Oxygen potentials of UO2 + x and (Th1 − yUy)O2 + x

Oxygen potentials of UO_{2 + x} and (Th_{1 ? y}U_y)O_{2 + x} (y = 0.2 and 0.4) were measured by means of thermogravimetry in the range of 1282 ≦ T ≦ 1373 K and 10su?19 ≦ P(O₂) ≦ 10^?1 Pa. The oxygen potentials of (Th_{1 ? y}U_y)O_{2 + x} plotted against the mean uranium valence (V_u) were seen to be more negative than those of UO_{2 + x} in the region of V_u < 4.08 and more positive than those of UO_{2 + x} in the region of V_u > 4.08. The partial molar enthalpies and entropies of oxygen for UO_{2 + x} and (Th_0.6U_0.4)O_{2 + x} showed that each has a maximum against $\text{O}\text{U}\text{or}\text{O}\text{M}$ ratio at about 2.001, but no clear maximum was seen for (Th_0.8U_0.2)O_{2 + x}. From the oxygen partial pressure dependences of x in UO_{2 + x} and (Th_{1 ? y}U_y)O_{2 + x}, the defect structure was discussed with a complex defect model consisting of oxygen vacancies and two different types of interstitial oxygens.     

First-principles study of structural, elastic and electronic properties of thorium dicarbide (ThC2) polymorphs

The comparative study of the structural, elastic, cohesive and electronic properties of three polymorphs (α-monoclinic, β-tetragonal and γ-cubic) of thorium dicarbide ThC₂ is performed within the density-functional theory. The optimized atomic coordinates, lattice parameters, theoretical density (ρ), bulk moduli (B), compressibility (β), as well as electronic densities of states, electronic heat capacity (γ) and molar Pauli paramagnetic susceptibility (χ) for all ThC₂ polymorphs are obtained and analyzed in comparison with available experimental data. The peculiarities of inter-atomic bonding for thorium dicarbide are discussed. Besides, we have evaluated the formation energies (E_f) of ThC₂ polymorphs for different possible preparation routes (namely for the reactions with the participation of simple substances (metallic Th and graphite) or thorium monocarbide ThC and graphite). The results show that the synthesis of the ThC₂ polymorphs from simple substances is more favorable - in comparison with the reactions with participation of Th monocarbide.     

A model for the carbon activity in nqnstoichiometric thorium carbide

The Rees formulation of nonstoichiometry is adopted together with a vacancy/single carbon atom/C₂ group model to calculate the hitherto largely unknown carbon activity in ThC_x for 0.0 ? x ? 2.0. The form of the carbon activity isotherms from ThC_0.9 to ThC_1.95 closely resembles the experimentally determined isotherms in the U/C system. Based on the occupation of two kinds of sites, the present formalism also generates an asymmetric shape of the miscibility gap between ThC and ThC₂ as found in both the real Th/C and U/C systems.     

Formation of columnar and equiaxed grains by the reduction of U3O8 pellets to UO2+x

The reduction of U₃O₈ pellets to UO_2+x has been investigated at 1300 °C in H₂, Ar and CO₂ gas atmospheres by TGA, SEM, and X-ray diffraction. The selected U₃O₈ pellet was prepared by sintering a U₃O₈ powder compact. The TGA results show that the reduction rate is fastest in H₂ gas, and X-ray diffraction indicates that U₃O₈ reduces to UO_2+x without any intermediate phase. The reduced pellet, UO_2+x, has a special grain structure that consists of equiaxed grains at the surface, columnar grains in the middle, and equiaxed grains in the center. The equiaxed grains and columnar grains are much smaller in H₂ gas than in Ar or CO₂ gas. The reducing gases significantly influence the morphology of the grain structure. This difference can be explained in terms of a relation between oxygen potential and critical nucleus size during the reduction.     

Thermodynamic properties of ThxU1−xO2 (0 < x < 1) based on quantum-mechanical calculations and Monte-Carlo simulations

Th_xU_1−xO_2+y binary compositions occur in nature, uranothorianite, and as a mixed oxide nuclear fuel. As a nuclear fuel, important properties, such as the melting point, thermal conductivity, and the thermal expansion coefficient change as a function of composition. Additionally, for direct disposal of Th_xU_1−xO₂, the chemical durability changes as a function of composition, with the dissolution rate decreasing with increasing thoria content. UO₂ and ThO₂ have the same isometric structure, and the ionic radii of 8-fold coordinated U⁴⁺ and Th⁴⁺ are similar (1.14 nm and 1.19 nm, respectively). Thus, this binary is expected to form a complete solid solution. However, atomic-scale measurements or simulations of cation ordering and the associated thermodynamic properties of the Th_xU_1−xO₂ system have yet to be determined. A combination of density-functional theory, Monte-Carlo methods, and thermodynamic integration are used to calculate thermodynamic properties of the Th_xU_1−xO₂ binary (ΔH_mix, ΔG_mix, ΔS_mix, phase diagram). The Gibbs free energy of mixing (ΔG_mix) shows a miscibility gap at equilibration temperatures below 1000 K (e.g., E_exsoln = 0.13 kJ/(mol cations) at 750 K). Such a miscibility gap may indicate possible exsolution (i.e., phase separation upon cooling). A unique approach to evaluate the likelihood and kinetics of forming interfaces between U-rich and Th-rich has been chosen that compares the energy gain of forming separate phases with estimated energy losses of forming necessary interfaces. The result of such an approach is that the thermodynamic gain of phase separation does not overcome the increase in interface energy between exsolution lamellae for thin exsolution lamellae (10 Å). Lamella formation becomes energetically favorable with a reduction of the interface area and, thus, an increase in lamella thickness to >45 Å. However, this increase in lamellae thickness may be diffusion limited. Monte-Carlo simulations converge to an exsolved structure [lamellae || ] only for very low equilibration temperatures (below room temperature). In addition to the weak tendency to exsolve, there is an ordered arrangement of Th and U in the solid solution [alternating U and Th layers || {1 0 0}] that is energetically favored for the homogeneously mixed 50% Th configurations. Still, this tendency to order is so weak that ordering is seldom reached due to kinetic hindrances. The configurational entropy of mixing (ΔS_mix) is approximately equal to the point entropy at all temperatures, indicating that the system is not ordered.     

Lattice parameters of (U, Pu, Am, Np)O2−x

The solid solutions of (U_{1−z−y’−y”}Pu_zAm_y’Np_y”)O_2−x (z = 0-1, y’ = 0-0.12, y” = 0-0.07) were investigated by X-ray diffraction measurements, and a database for the lattice parameters was updated. A model to calculate the lattice parameters was derived from the database. The radii of the ions present in the fluorite structure of (U, Pu, Am, Np)O_2−x were estimated from the lattice parameters measured in this work. The model represented the experimental data within a standard deviation of σ = ±0.025%.     

Oxygen potential and thermal conductivity of (U, Pu) mixed oxides

(U, Pu) mixed oxides, (U_1−yPu_y)O_2−x, with y = 0.21 and 0.28 are being considered as fuels for the Prototype Fast Breeder Reactor (PFBR) in India. The use of urania-plutonia solid solutions in PFBR calls for accurate measurement of physicochemical properties of these materials. Hence, in the present study, oxygen potentials of (U_1−yPu_y)O_2−x, with y = 0.21 and 0.28 were measured over the temperature range 1073-1473 K covering an oxygen potential range of −550 to −300 kJ mol⁻¹ (O/M ratio from 1.96 to 2.000) by employing a H₂/H₂O gas equilibration technique followed by solid electrolyte EMFmeasurement. (U_1−yPu_y)O_2−x, with y = 0.40 is being used in the Fast Breeder Test Reactor (FBTR) in India to test the behaviour of fuels with high plutonium content. However, data on the oxygen potential as well as thermal conductivity of the mixed oxides with high plutonium content are scanty. Hence, the thermal diffusivity of (U_1−yPu_y)O₂, with y = 0.21, 0.28 and 0.40 was measured and the results of the measurements are reported.     

Thermal studies on fluorite type ZryU1−yO2 solid solutions

Solid state reactions of UO₂ and ZrO₂ in mild oxidizing condition followed by reduction at 1673 K showed enhanced solubility up to 35 mol% of zirconium in UO₂ forming cubic fluorite type Zr_yU_1−yO₂ solid solution. The lattice parameters and O/M (M = U + Zr) ratios of the solid solutions, Zr_yU_1−yO_2+x, prepared in different gas streams were investigated. The lattice parameters of these solid solutions were expressed as a linear equation of x and y: a₀ (nm) = 0.54704 − 0.021x - 0.030y. The oxidation of these solid solutions for 0.1 ? y ? 0.2 resulted in cubic phase MO_2+x up to700 K and single orthorhombic zirconium substituted α-U₃O₈ phase at 1000 K. The kinetics of oxidation of Zr_yU_1−yO₂ in air for y = 0-0.35 were also studied using thermogravimetry. The specific heat capacities of Zr_yU_1−yO₂ (y = 0-0.35) were measured using heat flux differential scanning calorimetry in the temperature range of 334-860 K.     

Thermodynamic properties of the O-U system. II - Critical assessment of the stability and composition range of the oxides UO2+x, U4O9−y and U3O8−z

The published data concerned with the determination of the composition ranges of uranium oxides, UO_2+x, U₄O_9−y and U₃O_8−z, which have been determined using thermogravimetric, X-ray diffraction and electrochemical techniques are critically assessed. U₄O₉ and U₃O₈ have quite small domains of composition and the assessment of such data has carefully considered the uncertainties in the experimental determinations. In addition, the thermodynamic properties of U₄O₉ and U₃O₈, enthalpies of formation and transformation, entropies, and thermal capacities are analyzed and selected to build a primary data base for compounds.     

Crystal structure change of Li2+xTiO3+y tritium breeder under moist air

To investigate the aging behavior of Li_2+xTiO_3+y, the materials Li_2.0TiO_3+y, Li_2.2TiO_3+y and Li_2.4TiO_3+y were stored under moist air at ambient temperature. Under these conditions the weight of Li_2.0TiO_3+y increased by only 1% in 100 days, which provides evidence for the high chemical stability of Li₂TiO₃. In comparison, under the same storage conditions the weights of Li_2.2TiO_3+y and Li_2.4TiO_3+y increased by 14% and 18%, respectively. The observed weight gain is attributed to the uptake of water, and to the reaction of a by-product of Li_2+xTiO_3+y with water and carbon dioxide. The weight gain curves were evaluated by the Jander equation. From the analysis, it was possible to obtain the diffusion coefficient of water through Li₂TiO₃ particles at ambient temperature. The determined value was 2 × 10⁻¹⁷ m²/s.     

Effects of ion irradiation on the mechanical properties of SiNawOxCyHz sol-gel derived thin films

A study of the effects of ion irradiation of hybrid organic/inorganic modified silicate thin films on their mechanical properties is presented. NaOH catalyzed SiNa_wO_xC_yH_z thin films were synthesized by sol-gel processing from tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES) precursors and spin-coated onto Si substrates. After drying at 300 °C, the films were irradiated with 125 keV H⁺ or 250 keV N²⁺ at fluences ranging from 1 × 10¹⁴ to 2.5 × 10¹⁶ ions/cm². Nanoindentation was used to characterize the films. Changes in hardness and reduced elastic modulus were examined as a function of ion fluence and irradiating species. The resulting increases in hardness and reduced elastic modulus are compared to similarly processed acid catalyzed silicate thin films.     

Influence on the structural characteristic of defect solid solution GdxZr1−xO2−x/2 with 0.18 ? x ? 0.62 under longer term annealing studied by Gd Mössbauer spectroscopy

Three kinds of defect solid solution Gd_xZr_1−xO_2−x/2 with 0.18 ? x ? 0.62, including the three single crystal samples with x = 0.21, 0.26 and 0.30, were investigated by ¹⁵⁵Gd Mössbauer spectroscopy at 12 K. Difference in the structural characteristic under longer term annealing were confirmed by comparing the ¹⁵⁵Gd Mössbauer parameters of the polycrystalline samples sintered one time and twice at 1773 K for 16 h in air, respectively. The results indicated that the polycrystalline samples sintered twice have relatively equilibrated structure by comparing with the three single crystal samples. After being sintered twice, basically the local structure around the Gd³⁺ ions does not change, but the degree of the displacements of the six 48f oxygen ions from positions of cubic symmetry becomes slightly smaller, and distribution of the Gd³⁺ ions in the system becomes more homogeneous.     

Oxygen potentials of (th, u)o2+x solid solutions

The oxygen potential and the nonstoichiometry ($\text{x}$) for (Th_1?yU_y)O_2+x were measured in situ at 1000° 1&;#x0303;200°C using a solid electrolyte oxygen sensor and a thermobalance, respectively. The results showed that the oxygen potentials of (Th_1?yU_y)O_2+x are not a function only of the U valence in the investigated range of $\text{0.05?y?0.20}$. The oxygen potentials at 1200°C decrease negatively with increasing thorium content at a given U valence. The calculated activity coefficients of urania in the solutions indicate an increasing positive deviation from ideality with increasing U valence.     

Development of fabrication technologies for advanced breeding functional materials For DEMO reactors

Demonstration power reactors (DEMOs) require advanced tritium breeders and neutron multipliers that have high stability at high temperatures. Lithium titanate (Li₂TiO₃) is one of the most promising candidates among tritium breeders because of its tritium release characteristics. Li₂TiO₃ with additional Li (Li_2+xTiO_3+y) has increased stability in a reducing atmosphere at high temperatures. In this work, Li_2+xTiO_3+y pebbles were fabricated using the emulsion method, which is a sol–gel method. The raw material for the fabrication of Li_2+xTiO_3+y pebbles was synthesized from a mixture of LiOH·H₂O and H₂TiO₃ at specific ratios. The average diameter and the sphericity of the pebbles fabricated by the emulsion method were 1.40 mm and 1.02, respectively. In addition, beryllium (Be) intermetallic compounds (beryllides) are promising materials for advanced neutron multipliers. The results of the trial fabrications in this work showed that beryllides of Be–Ti and Be–V intermetallics could be synthesized using the plasma sintering method.     

Phase relations and linear thermal expansion of cubic solid solutions in the Th1−xMxO2−x/2 (M = Eu, Gd, Dy) systems

Cell parameters and linear thermal expansion studies of the Th-M oxide systems with general compositions Th_1−xM_xO_2−x/2 (M = Eu³⁺, Gd³⁺ and Dy³⁺, 0.0 ? x ? 1.0) are reported. The XRD patterns of each product were refined to specify the solid solubility limits of MO_1.5 in the ThO₂ lattice. The upper solid solubility limits of EuO_1.5, GdO_1.5 and DyO_1.5 in the ThO₂ lattice under conditions of slow cooling from 1673 K are represented as Th_0.50Eu_0.50O_1.75, Th_0.60Gd_0.40O_1.80 and Th_0.85Dy_0.15O_1.925, respectively. The linear thermal expansion (293-1123 K) of MO_1.5 and their single-phase solid solutions with thoria were investigated by dilatometery. The average linear thermal expansion coefficients () of the compounds decrease on going from EuO_1.5 to DyO_1.5. The values of for EuO_1.5, GdO_1.5 and DyO_1.5 containing solid solutions showed a downward trend as a function of the dopant concentration. The linear thermal expansion (293-1473 K) of the solid solutions investigated by high-temperature XRD also showed a similar trend.     

Thermodynamic and composition changes in UO2±x (x< 0.005) at 1950 K

The composition of UO_2±x has been determined in a narrow range on either side of the stoichiometric composition, as a function of the equilibrium oxygen pressure at a single temperature, 1950 K. Samples of UO_2−x cooled rapidly from 1950 K to room temperature disproportionate to stoichiometric UO₂ and U metal. The significance of these results to the commercial sintering of UO₂ fuel pellets is discussed.     

Hydrolysis of Thorium Nitrides and Carbonitrides

Studies were made on the hydrolysis by water and water vapor of thorium nitrides—ThN, Th₃N₄, Th₂N₂O—and carbonitrides. All the thorium nitrides and carbonitrides were found to decompose in water below 100°C, changing thereby into ThO₂. The order among the thorium nitrides in their propensity toward hydrolysis is: ThC_1-xN_x>ThN>Th₃N₄>Th₂N₂O. Upon hydrolysis, ThN produced NH₃ and H₂, but in the case of the higher nitrides no H₂ was found to evolve.

In the reaction between ThN and water vapor, no higher nitride was produced, in contrast to the case of UN. The difference in behavior between ThN and UN was studied from the standpoint of differences in crystallographic conditions for the transformation from mononitride into higher nitrides. Through the hydrolysis of ThC_1-xN_x in water vapor, products containing C-C, C-C-C and C-N bonds, such as ethane and amines, were found in smaller quantities than for the case of UC_1-xN_x. This fact as well as the difficulty of formation of higher nitrides has resulted in a fairly simple hydrolysis behavior of thorium nitrides.     

The use of HI-ERDA/RBS and NRA/RBS to depth profile N in GaAs1−xNx thin films

N profiles of several GaAs_1−xN_x epitaxial layers with different N mole fractions in the range 0 < x < 0.14 were obtained by using (1) heavy-ion elastic recoil detection analysis (HI-ERDA) along with Rutherford backscattering spectrometry (RBS) using a 35 MeV Si⁶⁺ beam, and (2) nuclear reaction analysis (NRA) with the ¹⁴N(α, p)¹⁷O reaction, also with RBS, using a 3.7 MeV ⁴He⁺ beam. The results from the two techniques are compared and the advantages, disadvantages and capabilities are discussed.     

Shell model calculation of enthalpy of solution of MgO in UO2+x

A computer program to evaluate the formation energies of defects in fluorite crystals is developed on the basis of the shell model Intrinsic defects of UO₂ are calculated with this program using the potential parameters reported by Catlow. The results are in agreement with those of Catlow. The present computer program is applied to a study of nonstoichiometry of the solid solutions of MgO and UO_2+x The calculation of the enthalpy of solution of MgO in UO_2+xis carried out on the basis of several defect models for charge compensation in the solid solution Mg_yU_1?yO_2±xThe calculated results are discussed in the light of our recent experimental work. It is proved that the present theoretical calculation predicts reasonably well the nonstoichiometric properties of the solid solution.     

Gd2Zr2O7 pyrochlore: Potential host matrix for some constituents of thoria based reactor’s waste

The present work explores the potential of Gd₂Zr₂O₇ for incorporation of ThO₂ and Al₂O₃ which are components of Advanced Heavy Water Reactors (AHWR) waste. XRD studies reveal that the compositions corresponding to y from 0.0 to 0.4 in Gd_2−yTh_yZr_2−yAl_yO₇ are single phasic in nature and beyond y > 0.4 the biphasic region starts. The solubility of thoria in Gd₂Zr₂O₇ pyrochlore could be enhanced by more than five times by simultaneous incorporation of alumina. The lattice parameter increases with increase in Th and Al content in the series. The r_A/r_B ratio increases with increase in Th and Al content in Gd₂Zr₂O₇ and in turn the degree order increases as has been seen by gradual increase in the intensity of superstructure peaks. Single phasic samples were investigated by Raman spectroscopy also. Thermal expansion behavior of single phasic samples was investigated by HT-XRD. In order to confirm the nature of the phases backscattered images have been recorded on all the samples.