Electrical conductivity of UO2-ThO2 solid solutions

The electrical conductivities of UO_2+x. ThO₂ and their solid solutions, in thermodynamic equilibrium with the gas phase, were measured as a function of temperature, and of oxygen partial pressure in the temperatnre range 800 to 1200°C. The slope of the plot log α versus 1/$\text{T}$ for UO_2+x and UO₂-rich solid solutions exhibits a single region, whereas in the ThO₂-rich solid solutions it exhibits two regions. The pressure dependence of the conductivity (σ) in the UO₂-rich solid solutions can be represented by σ ∝ [O_i] ∝ in the range of $\text{0.01 < x < 0.1}$. Here, O_i is an interstitial oxygen and the partial pressure of oxygen, and it varies with the ThO₂ content. At greater deviation from stoichiometry ($\text{x ? 0.1}$) the presence of U₄O₉ or (Th U)₄O₉ phases influences the conductivity data. In ThO$\text{2}$ or ThO₂-rich solid solutions. P-type conduction at high oxygen pressures is interpreted as arising from the incorporation of excess oxygen into oxygen vacancies.     

Modeling thermal conductivity in UO2 with BeO additions as a function of microstructure

New processing methods show promise for improved thermal conductivity in UO₂ by the incorporation of a highly-conducting material. Such composites are likely to have anisotropic microstructures which bring new challenges to thermal conductivity simulation but also significant potential for improvement in the thermal performance. This paper presents simulation results for the thermal conductivity of UO₂/BeO composites using statistical continuum mechanics. The results successfully capture the microstructural heterogeneity and predict the corresponding anisotropic thermal properties. The application of statistical continuum mechanics to materials design makes it possible to design novel anisotropic fuel pellets with enhanced thermal conductivity in a preferred direction.     

Electrical conductivity of UO2+x

Isothermal and isobaric conductivities of UO_2+x nave been measured as a function of oxygen pressure and temperature. Intrinsic disorder predominates at low-oxygen pressures. The pressure dependence of the conductivity can be expressed as in the intermediate oxygen pressure range in which more than one type of charge carrier predominate. At high oxygen pressures, it has been proposed that oxygen vacancy-interstitial trios in association with U⁺⁵ ions faciliate the fast transport of oxygen interstitials in the region.     

Reduction of UO2 by H2

The reactivity of H₂ towards UO₂²⁺ has been studied experimentally using a PEEK coated autoclave where the UO₂²⁺ concentration in aqueous solution containing 2 mM carbonate was measured as a function of time at p_H2∼40 bar. The experiments were performed in the temperature interval 74-100 °C. In addition, the suggested catalytic activity of UO₂ on the reduction of UO₂²⁺ by H₂ was investigated. The results clearly show that H₂ is capable of reducing UO₂²⁺ to UO₂ without the presence of a catalyst. The reaction is of first order with respect to UO₂²⁺. The activation energy for the process is 130 ± 24 kJ mol⁻¹ and the rate constant is k_298K=3.6×10⁻⁹ l mol⁻¹ s⁻¹. The activation enthalpy and entropy for the process was determined to 126 kJ mol⁻¹ and 16.5 J mol⁻¹ K⁻¹, respectively. Traces of oxygen were shown to inhibit the reduction process. Hence, the suggested catalytic activity of freshly precipitated UO₂ on the reduction of UO₂²⁺ by H₂ could not be confirmed.     

Frequency dependence of electrical conductivity and dielectric constant of UO2

The dielectric constant and electrical conductivity of single crystal and polycrystalline UO₂ are found to be frequency dependent. The dielectric constant measured at low frequencies is anomalously large at room temperature but decreases to a limiting value (~25) below about 130 K. A knee observed in the temperature dependence of the conductivity of polycrystalline UO₂ corresponds to a process having an activation energy of 0.15 eV.     

The effects of alpha-radiolysis on UO2 dissolution determined from batch experiments with Pu-doped UO2

The effects of alpha dose-rate on UO₂ dissolution were investigated by performing dissolution experiments with ²³⁸Pu-doped UO₂ materials containing nominal alpha-activity levels of ∼1-100 Ci/kg UO₂ (actual levels 0.4-80 Ci/kg UO₂), in 0.1 M NaClO₄ and in 0.1 M NaClO₄ + 0.1 M carbonate. Dissolution rates increased less than 10-fold for an almost 100-fold increase in doping level and fall within the range of predictions of the Mixed Potential Model (a detailed mechanistic model for used fuel dissolution). Dissolution rates were lower in carbonate-free solutions and enrichment of ²³⁸Pu on the UO₂ surface was suggested in carbonate solutions. Effective G values, defined as the ratio of the total amount of U dissolved divided by the maximum possible amount of U dissolved by radiolytically produced H₂O₂, increased with decreasing doping levels. This suggests that the dissolution reaction at high dose rates is limited by the reaction rate between UO₂ and H₂O₂, but becomes increasingly limited by the rate of production of H₂O₂ at lower dose rates.     

Integrity degradation of UO2 pellets subjected to thermal shock

Thermal shock behavior of UO₂ pellets has been investigated by means of out-of-pile experiments and a theoretical analysis which particularly emphasized the porosity effect on the thermal shock damage. In the experiments, specimens of porosity range 0.05–0.15 were thermal-shocked by heating and then quenching in a water bath at various quenching temperature differences (ΔT). Results showed that with increasing porosity, ΔT values cause a first damage (ΔT_c) and bring about destructive failure of the specimens increased, while the strength loss at ΔT_c was reduced. These findings suggested that the higher the porosity, the higher the pellet integrity during rise to power. Theoretical equations expressing the thermal shock damage were introduced. Good agreements were obtained between observed and predicted values.     

Kinetics of initial stage of sintering of UO2 and UO2 with Nd2O3 addition

The kinetics of initial stage sintering of UO₂ powder were reinvestigated, using Ar-10% H₂ atmosphere. The effect of the addition of neodynium oxide was studied. The results revealed that surface and grain boundary diffusion mechanisms act simultaneously. The values of activation energies were found to be $\text{48.48 ± 3.51 kcal/mole}$ in the temperature range 870–942°C and $\text{89.88 ± 9.87 kcal/mole}$ in the temperature range of 942–1030°C for UO₂, and $\text{115.61 ± 7.77 kcal/mole}$ in the temperature range 1030–1150°C for UO₂ + Nd₂O₃. An important decrease in the calculated diffusion coefficient occurs by the addition of Nd₂O₃.     

Optical properties of UO2 and PuO2

We perform first-principles calculations of electronic structure and optical properties for UO₂ and PuO₂ based on the density functional theory using the generalized gradient approximation (GGA) + U scheme. The main features in orbital-resolved partial density of states for occupied f and p orbitals, unoccupied d orbitals, and related gaps are well reproduced compared to experimental observations. Based on the satisfactory ground-state electronic structure calculations, the dynamical dielectric function and related optical spectra, i.e., the reflectivity, adsorption coefficient, energy-loss, and refractive index spectrum, are obtained. These results are consistent with the available experiments.     

The evaluation of temperature jump distances and thermal accommodation coefficients from measurements of the thermal conductivity of UO2 packed sphere beds

The results of thermal conductivity measurements on UO₂ 80 μm diameter microsphere beds in helium, argon and krypton over a range of temperatures and pressures are reported. These and previous results on UO₂ microsphere beds in helium, krypton and nitrogen obtained at Oak Ridge National Laboratory (ORNL) were analysed in terms of a previously developed model which enables thermal accommodation coefficients (TAC) and temperature jump distances to be evaluated. Values derived from ORNL results tend to be in reasonable agreement with the corresponding, rather sparse data reported previously. In particular, in the case of helium, TAC's are in reasonable agreement with the direct measurements of Ullman et al. and exhibit a similar decrease with increasing temperature. However the TAC and temperature jump distance values derived from the Harwell measurements are respectively larger and smaller than the comparable ORNL figures. Also, in the case of helium, the TAC increases with pressure in contrast with values obtained from the ORNL data which are pressure independent. These differences are attributed to the differing surface finishes that are achieved during the manufacturing processes employed in the two laboratories.     

Relationship between changes in the crystal lattice strain and thermal conductivity of high burnup UO2 pellets

Two kinds of disk-shaped UO₂ samples (4 mm in diameter and 1 mm in thickness) were irradiated in a test reactor up to about 60 and 130 GWd/t, respectively. The microstructures of the samples were investigated by means of optical microscopy, scanning electron microscopy/ electron probe micro-analysis (SEM/EPMA) and micro-X-ray diffractometry. The measured lattice parameters tended to be considerably smaller than the reported values, and the typical cauliflower structure which is often observed in high burnup fuel pellet is hardly seen in these samples. Thermal diffusivities of the samples were also measured by using a laser flash method, and their thermal conductivities were evaluated by multiplying the heat capacity of unirradiated UO₂ and sample densities. While the thermal conductivities of sample 2 showed recovery after being annealed at 1500 K, those of sample 4 were not clearly observed even after being annealed at 1500 K. These trends suggest that the amount of accumulated irradiation-induced defects depends on the irradiation condition of each sample. From the comparison of the changes in the lattice parameter and strain energy density before and after the thermal diffusivity measurements, it is likely that the thermal conductivity recovery in the temperature region from 1200 to 1500 K is related to the migration of dislocation.     

Sintering diagrams of UO2

Ashby's method of constructing sintering diagrams has been modified to obtain relative contribution diagrams directly from the computer. It is endeavoured here to study the interplay of sintering variables and mechanisms and determine the factors that affect the participation of mechanisms in UO₂. By studying the physical properties, it emerges that the order of inaccuracies is small in most cases and do not affect the diagrams. On the other hand, even a 10% error in activation energies, which is quite plausible, would make a significant difference to the diagram. The main criticism of Ashby's approach is that the numerous properties and equations used, communicate their inaccuracies to the diagrams and make them unreliable. Our study has considerably reduced the number of factors that need to be refined to make the sintering diagrams more meaningful.     

Fission-induced densification of UO2

A new technique has been developed to study fission-induced densification and hot-pressing of UO₂ at very low temperatures without complications from fracturing or other concurrent thermal effects. Thin disks of UO₂ of 0.22 to 20% enrichment and differing microstructural stability, were irradiated in the core of the CP — reactor at temperatures below 200°C and at pressures from 1 atm to 20.7 MPa. Results indicate that the pressurizing medium, NaK, had penetrated the open porosity at high pressure and impeded densification. To rationalize this effect, the previously proposed models for radiationinduced densification are critically reviewed. Modifications to models involving thermal sintering and hot-pressing, and pore resolution appear the most tenable. The former mechanism leads to predictions that fission-induced hot-pressing can occur, and ex-reactor sintering and hot-pressing should correlate with in-reactor densification. The proximity of pores and grain boundaries is also stressed. The effect of NaK logging is rationalized by changes in pore surface energy and by stabilization of small pores aginst complete resolution.     

Phasengleichgewichte in den systemen UO2-UO2,67-ThO2 und UO2 + x-NPO2

Solid-state chemical investigations have established that in the compositional range UO₂-UO_2.67-ThO₃ of the U-Th-O ternary system, the following single-phase domains exist: U₃O₈, which does not dissolve any ThO₂ in the solid state; an ordered M₄O₉ phase on the section between U₄O₉ and U₂Th₂O₉, below ≈ 1150 °C; and a phase with fluorite structure which occupies a large part of the system and which at 1250 °C is bounded by the compositions UO₂-UO_2.25 (U_0.43, ThO_0.57)O_0.25-ThO₃. The maximum O/M ratio of the “fluorite” phase is O:(U + Th) = 2.25. The highest oxidation valency of uranium is 5.30; this value falls as more thorium oxide is incorporated in the (U.Th)O_{2 + x} “fluorite” phase.     

The creep of UO2 fuel doped with Nb2O5

The creep of UO₂ containing small additions of Nb₂O₅ has been investigated in the stress range 0.5–90 MN/m² at temperatures between 1422 and 1573 K. The functional dependence of the creep rate of five dopant concentrations up to 0.8 mol% Nb₂O₅ has been examined and it was established that in all the materials the secondary creep rate could be represented by the equation /.εkT = Aσⁿexp(?Q/RT), where /.ε is the steady state creep rate per hour, Q the activation energy and A and n are constants for each material. It was observed that Nb₂O₅ additions can cause a dramatic increase in the steady state creep rate as long as the niobium ion is maintained in the Nb⁵⁺ valence state. Material containing 0.4 mol% Nb₂O₅ creeps three orders of magnitude faster than the pure material.Analysis of the results in terms of grain size compensated viscosity suggest that, like “pure” UO₂, the creep rate of Nb₂O₅ doped fuel is diffusion-controlled and proportional to the reciprocal square of the grain size. A model is developed which suggests that the increase in creep rate results from suppression of the U⁵⁺ ion concentration by the addition of Mb⁵⁺ ions, which modifies the crystal defect structure and hence the uranium ion diffusion coefficient.