Model representation of the physicochemical and diffusion properties during thermal extraction of gas from alloyed uranium dioxide

The release of the process gases H₂, H₂O, CO, CO₂, and Ar from alloyed samples of uranium dioxide by oxides of Nb, Ce, Al, Si, and Fe has been studied by means of thermal extraction in vacuum at 1873 K. The niobium concentration in U_1−yNb_yO₂ was 0.06–0.2 mass % (y = 0.001–0.004), the cerium content was 0–25 mass% (y = 0–0.3434), and the content of mullite or Indian red did not exceed 0.25 mass%, the oxygen potential Δ̄G_O2 of the surface of the samples ranged from −400 to −300 kJ/mole at 1873 K. It was determined that the specific gas release from alloyed samples of uranium dioxide depends on the content of niobium, cerium, and impurity carbon as well as the porosity and the radial gradient of the departure from stoichiometry. A model is proposed for the physicochemical and diffusion processes accompanying the thermal extraction of gas from alloyed uranium dioxide.     

Recommendations for calculating the thermal creep properties of uranium dioxide when analyzing fuel-element serviceability

Recommendations for calculating the thermal creep properties of uranium dioxide for fuel element serviceability analysis programs are developed on the basis of a physical model. The deformation processes in the model include diffusion and diffusion-controlled motion of dislocations. It is shown on the basis of the analysis of the thermodynamics of point defects in ionic crystals that the diffusion of ions is controlled by a vacancy mechanism and that the diffusion coefficient depends on the temperature and oxygen coefficient. The model includes the influence of temperature, stress, fuel density, grain size, and oxygen ratio on the creep rate. The relations obtained in the this work have made it possible to improve by approximately a factor of 10 the agreement between the calculations and experimental data as compared with the empirical relation used previously to describe the characteristics of creep.     

Atomistic modeling of the vibrational and thermodynamic properties of uranium dioxide, UO2

Modeling the thermal properties of uranium oxide is of immense interest to the nuclear industry. UO₂ belongs to the family of superionic conductors whose solid-state diffusion coefficients at high temperatures are comparable to that of liquids. We report lattice dynamics and molecular dynamics studies carried out on oxide UO₂ in its normal as well as superionic phase. Lattice dynamics calculations have been carried out using shell model in the quasiharmonic approximation. The calculated equilibrium structure, elastic constants, bulk modulus, phonon frequencies and specific heat are in excellent agreement with the reported experimental data. Pressure variation of the phonon dispersion and equation of state have also been predicted. Molecular dynamics simulations have been carried out to study the diffusion behavior and the thermodynamic properties in UO₂. The diffusion constant of O in UO₂ has been determined. The pair correlation functions, O-U-O bond angle and thermal amplitude of vibration for the oxygen atom provide a microscopic picture of the local structure thereby throwing light on the gradual increase in the disorder of the oxygen sub-lattice which is a signature of superionic transition. The calculated transition temperature of UO₂ is 2300 K, which compares well with experimental value of about 2600 K.     

The characteristics of fission gas release from monocrystalline uranium dioxide during irradiation

Release rates for ^85mKr, ⁸⁷Kr, ⁸⁸Kr, ¹³³Xe, ¹³⁵xe and ¹³⁸Xe were measured in the temperature range 700–1550°C. The data were analysed in terms of diffusion of the rare gases and their halogen precursors. The diffusion coefficients for xenon and iodine were found to be similar whilst krypton also had a similar mobility at ~1200°C but otherwise diffused more slowly. Bromine had a high mobility compared with the rare gases (X 200).     

Equation of state of uranium dioxide

An equation for the free energy of formation of $\text{UO}{}_2\text{(l)}$ is derived. Using this equation, a new calculational method for estimating consistent values for the vapor pressure of a liquid compound has been applied to uranium dioxide. Equations for the partial pressure of all vapor species are calculated. Values of the liquid density to the critical point are presented.     

Predicting the probability for fission gas resolution into uranium dioxide

Classical molecular dynamics simulations, using a set of previously established pair potentials, have been used to predict the minimum energy needed for krypton and xenon atoms to be resolved into uranium dioxide across a perfect (1 1 1) surface. The absolute minimum energy, E_min, is 53 eV for krypton and 56 eV for xenon atoms, significantly less than the 300 eV value often assumed in fuel modelling as the minimum energy required for gas resolution. The present values are, however, still sufficient to preclude thermal resolution at normal reactor temperatures. The discrepancies between the present and previous resolution energies are due to the significant variation in probabilities of absorption at different impact points on the crystal surface; we have mapped out the probability distribution for various impact sites across the crystal surface. The value of 300 eV corresponds to an 85% chance of resolution.     

Investigation of the properties of modified uranium dioxide

Experimental investigations have made it possible to develop a technology for obtaining uranium dioxide fuel pellets with anomalously high thermal conductivity at 600–800°C and higher. The thermal conductivity of pellets is increased, without and with the addition of very small amounts of tin or titaniun dioxide, by improving the deposition process, making it possible to obtain simultaneously “large” particles and nanoparticles followed by annealing of the deposit at optimal temperature. It is pointed out that there is an analogy with the temperature dependence of the thermal conductivity for single-crystal uranium dioxide. The radial heat flux method showed that the temperature gradient from the center to the periphery for modified pellets is approximately three times smaller than for pellets fabricated by the standard factory technology. The modified pellets contain uranium of different valence and have an unusual microstructure, characterized by the presence of close-packed grains. Small spherical pores 0.1–0.2 μm in diameter are observed inside grains; a minimal number of polyhedral pores 1–2 μm in size lie along the grain boundaries. __________ Translated from Atomnaya énergiya, Vol. 101, No. 5, pp. 347–352, November, 2006.     

The superplastic creep of uranium dioxide

In common with many other ceramics, uranium dioxide exhibits low creep ductility in both bending and compression loading modes. It is demonstrated that if the material and test conditions are suitably optimised, failure strains in compression of over 100% true strain are possible. The growth and reorientation of grain-boundary pores during such superplastic deformation are studied and a model to explain their behaviour is proposed.     

Pyrochemical reduction of uranium dioxide and plutonium dioxide by lithium metal

The lithium reduction process has been developed to apply a pyrochemical recycle process for oxide fuels. This process uses lithium metal as a reductant to convert oxides of actinide elements to metal. Lithium oxide generated in the reduction would be dissolved in a molten lithium chloride bath to enhance reduction. In this work, the solubility of Li₂O in LiCl was measured to be 8.8 wt% at 650 °C. Uranium dioxide was reduced by Li with no intermediate products and formed porous metal. Plutonium dioxide including 3% of americium dioxide was also reduced and formed molten metal. Reduction of PuO₂ to metal also occurred even when the concentration of lithium oxide was just under saturation. This result indicates that the reduction proceeds more easily than the prediction based on the Gibbs free energy of formation. Americium dioxide was also reduced at 1.8 wt% lithium oxide, but was hardly reduced at 8.8 wt%.     

Channeling studies of epitaxial uranium dioxide films

A quasi-ternary phase diagram was constructed for the UO₂-ThO₂-MgO-oxygen system at 1400°C in air on the basis of analyses using X-ray diffraction, electron probe microanalysis and metallography. Composition ranges were for the occurrence of single-phase or two-phases of fluorite-type cubic solid solutions.     

The evolution of gas from uranium dioxide

Conclusions We have examined the basic laws governing the character of gas evolution and the behavior of uranium dioxide during the operation of a fuel element prior to thorough burn-up. The behavior of the uranium dioxide and the evolution of gases depend directly on the distributions of energy evolution and temperature with respect to radius in the fuel element, and on the process of structure formation and burn-up in the different zones of the fuel. The evolution of gases from uranium dioxide during irradiation can be estimated on the assumption that all the gaseous fission products evolve from the columnar crystal zone and that evolution of gases from the equiaxial grain zone takes place by a mechanism of thermally activated diffusion.Sorption of gaseous fission products at the surface of the fuel can lead to errors in determining the quantity of gaseous fission products evolved from the uranium dioxide during postreactor determination by the fuel-can puncture method.Translated from Atomnaya Énergiya, Vol. 40, No. 5, pp. 390–395, May, 1976.