Kinetics of the UO2-C-N2 Reaction at 1700°C

The mechanism of the reaction of UO₂ with carbon in the presence of N₂ at 1700°C and the rate of formation of the carbonitride product were determined. Uranium carbonitride forms at specific O₂ and N₂ chemical potentials by reactions such as (1) UO₂( s ) + 0.67HCN( g )→UO_1.33N_0.45( s ) + 0.67CO( g ) + 0.11N₂( g ) + 0.335H₂( g ) and (2) UO_1.33N_0.45( s ) + 1.58HCN( g )→UO_0.25N_0.75( s ) + 1.33CO( g ) + 0.79H₂( g ) + 0.64N₂( g ). At P _H2=2×10^-5 atm, HCN formed, permitting a gas-phase transport of reactions not observed in the UO₂-C reaction. Reaction (1) is completed in 0.01 to 0.1 of the time for complete conversion to carbonitride; reaction (2), which proceeds as soon as oxynitride is available, is controlled by solid-state diffusion across the carbonitride layers after they become continuous on the entire specimen. The reaction rates and product compositions depend on the P _N2 and P_CO in the system.     

Study of Nonstoichiometry of 〈U1–zGdzO2±x〉

The extensive nonstoichiometry in the 〈U_{1– z} Gd _z O_{2± x} 〉 † phase was investigated experimentally and the data are represented by a chemical thermodynamic method. The experimental ranges of temperature, oxygen potential, and z were 1273 to 1773 K, 0 to −600 kJ/mol, and 0.1 to 0.8, respectively. For hypostoichiometry, ideal-solution thermodynamics for the equilibrium 3Gd_4/3O₂+ 4UO₂+ (O₂) = 6U_2/3Gd_2/3O_8/3 were used to represent the experimental data, while for hyperstoichiometry a nonideal solution was used for the equilibrium 4UO₂+ (O₂) = 2U₂O₅. The wide ranges in x and z led to an improvement of the previous analysis of literature data and led to partial molal Gibbs free energy values that are useful for any thermodynamic calculation involving the phase.     

Electrical Conductivity and Thermoelectric Power of Uranium Dioxide

The electrical conductivity and thermoelectric power of an uranium dioxide single crystal were measured between 908 and 1697 K and for P _O2 included between 10⁻²⁴ atm and the boundary UO_{2+ x} /U₄O₉. For T <1273 K and near stoichiometry, the electrical conductivity versus P _O2 shows a plateau characteristic of an extrinsic regime. For T ≥1273 K, the conductivity exhibits a minimum. The Seebeck coefficient ̄ _UO2 also exhibits a behavior change when the temperature is increased. For T <1200 K, ̄ _UO2 remains positive whatever the values of P _O2. For T>1200 K, ̄ _UO2 changes from negative to positive values when P _O2 increases. This set of results shows that the stoichiometric oxide presents a p extrinsic to intrinsic transition near 1273 K. At T ≤1273 K, the extrinsic conductivity values versus temperature confirm that the mobility of the holes occurs by a small polaron process, with an activation energy of 0.17 eV. At T >1273 K, the electrical conductivity minima are characteristic of a p → n transition. A band gap energy of 2.0 eV has been calculated from the temperature dependence of these conductivity minima values. Furthermore, this set of results has allowed us to determine the relative oxygen partial molar free energy of stoichiometric UO₂.     

Grain Growth in Sintered Uranium Dioxide: I, Equiaxed Grain Growth

Grain growth was investigated in a UO₂ sinter of 94%) theoretical density over the temperature range 1555° to 2440°C. The results were in close, but not exact, agreement with a theoretical expression describing grain growth with a poly-crystalline matrix. For the material studied the mean grain diameter D (μm) after annealing for t hours at a temperature T (°K) was given by the equation

where D₀ and K₀ are, respectively, the initial grain size and a proportionality constant. Uranium metal was found in all specimens annealed above 2000°C. This was taken as evidence that the UO₂ lattice can be oxygen-deficient at high temperatures.     

Thermodynamic Measurements and Modeling of <UC1-xOx>

Measurements were made of (CO) pressure over 1-x,O _x> at 1823, 1773, 1673, 1573, and 1473 K for various values of x , in the presence of 1.86). The data were used to model 1-x,O _x> as a solid solution of and , where Δ G^xs> is represented by an expansion in two terms, the first of which is temperature-dependent. The model allows for the accurate prediction of chemical activities and phase equilibria in regions also containing 1.86>, 2-y>, 1.86> - 2>, and 2-y> - (U).     

Thermal Conductivity of Nearly Stoichiometric Single-Crystal and Polycrystalline UO2

The thermal conductivities, λ, of single-crystal and polycrystalline UO₂ were measured from 80° to 420°K. The results indicate no observable difference in λ between single-crystal and polycrystalline UO₂, and both materials have broad peaks in λ at ∼220°K. The results were used with literature values to determine the effect of closed porosity on λ. The thermal conductivity of theoretically dense UO₂ is described phenomenologically from 80° to 1400°K, where conduction is dominated by the phonon component. The phonon conduction is analyzed by comparison with ThO₂. This analysis indicates that the high-temperature λ is limited by 3-phonon Umklapp scattering processes. Scattering by the disordered spins associated with the paramagnetic U ions contributes a large temperature-independent phonon scattering term. This mechanism has a mean free path of about 51 Å, which implies that grain boundaries and impurities have a relatively insignificant effect on the phonon conduction far above the antiferromagnetic-para-magnetic transition at ∼ 30°K. This implication agrees with the experimental results.     

Hot-Pressing of Si3N4 with Y2O3 and Li2O as Additives

The rates of densification and phase transformation undergone by α-Si₃N₄ during hot-pressing in the presence of Y₂O₃, Y₂O₃−2SiO₂, and Li₂0−2Si0₂ as additives were studied. Although these systems behave less simply than MgO-doped Si₃N₄, the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si₃N₄ phase.     

Volatility of UO2±x and Phase Relations in the System Uranium—Oxygen

The volatility of UO_2±x and the phase relations in the system uranium-oxygen were studied using thermogravimetric techniques. Chemical reactions describing the loss of uranium from UO_2±x at temperatures between 1100° and 2200°C in oxygen pressures between approximately 10² and 10⁻⁶ torr are proposed. Results were obtained requiring the consideration of UO₄(g) as the uranium-bearing vapor species above UO_2±x. Evidence supporting the existence of UO₄(g) included the volatilization of material with an oxygen-to-uranium ratio of 4 during the decomposition of UO_2+x(0.Z > × > 0) to near-stoichiometric UO₂ in vacuum above 1500°C and the dependence of the evaporation rate of the uranium dioxide on the oxygen pressure between 1200° and 1500°C. The equilibrium oxygen pressures over compositions between UO_2.02 and UO_2.63 in the UO_2+x and U₃O_8-y regions and over the boundary between these phases were measured between 1000° and 1600°C. The equilibrium oxygen-to-uranium ratio of UO_2±x was less than 2 above 1700°C in vacuum.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

Melting Behavior in the System UO2–PuO2

The melting points of UO₂ and PuO₂ in a helium atmosphere were found to be 2730°× 30° C and 2280°× 30° C, respectively. With the exception of a melting maximum at the composition 90 UO₂–10 wt% PuO₂, the liquidus exhibits good continuity and agrees well with that calculated from thermodynamic data. X-ray diffraction data on melted PuO₂ and UO₂-PuO₂ solid solutions indicate that oxygen is evolved during melting but that no reduction to a second-phase plutonium suboxide occurs. The oxygen-plutonium atomic ratio of melted PuO₂ is 1.62, so that the 2280° C temperature reported here is the result of a dissociation reaction and is considered to be a pseudo melting point. Lattice parameters of melted UO₂–PuO₂ samples vary linearly with composition but are approximately 0.2% greater than anticipated because of an oxygen deficiency.     

Fracture Toughness and Spalling Behavior of High-Al2O3 Refractories

The fracture energies and spalling resistance of high-Al₂O₃ refractories were studied. The fracture energies, γ _WOF and γ _NBT , were measured by the work-of-fracture and the notched-beam-test methods, respectively. Spalling resistance, as measured by the relative strength retained in a water quench, correlated well with the thermal-stress resistance parameter applicable to stable crack propagation under conditions of thermal shock, (γ _WOF /α² E ₀). Many of the refractories exhibited high ratios of γ_WOF to γ_NBT; such high ratios were shown analytically to maximize the parameter ( R ¹¹¹¹= E _0γWOF/S₁²) which describes the resistance to catastrophic spalling. The increase of crack length with increasing quenching temperature difference (Δ T ) was somewhat less than that predicted theoretically; the discrepancy was attributed to an increase of crack density with Δ T . In general, the results show that fracture energy is important in establishing the spalling resistance of high-Al₂O₃ refractories.     

Swelling of Hot-Pressed A12O3

Fully dense aluminas, prepared by hot-pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po₂= 10^-7 Pa). The reaction followed the relation p - po = -K log t, where p_o and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain-boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO₂ and SO₂ gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot-pressing. The consequences of internal gas-forming reactions to other processes such as high-temperature creep and sintering are also discussed.     

Simulation of Grain Growth and Pore Migration in a Thermal Gradient

The Potts Monte Carlo simulation was used to simulate microstructural evolution in uranium dioxide fuel rods. During service, grain growth, pore migration, and thermal segregation of the pores and UO₂ occur in the rods in a thermal gradient. In this investigation, we developed a model which simulates simultaneous grain growth, pore migration, and thermal segregation of the pores and UO₂ in a temperature gradient. Grain growth in a thermal gradient was simulated using the Monte Carlo Potts model technique developed by Anderson, Srolovitz, and co-workers. Pore migration was simulated using conserved dynamics with minimum-energy exchanges at a finite temperature. A temperature gradient was introduced into the model via interfacial mobility gradient. Finally, thermal segregation of the pores and UO₂ was achieved by introducing a heat of migration term, Δ E _t, which biased the motion of porosity to the high-temperature region. The development of this model is described and the incorporation of the proper physics of pore migration and thermal segregation is discussed.     

Compressive Deformation of Polycrystalline UO2

The deformation of polycrystalline stoichiometric UO₂ in compression exhibits a low-temperature (<1200°C) behavior that is distinct from its high-temperature behavior. The data for both temperature regions fit either an Arrhenius equation, =ν exp [-Δ H _(τ)/ RT ], or the relation = A τⁿ/ T exp [−Δ H ₀/ RT ]. At low temperatures, the activation energy and volume, the shape of the yield-stress-temperature curve, and the grain size-strength relation suggest a Peierls mechanism as rate-controlling in the deformation process. At high temperatures (≳ 1300°C), a different dislocation mechanism becomes rate-controlling for coarse-grained material, whereas very fine-grained (1 μm) material exhibits Nabarro-Herring deformation.     

Effect of Oxygen Potential on the Thermal Creep of Niobia Doped UO2

Compressive and tensile creep were measured on UO₂ doped with 0.4 wt % niobia at controlled oxygen potentials. Kinetic results were almost identical, irrespective of the nature of the applied stress, but differences in after-test microstructures were observed. Formation of "plastic" UO₂ with increases in strain rate of more than two orders of magnitude were recorded when oxygen potential changed from −560 to −410 kJ/mol; this was associated, primarily, with the formation of the Nb⁴⁺ ion. Over the same range of oxygen potential, creep activation energy decreased linearly from −425 to −225 kJ/ mol. In contrast, strain rate and activation energy for undoped UO₂ remain almost constant under these conditions.     

Elastic and Anelastic Response of Polycrystalline UO2-PuO2

A sonic resonance technique was used to investigate the room-temperature elastic and anelastic properties of physically mixed U_0.8PU_0.2O₂ as a function of density, stoichiometry, and cation homogeneity. The effect of porosity on the elastic moduli was linear and is described by E =2102.7 (1–2.03 P )± 13.5 Kbars for the Young's modulus, G =823.5(1–2.05 P )± 9.1 kbars for the shear modulus, and B = 1584.8(1–1.89 P )± 59.1 kbars for the bulk modulus, where P is the volume fraction porosity. Poisson's ratio was 0.28 and was not a function of porosity. The Debye temperature of U_0.8Pu_0.2O₂ computed from the Young's and shear moduli for theoretically dense specimens was 379°K. Variation of the O/M ratio from 1.968 to 2.006 produced no significant change in either the damping capacity or the elastic moduli of single-phase 80%UO₂-20% PuO₂ solid solutions. An approximate 24% decrease of the room-temperature Young's and shear moduli and an approximate increase by a factor of 14 in the internal friction were observed with gross modifications of plutonium cation homogeneity. Preliminary results suggest that internal friction measurements might be used to assay the homogeneity of UO₂-PuO₂ solid solutions.     

Reaction Pathways in the Synthesis of Photorefractive gamma-Bi12MO20 (M = Si, Ge, or Ti)

Reaction pathways in the synthesis of three photorefractive silicates—γ-Bi₁₂ SiO₂₀ (BSO), γ-Bi₁₂ GeO₂₀ (BGO), and gamma-Bi₁₂ TiO₂₀ (BTO)—were systematically investigated. The main results were as follows: (i) all the reactions of the form 6Bi₂O₃+ MO₂→> γ-Bi₁₂ MO₂₀ (SR1 for M = Si, SR2 for M = Ge, SR3 for M = Ti) in the solid state seemed to be diffusion-controlled processes and were affected by both temperature and time, where the reaction temperature increases in the order SR1 < SR2 < SR3; (ii) the metastable phases Bi₂ SiO₅ (tetragonal) in reaction SR1, Bi₂ GeO₅ (orthorhombic) in reaction SR2, Bi₄ Ti₃ O₁₂ (orthorhombic) in reaction SR3 may be formed and seemed to greatly accelerate the above-mentioned solid-state reaction processes; and (iii) for a continuous heating process, pure γ-Bi₁₂ SiO₂₀ and γ-Bi₁₂ GeO₂₀ could be produced before melting, whereas pure γ-Bi₁₂ TiO₂0 could not be produced, even if all the mixed phases had melted.     

Thermal Simulation of In-Reactor Microstructures in ThO2-UO2

A device was built to provide the capability for out-reactor simulation of the microstructures seen in irradiated fuel. Out-reactor simulation of nuclear heating makes possible the precise measurement of temperature gradients in the fuel and provides data for correlating microstructural alterations with temperature. The simulation apparatus was designed to produce controlled thermal gradients sufficient to produce both equiaxed and columnar grain growth in the specimens. Provision was made for calorimetric measurements from which thermal conductance could be determined. Micro-structural alterations in UO₂ and ThO₂-UO₂ materials were achieved, and thermal conductivity data were recorded. Microstructural alterations in the ThO₂-10 wt% UO₂ specimens were similar to those that other investigators have observed in UO₂ at lower temperatures.     

Grain Growth in Two-Phase Ceramics: Al2O3 Inclusions in ZrO

The effect of Al₂O₃ inclusions with a greater average size (0.6 μm) than the average particle size of the major phase powder (<0.1 μm) on grain gowth was examined by sintering ZrO₂/Al₂O₃ composites (0,3,5,10, and 20 vol%) at 1400°C and then heat-treating at temperatures up to 1700°C. Normal grain growth was observed for all conditions. The inclusions appeared to have no effect on grain growth until the ZrO₂ grain size was ∼1.5 times the average inclusion size. Grain growth inhibition increased with volume fraction of the Al₂O₃ inclusion phase. At temperatures 1600°C, the inclusions were relatively immobile and most were located within the ZrO₂ grains for volume fractions <0.20; at higher temperatures, the inclusions could move with the grain boundary to coalesce. Grain growth was less inhilited when the inclusions could move with the boundaries, resulting in a larger increase in grain size than observed at lower temperatures. Analogies between mobile voids, entrapped within grain at lower temperature due to abnormal grain growth during the last state of sintering, and the observations concerning the mobile inclusions are made suggesting that grain-boundary movement can "sweep" voids to grain boundaries and eventually of four-grain junctions, where they are more likely to disappear by mass transport.     

Kinetics and Mechanism of Hot Corrosion of γ-Y2Si2O7 in Thin-Film Na2SO4 Molten Salt

γ-Y₂Si₂O₇ is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10⁻⁶/K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y₂Si₂O₇ in thin-film molten Na₂SO₄ at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y₂Si₂O₇ exhibited good resistance against Na₂SO₄ molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na₂SO₄ and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y₂Si₂O₇ in Na₂SO₄ molten salt with flowing air was evaluated to be 255 kJ/mol.