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The boron isotope (10B) can be used as a neutron absorber in UO2 to control the reactivity of nuclear fuel pellets, however, the boron source can react with oxygen source in UO2 to form B2O3 that vaporize readily at temperatures above 1200 °C. Unfortunately, the sintering of UO2 fuel requires hours holding at high temperature (>1700 °C), resulting in an inevitable B loss during sintering and unpredictable B concentration in final product. It is challenging to incorporate boron through a conventional sintering method. In this work, we demonstrated that spark plasma sintering (SPS), a field assisted sintering technology, can effectively densify UO2 ​+ ​5 ​vol% ZrB2 composite fuel pellets by rapid consolidation at 1600 °C for a short duration of 5 min under an applied pressure of 40 MPa. Thermogravimetric analysis (TGA) measurements confirm that ZrB2 is fully retained inside the composite fuel pellets. Inside the composite fuel pellets, nano sized ZrB2 particles are uniformly distributed along the grain boundaries of the UO2 matrix. The ZrB2 particle transforms to a glassy B2O3 phase covering the sample surface and grain boundaries of UO2 matrix after a simple post-sintering annealing at 1000 °C in flowing Argon gas for 4 h. The formed glassy B2O3 slows down the diffusion of oxygen ions and postpones the onset temperature for oxidation of UO2 from 400 °C to 550 °C. This study demonstrates the capability of SPS, an advanced fuel manufacturing technique, to achieve a full retention of ZrB2 in UO2 oxide fuel and increase oxidation resistance through a simple post-sintering annealing. The reported work holds great engineering potential for development of advanced oxide fuel for nuclear application.

Hybrid halide perovskites display a great tunability of optoelectronic properties and environmental stability by controlling the halogen anions (e.g., I, Cl, and Br). However, their water interacti...

The Bouligand structure is a typical helicoidal architecture found in various crustaceans in which each layer rotates by a small angle relative to the adjacent one. We present a theoretical analysis to relate the rotation pitch angle to the mechanical performance (interlaminar shear property in this case) of laminas mimicking the Bouligand architecture. For a given laminate, there exists an optimal pitch angle that yields the lowest shear stress. Theoretical models are compared against experimental tests carried out on 32-layer helicoidal carbon fibre laminates with pitch angles of 6°, 9.1°, 12°, 25.7°. The 9.1° is the predicted optimal angle. The results show that this particular architecture with a pitch angle of 9.1° indeed attains the highest load-bearing capability and delays any catastrophic delamination. This provides a fundamental basis to the design of helicoidal structures mimicking the Bouligand architecture.

The development of advanced fuel fabrication technologies is important for developing accident-tolerant fuels and engineering fuels for safer and more effective nuclear energy systems. In this work, commercial-size uranium dioxide (UO2) fuel pellets with a theoretical density of 95% were consolidated by spark plasma sintering (SPS) at 1600°C for 5 min. Systematic investigations suggest uniform densification and stoichiometric UO2 with an ideal fluorite structure across the commercial-size fuel pellet, but with a distributed grain structure because of non-uniform distribution of temperature during sintering. This work demonstrates a great potential of using SPS for fabricating nuclear fuels at a cost-effective manner.

Single crystals of Langasite La 3 Ga 5 SiO 14 (LGS) were irradiated by 1 MeV Kr 2+ ions at temperature range from 298 to 898 K in order to simulate the damage effect of neutron radiation on Langasite, a candidate sensor material proposed as high temperature and pressure sensors in nuclear reactors. The microstructure evolution of LGS as functions of irradiation dose and temperature was followed by in-situ TEM observation through electron diffraction pattern. LGS is found to be sensitive to ion beam irradiation-induced amorphization from displacive heavy ions with a low critical dose of ∼0.5 ± 0.2 dpa (neutron fluence of (1.6 ± 0.6 ) × 10 19 neutrons/cm 2 ) at room temperature. The critical amorphization temperature, Tc, is determined to be 910 ± 10 K. Under simultaneous ionizing electron (300 keV, 45 nA) and displacive heavy ion irradiations (1-MeV Kr 2+ and flux of 6.25 × 10 11 ions/cm 2 ·s), LGS displayed greater stability of crystal structure against amorphization, possibly due to the electron radiation-induced recovery of displacive damage by heavy ions.

U3Si2, an advanced fuel form proposed for light water reactors (LWRs), has excellent thermal conductivity and a high fissile element density. However, limited understanding of the radiation performance and fission gas behavior of U3Si2 is available at LWR conditions. This study explores the irradiation behavior of U3Si2 by 300 keV Xe+ ion beam bombardment combining with in-situ transmission electron microscopy (TEM) observation. The crystal structure of U3Si2 is stable against radiation-induced amorphization at 350 °C even up to a very high dose of 64 displacements per atom (dpa). Grain subdivision of U3Si2 occurs at a relatively low dose of 0.8 dpa and continues to above 48 dpa, leading to the formation of high-density nanoparticles. Nano-sized Xe gas bubbles prevail at a dose of 24 dpa, and Xe bubble coalescence was identified with the increase of irradiation dose. The volumetric swelling resulting from Xe gas bubble formation and coalescence was estimated with respect to radiation dose, and a 2.2% volumetric swelling was observed for U3Si2 irradiated at 64 dpa. Due to extremely high susceptibility to oxidation, the nano-sized U3Si2 grains upon radiation-induced grain subdivision were oxidized to nanocrystalline UO2 in a high vacuum chamber for TEM observation, eventually leading to the formation of UO2 nanocrystallites stable up to 80 dpa.

Dense lanthanide titanates ranging from La2Ti2O7 to Yb2Ti2O7 were synthesized by spark plasma sintering with similar micro-sized microstructure, and their chemical durability and degradation mechanisms were investigated by semi-dynamic leaching tests. Different corrosion mechanisms from incongruent to congruent dissolution were observed with preferential elemental releases of lanthanides in A2Ti2O7 (A = La to Gd) and Ti in Er2Ti2O7 and Yb2Ti2O7. Initial leaching was controlled by surface dissolution and long-term dissolution shifted to a diffusion-controlled mechanism for titanate pyrochlores from Sm2Ti2O7 to Yb2Ti2O7. Both short-term and long-term leaching rates decreased from La2Ti2O7 to Yb2Ti2O7, and materials became more corrosion resistant with decreasing ionic radius ratios of the rare-earth elements and titanium. Amorphous passivation films of several nm-thick enriched with Ti formed on the surfaces of the corroded materials, and the formation mechanisms were governed either by surface reorganization with preferential release of lanthanides or dissolution-precipitation from saturated bulk solutions with preferential release of titanium. These results demonstrate that the ionic size plays a dominant role in determining chemical durability of lanthanide titanates and a strong correlation is established among chemical composition/structural variation, passivation film formation and corrosion mechanisms, and materials chemical durability.

Current challenges with uranium dioxide fuel degradation include fuel swelling, cracking and fission gas release, all of which reduce the operational life span of the fuel in commercial reactors. One important factor in prolonging the life span of uranium dioxide (UO2) fuel is developing a good understanding of the mechanical properties of the high burnup structure that forms at the periphery of the fuel pellet during the fuel cycle. Nanoindentation based testing techniques can probe the mechanical properties of small volumes of material and measure properties including hardness, elastic modulus, and creep. In this work, elevated temperature nanoindentation and nanoindentation creep testing is performed on UO2 samples with different microstructures, including nanocrystalline (NC) grains, as well as microcrystalline samples with different grain sizes and creep prestrains introduced at high temperatures. Tests allowed measuring hardness, elastic modulus and creep stress exponents up to 500°C. The test results indicate that for temperatures and stresses used here, NC UO2 had limited to no creep, with stress exponents greater than 10. For UO2 samples with creep prestrain and spark plasma sintered (SPS) UO2 with a 1.8 μm grain size the dominant creep mechanism at the high stresses and relatively low temperatures used is dislocation glide: the creep stress exponents of 3-4 at 500°C also matched well with recent literature values for macro scale compression creep tests. In addition, it was observed that the UO2 samples containing higher dislocation defect densities had lower creep stress exponents than conventional sintered UO2. The stress exponent measured on the conventional sintered UO2 at 500°C was ~ 7, which suggests that the deformation of UO2 at lower temperatures might be hindered by the energetic barriers to dislocation nucleation.

A nano-scale electrochemical sculpture (denoted here as NES) method has been developed to create TiO2 nanotube arrays on the surface of titanium sheets for improved bonding between titanium and carbon fiber reinforced epoxy composite laminates. For comparison, other surface treatments including anodization in NaOH, Na2SiO3, Na2C4H4O6 and EDTA electrolyte (denoted as NaTESi) and electrochemical etching in alkali solution (denoted as ALK) were carried out to investigate their effects on the surface morphology, wettability and the shear strength between titanium sheets and epoxy resin. The results from single-lap tests showed that the apparent shear strengths between titanium sheet and epoxy with NaTESi, ALK and optimized NES treatments have been improved by 37.2%, 53.9%, and 70.9%, respectively, in comparison with that of non-treated pristine samples. The samples in single-lap tests showed a mixture mode of cohesive and interfacial failures where the interfacial failure occurred at the epoxy/metal-oxide interface and the oxide films formed during the surface treatments remained intact. Utilizing the treated titanium sheets, fiber metal laminates (FMLs) were fabricated and their interlaminar shear strengths (ILSS) were found to increase by 43.5%, 56.1%, and 75.0%, respectively, compared to pristine samples, showing the promise of the nano-patterning technique.

Diffusion of fission gases in UO2 is studied at low burnups, before bubble growth and coalescence along grain boundaries (GBs) become dominant, using a 3-D finite element model that incorporates actual UO2 microstructures. Grain boundary diffusivities are assigned based on crystallography with lattice and GB diffusion coupled with temperature to account for temperature gradients. Heterogeneity of GB properties and connectivity can induce regions where concentration is locally higher than without GB diffusion. These regions are produced by “bottlenecks” in the GB network because of lack of connectivity among high diffusivity GBs due to crystallographic constraints, and they can lead to localized swelling. Effective diffusivities were calculated assuming a uniform distribution of high diffusivity among GBs. Results indicate an increase over the bulk diffusivity with a clear grain size effect and that connectivity and properties of different GBs become important factors on the variability of fission product concentration at the microscale.

In this work, we report an innovative approach synergizing multiple effects of microstructure control, oxide protection and phase transformation-induced mechanically toughening to develop advanced U3Si2 fuels with state-of-the-art materials properties. 3Y-TZP (3 mol% yttria doped stabilized tetragonal zirconia) additives are incorporated and uniformly distributed into the SPS-densified U3Si2 fuel matrix, enhancing materials fracture toughness up to 4.04 MPa m1/2. The 3Y-TZP incorporated U3Si2 composite pellets show significantly improved onset temperature of oxidation above 560 °C, which can be further improved to 617 °C with 3 vol% addition by post-sintering thermal annealing. The development of mechanically tough and oxidation-resistant U3Si2 with minimal additive represents a major step forward towards realizing the potential of high density U3Si2 as the leading fuel concept to increase accidence tolerance of nuclear energy systems.

Al-doped U3Si2 composite fuels with controlled microstructure were fabricated by spark plasma sintering that display greatly-improved oxidation resistance as compared with monolithic and Al-doped silicides prepared by standard powder metallurgy or arc melting. The effects of Al additives on the thermal-mechanical properties and oxidation resisance of the micron- and nano-sized U3Si2 composites were investigated. A minimal addition of 1.8 at% Al is effective to increase the onset oxidation temperature of as-fabricated U3Si2 pellets to 580 °C, which can be further increased to 610 °C by thermal annealing. The Al-doped U3Si2 composite fuels also display simultaneously higher hardness and fracture toughness than un-doped U3Si2. These results highlight an effective strategy by integrating minimal Al additives, microstructure control and post-thermal annealing to design advanced silicide fuels with excellent oxidation resistance, desired thermal-mechanical properties and maintained high fissile element density.

Uranium dioxide (UO2) composites with uranium diboride (UB2) and uranium tetraboride (UB4) have been proposed as advanced fuel candidates due to their high thermal conductivity, high melting point, high fissile density and their ability to incorporate a built-in burnable poison by tailoring the targeted 10B/11B ratio. As such, it is important to assess the fabrication, and thermal and micromechanical properties of such composites. In this work, UO2-UB2 and UO2-UB4 samples with boride phase fractions of 5, 15 and 30 wt% were fabricated to high densities (above 95% theoretical density) via spark plasma sintering (SPS). This enabled sintering at relatively low temperatures and short timescales. SPS also aided in maintaining the target phase fractions of the samples as reactions between the constituent phases were suppressed due to the short timescales and reducing environment during sintering. Thermal diffusivity measurements from 299 to 1273 K were conducted through laser flash analysis (LFA). The diffusivity increased as a function of boride weight fraction, and UB2 additions increased the thermal diffusivity of the composites more than UB4 additions. Assessment of the LFA results indicated that in-situ reactions between the UO2 and boride phases that suppress the thermal diffusivity occur above 800 K for all samples. Oxidation of the boride phase was proposed as the underlying reaction. This was supported by thermodynamic assessments from the literature, as well as microstructural, crystallographic, and nanoindentation characterization performed on these samples.

Dense Cr-doped U3Si2 composite fuels were manufactured by spark plasma sintering, and the effects of Cr addition on mechanical properties and oxidation resistance were investigated. Dynamic oxidation testing by thermogravimetric analysis revealed significantly improved oxidation resistance of U3Si2 with minimal doping of 3 wt% Cr. The onset oxidation temperature increased to above 550 °C in air and ∼520 °C in steam conditions for the 5 wt% and 10 wt% Cr-doped composites. Steam corrosion testing under 360 °C for 24 hours indicated well-maintained pellet integrity without pulverization for the 10 wt% Cr-doped U3Si2 pellet which only showed minor surface oxidation. The first promising results open up the possibility of designing and manufacturing metal additive-doped U3Si2 composite fuels with significantly-improved corrosion resistance as a potential candidate for accident tolerant fuels.

The present work focuses on an experimental investigation on the efficiency of the Z-pin pre-hole insertion technique in minimizing the in-plane damage while improving the delamination resistance of quasi-isotropic carbon fibre/epoxy laminates. The results indicate that the Z-pin pre-hole insertion causes much less crimped and/or broken fibres leading to a minimal strength reduction of 3.50% in tension and 4.80% in compression respectively for composite laminates containing 1 vol.% of carbon Z-pins. The Mode-I fracture toughnesses of the composites reinforced with ~1 vol.% carbon and Kevlar Z-pins are about 12 times higher than that of the control samples, suggesting an efficient crack arrest mechanism by the implanted pins. The substantially enhanced through-thickness properties together with the minimal reduction in in-plane performance warrant the pre-hole insertion technique as a practical method in mitigating the delamination in fibre reinforced laminates.

Among a variety of defects associated with composite technology, fibre wrinkles are known to be a major source of failure for fibre reinforced plastic and closely associated with the ever popular automated fibre placement technique. In the present work, with the aim to investigate their influence on the failure of carbon fibre reinforced plastics (CFRP), composite laminates containing fibre wrinkles of varying architectures have been designed and prepared utilizing gaps and overlaps based on the understanding of wrinkle formation mechanisms. The effects of these engineered wrinkles with different combination of gaps and overlaps on the mechanical performance of CFRP were systematically investigated. A maximum of 21% and 37% drop in the tensile and compressive strength were recorded for the most severe combination of gaps and overlaps. A simple analytical model was proposed to relate the wrinkle angle to the defect characteristic parameters and a good agreement with experimental results was obtained. Furthermore, numerical simulations were conducted to inform the failure mechanisms where different wrinkle models were generated using previously developed pre-processing tools. Good agreements with experimental observations were obtained in terms of ply waviness, strength and failure modes. These findings provide certain practical insights for controlling defects in composite manufacturing processes.

This work reports the synthesis and characterization of the U3Si2 and UO2 composites sintered by spark plasma sintering (SPS) with controlled microstructures for accident-tolerant fuel application. The U3Si2 and UO2 composites with different silicide and oxide ratios were synthesized by SPS at temperatures from 1000 to 1300 °C for 5 minutes. The microstructure and phase composition of the SPS densified composite fuels were characterized with scanning electron microscopy, X-ray diffraction (XRD), and energy dispersed spectroscopy (EDS). A systematic study of the thermal and mechanical properties was conducted using microhardness testing and laser flash apparatus, along with oxidation resistance measurements using thermogravimetric analysis (TGA). The results show that the synthesis of composite fuels can be achieved with a 90% theoretical density (TD) at 1000 °C and over 95% TD when sintered at 1300 °C. XRD and EDS results confirmed that the dominant phases in the composites are U3Si2 and UO2. Improved physical density generally leads to improved hardness, fracture toughness, thermal diffusivity, and onset temperature during the oxidation process. U3Si2 was found to play a dominant role in determining the mechanical and oxidation properties of the composite fuels, whereas UO2 had a more important impact on controlling the thermal diffusivity of the composites. The composite with 50 wt% UO2 sintered at 1300 °C displayed the onset oxidation temperature of 500 °C by dynamic oxidation testing using TGA at a ramp degree of 10 °C/min. The composite also achieved a high fracture toughness of ∼3.5 MPa m½. These results highlight the potential of composite fuel forms densified by SPS with simultaneously enhanced fissile element density, fracture toughness, thermal transport properties, and oxidation resistance.

Dense U3Si2 pellets with controlled grain structure and enhanced thermal-mechanical and oxidation properties are synthesized with spark plasma sintering (SPS). Microstructure and phase composition of the SPS densified pellets are characterized systematically using SEM, EDS, and XRD. Thermal-mechanical properties and oxidation behavior of the sintered silicide fuel pellets are analyzed by laser flash, indentation, and dynamic thermogravimetric analysis. Dense U3Si2 pellets are consolidated by combining high energy ball milling and rapid sintering by SPS, and the microstructure structures are controlled from micron-sized (∼5.7 μm grain size) for conventional silicide to a nanocrystalline matrix with an average grain size of ∼280 nm. A dominant phase of distorted U3Si2 was identified with lattice expansion due to residual thermal stress upon SPS consolidation and rapid cooling processes. Both micron-sized and nano-sized pellets show exceptional thermal transport properties, consistent with monolithic silicides reported in literature. The SPS-densified pellets possess simultaneously high hardness and fracture toughness. The SPS-densified silicide pellets also demonstrate exceptional oxidation performance with extended onset oxidation temperature above 500 °C and reduced oxidation kinetics, particularly for nano-sized pellets. A strong strain effect was proposed in which compressive stress in nano-sized pellets enhances the oxidation resistance of silicide fuels, as evidenced by the degradation of oxidation performance upon strain relaxation by isothermal annealing. The correlation among the sintering process – microstructure control – physical properties and fuel behavior is established. A new concept of strain engineering is proposed further properties optimization, enabling the development of potential oxidation and corrosion-resistant silicides with extended performance, the key technological challenge of U3Si2 as the leading concept of accident tolerant fuels.

The purpose of this work is to develop a model for normal grain growth in U3Si2. The average grain boundary energy was determined from previously published molecular dynamics simulations. The grain growth kinetics were quantified at various temperatures by annealing nanocrystalline samples. The mobility was determined by comparing phase field grain growth simulations to the experimental data. From these various methods, we found that the average grain size D in U3Si2 can be estimated over time t using the equation D 2 − D 0 2 = 2 α M γ t , where D 0 is the initial average grain size, the geometry factor α = 0.96 , the average grain boundary mobility M = 6.30 × 10 − 18 e − 0.33 [ eV ] k b T m 4 / ( Js ) with the Boltzmann constant k b and temperature T, and the average grain boundary energy has been found as a function of temperature, e.g. γ ¯ = 0.83 J/m2 at 673 K.

Uranium diboride (UB2) and uranium tetraboride (UB4) are candidate constituents for multi-phase accident tolerant fuel due to their anticipated high thermal conductivity. These fuels have high uranium density that contributes to fission, and by tailoring the ratio of 10B/11B, can also act as an integrated burnable poison. Understanding the thermophysical and mechanical properties of uranium borides, for which only limited data are available in the literature, is of importance to determine their accident tolerance. In this work UB2 and UB4 have been synthesized via arc melting and sintered to high densities via spark plasma sintering (SPS). High density samples, >90% theoretical density, were used to measure the thermal diffusivity and thermal expansion of UB2 and UB4 and, in conjunction with specific heat literature data, their thermal conductivities were calculated from 298 to 1773 K. Additionally, resonance ultrasound spectroscopy (RUS) and nanoindentation were performed to investigate the mechanical properties of the uranium borides. Our results are discussed in the context of available literature. Both UB2 and UB4 exhibit thermal conductivities higher than that of UO2, with UB2 having the highest. The thermal conductivity of UB2 increases with temperature above 874 K, while for UB4 there is a linear increase over the entire measured range. X-ray diffraction (XRD) results indicate that impurity phases were present in the fabricated materials, which could explain why literature density functional theory (DFT) results predict higher values. This suggests that if impurity phases or any microstructural defects can be eliminated then the thermal conductivity can be further increased.

Low thermal transport behavior along the radial direction of nuclear fuel pellets and pellet-cladding mechanical interaction significantly impact fuel performance and the safety of current nuclear energy systems. Here we report a new strategy of advanced fuel design in which highly thermally-conductive and mechanically-robust graphene nanoplatelets are incorporated into UO2 fuel matrix to improve fuel thermal-mechanical properties. The 2D geometry of the graphene nanoplatelets enables a unique lamellar structure upon fuel consolidation by spark plasma sintering. The thermal conductivity along the radial direction of the sintered fuel pellets at room temperature reaches 12.7 and 19.1 wm−1K−1 at 1 wt.% and 5 wt.% loadings of the graphene nanoplatelets, respectively, representing at least 74% and 162% enhancements as compared to pure UO2 fuel pellets. Indentation testing suggests great capability of the 2D graphene nanoplatelets to deflect and pin crack propagation, drastically improving the crack propagation resistance of fuel matrix. The estimated indentation fracture toughness reaches 3.5 MPa·m1/2 by 1 wt.% loading of graphene nano-platelets, representing a 150% improvement over 1.4 MPa·m1/2 for pure UO2 fuel pellets. Isothermal annealing of the composite fuel indicates that the graphene nano-platelet is able to retain its structure and properties against reaction with UO2 matrix up to 1150 °C.

Effective management of the long-lived radioactive 36Cl is necessary for the development of pyrochemical reprocessing of used fuels and also the molten salt reactors in which a large quantity of Cl-containing waste is encountered. Significant challenge exists in developing advanced waste form matrix with high Cl confinement capability and fabrication technology with minimized loss of Cl. In this work, Cl-bearing vanadinite apatite (Pb5(VO4)3Cl) powder samples are synthesized by solid state reaction at room temperature using high energy ball milling (HEBM), and dense pellets above 99% theoretical density can be consolidated by spark plasma sintering (SPS) at a temperature as low as 450 °C for 3 min. The effects of various sintering temperatures (300–800 °C) on physical density, microstructure, thermal stability and mechanical properties of the SPS densified pellets are investigated. Microstructure analysis shows that the average grain size of nanocrystalline ceramic is less than 190 nm when sintered at 450 °C, and the sintered microstructure is dominated by grain growth at higher sintering temperatures. No phase decomposition and significant chlorine loss can be identified during HEBM and low temperature SPS consolidation processes. Thermal gravimetric analysis (TGA) test indicates that the phase decomposition and chlorine loss occur only at an elevated temperature 830 °C for both HEBM-prepared powders and SPS densified pellets. Vanadinite pellets densified at 450 °C possess the maximum hardness of 4.6 GPa. The SPS-densified Cl-bearing vanadinite apatite displays high confinement of chlorine and a good thermal stability, and thus could be used as a potential nuclear waste form for the immobilization of the long-lived and highly volatile 36Cl.