caused by bubble nucleation and growth within the molten droplet. Based on the timescale, it is categorized into weak expansion and strong expansion phenomena. The expansion rate of droplets during the weak expansion process mostly ranges between 0.006 m/s and 0.4 m/s, while during the strong expansion process, it predominantly falls between 0.4 m/s and 1 m/s. An increase in ambient pressure results in the delayed occurrence and reduced frequency of the expansion–rupture phenomenon. When the ambient pressure exceeds 3 atm, the expansion phenomenon no longer occurs. In addition, a decrease in the initial diameter of the agglomerates lowers the overall occurrences of expansion–rupture. Analysis of the quenched particle cross-sections reveals that the size and density distributions of the pores within the boron agglomerates reduce gradually during the combustion process. In addition, the surface oxide layers of certain pores are removed during combustion. Estimates of the vaporization rates of (BO) n and B 2 O 3 support the speculation that the expansion of molten boron droplets is attributed to the evaporation of boron oxide layer. In this process, the molten boron acts as a heating surface to evaporate the boron oxide trapped inside the droplet, thus producing B 2 O 3 , B 2 O 2 , and BO, and leading to droplet growth and tearing of the liquid film. Analysis of bubble growth indicates that the merging of adjacent bubbles to reach the critical nucleation size is probable the starting point of the expansion process. These results can be helpful in promoting the combustion efficiency of boron within the combustion chamber. [ABSTRACT FROM AUTHOR]

With the increasing interest in small modular reactors or microreactors, developers are working to design and submit licensing approval requests of U–10Zr-fueled fast reactors. The developers and their proponents cite prior metal fuel experience (worldwide, but U.S. experience in particular for many developers) as the motivation and justification for their reactor concepts. The experience with metal fuel deployment in sodium-cooled fast reactors as well as the underlying irradiation testing database, provide a suitable basis for analytically justifying the use of metal fuel in new reactors. The evolution of metal fuel design and capability illustrates the importance of key fuel design parameters to consider in new applications of the prior experience: fuel smeared density, plenum-to-fuel volume ratio, the ratio of cladding radius to thickness, fuel composition, and cladding and duct materials. In-service operating and deployment conditions to be considered include fuel linear heat generation rate, fuel temperature, cladding temperature, peak burnup and peak fast fluence. Fuel designs and in-service conditions that are bounded by the database and experience are most easily addressed, but deviations from those previous parameters and conditions can be addressed by considering impacts on previously established behavior and applying other mitigating conservatisms, as appropriate. The authors recommend any new deployment proceed with fuel surveillance and monitoring to mitigate risk, application of conservative measures to address uncertainties, and a fuel qualification program that addresses a range of in-service operating conditions with production fuel. The work reported should be of interest to students and regulators unfamiliar with metal fuel in fast reactors. • Metal fuel deployment in SFRs can be technically supported by prior experience. • Evolution of metal fuel design illustrates the importance of fuel design parameters to consider. • Fuel designs bounded by the database and experience are most easily addressed. • Deviations from previous designs can be addressed by considering impacts and mitigations. • New deployment should include fuel surveillance and monitoring to mitigate risk. [ABSTRACT FROM AUTHOR]

In this study, the influence of temperature (40, 60 and 80 ∘C) and light exposure on the dissolution of ▪ particles in aqueous, oxalic acid (0.45 mol/l) is experimentally investigated. A novel experimental setup, consisting of a vessel with undistorted optical access, allows the recording of dissolving particles using a video system. During post-processing, particles are automatically identified and their size and morphology are determined. Binning particles into discrete time-steps during the reaction yields heavy-tailed distributions characterized by a single parameter estimated through a maximum-likelihood estimation of a doubly truncated power-law fit. For the first time, up to three consecutive phases could be identified during the dissolution process of iron oxide particles. The contribution of each phase to the overall dissolution process is determined by a complex interplay between particle fragmentation, different reaction mechanisms and solid product formation. The initial phase is characterized by the fragmentation of larger particles with a more complex morphology, resulting in greater surface area available for reaction. In the second phase, fragmentation is less apparent and the available particle surface area is reduced. The transition to the third phase relies primarily on the supply of short wavelength light, which leads to the formation of a solid product that can increase the tracked particle diameter. • Up to three consecutive phases during iron oxide particle dissolution. • First phase dominated by particle fragmentation leading to increasing surface area. • Second phase dominated by surface area reduction, less by particle fragmentation. • Third phase occurs only at low temperatures and short-wavelength light exposure. • The third phase is characterized by solid product formation. [ABSTRACT FROM AUTHOR]

Iron powder can be a sustainable alternative to fossil fuels in power supply due to its high energy density and abundance. Iron powder releases energy through exothermic oxidation (combustion), and stores back energy through its subsequent hydrogen-based reduction, establishing a circular loop for renewable energy supply. Hydrogen-based direct reduction is also gaining global momentum as possible future backbone technology for sustainable iron and steel production, with the aim to replace blast furnaces. Here, we investigate the microstructural formation mechanisms and reduction kinetics behind hydrogen-based direct reduction of combusted iron powder at moderate temperatures (400–500 °C) using thermogravimetry, ex-situ X-ray diffraction, scanning electron microscopy coupled with energy dispersive spectroscopy and electron backscatter diffraction, as well as in-situ high-energy X-ray diffraction. The influence of pre-oxidation treatment was studied by reducing both as-combusted iron powder (50 % magnetite and 50 % hematite) and the same powder after pre-oxidation (100 % hematite). A gas diffusion-limited reaction was obtained during the in-situ high-energy X-ray diffraction experiment, with successive hematite and magnetite reduction, and a strong increase in reduction kinetics with initial hematite content. Faster reduction kinetics were obtained during the thermogravimetry experiment, with simultaneous hematite and magnetite reduction. In this case, the reduction reaction was limited by a mix of phase boundary and nucleation and growth models, as analyzed by multi-step model fitting methods as well as by microstructural investigation. When not limited by gas diffusion, the pre-oxidation treatment showed almost no influence on the reduction time but a strong effect on the final microstructure of the reduced powder. [Display omitted] [ABSTRACT FROM AUTHOR]

• An SFR-based Small Modular Reactor (SMR), SALUS-100 is developed by KAERI. • The reactor features incorporate a long fuel-cycle core concept with a metallic alloy fuel. • The inherent safety and reliability features focuses on preventing severe accidents. • The key research and development activities for technology advancements are introduced. • The innovative design of SALUS-100 offers diverse applications in future SMR markets. The paper introduces the innovative development of an SFR-based Small Modular Reactor (SMR) known as SALUS-100, with a 100 MWe electrical output, developed by KAERI. This innovative design, derived from the previous TRU transmutation burner SFR concept, represents a significant step forward in establishing a new model for SMRs and offers diverse applications, including on– and off-grid electricity generation, district heating, and industrial heat supply. The reactor features of SALUS-100 incorporate an integrated pool-type primary system configuration and a long fuel-cycle core concept with a metallic alloy nuclear fuel. Its design concept prioritizes preventing severe accidents by capitalizing on the inherent safety and reliability features achieved through emphasizing passive design characteristics. The paper describes the engineering design and safety analyses of SALUS-100, highlighting advanced design concepts in reactor core, fuel, materials, and systems. Additionally, the key research and development endeavors, focusing on vital areas such as enhancing system design capability, validating designs, demonstrating safety, and innovating in metal fuel technology, are briefly discussed, emphasizing the potential of SALUS-100 in revolutionizing the SMR market and utilizing efficient nuclear energy in the future. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nitriding reaction and combustion of Al-Li alloy with tiny Li contents in N 2 was investigated. • Nitriding reaction temperature decreased from 853 °C for Al to 245 °C for Al-2.5Li alloy. • Intermediate nitriding products of Al-Li revealed key cracks and loose "island" structures. • Thermal nitriding reaction of Al-Li alloy was significantly faster than that of pure Al. • Metal Li can significantly improve the ignition and combustion performance of Al in N 2. • Combustion mechanism of Al-Li alloy and pure Al in N 2 were discussed and compared. For aluminum-based alloy particles, the addition of alloying element lithium (Li) with the properties of a high chemical activity and a low melting/boiling point can significantly improve the thermo-reactive properties of aluminum (Al) particles. To further reveal the combustion mechanism, this paper presents the nitriding reaction and combustion characteristics of Al-Li alloy in nitrogen (N 2) through thermogravimetric analysis and differential scanning calorimetry (TG-DSC) analysis at a low heating rate and laser-induced ignition experiments at a high heating rate, considering the influences of two kinds of Li contents (1.0 wt% @ Al-1.0Li and 2.5 wt% @ Al-2.5Li) on these characteristics and making a comparison to the counterpart Al. Al-Li alloy powders as-prepared by gas atomization method showed the texture of the Li-rich eutectic phase compounds on the surface by scanning electron microscope (SEM) characterization. The TG-DSC results demonstrated that the thermal nitriding reaction of Al-Li alloy in N 2 was significantly faster than that of pure Al. The nitriding reaction temperature decreased from 853 °C for pure Al to 245 °C for Al-2.5Li alloy with a reduction of 71 %. The final weight gain of Al-2.5Li alloy was almost three times higher than that of pure Al in N 2. The SEM photograph of intermediate nitriding products revealed that the cracks and loose "island" structures in the Li-rich zone on the Al-Li surface were the key to preventing the formation of a "passivation effect" into Al and promoting N 2 uptake during melting. In addition, Li also acts as a catalytic carrier and "oxygen absorber" to enhance the Al-N reaction. The combustion of single Al-Li alloy microparticles induced by laser showed two stages: preheating and rapid nitriding. Al-Li alloy particles were ignited more rapidly and showed typical gas-phase / surface combustion modes different from pure Al. According to these findings, the combustion mechanism was proposed to help guide the development of Al-based solid propellants containing Li and the synthesis of AlN/Al composites in nitrogen-rich environments. [ABSTRACT FROM AUTHOR]

In the present study, a novel in situ particle sizing approach is proposed and used to measure the characteristic timescales of micron-sized iron particle combustion under low-oxygen (10–17 vol%) dilution conditions. The particle size is determined by probing the light emission intensity of a burning particle during melting, which is proportional to the cross-section area of the particle projected to the camera. Detailed descriptions of the calibration, validation, and characterization of the experimental method are elaborated. With systematic measurements, we obtain one-to-one correlations between combustion timescales and single particle diameters at various diluted oxygen concentrations. Furthermore, we formally derive a theoretical model for heterogeneous combustion of growing (iron) particles in the diffusion-limited regime. The model suggests that the diffusion-limited burn time scales with the initial particle diameter squared (i.e., a new, generalized d 2 -law). Owing to accounting for the particle growth, the newly derived model suggests a significantly (1.66 times) shorter combustion duration compared to the conventional d 2 -law for shrinking particle combustion. It turns out that the new model agrees well with the experiment. This agreement also suggests that under low-oxygen dilution conditions, the combustion regime of iron particles during the intensive burning stage (i.e., from ignition to the peak particle temperature) is limited by external oxygen diffusion. • A novel particle sizing approach is proposed to study micron-sized iron particle combustion. • Size-resolved characteristic timescales of single iron particle combustion are measured. • A theoretical model for diffusion-limited, heterogeneous combustion duration of growing (iron) particles is derived. • Iron particles with diameters > 20 μ m burn in the diffusion-limited combustion regime at 10–17vol % O 2. [ABSTRACT FROM AUTHOR]

This study investigates the potential of LaYO 3 as a crucible material to prevent interactions between a U-10 wt% Zr melt containing highly reactive lanthanide elements and the substrate during casting. The stability of LaYO 3 phases at casting temperature was evaluated based on reaction phenomenon, chemical and physical properties. Sintered pellets were fabricated using both La 2 O 3 and Y 2 O 3 powders and structure of bodies were investigated using X-ray diffraction. The reaction behaviors of U Zr and U-Zr-RE melts with the LaYO 3 and Y 2 O 3 specimens were investigated using sessile drop method, and microstructures of the interfaces were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The thermodynamics of LaYO 3 were analyzed to demonstrate its reaction-preventing effect. Results indicate that LaYO 3 is more effective than Y 2 O 3 in reducing the interaction between the substrate and the melt, and it significantly decreases the partial reduction of the reaction-preventing layer. These findings suggest that LaYO 3 has the potential to be an effective crucible material for preventing interactions during casting. • A novel reaction-preventing material, LaYO 3 , has been developed for metal fuel casting applications. • LaYO 3 was fabricated using sintering, resulting in a perovskite structure that prevents reactions during casting. • The LaYO 3 enhanced the reaction-preventing effect more than the conventional material Y 2 O 3. • The interaction behavior of both LaYO 3 and Y 2 O 3 was investigated using sessile drop tests and thermodynamic analysis. [ABSTRACT FROM AUTHOR]

• The combustion of Al-Mg alloy in hot oxidizing flows is first studied in detail. • Effects of oxidizers on the combustion process of Al-Mg alloy are investigated. • Spectral measurements are conducted to determine important reaction pathways. • Quantified ignition temperature and consumption rates are provided. • SEM/EDS analysis are carried out to reveal key combustion processes. The use of aluminum-magnesium (Al-Mg) alloy particle as energetic additive in solid propellants was previously shown to have many advantages over pure Al particle, such as relatively low ignition temperature, high reaction rate and low particle agglomeration rate. In this paper, the combustion of Al-Mg alloy in hot H 2 O/O 2 /N 2 flows was experimentally studied using wires with a diameter of 200 µm. The employment of wires instead of particles provided a unique opportunity to obtain fundamental insights into the combustion process due to the spatial stabilization and large size of the sample. High-speed imaging showed that the combustion of Al-Mg alloy wire could be divided into three stages, namely pre-heating, ignition, and combustion. Spectral measurements suggested that the chemiluminescence emissions from Mg, MgO and MgOH dominated the collected spectra, in spite of only 3% Mg (by weight) existed in the alloy. Additionally, it was observed that moderate gaseous reactions could occur well before the breakup of the passive oxide coating, generating obvious fine oxide smokes. Moreover, consumption rates of the wire in different hot oxidizers were obtained and compared. It was shown that O 2 featured more significant promotion of the reaction than H 2 O. Nevertheless, without O 2 , much less metal-oxide particles were generated. Temperature measurements indicated that the ignition temperature lied within 2160 ∼ 2220 K, which was lower than the melting points of Al 2 O 3 (2350 K) and MgO (3125 K). Large single burning Al-Mg alloy droplets (∼200 µm diameter) were generated after the micro-explosion inside the wire. It was found that the combustion of Al-Mg alloy in both H 2 O/O 2 /N 2 and H 2 O/N 2 atmospheres were diffusion-controlled with a stand-off distance around 150 µm (stand-off ratios at ∼ 1.3). Finally, SEM and EDS measurements revealed that Al, Mg and O elements coexisted on the surface of the burnt wires. Nevertheless, it was observed that the oxidization of Mg started before Al, and the reaction of alloy was more intense when O 2 existed. This led to the generation of much thicker oxide layers and larger number of nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract Metal powder combustion has traditionally been studied to mitigate the risk of industrial accidents and to determine the contributions of metals as additives to the performance of energetic materials. Recently, there has been growing interest in exploring the potential of metal powders as recyclable, zero-carbon energy carriers as an alternative to the hydrocarbons known to contribute to climate change. The present work introduces, for the first time, a stabilized flat iron flame. The counterflow burner used in this work is comprised of an inverted ceramic nozzle which sits above, and is aligned axially with, a lower nozzle producing a laminar flow of particles suspended in an oxidizing gas. A stabilized methane flame sits inside the top nozzle and the hot combustion products impinge upon the two-phase flow from the bottom nozzle, creating a stagnation plane. Spherical iron powder, with 90% of the particles less than 2.5 µm in size, is pre-loaded into a piston and dispersed using mixtures of 30% and 40% oxygen balanced in argon. Flame speeds are measured using particle image velocimetry (PIV), while flame temperatures are determined using multicolour pyrometry. It is found that flame speeds range between 30 cm/s and 45 cm/s for both oxidizing mixtures. Despite having fuel loadings below stoichiometric concentrations, the observed particle combustion temperatures are close to the adiabatic flame temperature of the stoichiometric mixture, indicating combustion in the diffusion-controlled regime for these small particles. Finally, the independence of the flame speeds with respect to oxygen concentration suggests flame propagation in the discrete regime. [ABSTRACT FROM AUTHOR]

To address the resistance effect of protective oxide shell on the ignition of micron-sized aluminium (Al) and the particle agglomeration and residue deposition of Al-based fuels, aluminium-lithium (Al–Li) alloy by introducing tiny elemental Li could have a promising potential in substituting for Al. The paper presents a comparative study on the surface structure and the ignition and combustion characteristics between single Al-Li alloy and pure Al fuel microparticles in air. The Al-Li alloy with the content of 1.0 wt.% Li was prepared by gas-atomized method and characterized by multiple methods. The characterization of the surface morphology, elemental distribution, and residues of incomplete combustion show the dendritic structure of eutectic compounds like veins at the surface and interior of the Al-Li alloy microparticles. Compared to the dense oxide shell of Al, the oxide shell of Al-Li alloy provides many more pathways like cracks/openings for the Al and O 2 contact as inductively heated by a highly-focused laser beam, resulting in a reduction of ignition delay. The difference of ignition delay between the two demonstrated that the addition of Li element shortened the ignition delay, especially, over 80% for the particle size of > 45 µm. The ignition delay can be reduced by minimizing the particle size or enhancing the ignition power density. Owing to the variations in reactivity, melting, and boiling points compared to pure Al, the Al-Li alloy particles exhibit distinct gas-phase flame and micro-explosion behaviour. These differences result in considerably higher flame temperatures and more intense radiation. Finally, a sequence of reaction mechanisms that potentially control the ignition and combustion of single Al-Li alloy particles (as distinct from pure Al) is proposed, the physical and chemical properties of highly reactive Li are crucial to improve the combustion performance of Al. [ABSTRACT FROM AUTHOR]

The paper presents multi-parameter high-speed optical diagnostics of single iron particle oxidation in a hot laminar flow consisting of O 2 , H 2 O, CO 2 , and N 2. Prior to luminous combustion, micron-sized particles are visualized using diffuse-backlight-illumination with high temporal and spatial resolutions, enabling in situ particle sizing. The incandescent burning particles are then monitored by direct high-speed imaging. This approach allows for a successful determination of the multiple oxidation stages of individual iron particles, including the solid phase, melting, and liquid phase. Two characteristic timescales are quantified as a function of the resolved particle diameter at various O 2 volume fractions, namely the solid-phase oxidation time (SOT, defined as the time between the onset of rapid particle heating and melting) and the liquid-phase combustion time (LCT, defined as the duration between melting initiation and peak luminosity). SOT shows a linear dependence on the inverse O 2 mass fractions at relatively high O 2 levels but it tends to plateau when the O 2 content is below 20 vol. %. LCT is inversely proportional to the O 2 mass fraction, suggesting that liquid-phase oxidation of iron particles is limited by external oxygen diffusion. The experimental LCT quantitatively agrees with a theoretical mode developed for nonvolatile particle combustion in the diffusion-limited regime, considering O 2 as the only oxidant. This also implies that at the presence of O 2 , the oxidizing effect of H 2 O and CO 2 on diffusion-limited iron particle combustion is negligible. [ABSTRACT FROM AUTHOR]

Metallic fuel relocation experiments were conducted in which molten uranium was dropped into the cladding of a single fuel pin where eutectic interaction between fuel and cladding led to the release of eutectic fuel mixture into a narrow sodium channel. The thermal hydraulic relocation behavior of the molten uranium was analyzed in a previous study. Here, the extent of the eutectic formation was evaluated by scanning electron microscopy and energy-dispersive spectroscopy (SEM/EDS). The SEM/EDS analysis confirmed that cladding failure was caused by eutectic penetration. The eutectic penetration initiated around the boundary between cladding and uranium fuel, progressed into the cladding, and caused the cladding failure. Near the cladding failure region, the original cladding shape was unrecognizable, and many cracks, breakthroughs, and partial losses of the cladding were observed. The main components of the cladding failure region were UFe 2 and U 6 Fe, which are the key components in the eutectic penetration. [ABSTRACT FROM AUTHOR]

A theoretical model is considered to predict the minimum ambient gas temperature at which fine iron particles can undergo thermal runaway–the ignition temperature. The model accounts for Knudsen transition transport effects, which become significant when the particle size is comparable to, or smaller than, the molecular mean free path of the surrounding gas. Two kinetic models for the high-temperature solid-phase oxidation of iron are analyzed. The first model (parabolic kinetics) considers the inhibiting effect of the iron oxide layers at the particle surface on the kinetic rate of oxidation, and a kinetic rate independent of the gaseous oxidizer concentration. The ignition temperature is solved as a function of particle size and initial oxide layer thickness with an unsteady analysis considering the growth of the oxide layers. In the free-molecular limit (small particles), the thermal insulating effect of transition heat transport can lead to a decrease of ignition temperature with decreasing particle size; however, the presence of the oxide layer slows the reaction kinetics, and its increasing proportion in the small-particle limit can lead to an increase of ignition temperature with decreasing particle size. This effect is observed for sufficiently large initial oxide layer thicknesses. The continuum transport model is shown to predict the ignition temperature of iron particles exceeding an initial diameter of 30 µm to a difference of 3% or less (30 K or less) when compared to the prediction of the transition transport model. The second kinetic model (first-order kinetics) considers a porous, non-hindering oxide layer, and a linear dependence of the kinetic rate of oxidation on the gaseous oxidizer concentration. The ignition temperature is resolved as a function of particle size with the transition and continuum transport models, and the differences between the ignition characteristics predicted by the two kinetic models are identified and discussed. [ABSTRACT FROM AUTHOR]

India is pursuing a three stage nuclear power program employing its natural uranium and Thorium reserves. FBRs form the second stage of the program linking the first phase with natural U and third phase with Th fuels. The saga of FBR technology development in India is presented in this paper. The valuable experience in design and operation of Fast Breeder Test Reactor (FBTR) is recounted briefly. The R&D for the techno-economic demonstration of Prototype Fast Breeder Reactor (PFBR) is explained including technology development for addressing manufacturing challenges and engineering full-scale qualification. Finally a brief projection is made on the developments for FBR600 with improved safety and economics as well as metal fuel technology for future experimental and power reactors to follow. [ABSTRACT FROM AUTHOR]

Experiments dropping molten uranium into test sections of single fuel pin geometry filled with sodium were conducted to investigate relocation behavior of metallic fuel in the core structures of sodium-cooled fast reactors during a hypothetical core disruptive accident. Metallic uranium was used as a fuel material and HT-9M was used as a fuel cladding material in the experiment in order to accurately mock-up the thermo-physical behavior of the relocation. The fuel cladding failed due to eutectic formation between the uranium and HT-9M for all experiments. The extent of the eutectic formation increased with increasing molten uranium temperature. Voids in the relocated fuel were observed for all experiments and were likely formed by sodium boiling in contact with the fuel. In one experiment, numerous fragments of the relocated fuel were found. It could be concluded that the injected metallic uranium fuel was fragmented and dispersed in the narrow coolant channel by sodium boiling. [ABSTRACT FROM AUTHOR]

• Energy independent, fully automated Hydrogen generator of the original design is presented. • The average rate of hydrogen evolution was more than 30 ml/ sec *g Al at the hydrogen yield 95–99 %. • Hydrogen releases at the reaction of activated Al with water of initial temperature 70–100 °C. • Al was activated by Bismuth (3.5 wt%) and Tin (1.5 wt%) in a vibrating mill in dry argon atmosphere. • Hydrogen cost is estimated by 5–8 USD/kg H 2. The study reports a novel hydrogen generation system. The system comprises Al based hydrogen generating composition with a reduced content of activating additives and an original fully automated Al-water hydrogen generator with capsule supply of hydrogen generating composition. The hydrogen generating composition was synthesized by means of mechanochemical activation of aluminum powder. Bi − 3.5 wt%. and Sn − 1.5 wt%. were used as activating additives. The combination of the proposed hydrogen generating composition and the design features of the generator made it possible to provide a high rate of hydrogen evolution (more than 30 ml/ sec *g) and a hydrogen yield of 95–99 %. A high rate of hydrogen evolution was ensured by maintaining a temperature of at least 70 °C in the reaction zone. The heat of the aluminum oxidation reaction was utilized to maintain the necessary temperature. Based on the obtained experimental data, an assumption about the mechanism of the initial stage of the Al oxidation reaction was made. Some feasibility estimations of the hydrogen generation system are presented as well. [ABSTRACT FROM AUTHOR]

A new powder fueled ramjet configuration is put forward based on the previous studies on ramjet, and an experiment investigation is conducted to study the working process of the engine. A series of tests are conducted to investigate the combustion efficiency of the engine by changing the ram air flow inlet interface position and the fluidization gas mass flow rate. It is found that the engine achieves 24 s self-sustaining combustion and worked stably during fire tests. In addition, the powder feed system provides aluminum fuel particles continuously and steadily during the tests, and the piston velocity can be used as a function of the fuel mass flow rate. Optimized ram air flow inlet interface position is 250 mm to the combustion chamber front end. Furthermore, as the mass flow rate of fluidization gas rises from 5 to 15 g/s, the combustion efficiency of the engine significantly increases. The best combustion efficiency reaches 73.05% on the condition that the ram air flow inlet interface position is 250 mm to the combustion chamber front end and the mass flow rate of fluidization gas is 15 g/s. [ABSTRACT FROM AUTHOR]

Lithium combustion has been studied for several decades, with a primary focus on safety issues, such as lithium fires resulting from spills in nuclear reactors. Several studies have also considered the use of lithium as a fuel within propellants, or within propulsion systems that burn lithium in the atmospheric “air” of other planets. Lithium safety has typically been investigated through combustion of molten pieces of lithium or within pool fires. For propulsion applications, experiments were carried out using packed beds of lithium particles. A novel approach that has recently been proposed is the use of lithium as a recyclable fuel, or energy carrier that can compactly store renewable energy. In this scheme, lithium is burned with air, or power-plant exhaust, to generate heat for thermal power systems when power is needed. The solid-phase combustion products would be collected and recycled, via electrolysis, back into elemental lithium when excess renewable power is available. This paper summarizes the existing knowledge on lithium combustion. It presents the available findings on lithium combustion for large single pieces of lithium, on pool fires, reaction in packed beds, as well as the combustion of sub-mm sized particles and droplets which are needed for the use of lithium as an energy carrier. The combustion reactions of lithium with O 2 , H 2 O, CO 2 and N 2 are discussed. Modelling of lithium-particle combustion is at the early stages of development and available results are discussed. [ABSTRACT FROM AUTHOR]

• There graphene-based transition metal coordination polymers have been evaluated as the energetic catalysts in Al-based composites. • These catalysts are well doped in the PVDF/AP matrix as the oxidizer shell of Al-based composites prepared by a spray drying method. • The high energy level but low burn rate has been achieved for the Al@PVDF/AP in presence of these graphene-based catalysts. In this paper, energetic combustion catalysts doped core–shell Al@PVDF/AP composites have been fabricated via a spray-drying method, where three kinds of graphene-based energetic coordination polymers (GECPs, GO-CHZ-Co, GO-CHZ-Ni and GO-CHZ-Cu) are employed. These composites have been characterized comprehensively in terms of morphologies, thermal decomposition behaviors, energy density, combustion performances and condensed combustion products (CCPs). The results show that the GO-CHZ-Co and GO-CHZ-Cu could increases the heat of explosion for Al@PVDF/AP composites from 6885.2 J·g−1 to 7491.0 J·g−1 and 7390.1 J·g−1, respectively, which are 8.8 % and 7.3 % higher than the blank reference samples. Moreover, the burn rates for these composites decrease when GO-CHZ-Co or GO-CHZ-Ni is included, indicating that these composites are promising candidates with improved energy density even unusually with decreased burn rates. [ABSTRACT FROM AUTHOR]

Air-free electrolytic dealloying using non-aqueous electrolytes is a sustainable method for creating nanoporous Al (NP-Al) with highly reactive nanoscale-sized ligaments from Al-Mg parent alloys while simultaneously recovering the sacrificial Mg. However, this synthesis strategy is limited in versatility by the low number of available non-aqueous electrolytes that can conduct Mg ions and has so far only been proven to work with all-phenyl complex (APC) electrolyte derived from in situ reaction between two equivalents of Lewis base C₆H₅ClMg and one equivalent of Lewis acid AlCl 3 in tetrahydrofuran (THF) solvent. Here we demonstrated that a new electrolyte consisting of MgCl 2 /AlCl 3 /Mg(TFSI) 2 (or MACT) in glyme-based solvents recently introduced in the Mg-ion battery community can also be used to create NP-Al. This alternative electrolyte exhibits higher electrochemical performance than APC and can create ultrafine NP-Al with ≈ 6–14 nm ligament size, which is smaller than the ≈ 10–20 nm ligament size commonly achieved using APC. [Display omitted] [ABSTRACT FROM AUTHOR]

Metal fuels are attractive for solid/hybrid rocket propulsion and energy-conversion applications, because of their high energy densities. Boron powder (B), due to its high gravimetric (58.30 MJ·kg−1) and volumetric heats of combustion (136.44 kJ·cm−3), is ideally one of the most promising fuel candidates for fuel-rich solid propellant (SP). However, from an application perspective, amorphous B has drawbacks of high ignition temperatures and incomplete combustion, resulting in low energy-release rate and efficiency. Thus, there is growing interest in employing B-based composite energetic materials (B-CEMs) in SP, explosives, and pyrotechnics. The present work provides a comprehensive review of the advances made over the past few decades in the areas of preparation, combustion, and applications of B-CEMs. The preparation methods of various types of B-CEMs are introduced, and the physicochemical properties of B-CEMs are systematically discussed particularly with regards to achieving advantages over B and other metal powders in a broad range of applications. The ignition and combustion behavior of different energetic formulations with B-CEMs are reviewed. Finally, the existing problems and future challenges in our understanding of the field (prospects) are discussed. [ABSTRACT FROM AUTHOR]

To successfully transition from fossil-fuel to sustainable carbon-free energy carriers, a safe, stable and high-density energy storage technology is required. The combustion of iron powders seems very promising in this regard. Yet, little is known about their in-process morphological and microstructural evolution, which are critical features for the circularity of the concept, especially the subsequent reduction of the combusted oxide powders back to iron. Here, we investigated two iron powder combustion pathways, one in air and one with the assistance of a propane pilot flame. Both processes resulted in spherical hollow particles composed of a complex microstructure of wüstite, magnetite and/or hematite. Partial evaporation is indicated by the observation of nanoparticles on the micro-sized combustion products. The associated gas production inside the liquid droplet could be the origin of the internal porosity and micro-explosion events. Cracking at the end of the combustion process results in mostly open porosity, which is favorable for the subsequent reduction process. With this study, we aim to open the perspective of iron metal fuel from macroscopic combustion analysis towards a better understanding of the underlying microscopic thermodynamic, kinetic, microstructural and thermomechanical mechanisms. [Display omitted] [ABSTRACT FROM AUTHOR]

• Molten metal fuel was pressure injected into the coolant channel with pressure. • The Real Time X-ray Video System was able to capture the metal fuel relocation behavior. • The injected metal fuel was simultaneously dispersed downward and upward. • Relocation of metal fuels can potentially result in negative reactivity feedback effects. A pool-type sodium-cooled fast reactor (SFR) using metal fuels has a number of inherent safety features that can support benign consequences for design basis accidents (DBAs). Even in postulated severe accident conditions, the core is designed to remain under sub-critical condition in a passive coolable geometry. In case of the postulated severe accidents in SFRs, fuel relocation in the core region along the coolant channel is an important negative reactivity feedback factor that lowers the reactor power level and consequently eliminates the possibility of recriticality. Therefore, understanding the relocation behavior of fuels and the coolability of the relocated fuels in the postulated severe accident is one of the most important factors in the safety assessment of SFRs. In the present study, the relocation behavior of the metal fuel in a pin bundle geometry was investigated by injecting the metal fuels into the coolant channels with pressure. The first metal fuel relocation with pressure injection (RPI-1) experiment showed that many of the metal fuels levitated to upper plenum. In case of RPI-2 experiment, the Real Time X-ray Video System (RTXVS) was constructed and used to obtain a real time X-ray video of the experiment. This real time X-ray video clearly showed the relocation behavior of the pressure injected metallic uranium in the sodium coolant channel. Through this video, it was confirmed that part of the fuel was dispersed downward and part of the fuel was dispersed upward, and the relocation behavior of the injected fuel proceeded simultaneously in the downward and upward directions. It can be concluded that in case of pressure injection of the metallic fuel, there is a possibility that the metallic fuel can be quickly removed from the core region, resulting in a negative reactivity feedback effect. [ABSTRACT FROM AUTHOR]

Temporally, spatially, and thermally resolved measurements of in situ nanoparticle cloud formation during combustion of single micron-sized iron particles are carried out. The critical particle temperature for the cloud formation is identified using high-speed and high-magnification shadowgraphy, recording the evolution of a nanoparticle cloud, synchronized with two-color pyrometry probing the surface temperature of the burning micron particle. The finding could serve as an important criterion to minimize nanoparticle generation in the flame of iron-particle suspensions, making iron powders a cleaner and more recyclable fuel. [ABSTRACT FROM AUTHOR]

Exposure to high temperatures and irradiation results in interaction and interdiffusion between fuel and cladding constituents that can lead to formation of undesirable brittle or low-melting point phases. A diffusion couple study has been conducted to understand fuel-cladding interaction occurring between U–22Pu–4Zr (in wt%) fuel and pure Fe at elevated temperatures. The phases formed within fuel cladding chemical interaction (FCCI) layer have been characterized in the transmission electron microscope (TEM). The phases formed within FCCI layer have been identified as Fe 2 U (Fd-3m), FeU 6 (I4/mcm), Fe 2 Zr (Fd-3m), FeZr 2 (I4/mcm), Fe 2 Pu (Fd-3m), UZr 2 (P6/mmm), β-Zr (Im-3m), and ZrO 2 (Fm-3m). [ABSTRACT FROM AUTHOR]

Aluminum powders are commonly used in solid propellants to enhance the performance of space propulsion systems. During combustion, a fraction of the fuel metal particles, which emerge from the bulk, tends to merge into aggregates. These structures eventually leave the combustion surface in the shape of partially molten agglomerates which can reach the size of hundreds of microns. These condensed combustion products partake in nozzle expansion and hinder the delivered specific impulse of the rocket unit. The enhancement of original particle reactivity improves combustion quality and may reduce sensibly agglomerate size and relevant losses. More reactive aluminum fuel can be obtained by activation of micron-sized powders, without resorting to the use of nano-metals. One of the methods consists of a chemical treatment with a processing solution which alters the standard oxide layer at the surface of the particles. Such modifications grant lower ignition temperature and faster propellant burning rates but deplete a fraction of the active metal content, as a result of the chemical reaction. The present paper compares the features of three batches of aluminum particles which were treated with fluorine-based activating solutions of different concentrations. The batches were supplied in the frame of HISP FP7 European Project. The characterization focused on physical, chemical and thermal properties, looking at the reactivity of the samples and at the alterations introduced by the chemical processing. Finally, activated aluminum batches were tested in lab-scale propellants, monitoring the variation of ballistic properties with respect to a reference formulation. [ABSTRACT FROM AUTHOR]

Ignition of iron particles in an oxidizing environment marks the onset of self-sustained combustion. The objective of the current study is to quantitatively examine the ignition characteristics of fine iron particles (i.e., 1µm- to 100µm-sized) governed by the kinetics of solid-phase iron oxidation. The oxidation rates are inversely proportional to the thickness of the oxide layer (i.e., following a parabolic rate law) and calibrated using the experimentally measured growth of iron-oxide layers over time. Steady-state (i.e., Semenov's analysis) and unsteady analysis have been performed to probe the dependence of the critical gas temperature required to trigger a thermal runaway (namely, the ignition temperature T ign) on particle size, initial thickness of oxide layer, inert gas species, radiative heat loss, and the collective heating effect in a suspension of particles. Both analyses indicate that T ign depends on δ 0 , i.e., the ratio between the initial oxide layer thickness and particle size, regardless of the absolute size of the particle. The unsteady analysis predicts that, for δ 0 ≲ 0.003 , T ign becomes independent of δ 0. Under standard conditions in air, T ign is approximately 1080 K for any particle size greater than 5µm. The ignition temperature decreases as the thermal conductivity of the oxidizing gas decreases. Radiative heat loss has a minor effect on T ign. The collective effect of a suspension of iron particles in reducing T ign is demonstrated. The transition behavior between kinetic-controlled and external-diffusion-controlled combustion regimes of an ignited iron particle is systematically examined. The influences of initial oxide-layer thickness and particle temperature on the ignition delay time, τ ign , of iron particles are parametrically probed. A d 2 -law scaling between τ ign and particle size is identified. Possible sources of inaccuracy are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The ignition characteristics of boron particle agglomerates prepared by drying 2–3mm water slurry droplets of crystalline or amorphous boron powders were investigated by introducing suspended agglomerates into a flow of high temperature oxidizing gases (oxygen/nitrogen/water vapor) obtained by a combination of combustion and electrical heating. Monitoring of the agglomerate luminosity, spectra, and temperature revealed a dual-stage ignition process. The first stage ignition was observed at flow temperatures as low as 715K and was followed by agglomerate quenching if the oxygen concentration in the flow was below some critical value. As the oxygen concentration of the flow was increased beyond this critical value, the second stage ignition leading to full-fledge combustion was observed. The ignition characteristics of boron particle agglomerates were theoretically investigated by developing a numerical model that considers the oxygen concentration gradient inside the agglomerate in order to simulate the ignition process. The model results accurately captured the qualitative behavior of the first stage ignition and quenching as well as the dual-stage ignition processes. The model identified the closure of pores in the agglomerate due to the build-up of the boron oxide layer, which effectively seals off the large interior surface area of the agglomerate from the oxidizer, as being the mechanism responsible for the two-stage ignition phenomenon. The critical flow conditions in terms of flow temperature and ambient oxygen concentration for the second stage ignition were determined by this model. The experimental result for the critical ambient oxygen concentration for the second stage ignition in dry flow was determined to be 70%, which is fairly close to the model prediction of 79%. [Copyright &y& Elsevier]

Abstract: This study focuses on the criticality safety during the fabrication of fast reactor metal fuel pins comprising of the fuel type U–15Pu, U–19Pu and U–19Pu–6Zr in the Plutonium–Uranium Metal fuel Pin fabrication Facility (PUMP-F). Maximum amount of fissile mass which can be handled safely during master alloy preparation, Injection casting and fuel slug preparation following fuel pin fabrication were identified and fixed based on this study. In the induction melting furnace, the fissile mass can be limited to 10.5kg. During fuel slug preparation and fuel pin fabrication, fuel slugs and pins were arranged in hexagonal and square lattices to identify the most reactive configuration. The number of fuel slugs which can be handled safely after injection casting can be fixed to be 53, whereas after fuel fabrication it is 71. [Copyright &y& Elsevier]

Abstract: Pyro-processing fuel cycle technology is a promising technological candidate to realize advanced fuel cycle with reduced burdens of proliferation risk and long life radioactive waste. CRIEPI have been studying pyro-processing and metal fuel technology since 1986 for obtaining basic properties, process chemistry and engineering technologies. In this century, CRIEPI and JRC-ITU have jointly demonstrated the recovery of MAs from both irradiated metal fuels and spent MOX fuels. Since 2009, accumulation of the design basics through tests with engineering-scale pyro-processing equipment is carried out by CRIEPI. According to the results obtained in these studies, feasibility of pyro-processing fuel cycle technology was evaluated through estimation of Technology Readiness Level and cycle cost based on the design study of fuel cycle facility of 40 tonHM/y throughput. [Copyright &y& Elsevier]

Abstract: Sodium chlorate based mixtures for oxygen generators include metal fuels, which may produce instabilities and increase risk of fire. Eliminating or at least decreasing the metal content would improve the process stability and fire safety of the generators. In the present paper, thermal decomposition of sodium chlorate based mixtures with different concentrations of cobalt oxide catalyst and metals (iron and tin) is studied using thermogravimetric analysis. The effect of decreasing the catalyst particle size to nano scale is investigated. The possibility of decreasing the decomposition temperature of sodium chlorate by high-energy mechanical milling of its binary mixtures with cobalt oxide is explored. Thermodynamic calculations of the adiabatic combustion temperatures are conducted for the tested binary and triple compositions. The comparison of the characteristic decomposition temperatures, obtained in the experiments, with the predicted adiabatic combustion temperatures indicates that it is possible to achieve self-sustained propagation of the decomposition wave with no metal fuel or at least with a lower metal content. [Copyright &y& Elsevier]

Abstract: Minor actinide recycling in a fuel cycle is a potential technology to minimize the environmental radioactivity burden in waste disposal in the fast reactor era after 2050. Pyroprocessing technology with metal electrorefining expects that no additional process is required to separate minor actinides and short- cooled fuels can be treated due to no-use of organic solvent that degrades by radiation. Pyroprocessing has been explored for metal fuel cycle and nitride fuel cycle in Japan. Metal fuel fast reactor, which can achieve a high breeding ratio over 1.3, and its fuel cycle is a compact system by integrating pyroprocessing. Oxide fuel can be also treated by converting to metals by reduction. Separation of transuranium elements from high level liquid waste originating from aqueous reprocessing has been challenged. Verification of the process and development of an engineering scale device are the current interests for study. In addition, irradiation study of metal fuel with minor actinides currently much advances from the point of fundamental investigation. Accelerator-driven system (ADS) for transmutation of minor actinides is integrated with pyroprocessing for recycling system. The denitriding at anode and azotizing at cathode together with electrorefining have been fundamentally studied by use of plutonium. [Copyright &y& Elsevier]

Abstract: Molten salt electrorefining based pyrochemical reprocessing is ideally suited for treating the spent metallic fuels. Development of all aspects of the pyroprocess flow sheet is in progress at IGCAR to enable the commercialisation of the process. Following the initial laboratory scale studies, engineering scale studies have been taken up on the molten salt electrorefining process and the consolidation process for recovering the actinides from cathode deposit. Studies are also being carried out on the direct electrochemical reduction of actinide oxides, methods for the fabrication of waste forms and materials and coatings. [Copyright &y& Elsevier]

Abstract: RIAR has performed a set of developments and investigations of experimental metal fuel pins as applied to possible use in the BN reactors with heterogeneous oxide-metal core of various types: BEAH (by FA heterogenization), IFAH (intra FA heterogenization), IFEH (intra fuel element heterogenization). This set covers calculated, process, out -of-pile, in-pile and postirradiation examinations of metal fuel, fuel pins and FA of various types and purposes. Main radiation-thermal effects have been studied both in fuel specimens and in fuel pins. Specifically, effect of radiation growth and radiation swelling of metal fuel doped -free and doped uranium, and uranium alloys with plutonium) and physical-chemical interaction between fuel and steel claddings on parameters of the fuel pin reliability during irradiation in reactors MIR, BOR-60 and BN- 350 have been investigated. The experiment confirmed the effectiveness of the developed approaches and technique of providing reliability of metal doped-free fuel pins under the conditions similar to possible conditions in the heterogeneous core of the BN type: BFAH, IFAH, IFEH. [Copyright &y& Elsevier]

The combustion behavior of single laser-ignited iron particles is investigated. Transient particle radiant intensities at 850 nm and 950 nm are measured by post-processing recorded high-speed camera images using an in-house developed particle tracking program. Then, the time-resolved particle temperature is obtained based on two-color pyrometry. A plateau-like stage shortly after the ignition is repeatedly observed, and identified as iron particle melting by the measured temperature and the estimated melting time. Besides, an abrupt brightness jump near the end of combustion is observed for most burning particles, while a small portion of the particles (< 10%) show a second plateau-like stage instead. The particle temperature right after the brightness jump (1880 ± 70 K) is almost identical to that during the second plateau-like stage. This temperature corresponds to two phase-change temperatures in the Fe-O phase diagram: i) L2 < = > Fe 3 O 4 (s) at 1869 K (congruent melting) and ii) L2 < = > Fe 3 O 4 (s) + O 2 (g) at 1855 K (eutectic reaction), where L2 represents a liquid iron oxide. Based on this, the presence of the brightness jump (spear point) is explained by a sudden solidification of supercooled iron oxide droplets with an atomic O/Fe ratio larger than or close to 4/3. Particles' near-peak temperatures are also measured based on time-integrated spectra. The results indicate that the near-peak temperature increases first fast and then slowly with an increase of oxygen concentration. At higher oxygen concentrations, smaller particles have a slightly lower temperature. The effect of particle size on the near-peak temperature is negligible at lower oxygen concentrations due to weaker radiation. The morphology of combusted particles is examined by micrography. Some burned particles appear as hollow thin-shell spheres at all adopted oxygen concentrations. Additionally, nano oxides are found at 13–51% oxygen concentrations. Less traces of nano oxides were observed at reduced oxygen concentrations. The nano-oxide formation mechanisms are analyzed based on thermochemical equilibrium calculations. [ABSTRACT FROM AUTHOR]