Alloy Fuels Research Articles

Structural, Electronic, and Mechanical Properties of α(U) and α(U-Zr) Alloy Fuels

Uranium-zirconium (U-Zr) alloy fuels have been taken into consideration for fast reactors because of their superior reactor safety, high uranium (U) density, and excellent thermal conductivity. In this paper, the structural and mechanical properties of metallic U and U-Zr alloy fuels are calculated at the atomic level by density functional theory–based calculations with Hubbard U (density functional theory + U) corrections. Several structure-property relations, such as lattice volume, bulk modulus, Young’s modulus, shear modulus, electronic density of states, etc. are calculated for U metal and U-Zr alloy fuels. In addition, the Zr content in metallic U fuel is adjusted from roughly 5 at. % to 15 at. % in order to determine how the Zr content affects the characteristics of U-Zr fuel material. A linear relationship between the volume and Zr concentrations is observed. The concentration of Zr in the fuel affects the mechanical characteristics of U-Zr alloy fuel. It is also observed that the electronic structure of the α-U phase is not changed significantly in the presence of Zr. Our computations provide insight into how U-Zr alloy fuels behave in reactor conditions.

Combustion and energy performance of multiple aluminum-based alloy particles

The long ignition delay time, high ignition temperature and agglomeration prevent the aluminum (Al) particles from fully releasing the chemical energy stored within them during combustion. Al-based alloy fuels with high energy density and excellent combustion performance have become a potential candidate for replacement with pure Al. Therefore, understanding the combustion and energy performance of Al-based alloys is beneficial for the development of Al-based energetic materials. Herein, the combustion and energy performance of pure Al and three Al-based alloys were comparatively investigated by means of a thermal analysis system, an ignition and combustion test apparatus, and a variety of physicochemical property characterizations. The results showed that all three Al-based alloys contained at least one intermetallic compound. Al-boron (B) and Al-magnesium (Mg)-B alloys had higher theoretical energy densities than pure Al, while Al-Mg did not. The thermal oxidation properties of all three alloys were significantly better than those of pure Al. Compared with pure Al, the three alloys showed higher burning rate, shorter ignition delays, more vigorous combustion, and higher burnout at room pressure in air. This is still true after adding AP. According to the compositions of the condensed combustion products, the possible chemical reactions during the combustion of the alloys were speculated. Grasping the combustion mechanisms of the alloys is challenging due to the relative lack of thermodynamic data on intermetallic compounds. Finally, a comprehensive comparison of the combustion processes of pure Al and alloys was made. Although the Al-Mg alloy does not have the same theoretical energy density as pure Al, the more outstanding combustion performance and fewer two-phase flow losses may make up for the gap in theoretical energy density.

Stable aluminum-lithium alloy fuels for solid propellants by facile surface modifying

As a highly reactive metal fuel, aluminum–lithium (Al-Li) alloy is an ideal candidate for replacing Al powders in solid propellants. However, the Li solid solution and AlLi compounds on the surface of Al-Li alloy powders display high reactivity, which can react with a large number of organic and inorganic reagents. Thus, the incompatibility of Al-Li alloy with other components of propellants makes it difficult to be further applied in propellants. In this study, surface-modified Al-Li alloy powders (Li content of 5 wt%) with 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS17) were prepared by a facile self-assembly method, which made the alloy powders super-hydrophobic. The results of SEM, XPS and FIB-TEM show that the surface of Al-Li alloy powder is well-coated by an evenly distributed FAS17 layer. Due to the fluoroalkyl on the surface of the powders, the water contact angle increased by 91°and the intensity of the reaction between alloy powders and water is significantly reduced, which results in improved compatibility with other components of propellants.Meanwhile, the propellant grain of surface-modified powders had fewer defects than the sample before coating, and showed original excellent combustion performance. The average combustion pressure and pressurization rate increased by ∼114 kPa and ∼8613 kPa/s, and the steady burning rate increased by 0.24 mm/s. Therefore, the surface-modified Al-Li alloy powders with FAS17 are expected to be an attractive candidate as a highly active and stable fuel in solid propellants.

Formation of uranium nitride nanoparticles via mechanical alloying of uranium-molybdenum alloy fuels in gaseous nitrogen

Uranium-molybdenum (U-Mo) alloys show promise as a nuclear fuel system due to their high thermal conductivity and fuel loading capability. However, U-Mo systems are susceptible to irradiation induced swelling ultimately affecting the cladding via mechanical and chemical interactions. To address these shortcomings, this research investigated the formation of uranium mononitride (UN) nanoparticles within a 90 wt% U/ 10 wt% Mo (U-10Mo) matrix to act as a prospective defect sink for fission products at nanometric hetero-interfaces. To promote the formation of UN, U-10Mo powders were mechanically alloyed under a high purity nitrogen atmosphere. Variations of the milling process investigated included media size, duration of milling, and number of times the milling jar was re-aerated with nitrogen gas. Characterization of the fuel microstructure was completed using light element analysis, X-ray diffraction, scanning and transmission-electron microscopy, electron energy loss spectroscopy, and atom probe tomography. UN nanoparticles measuring 1–5 nm in radius were observed in the U-Mo matrix as early as 1 h into the mechanical alloying process. Milling time in excess of 10 h was found to lead to deleterious effects induced by the stainless-steel milling media.

Reaction characteristics of Al–Mg alloy fuels with heterogeneous oxidation shell structures

Al–Mg alloy fuels are the most widely used Al-based alloy fuel in energetic material systems owing to their low ignition temperature and fast reaction. However, their surface oxide layer structure and oxidation, ignition, and combustion mechanisms differ from those of single-metal fuels. Furthermore, insufficient information has been reported on Al–Mg alloy fuels. In this study, Al–Mg alloy fuels were formed by introducing Mg into Al via centrifugal atomization. The microstructure and thermal reaction characteristics of the Al–Mg alloy fuel were evaluated. The ignition and combustion characteristics of the Al–Mg alloy fuels were monitored via single-particle ignition. When the Mg content was increased from 5 % to 40 %, a typical core–shell structure formed, the oxide shell thickness increased from 4.50 to 6.41 nm, and the content of the internal alloy compounds gradually increased. The surfaces of the Al–Mg alloy fuel particles with different Mg contents exhibited heterogeneous oxidation shells. Compared to Al powder, the Al–Mg alloy fuel exhibited better melting and oxidation peak temperatures and higher oxidation weight gain. As the Mg content was increased, the combustion of the Al–Mg alloy fuel intensified with more severe microexplosions and a shorter combustion time. The microexplosion of the Al–Mg alloy fuel was elucidated by theoretical calculations, and its combustion mechanism was proposed. The detailed description of the oxide layer structure, oxidation, and combustion of the Al–Mg alloy fuel provides a reliable explanation of its energy release mechanism in energetic material systems.

CsCl enrichment during solidification of molten LiCl–KCl–CsCl salt mixture

Metallic alloy fuels from fast reactors will be reprocessed by a non-aqueous electrochemical technique known as electrorefining. Electrorefining of spent metal fuel results in the accumulation of heat generating fission products especially 137Cs in the electrolyte, which is a eutectic salt mixture of 44 wt% LiCl and 56 wt% KCl. The fission products need to be removed from the salt so as to reduce the decay heat load and contamination thereby facilitating reuse of the salt and minimizing waste generation. Melt crystallization technique is a simple separation process being pursued for separation of fission products. This is a single step process which utilizes the difference in solubility of fission products in the solid and liquid phase. Crystallization involves purification of a substance from a liquid mixture by solidification of the desired component. Numerical modeling of separation of CsCl from a LiCl–KCl eutectic mixture by solidification was carried out using ANSYS Fluent (19.2). The enrichment of CsCl during solidification, its distribution in molten salt and segregation time were evaluated. Further, solidification experiments were conducted using the molten salt mixture and the predicted numerical results were validated.

Synthesis, Characterization and Dissolution behavior of Nitrides of U-Zr and U-Pu-Zr metallic alloy fuels for Aqueous reprocessing

The aqueous based PUREX process is an alternate choice to pyro-reprocessing of metallic alloy fuels (U-Zr/U-Pu-Zr). Direct dissolution of metallic alloy fuels in a nitric acid medium is not feasible due to vigorous reaction exhibited by the alloys when exposed to this medium. Conversion of metallic alloys into their oxides is a challenging method due to poor dissolution of oxides in nitric acid. Excellent dissolution of nitrides compared to oxides of metallic alloy fuels lead to exploit the nitride route for aqueous reprocessing. Hence, a novel method was adopted to prepare the nitrides of unirradiated U-Zr and U-Pu-Zr alloys via hydridation using UH3, as source-cum-getter instead of hydrogen cylinder. In this context, nitrides of unirradiated U-Zr/U-Pu-Zr metallic alloys were prepared by nitridation via hydridation. Initially, nitrides of U and Zr metals were synthesized and the formation of ZrN and U2N3 and UN phases were confirmed by powder X-ray diffraction measurements. Subsequently, nitrides of unirradiated U-Zr and U-Pu-Zr alloys were synthesized and characterized by X-ray diffraction, scanning electron microscopy and particle analyzer. The dissolution behavior of nitrides of Zr, U, U-Zr and U-Pu-Zr alloys were examined in nitric acid (12 M) medium under reflux conditions at 383 K for 8 h. The present study highlights the potential of the nitride route for efficient dissolution of actinides (U and Pu) and it is very important to develop flow sheets for the aqueous reprocessing of metallic alloys fuels.

Study on a novel Al-B-Li alloy powder with high enthalpy of combustion

ABSTRACT To develop aluminum-based alloy fuels with high energy release levels and good storage stability, the series of Al-B-Li alloy powders were prepared using high-temperature gas atomization. Al-4B-5Li meets the batch preparation requirements, with the mass combustion enthalpy of 31,101 J/g, exceeding the theoretical value of pure Al powder. XRD, SEM/EDS, XPS, TG-DTA, and other methods were used to characterize the alloy powder. The results showed that at 680℃~800°C, “channels” favorable for oxygen diffusion are formed inside the particles, which promotes the complete oxidation of Al-4B-5Li at 1300°C. Al-4B-5Li exhibits a total heat release of 6317 mJ/mg and an oxidation weight gain ratio of up to 95.0%, compared to just 8.39% for pure Al powder. In addition, Al-4B-5Li has good storage stability and lower energy attenuation compared to Al-5Li. After being placed in air for two weeks, the mass combustion enthalpy still exceeds 29,500 J/g, with a decay rate of only 4.52%. And the attenuation rate of Al-5Li is 10.17%. Al-4B-5Li has excellent combustion heat, thermal oxidation performance, and storage stability, and has good application prospects in the field of energetic materials.

Comparative Study on Combustion Behavior of Aluminum-Based Alloy Fuels and Aluminum Powder in Solid Propellants

In order to reduce the ignition temperature and improve the combustion efficiency of aluminum powder, three aluminum-based alloy fuels, Al–Mg, Al–Zn, and Al–Si–Mg, were prepared by the atomization method. The oxidation, ignition, and combustion performance of alloy fuels were investigated, and the results showed that, using pure aluminum powder as a reference, the weight gain of alloy fuels increased from 10% to 84%, the reaction activation energy decreased from 582 kJ·mol−1 to 208 kJ·mol−1, the alloy fuels containing Mg had good ignition response, and aluminum-based alloy fuels showed high calorific value and efficient combustion as a whole. In order to investigate the combustion behavior of alloy fuels in the solid propellant, tests were conducted on the mechanics, safety, process, and combustion properties of propellant according to the national standard, and the test results showed that, compared with the propellant made of aluminum powder with same quality, the propellant made of alloy has better mechanical properties, higher frictional sensitivity, lower electrostatic sensitivity, comparable process performance, and increased combustion calorific value and combustion speed. Engine test results confirmed that Al–Zn and Al–Si–Mg alloy fuels could effectively improve the specific impulse efficiency of the solid propellant and reduced the residual rate of the engine.

Modeling and Validation Studies on Solidification of Sodium Nitrate and LiCl‐KCl

AbstractTernary alloy fuel will be used as driver fuel for future fast reactors. Reprocessing of this spent alloy fuels will be carried out by pyrochemical reprocessing which is based on molten salt electrorefining at high temperature. During electrorefining, fission products get accumulated in the electrolyte along with some actinides in the form of their chlorides. The presence of fission products in the salt will add to the radioactivity levels and hence have to be stripped off. Melt crystallization is pursued to remove fission products from contaminated salt and to reuse the purified salt for electrorefining. Numerical modeling and experimental studies on tracking the phases during solidification of molten sodium nitrate salt were carried out. The numerical predictions closely matched with the experimental results. A resistance measurement technique was developed to accurately track the solidification interface so as to validate the numerical predictions.

Investigation of Radioisotopes Produced in the Core of a Navy Nuclear Reactor with Low- and High-Enriched Uranium Flues During One Cycle

ABSTRACT Today, nuclear reactors, especially small reactors, have been significantly welcomed in different countries and have various applications, including use as desalination plants, naval propulsions, etc. During the operation of the reactor, different types of radioisotopes are produced, some of which have very high activity and half-life. The type and amount of radioisotopes produced depends on the type of fuel used, the level of enrichment and the operating time of the reactor. The atomic density and activity of the most important radioisotopes for a typical reactor with two types of high and low enrichment fuel have been calculated in terms of time using MCNPX and CINDER codes. The activity of some of these radioisotopes is high and it is necessary to pay attention to possible environmental hazards. In this research, using the MCNPX Monte Carlo code, the core of a small nuclear reactor proposed as a Virginia class submarine propulsion with a power of 150 MWth was modeled. Using the BURN and KCODE cards in this code as well as the CINDER code, the fuel burnup calculations of the core of this naval reactor have been performed. For this reactor, there are two types of metal alloy fuels, including the high-enrichment UO2-Zr fuel and the low-enrichment U-10Mo fuel, and fuel burnup calculations have been performed for both fuels. After calculations, different types of radioisotopes were identified and changes in atomic density and activity quantities were calculated for the most important of these radioisotopes with high half-life during cycle and compared.

Point Defects Stability, Hydrogen Diffusion, Electronic Structure, and Mechanical Properties of Defected Equiatomic γ(U,Zr) from First-Principles.

At present, many experimental fast reactors have adopted alloy nuclear fuels, for example, U-Zr alloy fuels. During the neutron irradiation process, vacancies and hydrogen (H) impurity atoms can both exist in U-Zr alloy fuels. Here, first-principles density functional theory (DFT) is employed to study the behaviors of vacancies and H atoms in disordered-γ(U,Zr) as well as their impacts on the electronic structure and mechanical properties. The formation energy of vacancies and hydrogen solution energy are calculated. The effect of vacancies on the migration barrier of hydrogen atoms is revealed. The effect of vacancies and hydrogen atom on densities of states and elastic constants are also presented. The results illustrate that U vacancy is easier to be formed than Zr vacancy. The H interstitial prefers the tetrahedral site. Besides, U vacancy shows H-trap ability and can raise the H migration barrier. Almost all the defects lead to decreases in electrical conductivity and bulk modulus. It is also found that the main effect of defects is on the U-5f orbitals. This work provides a theoretical understanding of the effect of defects on the electronic and mechanical properties of U-Zr alloys, which is an essential step toward tailoring their performance.

Mixer-settler runs with tri-iso-amyl phosphate and tri-n-butyl phosphate for the aqueous reprocessing of U–Zr alloy fuels

Mixer-settler runs with tri-iso-amyl phosphate and tri-n-butyl phosphate for the aqueous reprocessing of U–Zr alloy fuels

GTAW Welded Inconel 625 Alloy Fuel Cladding for the Canadian SCWR: Microstructure and Mechanical Property Characterization

Abstract Inconel 625 is considered one of the candidate materials for reactor fuel cladding in the Canadian supercritical water reactor (SCWR) design. Gas tungsten arc welding (GTAW) is being evaluated as a joining technique for SCWR fuel cladding since this method is widely used to join components in the power and nuclear industry. During the GTAW process, the welding thermal cycle produces different types of microstructures in both the heat-affected zone (HAZ) and fusion zone (FZ) that affect the material's mechanical properties. A series of welding experiments at various weld conditions were performed using an automatic GTAW orbital process on Inconel 625 alloy tubing. Simple analytical heat conduction and grain growth models were developed to predict weld temperature profiles and metallurgical transformations. Weld characterization included mechanical tests, optical microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) elemental analysis, and microhardness measurements. Weld microstructural characterization revealed that a characteristic dendritic structure was formed in the FZ, while the HAZ exhibited larger equiaxed grains than those found in the base material (BM). SEM-EDS analysis showed no distinct alloying element segregation in both the HAZ and FZ. Welds produced with heat inputs of about 3.00 J/mm3 presented similar mechanical properties as those observed in the BM. In these welds, grain growth was homogenously minimized in the FZ. It is concluded that the effective welding heat input control can optimize the weld microstructure and the weld mechanical properties in Inconel 625 tubing used as Canadian SCWR reactor fuel cladding.

Demonstration of Aqueous Reprocessing of U-Zr and U-Pu-Zr Metallic Alloy Fuels Using an Ejector Mixer-settler with Tri-n-Butyl Phosphate (TBP) as the Extractant

ABSTRACT U-Zr and U-Pu-Zr metallic alloy fuels have been employed in fast reactors in the early days and these fuels have been considered for use in future fast reactors in India. It is proposed to develop an aqueous method based on the PUREX process for the reprocessing of spent metallic fuels. In the present study, an ejector mixer-settler was employed to demonstrate the feasibility of aqueous reprocessing of U-Zr and U-Pu-Zr metallic alloy fuels with tri-n-Butyl Phosphate (TBP) as the extractant. Continuous counter-current mixer settler runs were separately carried out with U-Zr and U-Pu-Zr feed solutions using 1.1 M TBP/n-DD solvent. Quantitative extraction of U and U + Pu was achieved within five stages leaving zirconium in the raffinate in the case of U-Zr and U-Pu-Zr feed solutions, respectively. Quantitative back extraction of uranium from loaded organic phase was achieved using 0.01 M HNO3 in the case of U-Zr mixer-settler runs. Dual strip solutions such as 2 M and 0.01 M HNO3 were used for the quantitative stripping of U and Pu from the loaded organic phase in the case of U-Pu-Zr mixer-settler runs. The proposed flow sheets demonstrate that the aqueous reprocessing of metallic fuels using a PUREX-based method is feasible.

Evolution of the Structural-Phase State of E110 Alloy Fuel Rod Claddings at High Temperatures and Stresses

The results of microstructural studies of the fragments of E110 alloy fuel rod cladding specimens based on sponge and electrolytic zirconium after the operation as a part of VVER-1000 fuel assemblies with the subsequent creep tests under axial loading are presented. It was shown that, during the creep tests, the studied specimens showed no changes in the chemical composition, average size, and bulk density of the second phases, including radiation-induced ones. It was found that, during the creep tests, dislocation loops were annealed, i.e., an increase occurred in their average size with a simultaneous decrease in bulk density. It was shown that the specimens of fuel rod claddings made of an alloy based on electrolytic zirconium demonstrated more significant creep resistance compared to the sponge-based zirconium alloy specimens, which is related to a higher density of globular β-Nb precipitates in the irradiated electrolytic zirconium specimens.

Defect cluster and nonequilibrium gas bubble associated growth in irradiated UMo fuels – A cluster dynamics and phase field model

Irradiation examination shows that gas bubble swelling kinetics are much faster after irradiation-induced recrystallization than that prior to recrystallization in U-10 wt% Mo alloy (UMo) fuels. These kinetics imply that gas bubbles in coarse grains and small recrystallized grains have different growth behavior. For the first time, researchers developed a phase-field model of gas bubble evolution integrating microstructure-dependent cluster dynamics to study the gas bubble swelling behavior in the recrystallization zone of UMo fuels. Generation, diffusion, reaction, sink, emission, and clustering of vacancies and interstitials are described by the cluster dynamics model while a phase-field model is used to describe the evolution of nonequilibrium gas bubbles including nucleation and growth. With the coupled model, the effect of defect generation rate, clustering rate, interstitial emission, and sink rates on grain boundaries on the gas bubble evolution are systematically simulated. A set of model parameters (defect generation rate, clustering rate, interstitial emission, and sink rates) is determined by comparing measured and simulated gas bubble swelling kinetics. The results demonstrate that interstitial clustering is one of the important physical mechanisms that results in fast gas bubble swelling kinetics in the recrystallization zone. The developed model can also be extended to study the associated growth of defect and second-phase precipitates often observed in irradiated materials.

Atomistic calculations of the surface energy as a function of composition and temperature in [formula omitted] U–Zr to inform fuel performance modeling

Uranium-zirconium alloy fuels are candidates for advanced sodium cooled fast reactors due to their high uranium density, high thermal conductivity, inherent safety and ability to incorporate minor actinides into the fuel. Unlike traditional ceramic UO2 fuel, U–Zr alloys swell rapidly and substantially, but the actual mechanistic process of swelling, including the rate of swelling, is not well understood. Fuel performance models are being developed to describe the swelling process, but these models currently lack the requisite underlying physics and fundamental property data to be truly predictive. In this work, molecular dynamics simulations are utilized to investigate a number of bulk thermophysical properties in γU-Zr, the void surface energy as a function of temperature and composition, and the void free energy. Finally, the effect of surface energy on fuel swelling behavior is demonstrated via finite element based fuel performance simulations, emphasizing the importance of the inclusion of accurate fundamental material properties.

Revealing the Role of Surface Composition on the Particle Mobility and Coalescence of Carbon-Supported Pt Alloy Fuel Cell Catalysts by In Situ Heating (S)TEM

Thermal annealing is an indispensable process during the preparation and structural ordering of Pt alloy fuel cell catalysts, which exhibit superior electrocatalytic activities as compared to Pt catalysts and thus enable decreased Pt usage. However, thermal annealing usually induces detrimental particle sintering, which greatly offsets the performance enhancement. Although the mechanisms of particle sintering of monometallic Pt catalysts have been well studied, knowledge on the key factors controlling the particle sintering of Pt alloy catalysts is still very poor. Herein, we perform in situ heating (scanning) transmission electron microscopy of carbon-supported low-Pt alloy catalysts (PtFe3, PtCo3, and PtNi3) and reveal that the surface composition plays a key role in both the particle mobility and the coalescence process of the supported low-Pt nanoparticles (NPs) under high temperatures. A surface enrichment of the less-noble transition metals not only induces a faster particle coalescence due to enhanced surface diffusion, but also causes a higher mobility of the NPs on the carbon support due to a strong chemical interaction between the less-noble transition metals and the carbon support. In contrast, the Pt richer surface results in a lower NP mobility as well as slower surface diffusion across contact NPs, which contributes to a higher antisintering capability. Our results suggest that controlling the surface composition, for example, by engineering the elemental growth kinetics during nanoparticle synthesis, is critical for controlling the particle sintering of Pt alloy catalysts during thermal annealing.

Evolution of the PtNi Bimetallic Alloy Fuel Cell Catalyst under Simulated Operational Conditions.

Comprehensive understanding of the catalyst corrosion dynamics is a prerequisite for the development of an efficient cathode catalyst in proton-exchange membrane fuel cells. To reach this aim, the behavior of fuel cell catalysts must be investigated directly under reaction conditions. Herein, we applied a strategic combination of in situ/online techniques: in situ electrochemical atomic force microscopy, in situ grazing incidence small angle X-ray scattering, and electrochemical scanning flow cell with online detection by inductively coupled plasma mass spectrometry. This combination of techniques allows in-depth investigation of the potential-dependent surface restructuring of a PtNi model thin film catalyst during potentiodynamic cycling in an aqueous acidic electrolyte. The study reveals a clear correlation between the upper potential limit and structural behavior of the PtNi catalyst, namely, its dealloying and coarsening. The results show that at 0.6 and 1.0 VRHE upper potentials, the PtNi catalyst essentially preserves its structure during the entire cycling procedure. The crucial changes in the morphology of PtNi layers are found to occur at 1.3 and 1.5 VRHE cycling potentials. Strong dealloying at the early stage of cycling is substituted with strong coarsening of catalyst particles at the later stage. The coarsening at the later stage of cycling is assigned to the electrochemical Ostwald ripening process.