Uranium Mononitride Research Articles

Oxidation and passivation of the uranium nitride (001) surface

The oxidation of uranium mononitride is a potential issue when considering this material as a nuclear fuel. This work investigates the rate and mechanism of this reaction at room temperature, giving insight into fuel surface oxidation during manufacture and storage. Chemical and structural investigation of the ideal surface of an epitaxial (001) UN thin film, shows that a UO2+xNy layer forms on the surface, highlighting the need for better understanding of the U-N-O system. Below this, a U2N3 interlayer is detected. The topotactic formation of these layers is predicted to play a critical role in the passivation measured during long-duration structural investigation.

First principles investigation of the electronic-thermal transport of ThN, UN, and ThC

The resistivity and electronic-thermal conductivity of ThN, ThC and UN were compared. It was found that the new electron–phonon physics from first principles code Electron-Phonon coupling using Wannier functions (EPW, v. 5.4) code as implemented in Quantum Espresso (QE, v. 6.8) code based on density-functional theory predicts higher electronic-thermal conductivity for thorium and uranium mononitride than for thorium carbide in agreement with experiment. QE combined with EPW code were used to calculate integrated electron–phonon coupling strength, the Eliashberg transport coupling function, and electron–phonon scattering rates. The electronic resistivity was calculated using Ziman’s formula for metals and compared to the values derived from first principles using Boltzmann transport theory. The number of mobility electrons were obtained by equating the calculated resistivity at 1000 K. Estimation showed that in ThN the number of electronic carriers is two times larger and both electron–phonon coupling strength and electron scattering rates are lower, which led to about four times higher electronic thermal conductivity than calculated for ThC. Although a good agreement with experiment was found for ThN and ThC (at high temperatures), the current version of EPW code for non-magnetic solids overestimates the electronic thermal conductivity of UN and underestimates resistivity.

Energetics of oxidation and formation of uranium mononitride

Uranium mononitride (UN) is an advanced nuclear fuel currently being considered for use in several generation IV fast and thermal neutron spectrum core designs, with additional applications to thermal and electric nuclear propulsion reactors. To better understand the thermal behavior and thermodynamic stability of UN, we investigated the bulk thermal oxidation process and thermochemical reactions, including the enthalpy of oxidation and standard enthalpy of formation, by conducting thermalgravimetric analysis – differential scanning calorimetry coupled with mass spectrometry (TGA-DSC-MS), and high temperature transposed temperature drop and oxide melt drop solution calorimetry. The bulk oxidation of UN (containing a small amount of α-UN1.5+x) in air was found to follow a step-wise process characterized by consecutive oxidative reactions UN-UN1.5+x-UO2 → UO2-UO3Nk → UO3Nk → UO3 → U3O8. TGA results support that the UO2 – U2N3+x passivating layer delays the onset of rapid bulk oxidation of UN in air up to 662 K. Synchrotron X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) analyses were performed to characterize UN and its final oxidized product. The standard enthalpy of formation (ΔH°f) of UN was determined to be –144.4 ± 5.9 kJ/mol·atom, in good agreement with previously determined values from Pt encapsulation and bomb calorimetric experiments. Lastly, a negative linear correlation between ΔH°f and the N/U molar ratio was established based on the thermochemical data obtained in this work and previously reported enthalpies of formations of β-UN1.5-x and α-UN1.5+x.

Diffusion of krypton and xenon in uranium mononitride; a Density Functional Theory Study

Uranium mononitride is a strong candidate for an advanced nuclear fuel. In this work we use density functional theory to model the diffusivity of matrix U atoms, as well as the noble gas (Ng) fission products Kr and Xe, at the atomic scale under three stoichiometric conditions (U-rich, stoichiometric, N-rich). U self-diffusion is found to be dependent on stoichiometry, being largest under N-rich conditions. The U formation entropy significantly affects the U self-diffusion coefficient, indicating that it is necessary to consider the vibrational properties of the system to accurately describe diffusion properties in UN. The calculated Kr and Xe diffusion coefficients from the U vacancy (VU)-assisted mechanism are much larger than by interstitial mechanisms under the three stoichiometric conditions studied. The two mechanisms show opposite stoichiometric dependence, with the former increasing from U-rich to N-rich conditions and the latter decreasing. Kr moves significantly more quickly than Xe via the interstitial mechanism due to the larger atomic radius of the latter, while the effect of atomic size on the VU-assisted mechanism is negligible, indicating similar diffusivity of Kr and Xe in UN. The good agreement with experiment of our calculated Xe monovacancy-assisted diffusion coefficient indicates that the monovacancy-assisted mechanism governs the Ng diffusion in UN, and supports the accuracy of our theoretical model.

Simulation of uranium mononitride spent fuel: A crystallographic approach

Uranium mononitride (UN) is an attractive fuel for a range of reactors, including, with adequate protection against reaction with water, for large scale and small modular light water reactors. The study of the speciation of fission products (Fps) in spent UN fuel is required to understand their properties such as phase stability and retention during long term storage and disposal. The present study reviews and applies Hume-Rothery rules to predict this speciation. This law provides an estimate of the way UN may form solid solutions with actinide (An) and fission product mononitrides from Beginning of Life (BoL) to End of Life (EoL). Composition at EoL is estimated for a high burnup fuel (60 MW d kg−1) using the FISPIN fuel inventory code. Many Fps are trivalent (Ln, Y, Zr, Nb) and are expected to recrystallize in face centred cubic (fcc) solid solutions with UN. Other Fps are divalent (Ba, Sr) and monovalent (Cs, Rb) and some elements are non-valent (Mo, Tc, Ru, Rh, Pd) as well as noble gases (Xe, Kr), and halides (I, Br) which may form nano-precipitates. Actinides formed by neutron capture of 238U are all An3+ which are also expected and found in fcc solid solution with UN.The spent fuel is consequently formed of a large fraction of solid soluble nitrides (U, An, Ln, Y, Zr, Nb)N forming a single rather homogeneous phase according to the Hume-Rothery rules. This phase will also follow Vegard's law. Because of the low temperature of the fuel, the Fps are expected to be soluble or nano-dispersed precipitates. Microscopic precipitated phases are not expected.

Analysis of SMART reactor core with uranium mononitride for prolonged fuel cycle using OpenMC

Abstract The neutronics performance and safety characteristics of Uranium mononitride (UN) fuel for System-Integrated Modular Advanced Reactor (SMART) has been investigated to discern the potential for non-proliferation, waste, and accident tolerance benefits of UN fuel. The neutronic evaluation of UN fuel for SMART reactor has been carried out under normal operation using OpenMC and compared with Uranium dioxide (UO2) in terms of fuel cycle length, reactivity coefficients, Fuel depletion (burnup), thermal flux, and fission product activity. The power peaking factor (PPF) has been compared at the beginning of the fuel cycle (BOC), mid of the fuel cycle (MOC), and at the end of the fuel cycle (EOC). Results indicate that the UN fuel can be operated beyond the designed fuel cycle length of the SMART reactor, which induces the positive reactivity at the end of the fuel cycle of about 4625 pcm. However, the UO2 showed negative reactivity after three years. The total fission product activity at the end of the fuel cycle (3.5 years) for UO2 and UN has been founded 1.003 × 1020 Bq and 1.023 × 1020 Bq, respectively.

High volume packing fraction TRISO-based fuel in light water reactors

For a decade, fully ceramic microencapsulated (FCM) fuel, containing tri-structural isotropic (TRISO) fuel particles in a silicon carbide (SiC) matrix, has been investigated as an accident-tolerant fuel for light water reactors (LWRs). Other examples exist of TRISO-based concepts for LWR fuels with different matrix materials. Previous studies assumed TRISO particle volume packing of approximately 0.44 in SiC (or another) matrix, the highest realistic packing fractions possible with conventional manufacturing. Recent advances in advanced manufacturing have yielded the development and demonstration of a fuel form that consists of conventionally manufactured TRISO particles in a 3D-printed SiC matrix with significantly higher possible TRISO packing fractions (0.5–0.7). This increased uranium loading enhances the viability of using TRISO-based particle fuel forms in LWRs. The viability of high-packing-fraction TRISO-based particle fuel forms in LWRs is assessed from the perspective of fuel cycle length, achievable fuel burnup, reactivity coefficients, and fuel cycle performance. Higher-packing-fraction TRISO-based fuel enables either longer cycle lengths (by ∼25% at a packing fraction of 0.55 relative to 0.44) at a constant enrichment or decreased enrichments (by ∼25% at a packing fraction of 0.55 relative to 0.44) at a constant cycle length. Studies of different fuel kernel types (uranium nitride, uranium oxycarbide, and uranium carbide) yield similar results, although the cycle length of uranium oxycarbide is shorter than for uranium nitride or uranium carbide (due to the lower density of uranium oxide). This work also characterized the production of 14C resulting from neutron absorption in 14N during operation for uranium mononitride fuel kernels; the ratio of 14C/N was 1–2 at. % at discharge. For the fuel cycle evaluation, the activity of spent nuclear fuel and high-level waste at 100 and 100,000 years was lower for high-packing-fraction fuels than for conventional LWR fuel. Environmental impact metrics were similar overall, but higher on the front end of the fuel cycle and lower on the back end of the fuel cycle. Reactivity coefficients of higher-packing-fraction TRISO-based fuel were reasonable compared with those of conventional fuels.

Development and application of a uranium mononitride (UN) potential: Thermomechanical properties and Xe diffusion

Atomic-scale modeling of thermophysical and defect properties of uranium mononitride (UN) plays an important role in establishing a better understanding and improved models of UN fuel performance. Having an accurate interatomic potential is crucial for generating reliable data at finite temperatures using molecular dynamic simulations. We report a new interatomic potential for UN, based on a combination of many-body and pairwise interactions, a simple form that we later show could be easily adapted to include Xe–U and Xe–N interactions, i.e., generating a U–N–Xe interatomic potential. The potential was fitted to experimental thermal expansion and single crystal elastic constants, as well as Frenkel, Schottky, anti-Schottky, and antisite pair reaction energies from density functional theory (DFT) calculations. Using the potential, we successfully reproduced experimental lattice parameters, thermal expansion, single crystal elastic constants, and temperature dependent heat capacity. The potential also performs reasonably well in reproducing the energy of the aforementioned stoichiometric defect reactions and defect migration barriers calculated using DFT. However, the potential underestimates the energy difference between the tetrahedral and dumbbell uranium interstitials, and a more complex potential form might be needed to overcome this issue. The potential was also used to predict UN single crystal elastic constants and elastic properties at different temperatures, showing that UN becomes softer and more compressible with increasing temperature. We also compare our potential against literature data from previous empirical potentials, demonstrating similar or better behavior depending on the property of interest. To enable the simulation of Xe in UN a Buckingham potential has been fitted to DFT-derived Xe incorporation energies. The potential was then used to determine the activation energy for Xe diffusion due to various Xe-containing defects, with {XeU:VU} exhibiting the lowest activation energy.

First-principles investigation of uranium mononitride (UN): Effect of magnetic ordering, spin-orbit interactions and exchange correlation functional

Uranium mononitride (UN) is a promising nuclear fuel that combines the advantageous properties of readily used UO 2 and uranium alloys . Various properties of UN have been previously studied using different density functional theory (DFT) methodologies; however, there are still inconsistencies when it comes to the dynamical stability and defect properties of UN. We address these inconsistencies by studying the UN phonons and defect properties using DFT calculations employing two generalized gradient approximation (GGA) exchange-correlation functionals: PBE and AM05, with and without an added on-site Coulomb repulsion term (+U). Furthermore, we investigate the importance of spin-orbit coupling (SOC) when calculating the properties of UN. We use the different methodologies to determine the preference of UN to have antiferromagnetic (AFM) ordering, as seen in experiments, or ferromagnetic (FM) ordering of the uranium spins. We compare the crystallographic properties , density of states, the DFT X-ray photoelectron spectra and phonon dispersions calculated using the different methodologies. We demonstrate that GGA + U reproduces the AFM ordering in UN, but the crystal structure is dynamically unstable. We also show that magnetic ordering is important in finding the lowest energy defective structure, and that SOC has a distinct influence on the energy of the different uranium interstitial defects . Lastly, we discuss the point defect formation energies under U-rich and N-rich conditions, and the stoichiometric formation energies calculated with the different methodologies, providing insight into the observed tendency for forming hypostoichiometric UN.

Steam oxidation of uranium mononitride in pure and reducing steam atmospheres to 1200 °C

The present study addresses the lack of steam oxidation kinetic data for high purity uranium mononitride (UN), assesses the sensitivity of the dynamic response of UN to the density of the monolithic sample exposed to flowing steam, as well as observes the variation in the oxidation kinetic response of UN to the presence of a common fabrication contaminant, carbon. In addition, the mass spectrometry performed on the evolved gas during the reaction with steam informs the reactions thermochemistry for UN with flowing steam during a thermal ramp and during an isothermal exposure at 500 °C. It is concluded here that high purity, high density samples, (≥94% theoretical density), have a significantly delayed onset of reaction with steam compared to values previously reported, the kinetic reaction is retarded in a reducing steam atmosphere (steam + 100 ppm H2), and the reaction products are predominantly ammonia (NH3 gas) and UO2 at temperatures less than 700 °C, above which UO2+x (0.2 ≥ x ≥ 0.0) begins to form and NH3(g) dissociates.

Compatibility of UN with refractory metals (V, Nb, Ta, Cr, Mo and W): An ab initio approach to interface reactions and diffusion behavior

Uranium mononitride (UN)-uranium dioxide (UO2) composites are being considered as an innovative advanced technology fuel option for light water reactors, where an optimal balance between the chemical advantages of UO2 and the thermal and neutronic properties of UN is struck. However, the effect and extent of chemical interactions between UN and UO2 during sintering and operation are still open issues of importance. A possibility to avoid these interactions is to protect the UN phase before sintering the UN-UO2 composites by encapsulating the UN. This protective material must have a high melting point, high thermal conductivity, and reasonably low neutron cross-section. Among many candidates, the use of refractory metals is a promising option. In this study, density functional theory calculations (DFT) were performed to study the interactions and kinetics at the UN-X interfaces respectively (X = V, Nb, Ta, Cr, Mo, and W). The diffusion behaviors in UN and in the metal were studied using the self-consistent mean field (SCMF) theory. Generally, the diffusion of metal atoms in UN is slow compared to the diffusion of N atoms in the metals. Furthermore, the DFT calculations predict that Ta and V may react with UN to form UTaN2 and V8N at the UN-X interfaces, respectively. In some cases, the formation of these phases also promotes the formation of point defects in the UN and metal phases. The interaction between W and Mo with the UN phase is largely prohibited. According to this work, Mo and W can be regarded as highly promising candidate materials for the fabrication of stable UN-UO2 composite fuel.

Dislocation Loops in Proton Irradiated Uranium-Nitrogen-Oxygen System

In this study, we investigated the type of dislocation loops formed in the proton-irradiated uranium-nitrogen-oxygen (U-N-O) system, which involves uranium mononitride (UN), uranium sesquinitride (α-U 2 N 3 ), and uranium dioxide (UO 2 ) phases. The dislocation loop formation is examined using specimens irradiated at 400°C and 710°C. Based on the detailed transmission-based electron microscopy characterization with i) the morphology-based on-zone and ii) the invisibility-criterion based two-beam condition imaging techniques, only a single type of dislocation loop in each phase is found: a/2⟨110⟩, a/2⟨111⟩, or a/3⟨111⟩ dislocation loops in UN, α-U 2 N 3 , and UO 2 phases, respectively. Molecular statics calculations for the formation energy of perfect and faulted dislocation loops in the UN phase indicate a critical loop size of ∼ 6 nm, above which perfect loops are thermodynamically favorable. This could explain the absence of faulted loops in the experimental observation of the irradiated UN phase at two temperatures. This work will enhance the understanding of irradiation induced microstructural evolution for uranium mononitride as an advanced nuclear fuel for the next-generation nuclear reactors.

Uranium nitride (UN) pellets with controllable microstructure and phase – fabrication by spark plasma sintering and their thermal-mechanical and oxidation properties

Dense uranium mononitride (UN) pellets with controlled microstructures and tailored grain size from large-grained to a few microns are synthesized by spark plasma sintering (SPS) combined with high energy ball milling . The impacts of the sintering conditions on fuel microstructure, grain size, physical density, and phase behavior are systematically investigated, and the thermal-mechanical properties and oxidation behavior of the SPS densified UN pellets are characterized. Higher sintering temperatures and longer ball milling durations and thus finer starting UN powders promote sintering and densification , and dense UN pellets above 95% theoretical density can be achieved by SPS at 1873 K for 10 min. UN phase purity is maintained in the SPS-densified pellets sintered at a lower temperature and short duration. A phase heterogeneity with secondary UO 2 or uranium sesquinitride (U 2 N 3 ) occurs for the UN pellets sintered at higher temperatures using finer UN powders. The hardness and fracture toughness of the SPS-densified UN pellets increase with smaller grain sizes and higher densities to 7.9 GPa and 3.5 MPa m 1/2 , respectively. Both small (1–2 μm) and large grain-sized (30–50 µm) UN pellets exhibit good thermal conductivity . Dynamic oxidation testing by a thermogravimetric analyzer in air shows that the onset temperature for oxidation varies with microstructure and phase heterogeneity of the SPS densified UN pellets. Particularly, the smaller-grained (micron-sized) UN pellets containing uranium oxides and U 2 N 3 display lower weight gain and significantly-reduced oxidation kinetics , and full oxidation completes at a temperature above 1173 K when tested with a ramp rate of 10 K/min.

Incorporation of Kr and Xe in Uranium Mononitride: A Density Functional Theory Study

Incorporation of Kr and Xe in Uranium Mononitride: A Density Functional Theory Study

5f -electron magnetism in single crystal UN probed by N14 NMR

Spin susceptibility and low-frequency dynamics of uranium $5f$ electrons have been investigated by nuclear magnetic resonance (NMR) on the $^{14}\mathrm{N}$ nuclei in paramagnetic and magnetically ordered phases for single crystalline and polycrystalline samples of uranium mononitride (UN). NMR spectra, shifts of the $^{14}\mathrm{N}$ NMR lines, and the spin-lattice relaxation times ${T}_{1}$ have been obtained in the temperature range $T=10--760\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in magnetic field $B=92.8\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$. It is shown that in the UN paramagnetic phase, temperature dependence of the $^{14}\mathrm{N}$ NMR line shift is proportional to the spin susceptibility of the uranium $5f$ electrons. Joint analysis of NMR and magnetic susceptibility data allows us to determine temperature dependence of spin fluctuation energy ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{nmr}}(T)$ of the uranium $5f$ electrons and to demonstrate that its temperature variation is close to $\mathrm{\ensuremath{\Gamma}}(T)\ensuremath{\propto}{T}^{0.5}$ dependence which is characteristic of the concentrated Kondo systems above the coherent state formation temperature. In the magnetically ordered UN phase the $^{14}\mathrm{N}$ NMR spectra consist of several lines that can be explained in terms of the model of type $I$ antiferromagnetic order corresponding to $1k$ structure in the presence of magnetic domains.

Fermi-surface reconstruction at the metamagnetic high-field transition in uranium mononitride

We report on the electronic and thermodynamic properties of the antiferromagnetic metal uranium mononitride with a N\'eel temperature $T_N\approx 53\,$K. The fabrication of microstructures from single crystals enables us to study the low-temperature metamagnetic transition at approximately $58\,$T by high-precision magnetotransport, Hall-effect, and magnetic-torque measurements. We confirm the evolution of the high-field transition from a broad and complex behavior to a sharp first-order-like step, associated with a spin flop at low temperature. In the high-field state, the magnetic contribution to the temperature dependence of the resistivity is suppressed completely. It evolves into an almost quadratic dependence at low temperatures indicative of a metallic character. Our detailed investigation of the Hall effect provides evidence for a prominent Fermi-surface reconstruction as the system is pushed into the high-field state.

Challenges and opportunities to alloyed and composite fuel architectures to mitigate high uranium density fuel oxidation: Uranium mononitride

The challenges and opportunities to alloyed and composite fuel architectures designed and intended to mitigate oxidation of the fuel during a cladding breech of a water-cooled reactor are discussed in three review manuscripts developed in parallel, with the presented article focused on the oxidation performance of uranium mononitride. Several high uranium density fuels are under consideration for deployment as accident tolerant and/or advanced technology nuclear reactor fuels, including one on each: UN, U 3 Si 2 , UC and UB 2 . Presented here is the research motivation for the incorporation of additives, dopants , or composite fuel architectures to improve the oxidation/corrosion behavior of high uranium density nuclear fuels for use in LWRs. Furthermore, this review covers the literature on the degradation modes, thermodynamics, and oxidation performance of pure UN and UN-compounds as well as reported alloyed and composite architectures.

Toward an In-Depth Material Model for Cermet Nuclear Thermal Rocket Fuel Elements

The development and qualification of nuclear thermal propulsion (NTP) fuel element technologies would be aided by an in-depth model of material response and failure modes at operating conditions. Integrated computational materials engineering techniques have the potential to provide such a model, as demonstrated here through three case studies focused on a tungsten–uranium mononitride (UN) cermet fuel. The first case focuses on the erosion of tungsten (also named wolfram), a nominal coating/cladding and fuel element matrix material, in hot hydrogen. Ab initio techniques are used to calculate erosion rates and thermal expansion at NTP operating conditions. The second focuses on the stability of UN fuels at high temperature and in the presence of hydrogen. Phase diagram techniques augmented with ab initio thermodynamic data reveal potential instabilities and decomposition pathways at high hydrogen concentrations. The third focuses on using microstructure information to predict high-temperature mechanical response and failure of tungsten. Combined finite element and discrete dislocation dynamics techniques provide mechanical properties in agreement with experimental methods. The integration of these techniques for an all-encompassing material model is discussed.

Phase and defect evolution in uranium-nitrogen-oxygen system under irradiation

Uranium mononitride (UN) with 5 wt.% uranium dioxide (UO2) is used as a model system to study the phase and defect evolution under proton irradiation in nitride-oxide composite. Phase composition, crystallographic orientation relationships (ORs) and dislocation loops were characterized using X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy techniques. Proton-irradiation at elevated temperatures promoted the transformation of UN into uranium sesquinitride (U2N3) and UO2 phases. U2N3 and UO2 formed a fully coherent structure with two ORs: {002}U2N3‖{002}UO2 and [001]U2N3‖[001]UO2; U2N3{101}‖UO2{101} and U2N3[101]‖UO2[101] due to low lattice misfit (2.3%) and low interfacial energy (127 mJ/m2). Observed oxidation of UN and coherent interface are consistent with density-functional theory calculations which suggest lower energy for oxidized configuration and low energy of the interface. The dislocation loops grew while their number density decreased with the temperature and dose. The loop size was over three times larger in two nitride phases than that in UO2, while the number density was one order of magnitude higher in UO2 than in nitride phases. Loop density and diameter were analyzed using a kinetic rate theory that considers stoichiometric loop evolution. This analysis led to the conclusion in all compounds loop growth is governed by mobility of uranium interstitials, and enabled measurement of diffusion coefficients of uranium interstitials and non-metal interstitials and vacancies. This analysis provided a comparative study of early stage of microstructure evolution under irradiation which has implications for use of this mixture as advanced fuel in nuclear energy systems.

Thermal conductivity of gadolinium added uranium mononitride fuel pellets sintered by spark plasma sintering

Thermal conductivity of spark plasma sintered UN/Gd2O3 and UN/GdN composite pellets with various compositions of Gd2O3 (5.0 to 15.0 wt%) and GdN (3.5 to 38.4 wt%) was measured from 25 °C to 1000 °C using laser flash analyzer (LFA). Overall thermal conductivity of both composite pellets decreases with increasing Gd compositions for the entire temperature range measured. The UN/GdN pellets sintered at high temperature (2000 °C) contained solid-solution (U1-x,Gdx)N phase and exhibited 20–65 % higher thermal conductivity than the UN/Gd2O3 pellets. We also confirmed a two-phase structure in low temperature (1800 °C) sintered UN/GdN pellets, which exhibited 14–25 % higher thermal conductivity than solid-solution UN/GdN pellets. This result may indicate that relatively higher thermal conductivity of UN/GdN composite as burnable absorber fuel could be further enhanced with an optimized microstructure.