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.     

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.     

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.     

Vaporization behavior and Gibbs energy of formation of Rb2ThO3

The thermodynamic stability of rubidium thorate, Rb₂ThO₃(s), was determined from vaporization studies using the Knudsen effusion forward collection technique. Rb₂ThO₃(s) vaporized incongruently and predominantly as Rb₂ThO₃(s)=ThO₂(s) + 2Rb(g) + 1/2 O₂(g). The equilibrium constant K=p_Rb²·p_O2^1/2 was evaluated from the measurement of the effusive flux due to Rb vapor species under the oxygen potential governed by the stoichiometric loss of the chemical component Rb₂O from the thorate phase. The Gibbs energy of formation of Rb₂ThO₃ derived from the measurement and other auxiliary data could be given by the equation, Δ_fG°(Rb₂ThO₃,s)=−1794.7+0.42T ± .     

Thermal conductivities of hypostoichiometric (U, Pu, Am)O2−x oxide

The thermal conductivities of (U_0.68Pu_0.30Am_0.02)O_2.00−x solid solutions (x = 0.00-0.08) were studied at temperatures from 900 to 1773 K. The thermal conductivities were obtained from the thermal diffusivities measured by the laser flash method. The thermal conductivities obtained experimentally up to about 1400 K could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(x) = 3.31 × x + 9.92 × 10⁻³ (mK/W) and B(x) = (−6.68 × x + 2.46) × 10⁻⁴ (m/W). The experimental A values showed a good agreement with theoretical predictions, but the experimental B values showed not so good agreement with the theoretical ones in the low O/M ratio region. From the comparison of A and B values obtained in this study with the ones of (U,Pu)O_2−x obtained by Duriez et al. [C. Duriez, J.P. Alessandri, T. Gervais, Y. Philipponneau, J. Nucl. Mater. 277 (2000) 143], the addition of Am into (U, Pu)O_2−x gave no significant effect on the O/M dependency of A and B values.     

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.     

Microstructural evolution in irradiated uranium-bearing delta-phase oxides A6U1O12 (A = Y, Gd, Ho, Yb, and Lu)

Irradiation-induced microstructural evolution in uranium-bearing delta-phase oxides of A₆U₁O₁₂ (A = rare earth cations) were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Polycrystalline Y₆U₁O₁₂, Gd₆U₁O₁₂, Ho₆U₁O₁₂, Yb₆U₁O₁₂, and Lu₆U₁O₁₂ samples were irradiated with 300 keV Kr⁺⁺ to a fluence of 2 × 10²⁰ ions/m² at cryogenic temperature (∼100 K). The crystal structure of these compounds was determined to be an ordered, fluorite derivative structure, known as the delta-phase, a rhombohedral symmetry belonging to space group . Experimental results indicate that all these compounds are resistant to amorphization to a displacement damage dose of ∼60 displacements per atom. In these experiments, we sometimes observed an irradiation-induced order-to-disorder phase transformation, from an ordered rhombohedral to a disordered fluorite structure.     

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.     

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.     

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.     

Secondary ion emission from Cu(1 0 0) surfaces with atomic adsorbates (N, O, Cl, S and Br)

Several targets that consist of atomic species X (X = N, O, Cl, S, Br) adsorbed at hollow sites on the Cu(1 0 0) surface have been examined with low-fluence secondary ion mass spectrometry (SIMS). The positive and negative secondary ion (SI) abundance distributions, which show a range of characteristics, have been discussed with the aid of thermochemical data derived from ab initio calculations. In positive SIMS, CuX⁺ is never observed, while the only heteronuclear (mixed-atom) SI that is observed for all five systems is Cu₂X⁺. In negative SIMS, the dominant heteronuclear species for all systems is , except for N/Cu(1 0 0), which produces no , ions. Cu⁻ emission is observed only for O/Cu(1 0 0). By analogy with results from laser ablation studies of O/Cu targets, it is conjectured that Cu⁻ is a daughter product of the gas-phase dissociation of polyatomic Cu-O anion clusters.     

H2 inhibition of radiation induced dissolution of spent nuclear fuel

In order to elucidate the effect of noble metal clusters in spent nuclear fuel on the kinetics of radiation induced spent fuel dissolution we have used Pd particle doped UO₂ pellets. The catalytic effect of Pd particles on the kinetics of radiation induced dissolution of UO₂ during γ-irradiation in containing solutions purged with N₂ and H₂ was studied in this work. Four pellets with Pd concentrations of 0%, 0.1%, 1% and 3% were produced to mimic spent nuclear fuel. The pellets were placed in 10 mM aqueous solutions and γ-irradiated, and the dissolution of was measured spectrophotometrically as a function of time. Under N₂ atmosphere, 3% Pd prevent the dissolution of uranium by reduction with the radiolytically produced H₂, while the other pellets show a rate of dissolution of around 1.6 × 10⁻⁹ mol m⁻² s⁻¹. Under H₂ atmosphere already 0.1% Pd effectively prevents the dissolution of uranium, while the rate of dissolution for the pellet without Pd is 1.4 × 10⁻⁹ mol m⁻² s⁻¹. It is also shown in experiments without radiation in aqueous solutions containing H₂O₂ and O₂ that ?-particles catalyze the oxidation of the UO₂ matrix by these molecular oxidants, and that the kinetics of the catalyzed reactions is close to diffusion controlled.     

Oxygen non-stoichiometries in (Th0.7Ce0.3)O2−x

Oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x oxide solid solutions was investigated from the viewpoint of Ce reduction. The oxygen non-stoichiometry was experimentally determined by means of thermogravimetric analysis as a function of oxygen potential at 1173, 1273 and 1373 K. Features of the isotherms of oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x similar to those in oxygen non-stoichiometric actinide and lanthanide dioxides were observed. The oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x was compared with those of CeO_2−x and (U_0.7Ce_0.3)O_2−x. It was concluded that the Ce reduction has some relation to defect forms and their transformations in the solid solutions.     

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.     

Absorption and photoluminescence study of Al2O3 single crystal irradiated with fast neutrons

Colour centers formation in Al₂O₃ by reactor neutrons were investigated by optical measurements (absorption and photoluminescence). The irradiation’s were performed at 40 °C, up to fast neutron (E_n > 1.2 MeV) fluence of 1.4 × 10¹⁸ n cm⁻². After irradiation the coloration of the sample increases with the neutron fluence and absorption band at about 203, 255, 300, 357 and 450 nm appear in the UV-visible spectrum. The evolution of each absorption bands as a function of fluence and annealing temperature is presented and discussed. The results indicate that at higher fluence and above 350 °C the F⁺ center starts to aggregate to F center clusters (F₂, F₂⁺ and ). These aggregates disappear completely above 650 °C whereas the F and F⁺ centers persist even after annealing at 900 °C. It is clear also from the results that the absorption band at 300 nm is due to the contribution of both F₂ center and interstitial ions.     

Compactibility and sintering behavior of nanocrystalline (Th1−xCex)O2 powders synthesized by co-precipitation process

Thoria (ThO₂) based ceramic material is a versatile and very important matrix for immobilization of plutonium and other tetravalent actinides either as a burning or a deposition material for final disposal. The aim of this study was to investigate the influence of the actinide concentration (simulated with cerium), the fabrication conditions and the properties of the produced powders on the compactibility and sinterability of the final products. The (Th_1−xCe_x)O₂ powders with ceria concentration varying from 5 to 50 mol% were synthesized by co-precipitation method. The pellets were then compacted from calcined and ground powders at pressures varying from 250 to 750 MPa. The produced pellets had a homogenous grain size and sintered densities of 0.88% to 0.95% TD, respectively.     

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.     

Structural study of Pu1−xAmxO2 (x = 0.2; 0.5; 0.8) obtained by oxalate co-conversion

This study describes the synthesis and the characterisation of Pu_1−xAm_xO₂ (x = 0.2; 0.5; 0.8) mixed oxides obtained by oxalate co-conversion. We studied the self-irradiation effect in these compounds at the structural scale. We determined, for each composition, the initial lattice parameter and the equation describing its variation versus time and displacements per atom. Similarly to other α emitting compounds, it was observed a fast lattice parameter expansion rate, followed by a stabilisation at a maximum value. The observations also showed that the initial expansion rate varies according to the Am content and the maximum value to the Pu content. However, for all compositions, the lattice parameter relative variations are the same.     

Calorimetric measurements on (U,Th)O2 solid solutions

Enthalpy increments of urania - thoria solid solutions, (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ were measured by drop calorimetry in the temperature range 479 - 1805 K. Heat capacity, entropy and Gibbs energy function were computed. The heat capacity measurements were carried out also with differential scanning calorimetry in the temperature range 298 - 800 K. The heat capacity values of (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ at 298 K are 59.62, 61.02, 63.56 J K⁻¹ mol⁻¹, respectively. The results were compared with the data available in the literature. From the study, the heat capacity of (U,Th)O₂ solid solutions was shown to obey the Neumann - Kopp’s rule.     

Radiation induced defects in BaBPO5:Ce and their role in thermally stimulated luminescence reactions: EPR and TSL investigations

Defect centers induced by gamma irradiation in Ce doped BaBPO₅ were investigated using EPR spectroscopy. From EPR studies, three phosphorous centered radicals were characterized on the basis of observed ³¹P hyperfine splitting and g values as , and radicals. In addition to this, two types of boron oxygen hole centers (BOHC) and O⁻ were also formed at room temperature. An intense broad signal in sample annealed in argon (g_⊥ = 1.9258 and g_‖ = 1.8839) was assigned to Ce³⁺ ions associated with the electron trapped at anion vacancy or nearby lattice defect. TSL studies showed two glow peaks, a relatively weaker one at 425 K and an intense one at 575 K. Spectral studies of the TSL glow peaks have shown that Ce³⁺ ion acts as emission center. From the temperature dependence of the EPR spectra of gamma irradiated samples, the glow peaks at 425 K and 575 K were attributed to thermal destruction of /O⁻ and BOHC, respectively, by trapping of electrons from elsewhere. The energy released in electron hole recombination process is used for the excitation of Ce³⁺ ions resulting in these glow peaks at 425 K and 575 K. The spectral studies of the TSL glow peaks have shown emission at 330 nm indicating Ce³⁺ acts as the luminescent centre. The trap depth and the frequency factor for the 425 K and 575 K peaks were determined using different heating rates method.