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.     

Light ion irradiation effects on stuffed Lu2(Ti2−xLux)O7−x/2 (x = 0, 0.4 and 0.67) structures

We have recently synthesized “stuffed” (i.e., excess Lu) Lu₂(Ti_2−xLu_x)O_7−x/2 (x = 0, 0.4 and 0.67) compounds using conventional ceramic processing. X-ray diffraction measurements indicate that stuffing more Lu³⁺ cations into the oxide structure leads eventually to an order-to-disorder (O-D) transition, from an ordered pyrochlore to a disordered fluorite crystal structure. At the maximum deviation in stoichiometry (x = 0.67), the Lu³⁺ and Ti⁴⁺ ions become completely randomized on the cation sublattices, and the oxygen “vacancies” are randomized on the anion sublattice. Samples were irradiated with 400 keV Ne²⁺ ions to fluences ranging from 1 × 10¹⁵ to 1 × 10¹⁶ ions/cm² at cryogenic temperatures (∼77 K). Ion irradiation effects in these samples were examined by using grazing incident X-ray diffraction. The results show that the ion irradiation tolerance increases with disordering extent in the non-stoichiometric Lu₂(Ti_2−xLu_x)O_7−x/2.     

Release kinetics of tritium generated in lithium-enriched Li2+xTiO3 by thermal neutron irradiation

Tritium release kinetics for Li_2+xTiO₃, the lithium-enriched Li₂TiO₃, was investigated by isochronal and isothermal annealing experiments. Tritium release by isochronal annealing showed that the dominant release stage was found at around 600 K. An additional release stage at lower temperature side was appeared with increasing excess lithium, which was attributed to the release of tritium trapped in Li₄TiO₄ structure. The dominant release stage was considered to a release of tritium trapped by irradiation defects. Isothermal annealing experiments indicated that tritium releases were controlled by diffusion process. The diffusion coefficient of Li_2.0TiO₃ was one order of magnitude as large as those of Li_2.2TiO₃ and Li_2.4TiO₃, although their activation energies were almost the same. These results showed that rate-determining step was the diffusion process of tritium in Li₂TiO₃ structure for Li_2+xTiO₃ and excess lithium would make diffusion coefficient smaller. Simulation of tritium-TDS spectra for Li_2.0TiO₃ has clarified that the TDS spectrum forLi_2.0TiO₃ can be demonstrated by using Arrhenius diffusion parameters obtained by isothermal annealing experiment in the present study.     

Structural characterization of Nd-doped Hf-zirconolite Ca1−xNdxHfTi2−xAlxO7 ceramics

Because of its high incorporation capacity and of the high thermal neutron capture cross-section of hafnium, Hf-zirconolite (CaHfTi₂O₇) ceramic can be envisaged as a potential waste form for minor actinides (Np, Am, Cm) and plutonium immobilization. In this work, Nd-doped Hf-zirconolite Ca_1−xNd_xHfTi_2−xAl_xO₇ (x = 0; 0.01 and 0.2) ceramics have been prepared by solid state reaction. Neodymium has been used as trivalent actinide surrogate. The ceramic samples structure has been studied by X-ray diffraction and refined by the Rietveld method. This revealed that Nd³⁺ ions only enter the Ca site, whereas part of Hf⁴⁺ ions substitute titanium into Ti(1) sites and Al³⁺ ions mainly occupy the Ti(2) split sites and Ti(3) sites of the zirconolite structure. Using various spectroscopic techniques (electron spin resonance, optical absorption and fluorescence), the environment of Nd³⁺ cations in Hf-zirconolite has been studied and compared with that of Nd³⁺ cations in Zr-zirconolite (CaZrTi₂O₇). Different local environments of Nd³⁺ cations have been detected in Hf-zirconolite that can be attributed to the existence of an important disorder around Nd in the Ca site probably due to the statistical occupancy of the next nearest cationic site of neodymium (a split Ti site) by Ti⁴⁺, Al³⁺ cations and vacancies. No significant differences were observed concerning Nd³⁺ cations environment and distribution in Hf- and Zr-zirconolite ceramics.     

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.     

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.     

Eutectic crystallization in the FeO1.5-UO2+x-ZrO2 system

Results of the investigation of the FeO_1.5-UO_2+x-ZrO₂ system in air are presented. The eutectic position and the content of the phases crystallized at this point have been determined. The temperature and the composition of the ternary eutectic are 1323 ± 7 °С and 67.4 ± 1.0 FeO_1.5, 30.5 ± 1.0 UO_2+x, 2.1 ± 0.2 ZrO₂ mol.%, respectively. The solubilities of FeO_1.5 and ZrO₂ in the UO_2+x(FeO_1.5, ZrO₂) solid solution correspond to respectively 3.2 and 1.1 mol.%. The solubilities of UO₂ and ZrO₂ in FeO_1.5 are not significant. The existence of a solid solution on the basis of U(Zr)FeO₄ compound is found. The ZrO₂ solubility in this solid solution is 7.0 mol.%.     

Raman spectroscopy characterization of actinide oxides (U1−yPuy)O2: Resistance to oxidation by the laser beam and examination of defects

Structural changes in four (U_1−yPu_y)O₂ materials with very different plutonium concentrations (0 ? y ? 1) and damage levels (up to 110 dpa) were studied by Raman spectroscopy. The novel experimental approach developed for this purpose consisted in using a laser beam as a heat source to assess the reactivity and structural changes of these materials according to the power supplied locally by the laser. The experiments were carried out in air and in water with or without hydrogen peroxide. As expected, the material response to oxidation in air depends on the plutonium content of the test oxide. At the highest power levels U₃O₈ generally forms with UO₂ whereas no significant change in the spectra indicating oxidation is observed for samples with high plutonium content (²³⁹PuO₂). Samples containing 25 wt.% plutonium exhibit intermediate behavior, typified mainly by a higher-intensity 632 cm⁻¹ peak and the disappearance of the 1LO peak at 575 cm⁻¹. This can be attributed to the presence of anion sublattice defects without any formation of higher oxides. The range of materials examined also allowed us to distinguish partly the chemical effects of alpha self-irradiation. The results obtained with water and hydrogen peroxide (a water radiolysis product) on a severely damaged ²³⁸PuO₂ specimen highlight a specific behavior, observed for the first time.     

Analysis of oxygen potential of (U0.7Pu0.3)O2±x and (U0.8Pu0.2)O2±x based on point defect chemistry

Stoichiometries in (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x were analyzed with the experimental data of oxygen potential based on point defect chemistry. The relationship between the deviation x of stoichiometric composition and the oxygen partial pressure P_O2 was evaluated using a Kröger-Vink diagram. The concentrations of the point defects in uranium and plutonium mixed oxide (MOX) were estimated from the measurement data of oxygen potentials as functions of temperature and P_O2. The analysis results showed that x was proportional to near the stoichiometric region of both (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x, which suggested that intrinsic ionization was the dominant defect. A model to calculate oxygen potential was derived and it represented the experimental data accurately. Further, the model estimated the thermodynamic data, and , of stoichiometric (U_0.7Pu_0.3)O_2.00 and (U_0.8Pu_0.2)O_2.00 as −552.5 kJ·mol⁻¹ and −149.7 J·mol⁻¹, and −674.0 kJ · mol^-1 and −219.4 J · mol⁻¹, respectively.     

Band gap controlled H loss from passivated Hg1−xCdxTe (MCT) wafers under intense electronic excitations

We report here loss of H monitored by on-line elastic recoil detection analysis (ERDA) technique from passivated Hg_1−xCd_xTe (MCT) wafers due to irradiation by 80 MeV Ni⁹⁺, 120 MeV Au¹⁵⁺ and 200 MeV Ag¹⁰⁺. The loss of H is more in case of the wafer irradiated by Ag ions as compared to other two because of higher electronic energy loss (S_e). For same S_e value, H loss is more in case of the wafer having x = 0.29 as compared to the one having x = 0.204. This is due to higher band gap of the former as compared to the later, which is an important data for proper use of these materials as IR detector in intense radiation zone. These results are explained on the basis of thermal spike model of ion-solid interaction.     

Oxygen potential measurements of Am0.5Pu0.5O2−x by EMF method

The dependence of the oxygen potentials on oxygen non-stoichiometry and temperature of Am_0.5Pu_0.5O_2−x has been obtained by the electromotive force (EMF) method with the cell: (Pt) air |Zr(Ca)O_2−x| Am_0.5Pu_0.5O_2−x (Pt). The x value of Am_0.5Pu_0.5O_2−x was changed at 1333 K over 0.02 < x ? 0.25 by the coulomb titration method. The temperature dependence of the oxygen potential was also measured over the range of 1173-1333 K. It was found that the oxygen potential decreased from −80 to −360 kJ mol⁻¹ with increasing x from 0.021 to 0.22 at 1333 K and that it remained almost constant at −360 kJmol⁻¹ around x = 0.23. It was concluded that Am_0.5Pu_0.5O_2−x should be composed of the single fluorite-type phase over 0.02 < x ? 0.22 and the mixed phases of fluorite-type and (Am, Pu)₉O₁₆ at around x = 0.23.     

Formation of planar waveguide in BiB3O6 crystal by MeV carbon implantation

Non-leaky planar waveguide structure has been fabricated in x-cut BiB₃O₆ crystal by 6 MeV C³⁺ ion implantation at a dose of 1 × 10¹⁴ ions/cm². The effective refractive indices of the waveguide are measured at a wavelength of 632.8 nm. We perform a computer code based on the finite difference method to reconstruct the refractive index profiles of n_x and n_y of this waveguide. The beam propagation method is used to calculate the electric and magnetic field profiles in the waveguide region from the reconstructed refractive index profiles. Our simulated data show that the refractive index increased waveguide layer can confine the mode completely.     

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%.     

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 equilibria in the FeO1+x-UO2-ZrO2 system in the FeO1+x-enriched domain

Experimental results of the investigation of the FeO_1+x-UO₂-ZrO₂ system in neutral atmosphere are presented. The ternary eutectic position and the composition of the phases crystallized at this point have been determined. The phase diagram is constructed for the FeO_1+x-enriched region and the onset melting temperature of 1310 °C probably represents a local minimum and so will be a determining factor in this system and its application to safety studies in nuclear reactors.     

Thermodynamic investigations of ThO2-UO2 solid solutions

Heat capacities and enthalpy increments of solid solutions Th_1−yU_yO₂(s) (y = 0.0196, 0.0392, 0.0588, 0.098, 0.1964) and Simfuel (y = 0.0196) were measured by using a differential scanning calorimeter and a high temperature drop calorimeter. The heat capacities were measured in two temperature ranges: 127-305 K and 305-845 K and enthalpy increments were determined in the temperature range 891-1698 K. A heat capacity expression as a function of uranium content y and temperature and a set of self-consistent thermodynamic functions for Th_1−yU_yO₂(s) were computed from present work and the literature data. The oxygen potentials of Th_1−yU_yO_2+x(s) have been calculated and expressed as a polynomial functions of uranium content y, excess oxygen x and temperature T. The phase diagram, oxygen potential diagram of thorium-uranium-oxygen system and major vapour species over urania thoria mixed oxide have been computed using FactSage code.     

Oxygen potential of (U0.88Pu0.12)O2±x and (U0.7Pu0.3)O2±x at high temperatures of 1673-1873 K

The oxygen potential of (U_0.88Pu_0.12)O_2±x (−0.0119 < x < 0.0408) and (U_0.7Pu_0.3)O_2±x (−0.0363 < x < 0.0288) was measured at high temperatures of 1673-1873 K using gas equilibrium method with thermo gravimeter. The measured data were analyzed by a defect chemistry model. Expressions were derived to represent the oxygen potential based on defect chemistry as functions of temperature and oxygen-to-metal ratio. The thermodynamic data, and , at stoichiometric composition were obtained. The expressions can be used for in situ determination of the oxygen-to-metal ratio by the gas-equilibration method. The calculation results were consistent with measured data. It was estimated that addition of 1 wt.% Pu content increased oxygen potential of uranium and plutonium mixed oxide by 2-5 kJ/mol.     

Surface desorption and bulk diffusion models of tritium release from Li2TiO3 and Li2ZrO3 pebbles

The release of tritium from Li₂TiO₃ and Li₂ZrO₃ pebbles, in batch experiments, is studied by means of temperature programmed desorption. Data reduction focuses on the analysis of the non-oxidized and oxidized tritium components in terms of release limited by diffusion from the bulk of ceramic grains, or by first or second order surface desorption. By analytical and numerical methods the in-furnace tritium release is deconvoluted from the ionization chamber transfer functions, for which a semi-empirical form is established. The release from Li₂TiO₃ follows second order desorption kinetics, requiring a temperature for a residence time of 1 day (T_1dRes) of 620 K, and 603 K, of the non-oxidized, and the oxidized components, respectively. The release from Li₂ZrO₃ appears as limited by either diffusion from the bulk of the ceramic grains, or by first order surface desorption, the first possibility being the more probable. The respective values of T_1dRes for the non-oxidized component are 661 K, according to the first order surface desorption model, and 735 K within the bulk diffusion limited model.     

Solubility of ThO2 in Gd2Zr2O7 pyrochlore: XRD, SEM and Raman spectroscopic studies

Solubility of ThO₂ in gadolinium zirconate pyrochlore, a potential host for radioactive materials, has been investigated. The phase relations in Gd_2−xTh_xZr₂O_7+x/2 (0.0 ? x ? 2.0) systems have been established under the slow-cooled conditions from 1400 °C. XRD studies reveal that the compositions corresponding to x = 0.0-0.075 are single phasic in nature and beyond x ? 0.1 the biphasic region starts. The first biphasic region comprising of pyrochlore and thoria exist from x = 0.1-0.8, and from x = 1.2 another biphasic region consisting of gadolinia stabilized zirconia (GSZ) and thoria appears which persists till x = 1.6. The end member (i.e. x = 2.0) of the series is found to be a mixture of monoclinic ZrO₂ and thoria. Interestingly, gadolinia which has wide solubility in thoria, did not show any miscibility in thoria in the presence of zirconia. Irregular grains of Gd_1.8Th_0.2Zr₂O_7.1 as shown in SEM supports its biphasic nature. Raman spectra of heavily thoria doped (x = 0.1 and 0.2) samples, indicates the presence of Zr-O₇ mode which implies the samples are highly disordered in nature.     

Optical properties of InxGa1−xP/InP grown at high fluence Ga implantation on InP using focused ion beam

Single-crystalline InP(1 0 0) substrate was implanted by 30 keV Ga⁺ ions with fluences of 1 × 10¹⁶-1.5 × 10¹⁷ cm⁻² followed by post-annealing treatment at 750 °C to recover implantation-induced structural defects and activate dopants into the lattices. The optical property, composition, and microstructure of the Ga⁺-implanted InP were studied by Raman spectroscopy and transmission electron microscopy (TEM). Raman spectra show that the In_xGa_1−xP phase is formed at a critical fluence of 7 × 10¹⁶ cm⁻². The newly grown phase was identified with the appearance of Ga rich TO_InP and In rich TO_GaP modes of a random alloy in the 1 bond-2 phonon mode configuration along with TEM structural identification.