High-temperature specific heat of UO2 ThO2, and A12O3

The high-temperature specific heat of solid UO₂, ThO₂, and Al₂O₃ can be represented by an equation of the form $\text{C}{}_{\mathrm{p(s)}}\text{= 3}{}_{\mathrm{n}}\text{RF(?}{}_{\mathrm{D}}\text{/T) + dT}{}^3$, (1) where ?_D is the Debye temperature, F(?_D/T) is the Debye function, $\text{d}$ represents contributions of the anharmonic vibrations within the lattice, and $\text{n}$ denotes the number of atoms per molecule. In the liquid the corresponding equation is $\text{C}{}_{\mathrm{p(1)}}\text{= 3}{}_{\mathrm{n}}\text{RF(?}{}_{\mathrm{D}}\text{/T) +}\text{h}\text{T}{}^2$, (2) where h is the anharmonic term. It is shown that for Al₂O₃ and UO₂, where experimental data for the liquid phase are also available, $\text{d}\text{h}$ has the same value, Indicating that both materials behave identically. If we compare the thermodynamic relationship $\text{C}{}_{\mathrm{p}}\text{? C}{}_{\mathrm{v}}\text{= Vα}{}^2\text{KT}$, (3) where V is the volume, $\text{α}$ the volume expansion coefficient, and $\text{K}$ the bulk modulus, with equation (1), It follows that d must be equal to $\text{Vα}{}^2\text{K}\text{T}{}^2$; the value of $\text{Vα}{}^2\text{K}\text{T}{}^2$ is calculated in the temperature region where d was obtained; within experimental error they are equal.     

Transport properties of U0.7Ce0.3O2 − x

We have determined a number of transport properties of $\text{U}{}_{0.7}\text{Ce}{}_{0.3}\text{O}{}_{\mathrm{2-x}}$ at 1273 K for various deviations from stoichiometry and compared them with available results on $\text{(UPu)O}{}_{\mathrm{2\; ?\; x}}$. They are: the electrical conductivity, Seebeck coefficient, effective charge number and chemical diffusion coefficient.A very characteristic behaviour is observed for the electronic properties of $\text{(UCe)O}{}_{\mathrm{2\; ?\; x}}$. A p-type conduction for all the studied deviations from stoichiometry (up to $\text{x = 3 × 10}{}^{\mathrm{?2}}$) is interpreted in terms of a high electronic disorder in the stoichiometric compound. Electronic disorder at stoichiometry is probably less important in $\text{(UPu)O}{}_{\mathrm{2\; ?\; x}}$, which presents a sharp p-n transition at $\text{x = 5 × 10}{}^{\mathrm{?3}}$.Ionic transport properties obtained on $\text{(UCe)O}{}_{\mathrm{2\; ?\; x}}$ indicate an approximate proportionality between the ionic conductivity resulting from oxygen ions transport and the deviation from stoichiometry. Results available on $\text{(UPu)O}{}_{\mathrm{2\; ?\; x}}$ do not appear to be compatible with ours and some arguments are presented which cast doubt on their validity.     

Plastic flow of ordered Zr3Al

Room-temperature tensile experiments established that ordered Zr₃Al of the Ll₂ type obeys the relationship $\text{σ}{}_{\mathrm{?}}\text{= σ}{}_{\mathrm{0,?}}\text{+ kd}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ where σ_? is the flow stress at a given strain ?, σ_0,? is a strain-dependent frictional stress, $\text{d}$ is the average grain diameter, and $\text{k}$ is a strain-independent constant of magnitude $\text{(2.8 ± 0.3) kg/mm}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$. Zr₃Al flows by fine, planar slip, and is susceptible to intergranular cracking.     

Study of homogenisation and cation interdiffusion in mixed UO2-PuO2 compacts by X-ray diffraction

Homogenisation in mixed UO₂-PuO₂ compacts has been studied by X-ray diffraction. It is observed that the homogenisation proceeds, mainly, by the assimilation of UO₂ into PuO₂. This near one-way flow of material, from UO₂ to PuO₂, is shown to be due to high activity (large BET surface area) of the PuO₂ powder as compared with that of the UO₂ powder.An X-ray line profile analysis method of determining various mixed composition fractions in sintered mixed compacts has been used to evaluate homogeneity in terms of the fraction of UO₂ that has gone into PuO₂. A concentric core-shell diffusion model, in which UO₂ forms a solute core and PuO₂ forms a solvent shell, was used to determine cation interdiffusion coefficients from the homogenisation data. The temperature dependence of cation diffusivity in the range 1573–1873 K is obtained as $\text{D = 2.55 × 10}{}^{\mathrm{?11}}\text{exp(?2.22 × 10}{}^5\text{/8.31 T)}$ m²/s. The low value (222 kJ/mole) of activation energy for cation interdiffusion is attributed to the hypostoichiometry of the mixed compacts studied.The diffusivity values at 1573 and 1673 K separately give an activation energy of 126 kJ/mole, which suggests grain-boundary diffusion as the primary mechanism of homogenisation in this temperature range.     

Density,thermal expansion of stainless steel and interfacial properties of UO2-stainless steel above 1690 K

The sessile drop method was used for the determination of the density in liquid state. The results for stainless steel 1.4970 using uranium dioxide as substrate material in the temperature range 1690 K (liquidus temperature $\text{T}{}_1$) $\text{< T < 2120 K}$ are $\text{ρ = 6.82 × 10}{}^3\text{? 10.25 × 10}{}^{\mathrm{?1}}\text{(T ? T}{}_1\text{) kg/m}{}^3$, and $\text{α = 1.50 × 10}{}^{\mathrm{?4}}\text{K}{}^{\mathrm{?1}}$. Below 1690 K the linear thermal expansion is given by $\text{Δl/l}{}_0\text{= 0.002}{}_{04}\text{+ 7.11}{}_0\text{× 10}{}^{\mathrm{?6}}\text{T + 7.734 × 10}{}^{\mathrm{?9}}\text{T}{}^2$. Using the same method but not correlated with the density measurements the following interfacial properties of the system UO₂-stainless steel have been determined: surface energy of liquid steel $\text{γLv = 1.19 ? 0.57 × 10}{}^{\mathrm{?3}}\text{(T ? T}{}_1\text{) J/m}{}^2$ and interfacial energy of liquid steel against UO₂$\text{γSL = 1.57 ? 2.01 × 10}{}^{\mathrm{?3}}\text{(T ? T}{}_1\text{) J/m}{}^2$, the results yield a contact angle $\text{θ = 0°}$ at $\text{T= 2515 K}$. Using literature data for the compressibility of liquid UO₂, an estimate of the surface energy of UO₂ in liquid state was performed. The estimated value at the melting point is: $\text{γLV = 0.522 J/m}{}^2$. The mean value of the experimental data given by several authors is $\text{0.513 ± 0.085 J/m}{}^2$. The estimated temperature dependence of the surface energy of liquid UO₂ is given by $\text{dγLV/dT = ?0.19 × 10}{}^{\mathrm{?3}}\text{J/m}{}^2$.     

Heterodiffusion de l'or,de l'argent et du cuivre dans les phases cubiques δ et ε du plutonium

The solute diffusion at infinite dilution of ¹⁹⁸Au and ^110mAg in cubic phases of Pu has been studied using the serial sectroning method. The solute diffusion coefficients in the b.c.c. ? phase can be expressed by: $\text{D}{}^{\mathrm{Au}}{}_{\mathrm{?Pu}}\text{= 5,7 × 10}{}^{\mathrm{?5}}$ exp(?10300/RT) cm²/s and $\text{D}{}^{\mathrm{Ag}}{}_{\mathrm{?Pu}}\text{= 4,9 × 10}{}^{\mathrm{?5}}\text{exp(?9600/RT) cm}{}^2\text{/s}$. The solute diffusion mechanism is interstitial of the dissociative type in both cases. These experiments confirm the activated interstitial model which has been proposed for self diffusion of ?Pu. Indeed the solute diffusion coefficients of Au and Ag are near of the self diffusion coefficients of Pu. The mechanisms are therefore interstitial in both cases. In the f.c.c. δ phase of Pu where self diffusion takes place by a vacancy mechanism, the solute diffusion coefficients of Au and Ag are near of the self diffusion coefficients of δ Pu. Solute diffusion takes place also by a vacancy mechanism. On the other hand, the extrapolation at infinite dilution of experiments of solute diffusion of Cu in ?Pu (Matano-Wagner coupling) gives the following results: $\text{D}{}^{\mathrm{Cu}}{}_{\mathrm{?Pu}}\text{= 1 × 10}{}^{\mathrm{?3}}\text{exp(?12300/RT)}$ cm²/s. The solute diffusion mechanism is interstitial of the dissociative type. In the ? phase the smaller the atomic radius the faster the migration: r_Co < r_Cu < r_?Pu < r_Ag = r_Au, and D^Co_?Pu > D^Cu_?Pu >D^Pu_?PU > D^Ag_?Pu ≈ D^Au_?Pu.     

Phase relation and defect structures of nonstoichiometric U4O9±y and UO2+x at high temperatures

Phase relations in the composition range from $\text{UO}{}_{\mathrm{2+x}}$ to $\text{U}{}_3\text{O}{}_{\mathrm{8?z}}$ were studied by electrical-conductivity measurements and X-ray diffraction in the ranges $\text{1025°C ? T ? 1140°C}$ and . The plot of log σ versus log showed straight lines with distinct slopes, which corresponded to four regions ($\text{UO}{}_{\mathrm{2+x}}$, $\text{U}{}_4\text{O}{}_{\mathrm{9?y}}$, $\text{U}{}_4\text{O}{}_{\mathrm{9+y}}$ and $\text{U}{}_3\text{O}{}_{\mathrm{8?z}}$). The existence of the hyperstoichiometric $\text{U}{}_4\text{O}{}_{\mathrm{9+y}}$ phase was suggested in the temperature range from 1025 to 1126°C. The peritectoid temperature ($\text{U}{}_4\text{O}{}_{\mathrm{9\pm y}}\text{= UO}{}_{\mathrm{2+x}}\text{+ U}{}_3\text{O}{}_{\mathrm{8?z}}$) was estimated to be present between 1126 and 1131°C. The partial free enthalpies and entropies for the two-phase equilibrium ($\text{U}{}_4\text{O}{}_{\mathrm{9+y}}\text{+ U}{}_3\text{O}{}_{\mathrm{8?z}}$, and $\text{U}{}_4\text{O}{}_{\mathrm{9?y}}\text{+ UO}{}_{\mathrm{2+x}}$) were calculated and compared with previous results. From the dependence of the electrical conductivity on the oxygen partial pressure the nonstoichiometric defect structures of $\text{UO}{}_{\mathrm{2+x}}$ and $\text{U}{}_4\text{O}{}_{\mathrm{9\pm y}}$ were interpreted as consisting of doubly charged oxygen interstitials (O_i'') and doubly charged oxygen vacancies (V_O''). At room temperature, the homogeneity range of the U₄O₉ phase was investigated with a Debye-Scherrer camera.     

Transition probabilities for the D and B′ vibrational levels to the X vibrational levels and continuum of H2

The electronic dipole-moment functions are used in calculating the transition probabilities for the individual bands of the $\text{D 3pπ}{}^1\text{Π}{}_{\mathrm{<mtext>u</mtext>}}\text{?X}{}^1\text{Σ}{}_{\mathrm{<mtext>g</mtext>}}{}^{\mathrm{+}}$ and $\text{B′ 3pσ}{}^1\text{Σ}{}_{\mathrm{<mtext>u</mtext>}}{}^{\mathrm{+}}\text{?X}{}^1\text{Σ}{}_{\mathrm{<mtext>g</mtext>}}{}^{\mathrm{+}}$ systems of H₂, and for the D and B′ vibrational levels to the $\text{X}{}^1\text{Σ}{}_{\mathrm{<mtext>g</mtext>}}{}^{\mathrm{+}}$ continuum.     

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.     

Consideration of the simulation of in-pile radial temperature profiles in mixed-oxide fuel pins

Temperature profiles similar to those existing in fuel rods under irradiation have been simulated by passing electrical current through cylindrical pellets. The comparison between calculated and measured temperatures in pellets heated in a thermal gradient shows: (1) The values of thermal conductivity obtained by different authors in isothermal experiments and extrapolated to temperatures up to 2700°C are not in agreement. Therefore, the calculation of the temperature of the fuel leads to errors which vary between 1 and 16% depending on the data for λ used. For central temperatures above 1900°C the values of Schmidt better suit the calculations, especially if the oxygen content of the fuel is smaller than 2.00. (2) The published data of electrical conductivity are in open disagreement. The values of activation energy are generally higher than those deduced from the present investigation. It has been assumed that the activation energy $\text{E}$ in the equation $\text{σ = A exp(?E(T)/kT)}$ varies with temperature as $\text{E(T)= E}{}_0\text{(1?1.94 × 10}{}^{\mathrm{?4}}\text{T)}$ when $\text{E}{}_0\text{= 0.58}$ eV if O/M = 2.00, and $\text{E(T) = E}{}_0\text{(1?2.21× 10}{}^{\mathrm{?4}}\text{T)}$ when $\text{E}{}_0\text{=0.91}$ eV if O/M = 1.94.     

Phase relations and crystal chemistry in the ternary PrO1.5-UO2-O2 system

Phase relations and defect structures were studied in a PrO_1.5-UO₂-O₂ ternary system in the temperature range from 1200 to 1500°C. Phases and compositions of the products heated in either air, helium or vacuum were examined by $\text{X-}\text{ray}$ diffraction and chemical analysis, respectively. The regions of existence of solid solution having fluorite type structure, of rhombohedral phase and of A-type rare earth sesquioxide phase were determined. It was found that these phase relations could be classified by the mean valency of uranium and the type of oxygen defects. The change of cubic lattice parameter of the solid solution, $\text{Pr}{}_{\mathrm{y}}\text{U}{}_{\mathrm{1?y}}\text{O}{}_{\mathrm{2+x}}$, in single phase region ($\text{0 ≤ y ≤ 0.7}$) was expressed as linear equations of $\text{x}$ and $\text{y}$: $\text{a = 5.4704?0.127x ?0.007y, for x ≥ 0}$ and $\text{a = 5.4704?0.397x?0.007y, for x < 0}$. The change of lattice parameters was discussed in some detail.     

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.     

Fission-enhanced self-diffusion of uranium in UO2 and UC

In-pile self-diffusion measurements in stoichiometric UO₂ sinters and single crystals and in arc-cast stoichiometric UC have been performed using the thin layer condition and ²³³U as tracer. The nominal irradiation temperature was 900°C. The resulting diffusion coefficients $\text{D1}$ of $\text{1.5 × 10}{}^{\mathrm{?16}}\text{cm}{}^2\text{· sec}{}^{\mathrm{?1}}$ for UO₂ and $\text{2.2 × 10}{}^{\mathrm{?17}}\text{cm}{}^2\text{· sec}{}^{\mathrm{?1}}$ for UC for a fission rate $\text{S}$ of 1 × 10¹³f/cm³ · sec represent radiation enhanced diffusion and are higher by factors of 10³ to 10⁴ than (extrapolated) coefficients of thermal diffusion. The data are of immediate relevance for understanding and predicting such important quantities as in-pile sintering and densification, diffusion controlled creep and fission gas behavior in the outer zones of the fuel. They are at the upper limit of expected values.     

Thermodynamic behavior of (U,Pu) mixed-oxide fuels

Transpiration experiments were performed at temperatures between 1000 and 1700°C to study the thermodynamics and defect structure of hypostoichiometric UO₂-20 wt % PuO₂ solid-solution systems. The oxygen partial pressures were established by using flowing $\text{H}{}_2\text{/H}{}_2\text{O}$ mixtures. After equilibration, the quenched products were analyzed by chemical, X-ray, neutron-diffraction and metallographic techniques. The versus temperature curves for the mixed oxide with different oxygen-to-metal ratios were plotted.Based on X-ray, neutron diffraction and metallographic data, it was concluded that the 20 wt % PuO₂ mixed oxide exists as a single phase under normal conditions, even at an oxygen-to-metal ratio as low as 1.92.The data from density measurements and neutron-diffraction analysis indicated that the predominant defects in the hypostoichiometric UO₂-20 wt % PuO₂ are anion vacancies.     

Chemical thermodynamic representations of 〈PuO2−x〉 and 〈U1−zPuzOw〉

All available oxygen potential-temperature-composition data for the calcium fluorite-structure 〈PuO_2?x〉^★★ phase were retrieved from the literature and utilized in the development of a binary solid solution representation of the phase. The data and phase relations are found to be best described by a solution of [Pu_{$\text{4}\text{3}$}O₂] and [PuO₂] with a temperature dependent interaction energy. The fluorite-structure 〈U_1?zPu_zO_w〉 is assumed to be represented by a combination of the binaries 〈PuO_2?x〉 and 〈UO₂ ± x〉, and thus treated as a solution of [$\text{Pu}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{O}{}_2$], [PuO₂], [UO₂], and either [U₂O_4.5] or [U₃O₇]. The resulting equations well reproduce the large amount of oxygen potential-temperature-composition data for the mixed oxide system, all of which were also retrieved from the literature. These models are the first that appear to display the appropriate oxygen potential-temperature-composition and phase relation behavior over the entire range of existence for the phases.     

An electrical conductivity and X-ray study of a high-temperature transition in U4O9

The high temperature transition in U₄O₉ has been studied by electrical conductivity measurements and X-ray diffraction. From the electrical conductivity measurements, a similar variation of log σ$\text{T}$ with reciprocal temperature to that in the transition range near room temperature is observed in the temperature range from about 300 to 800°C. Like the low temperature transition, a small lattice contraction is also observed in that temperature range by means of X-ray diffractometry, and the transition temperature increases from 530 to 620°C with increasing O/U ratio. After the transition the intensity of $\text{4a}{}_0$ superlattice reflections increases, but that of $\text{8a}{}_0$ superlattice reflections disappears. The mechanism of this high temperature transition is considered to be a second-order transition of the order—disorder type based on the configurational change of U⁴⁺ and U⁵⁺ with the shift of some portions of the lattice oxygen atoms from the lattice sites to the interstitial positions. The phase diagram of U₄O₉ is presented on the basis of the electrical conductivity and X-ray data.     

Existence of hypostoichiometric chromium sesquioxide at low oxygen partial pressures

The nonstoichiometric composition of $\text{Cr}{}_2\text{O}{}_{\mathrm{3\pm x}}$ was measured by means of thermogravimetry in the range of $\text{1173 ≦ T/K ≦ 1318}$ and . The compositional deviation from stoichiometry, $\text{x}$, in the hyperstoichiometric Cr₂O_3+x phase was observed to be smaller than 2 × 10^?4, irrespective of temperatures, provided that the hyperstoichiometric Cr₂O_3+x exists. The existence of the hypostoichiometric Cr₂O_3?x phase was first established in this study in the region of low oxygen partial pressure below 10^?5 Pa. From the oxygen partial pressure dependence of $\text{x}$ in Cr₂O_3?x, the defect structure was discussed with the neutral chromium interstitials in the composition near stoichiometry and with the triply charged ones far from stoichiometry. The partial molar enthalpy and entropy of oxygen of Cr₂O_3?x showed the complex compositional dependences, suggesting the change of the type of the predominant defect.     

Steady-state creep of zircaloy-4 fuel cladding from 940 to 1873 K

Steady-state creep rates of as-received zircaloy-4 fuel cladding have been determined from 940 to 1073 K in the α-Zr range, from 1140 to 1190 K in the mixed (α + β) phase region and from 1273 to 1873 K in the β-Zr phase region. Strain rates of between 10^?6 and 10^?2/s were determined under constant uniaxial load conditions. Assuming that creep rates can be described by a power law-Arrhenius equation, the creep rate for α-phase zircaloy-4 is given by: $\text{ge}{}_{\mathrm{ss}}\text{?}\text{= 2000 σ}{}^{5.32}\text{exp}\text{(?284 600/kT) s}{}^{\mathrm{?1}}$; for the β-phase zircaloy-4 by: $\text{ge}{}_{\mathrm{ss}}\text{?}\text{= 8.1 σ}{}^{3.79}\text{exp}\text{(?142 300/kT) s}{}^{\mathrm{?1}}$; and for the mixed (α + β) phase of zircaloy-4 (for creep rates ?3 × 10^?3 s^?1) by: $\text{ge}{}_{\mathrm{ss}}\text{?}\text{= 6.8 × 10}{}^{\mathrm{?3}}\text{σ}{}^{1.8}\text{exp}\text{(?56 600/kT) s}{}^{\mathrm{?1}}$. For the both the α and β phases, the activation energies for creep are in agreement with those of self-diffusion. For the mixed (α + β) phase region, the low creep rate range is controlled by grain boundary sliding at the α/(α + β) phase boundary.     

Populations of np terms in deuterium atoms emergent from carbon foils bombarded with D+, D2+ and D3+ ions

We have measured relative beam-foil populations of 2p, 3p, and 4p terms in D⁰ as a function of the projectile energy $\text{(20 ?}\text{E}\text{M}\text{? 500}\text{keV}\text{amu}\text{)}$ for D⁺, D₂⁺, and D₃⁺ ions impinging on carbon foils of various thicknesses (? 2–20 $\text{μg}\text{cm}{}^2$).With D⁺ projectiles, the np populations reach their equilibrium values even in the thinnest foils used. We compare the dependence on energy of these populations to the equilibrium neutral fraction variation for hydrogen (deuterium) beams emerging from a carbon foil and deduce some information concerning beam-foil populations.When molecular projectiles pass through very thin foils, well known molecular effects appear which depend on the dwell time, t, i.e., the time spent by the projectile in the foil. In this work we consider only the long-dwell-time region t > 2 × 10^?15s. We study the variation of R_α = I^molec/I^atom (I^molec and I^atom are the Ly-α intensities per incident deuteron (proton) observed with molecular and atomic projectiles of the same velocity, respectively) with the projectile energy per nucleon ($\text{E}\text{M}$) and the thickness (T) of the foil. For a foil of given thickness, R_α increases with $\text{E}\text{M>}$ and reaches a saturation value R_∞ which decreases when T increases. These results, in agreement with our previous measurements using hydrogen projectiles, indicate that t is not the only parameter relevant to molecular effects. Comparisons are reported between $\text{R>}{}_{\mathrm{\alpha }}\text{(E}\text{M>}$) values obtained (a) with H₂⁺ and D₂⁺ projectiles and (b) with D₂⁺ and D₃⁺ projectiles, using foils of various given thicknesses. Ratios $\text{R}{}_{\mathrm{\beta }}\text{(E}\text{M}$) and $\text{R}{}_{\mathrm{\gamma }}\text{(E}\text{M}$) are also measured using Ly-β and Ly-γ radiations and compared to $\text{R}{}_{\mathrm{\alpha }}\text{(E}\text{M}$) values. An interpretation for some of our results is proposed.     

Electrical conductivity measurement and thermogravimetric study of pure and niobium-doped uranium dioxide

The electrical conductivity and nonstoichiometric composition of UO_2+x and (U_1?yNb_y)O_2+x (y = 0.01, 0.05 and 0.10) were measured in the range $\text{1282 ≦ T ≦ 1373}$ K and Pa by tie four inserted wires method and thermogravimetry, respectively. The electrical conductivity of (U_1?yNb_y)O_2+x plotted against the oxygen partial pressure indicated a minimum corresponding to the transition between $\text{n}$- and $\text{p-}\text{type}$ cone uction. The band-gap energy of (U_1?yNb_y)O_2+x was calculated to be $\text{(248 ± 12)}$ kJmol.^?1, independent of niobium content, which is nearly the same as that of UO_2+x. From the oxygen partial pressure dependences of both the electrical conductivity and the deviation $\text{x}$ of UO_2+x and (U_1?yNb_y)O_2+x, the defect structures in these oxides were discussed with the complex defect model consisting of oxygen vacancies and two kinds of interstitial oxygens.