Crystals Of UO2 Research Articles

Photoluminescence with a Quasi-White Spectrum in LiF–UO2 Crystals with Color Centers

Photoluminescence with a Quasi-White Spectrum in LiF–UO2 Crystals with Color Centers

Modeling the UO2 reduction process

Methods of molecular dynamics and DFT calculations have been used to study the reduction mechanisms of UO2 as the most representative part of spent nuclear fuel to metallic uranium. It is shown that the critical softening of the combined modulus of elasticity C11-C12 to zero is the reason for the destruction of the UO2 crystal as a result of the removal of oxygen from it. This destruction is accompanied by an order-disorder phase transition in the oxygen subsystem of the crystal under consideration. DFT calculations indicate a continuous decrease in the band gap as oxygen is removed from the UO2 crystal. When the system reaches the composition U2O3, the band gap disappears and the system becomes electrically conductive. The appearance of the dielectric-conductor transition explains the realization of the FFC Cambridge process during the recovery of spent nuclear fuel. The passage of Li+ and Cl– ions of the LiCl melt through cylindrical channels in a UO2 crystal with cross-sectional radii from 0.25 up to 2 nm has been studied. The strength of the external electric field required for the passage of these channels decreases with an increase in the channel cross section, and the number of Cl– ions entering the channel increases. On the walls of the channels that pass ions with charges of both signs, colonies of adsorbed Cl– and Li+ atoms appear separated from each other, between which strong electric fields are formed. The existence of such fields can cause Li+ ions to move deep into the material being reduced.

Electrochemical recovery of UO2 from LiCl-KCl-UO2F2 molten salt

To recover UO2 from LiCl-KCl-UO2F2 molten salt, the electrochemical behavior of UO22 + and nucleation mechanism of UO2 were studied by a series of electrochemical techniques. The results of cyclic voltammetry and square wave voltammetry showed that UO2 was formed through two-step reduction process of UO22 + involving one electron transfer. The disproportionation reaction of UO2+ was investigated by calculating electric charge, demonstrating the disproportionation reaction occurred as 2UO2+=UO2+UO22 +. The exchange current densities of UO2+/UO2 couple were determined at various temperatures by Tafel technique, and their values increased with the increasing temperature, which was in accordance with the Arrhenius equation, therefore, the activation energy of the reaction was estimated to be 32.31 kJ mol−1. Meanwhile, the nucleation mechanism of UO2 was investigated to follow the progressive nucleation by chronoamperometry. Furthermore, potentiosatic electrolysis was performed at various applied potentials, and the results indicated that UO2 had a regular octahedral microstructure, and the applied potential affected the size of UO2 crystal. In addition, UO2 was prepared by pulse electrolysis at 2.02 V in duty ratios of 75 % and 60 %, the results of SEM showed that pulse electrolysis could avoid the formation of dendrite.

Uranium Photoluminescence Excited in the Charge Transfer Band of Unirradiated LiF–UO2 Crystals

Uranium Photoluminescence Excited in the Charge Transfer Band of Unirradiated LiF–UO2 Crystals

Studies on the Electrodeposition of UO2 Production in UO2Cl2-LiCl-KCl Melt

UO2 is an important nuclear fuel, also has potentials in the field of functional materials. In this work, the electrochemical behavior of UO2 2+ in LiCl-KCl molten salt (500 °C) and the effects of reduction potential (−0.1 to −1.0 V) and uranyl concentration (0.5 wt% to 4.0 wt%) on morphology and grain size of electrolytic products were studied. The reduction potential was found playing a decisive role in the morphology of cathodic products. When the potential was ≤−0.4 V (vs Pt(II)/Pt), dendritic and cracked characteristic of the UO2 crystal appeared. The average current density had a positive correlation with the grain size of cathodic products, and it increased first and then decreased with the increase of overpotential, having a maximum at −0.6 V (vs Pt(II)/Pt). On the other hand, the uranyl concentration also had a positive correlation with the grain size of the cathodic product without changing the integrity of grain morphology. For the preparation of large and granular UO2 crystal, the reduction potential of −0.2to−0.3 V (vs Pt(II)/Pt), as well as the uranyl concentration of greater than 2.0 wt% were recommended, and UO2 crystal with average grain size greater than 500 μm and high uniformity could be finally obtained.

Point light detection with UO2

UO2's electrical properties and U's inherently high fission energy release make a UO2-based direct conversion neutron detector a tantalizing possibility, but the evolution of even rudimentary devices is inhibited by the absence of large, high quality single crystals of UO2. Hydrothermally grown UO2 is incorporated into a prototype device here, and laser light is used as a surrogate for neutron-induced fission events. Excitation sources with wavelengths of 325, 488, 532, and 633 nm are incident on the device for a variety of timescales to test the detection responses, resolution, and saturation behaviors. The non-amplified changes in the current are presented, including when the illumination's diameter, power, and location are altered. Photocurrent changes follow a positively correlated linear trend with power and device efficiency is inversely correlated with illumination wavelength. The impact of illumination time on the detection efficiency is discussed. Illumination event detection is independent of illumination surface area size and largely independent of the illumination location. The ramifications of these results with regard to the development of UO2 as a neutron detector are considered.

Investigation of phonon properties and thermal behavior of UO2 crystal using density-functional theory

Understanding thermal behavior and processes underlying the heat transport of UO2 nuclear fuel in nuclear reactor plays a key role in predicting the efficiency of the fuel. If the heat transport, which is an important parameter in temperature distribution of the fuel, does not occur properly, the continuous increase of temperature would lead to the melting of the fuel and therefore, environmental hazards. In this work, by using a non-spin-polarized calculation for the simple description of the paramagnetic state and ignoring the Hubbard correction, the thermal properties and phonon properties of bulk UO2 are calculated. These calculations are based on the density-functional theory (DFT) and density-functional perturbation theory (DFPT). To determine the lattice-vibration properties by the finite-displacement method, we have calculated the second-order and third-order force constants based on which such quantities as constant-volume specific heat, Gruneisen parameter, three-phonon scattering rate, scattering rate due to different levels of isotopic enrichment, and cumulative thermal conductivity are calculated. The results of the calculated specific heat based on the harmonic approximation show a good agreement with the experimental values, specifically for temperatures lower than 400 Kelvin. The results obtained for three-phonon scattering rate reveal that the scattering rate increases with temperature, thereby leading to the decrease of thermal conductivity. The results related to different levels of isotopic enrichments do not show any sensible changes in the scattering rates.

Plasticity of UO2 studied and quantified via elevated temperature micro compression testing

Small scale mechanical testing has many beneficial features including the ability to sample the properties of a single crystal over a wide temperature range. This manuscript presents the results of high temperature, in-situ scanning electron microscope microcompression testing of a single crystal of UO2. Observations indicated that the UO2 microcompression specimens deformed in a ductile fashion at temperatures of 350 °C and above. The slip traces on the micro-compression specimens after deformation appeared on the {100}1/2<110> system at the temperatures and orientation tested. Lastly, the Peierls stress of UO2 is calculated using the microcompression data.

Optimizing the orbital occupation in the multiple minima problem of magnetic materials from the metaheuristic firefly algorithm.

We present the use and implementation of the firefly algorithm to help in scanning the multiple metastable minima of orbital occupations in density functional theory (DFT) plus Hubbard U correction and to identify the ground state occupations in strongly correlated materials. We show the application of this implementation with the Abinit code on KCoF3 and UO2 crystals, which are typical d and f electron systems with numerous occupation minima. We demonstrate the validity and performance of the method by comparing with previous methodologies. The method is general and can be applied to any code using constrained occupation matrices.

Assessing UO2 sample quality with μ-Raman spectroscopy

μ-Raman spectroscopy of UO2 has demonstrated its value as a tool in nuclear forensics and for analyzing nuclear fuels, but its ability to evaluate UO2 sample quality has been relatively unexplored. To investigate this potential, a three-pronged study that uses Raman spectroscopy to analyze 1) three distinct qualities of UO2 single crystals with two different excitation lasers, 2) a high quality single crystal UO2 sample that is progressively damaged with a laser, and 3) a single crystal of UO2 grown on top of a ThO2 substrate was undertaken. The results demonstrate that the peak height ratio between the second order longitudinal optical phonon signal near 1145 cm−1 and the T2g peak located near 450 cm−1 is directly correlated with crystal quality when a 532 nm excitation source is used. Furthermore, this observed pattern relies upon the two most intense peaks that are present in the Raman spectra, making it more robust to lower signal-to-noise ratios than similar trends present in the peak height ratio between the first order longitudinal optical phonon signal near 565 cm−1 and the T2g peak. Both peak height ratios are more sensitive indicators of the crystal quality than the width ratios of the same signals or the associated wavenumber shift in the T2g peak location. The correlation between the 2LO and T2g peak heights and the overall crystal quality offers the potential to directly assess and compare the quality of different samples of single crystal UO2.

Synthesis and preservation of graphene-supported uranium dioxide nanocrystals

Graphene-supported uranium dioxide (UO2) nanocrystals are potentially important fuel materials. Here, we investigate the possibility of synthesizing graphene-supported UO2 nanocrystals in polar ethylene glycol compounds by the polyol reduction of uranyl acetylacetone under boiling reflux, thereby enabling the use of an inexpensive graphene precursor graphene oxide into a one-pot process. We show that triethylene glycol is the most suitable solvent with an appropriate reduction potential for producing nanometer-sized UO2 crystals compared to monoethylene glycol, diethylene glycol, and polyethylene glycol. Graphene-supported UO2 nanocrystals synthesized with triethylene glycol show evidence of heteroepitaxy, which can be beneficial for facilitating heat transfer in nuclear fuel particles. Furthermore, we show that graphene-supported UO2 nanocrystals synthesized by polyol reduction can be readily stored in alcohols, impeding oxidation from the prevalent oxygen in air. Together, these methods provide a facile approach for preparing and storing graphene-supported UO2 nanocrystals for further investigation and development under ambient conditions.

Synthesis and structure of crystals of UO2(C2H5COO)2·1.5L (L = methylurea or N,N’-dimethylurea)

The compounds UO2(C2H5COO)2·1.5L, where L is methylurea (Meur) or N,N’-dimethylurea (s-Dmur), were synthesized and studied by IR spectroscopy and single crystal X-ray diffraction. Irrespective of the kind of amide L, the structure of the compounds consists of two mononuclear uranium-containing complexes: cationic [UO2(prop)L3]+ and anionic [UO2(prop)3]–, where prop is propionate ion. Both compounds belong to crystal-chemical group AB01M31+ AB01M301 (A = UO22+, B01 = prop–, M1 = L) of uranyl complexes. The composition of 15 stable uranyl complexes that can be formed in the UO22+, B01–prop––L–H2O system was predicted on the basis of the characteristics of the Voronoi–Dirichlet polyhedra from the standpoint of the 18-electron rule.

5 f -Shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations

Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5 f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5 f -shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4–5 eV.

First uranyl methacrylate complexes: Synthesis and structure

Crystals of UO2(mac)2 · 2H2O (I), UO2(mac)2 · 1.5Dmur (II), and UO2(mac)2 · Tmur (III) (mac− is methacrylate ion C3H5COO−, Dmur is N,N-dimethylurea, Tmur is tetramethylurea) have been synthesized and studied by X-ray crystallography and IR spectroscopy. The structural units of I are mononuclear [UO2(mac)2(H2O)2] complexes classified with crystal-chemical group AB 2 01 M 2 1 (A = UO 2 2+ B01 = mac−, M1 = H2O). Structure II contains two types of mononuclear uranium-containing complexes: cationic [UO2(mac)(Dmur)3]+ and anionic [UO2(mac)3]−. The crystal-chemical formula of II is AB01M 3 1 + AB 3 01 (A = UO 2 2+ , B01 = mac−, M1 = Dmur). The basic structural units of crystal structure III are [UO2(mac)2(Tmur)]2 dimers belonging to crystal-chemical group AB2B01M1 of uranyl complexes (A = UO 2 2+ , B2 and B01 = mac−, M1 = Tmur). The composition and possible structures of 14 stable uranyl complexes that can form in the UO 2 2+ -mac−-L-H2O system (L is Dmur or Tmur) have been determined on the basis of Voronoi-Dirichlet polyhedron characteristics with the use of the 18-electron rule. It has been demonstrated that, in crystals I–III, uranium-containing complexes are combined into a framework through a system of hydrogen bonds and electrostatic and noncovalent interactions.

Novel succinate-containing coordination polymers [UO2(C4H4O4) · L] (L = C3H6N2O or C4H9NO): Synthesis and structure

Synthesis, IR spectral study, and X-ray diffraction analysis of crystals of UO2(C4H4O4) · C3H6N2O] (I) and [UO2(C4H4O4) · C4H9NO] (II) were performed. Both compounds are crystallized in monoclinic syngony with unit cell parameters: a = 7.7582(1) A, b = 10.1601(2) A, c = 14.6234(3) A, β = 103.753(1)°, space group P21/c, Z = 4, R = 0.0219 (I); a = 8.2894(4) A, b = 12.3667(5) A, c = 12.0728(4) A, β = 96.464(2)°, space group P21/n, Z = 4, R = 0.0330 (II). Complexes [UO2(C4H4O4)L] (L = C3H6N2O or C4H9NO) in the crystals of I and II are related to crystal-chemical group A(K21-4)M1 (A = UO22+, K21-4 = C4H4O42−, M1 = L) of uranyl complexes and have layered structure. The effect of succinate ion conformation on the topology of all known complexes [UO2(C4H4O4)L] is discussed.

X-Ray diffraction and IR-spectroscopic studies of UO2(n-C3H7COO)2(H2O)2 and Mg(H2O)6[UO2(n-C3H7COO)3

Single crystals of UO2(n-C3H7COO)2(H2O)2 (I) and Mg(H2O)6[UO2(n-C3H7COO)3]2 (II) are synthesized. Their IR-spectroscopic and X-ray diffraction studies are performed. Crystals I are monoclinic, a = 9.8124(7) A, b = 19.2394(14) A, c = 12.9251(11) A, β = 122.423(1)°, space group P21/c, Z = 6, and R = 0.0268. Crystals II are cubic, a = 15.6935(6) A, space group \(Pa\bar 3\), Z = 4, and R = 0.0173. The main structural units of I and II are [UO2(C3H7COO)2(H2O)2] molecules and [UO2(C3H7COO)3]− anionic complexes, respectively, which belong to AB201M21 (I) and AB301 (II) crystal chemical groups of uranyl complexes (A = UO22+, B01 = C3H7COO−, and M1 = H2O). A crystal chemical analysis of UO2L2 · nH2O compounds, where L is a carboxylate ion, is performed.

Radiation damage in urania crystals implanted with low-energy ions

Implantations with low-energy ions (470-keV Xe and 500-keV La with corresponding ion range Rp∼85nm and range straggling ΔRp∼40nm) have been performed to investigate both radiation and chemical effects due to the incorporation of different species in UO2 (urania) crystals. The presence of defects was monitored in situ after each implantation fluence step by the RBS/C technique. Channelling data were analysed afterwards by Monte-Carlo simulations with a model of defects involving (i) randomly displaced atoms (RDA) and (ii) distorted rows, i.e. bent channels (BC). While increasing the ion fluence, the accumulation of RDA leads to a steep increase of the defect fraction in the range from 4 to 7dpa regardless of the nature of bombarding ions followed by a saturation plateau over a large dpa range. A clear difference of 6% in the yield of saturation plateaus between irradiation with Xe and La ions was observed. Conversely, the evolutions of the fraction of BC showed a similar regular increase with increasing ion fluence for both ions. Moreover, this increase is shifted to a larger fluence in comparison to the sharp increase step of RDA. This phenomenon indicates a continuous structural modification of UO2 crystals under irradiation unseen by the measurement of RDA.

Molecular dynamics simulation of the surface properties of nanocrystalline uranium dioxide

The molecular dynamics method is used to simulate the nanosized UO2 crystals. The phase-transition temperatures are calculated for the nanosized crystals of the uranium dioxide. It is demonstrated that the melting point and the temperature of the transition to the superionic state (melting of the anion sublattice) of the crystals decrease with decreasing sizes. In particular, melting point (T m ∼ 2300 K) for the cubic nanocrystal with a size of 3.3 nm is lower than the melting point of the single crystal by almost 1000 K. The calculated surface energies are in agreement with the experimental results. The dependence of the surface energy on the size of the UO2 nanocrystals is obtained. The effect of the nanocrystal temperature on the surface energy is studied. The temperature dependence of the thickness of the melt layer is obtained in the framework of the model of the heterogeneous melting. The parameters and dependences can be used for the further analysis of the microstructure properties of nuclear fuel in working systems.

Defect disorder in UO2

A defect disorder model has been developed to determine equilibrium off-stoichiometry and its spatial variations in UO2 crystals. The model gives the concentrations of atomic defects and electronic carriers as functions of oxygen partial pressure and temperature in the bulk and near crystal surfaces subject to an oxygen environment. Energetic parameters from published density functional theory calculations have been integrated into the defect disorder model for an accurate determination of the defect density and off-stoichiometry. The ionosorption theory has been used to couple the oxygen environment with the defect state in the crystal as we solved for the defect disorder near crystal surfaces. Contrary to the common belief that hyper-stoichiometry of UO2 is dominated by oxygen interstitials, the current model predicts that this regime is rather dominated by uranium vacancies. The model predictions also show that, in the presence of surfaces, the point defect concentrations vary by orders of magnitude in the subsurface region relative to the bulk region.

High temperature chromium volatilization from Cr2O3 powder and Cr2O3-doped UO2 pellets in reducing atmospheres

Chromium volatilization from Cr2O3 powder and Cr2O3-doped UO2 pellets during sintering in reducing atmospheres has been studied by thermogravimetry (TG) coupled with differential thermal analysis (DTA) up to 1700°C. The sintering of Cr2O3-doped UO2 pellets was also followed by dilatometry. Oxygen partial pressures in the range 10−20–10−11atm (10−15–10−6Pa) have been fixed in all the experiments thanks to mixtures of hydrogen and carbon dioxide. A linear heating rate of 20°Cmin−1 was applied to all the experiments. The dopant amount was in the range 0.18–0.9mol%Cr in UO2. For all the oxygen potentials, the mass loss of Cr2O3 powder was found to start at temperatures as high as 1470°C due to Cr2O3 dissociation, the lower the oxygen potential, the lower the starting temperature and the higher the volatilized amount. For intermediate oxygen potentials, an exothermic DTA peak observed during cooling, from 1700°C to room temperature, attested for the crystallization of a liquid phase which was attributed to CrO(l) according to thermodynamic predictions. Then, the dilatometry and TG studies allowed following the behavior of Cr2O3-doped UO2 pellets. The mass loss at temperatures higher than 1470°C was attributed to chromium volatilization for all the doped samples. During the sintering of doped UO2 pellets, the liquid phase CrO(l) seemed to appear at a lower oxygen potential than in Cr2O3 powder, which probably contributed to enhance the densification rate. For the highest dopant amount, 0.9mol%Cr, the volatilization process was found to be rather similar to that of Cr2O3 powder, due to the part of chromia not solubilized in the UO2 crystal. Moreover, as the initial pellets were not dense, as long as the pellet porosity remained open, the chromia particles were able to dissociate as in the Cr2O3 powder. Thus the volatilization of chromium from doped UO2 pellets under sintering in reducing atmospheres must be understood as the result of several phenomena whose contribution depends on temperature, oxygen potential and heating rate: before the porosity closure, both dissociation of chromia into Cr(g) and oxygen from excess Cr2O3 particles, and Cr volatilization from doped UO2 particles; then chromium volatilization from the doped UO2 ceramic during further densification process.