Phase diagram and thermodynamic properties of the system MnCl2-UCl4, prediction and investigation results

The MnCl₂-UCl₄ system was studied employing differential thermal analysis, common thermal analysis and high-temperature cryometry. From the results obtained, both the phase diagram and the thermodynamic characteristics of liquid phases of the system have been determined. The results were discussed in terms of regularities found in the phase equilibria of binary salt systems with common anion and compared with those for other systems of the type MCl_n-UCl₄. It appeared that the results confirmed earlier predictions on the title system.     

Thermodynamic modelling of LiF-LnF3 and LiF-AnF3 phase diagrams

The phase diagrams of the LiF-LnF₃ series, where Ln = La-Sm, and of LiF-AnF₃, where An = U, Pu, have been optimized using Redlich-Kister functions. The phase diagrams of LiF-AmF₃ and LiF-PuF₃-AmF₃ have been calculated. The necessary Gibbs energy functions for americium trifluoride were defined by use of a semi-empirical method. The excess Gibbs energy terms, which are expressed as Redlich-Kister polynomials and describe the effect of interaction between the two fluoride components in the liquid phase, were obtained by translating the trends observed in the lanthanide trifluoride series into the actinide series. A single eutectic has been found in the LiF-AmF₃ system with the eutectic point at ?33 mole% AmF₃ and at ?951 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.     

Progress in the thermodynamic modelling of the O-U binary system

The fuel of civil nuclear plants, UO₂, melts at 3120 K. During an hypothetical severe accident, urania, submitted to high temperatures and various oxygen potentials, presents a wide non-stoichiometry range: the melting temperature of UO_2±x, related to oxygen potential, decreases in all cases. In this scenario, urania could react with other materials, firstly zircaloy, and the melting temperature of (U, Zr)O_2±x still decreases. That is why the critical assessment of the O-U binary system including the non-stoichiometry range of urania, is a major step to a correct thermodynamic modelling of multicomponent systems for nuclear safety. The very numerous experimental information has been compiled and analysed. The associate model was used for the liquid phase, and a sublattice model for UO_2±x; U₄O_9−y, U₃O₈ and UO₃ were treated as stoichiometric. Phase diagram and thermodynamic properties have been calculated from the optimised Gibbs energy parameters. The calculated consistency with the experimental ones is quite satisfactory.     

Thermodynamics of the O-U system. I - Oxygen chemical potential critical assessment in the UO2-U3O8 composition range

A critical assessment of oxygen chemical potential of UO_2+x, U₄O₉ and U₃O₈ oxide non-stoichiometric phases as well as of diphasic related domains has been performed in order to build up primary input data files used in a further optimization procedure of thermodynamic and phase diagram data for the uranium-oxygen system in the UO₂-UO₃ composition range. Owing to the fact that original data are very numerous, more than 500 publications, a twofold process is used for the assessment - (i) first a critical selection of data is performed for each method of measurement together with a careful estimate of their uncertainties, (ii) second a reduction of the total number of data on the basis of a chart with fixed intervals of temperature and composition that allows a comparison to be made of the results from the various experiments. Results are presented for chemical potentials of oxygen with their associated uncertainties.     

Density measurement of amorphous SixGe1 − x alloys

The atomic density of amorphous Si_xGe_{1 − x} alloys (x = 1, 0.85, 0.67, 0.50, 0.20 and 0) has been measured. Mono-crystalline Si_xGe_1−x layers Were implanted with 1.50–2.75 MeV Si²⁺ and Ge²⁺ ions to produce the amorphous material. Using surface profilometry and RBS/channeling, it was found that amorphous alloys are less dense than the crystalline alloys, and that Vegard's law underestimates the a-Si_xGe_1−x density.     

2 MeV Si ion implantation damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3 μm thick relaxed Si_1−xGe_x layers with 2 MeV Si⁺ ions has been measured as a function of Ge content (x = 0.04, 0.13, 0.24 or 0.36) and Si dose in the dose range 10¹⁰–10¹⁵ cm⁻². The accumulation of damage with increasing dose has been studied as a function of Ge content by Rutherford Backscattering Spectrometry, Optical Reflectivity Depth Profiling and Transmission Electron Microscopy and an increased damage efficiency in Si_1−xGe_x with increasing x is observed. The characteristics of implantation-induced defects have been investigated by Electron Paramagnetic Resonance. The results are discussed in the context of a model of the damage process in SiGe.     

Rapid thermal annealing of arsenic implanted relaxed Si1−xGex

The electrical activity and redistribution during rapid thermal annealing (RTA) of high concentrations of As implanted into epitaxially grown, relaxed Si_1−xGe_x for x≤0.5 have been studied as a function of composition x and RTA parameters. At a given RTA temperature the maximum carrier concentration decreases and the redistribution increases with increasing x. Maximum carrier concentrations and junction depths as a function of composition and RTA parameters are given.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Chlorination of UO2, PuO2 and rare earth oxides using ZrCl4 in LiCl-KCl eutectic melt

A new chlorination method using ZrCl₄ in a molten salt bath has been investigated for the pyrometallurgical reprocessing of nuclear fuels. ZrCl₄ has a high reactivity with oxygen but is not corrosive to refractory metals such as steel. Rare earth oxides (La₂O₃, CeO₂, Nd₂O₃ and Y₂O₃) and actinide oxides (UO₂ and PuO₂) were allowed to react with ZrCl₄ in a LiCl-KCl eutectic salt at 773 K to give a metal chloride solution and a precipitate of ZrO₂. An addition of zirconium metal as a reductant was effective in chlorinating the dioxides. When the oxides were in powder form, the reaction was observed to progress rapidly. Cyclic voltammetry provided a convenient way of establishing when the reaction was completed. It was demonstrated that the ZrCl₄ chlorination method, free from corrosive gas, was very simple and useful.     

Elastic and electronic properties and stability of SrThO3, SrZrO3 and ThO2 from first principles

First-principle calculations in the framework of the full-potential linearized-augmented-plane-wave method (FLAPW, as implemented into the WIEN-2k code) have been performed to understand the structural, elastic, cohesive and electronic properties of the meta-stable cubic strontium thorate SrThO₃. The optimized lattice parameters, elastic parameters, formation energies, densities of states, band structures and charge density distributions are obtained and discussed in comparison with those of cubic SrZrO₃ and ThO₂.     

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.     

Phase relations in the quaternary Fe-Pu-U-Zr system

The phase diagram in the quaternary Fe-Pu-U-Zr system was established at 923 K in the uranium-rich region to understand better the compatibility between the metal fuel and stainless steel cladding in a fast reactor. The experimental phase relation data obtained in this study was applied to the thermodynamic methodology for construction of the phase diagram. The calculated phase diagram was consistent and well within the experimental data. The applicability of the thermodynamic model to other temperatures was confirmed by comparing the present results of differential thermal analysis with the calculated phase diagrams. These consistencies mean that both the thermodynamic model and the assessed parameters in the binary and ternary subsystems developed so far are reasonable. The calculated phase diagram established in this study was also in good agreement with the analysis of the diffusion zone in tests on Pu-U-Zr/Fe couples. This suggests that the diffusion zone formed at the fuel-cladding interface in the reactor system can be assessed using the phase diagram in the quaternary Fe-Pu-U-Zr system.     

Characterization and densification studies on ThO2-UO2 pellets derived from ThO2 and U3O8 powders

ThO₂ containing around 2-3% ²³³UO₂ is the proposed fuel for the forthcoming Indian Advanced Heavy Water Reactor (AHWR). This fuel is prepared by powder metallurgy technique using ThO₂ and U₃O₈ powders as the starting material. The densification behaviour of the fuel was evaluated using a high temperature dilatometer in four different atmospheres Ar, Ar-8%H₂, CO₂ and air. Air was found to be the best medium for sintering among them. For Ar and Ar-8%H₂ atmospheres, the former gave a slightly higher densification. Thermogravimetric studies carried out on ThO₂-2%U₃O₈ granules in air showed a continuous decrease in weight up to 1500 °C. The effectiveness of U₃O₈ in enhancing the sintering of ThO₂ has been established.     

A critical examination of the thermodynamics of water adsorption on actinide oxide surfaces

The reversible adsorption of water from actinide oxide surfaces is examined from several viewpoints in this article. A reinterpretation and critical look at the previously published thermodynamic values for desorption of water from PuO₂ [J. Phys. Chem. 77 (1973) 581] are reexamined in light of more recent mathematical treatments of thermal desorption data from high surface area materials. In addition, the time and temperature dependent process of water adsorption/desorption in closed system experiments is examined using chemical kinetics modeling. A simple experimental method and mathematical treatment of determining adsorption enthalpies based upon a closed system is also described. The desorption enthalpy for reversibly adsorbed water from PuO₂ is determined to be a function of adsorbate coverage with values ranging from 51 to 44 kJ mol⁻¹ for coverages of one to several monolayers (MLs). Consistent desorption enthalpy values are obtained using either approach thus highlighting the importance of proper interpretation of adsorption parameters determined from high surface area powders. Reversible adsorption/desorption equilibrium of water with actinide oxide materials is discussed from the practical standpoint of storage and subsequent pressurization of containers. These results obtained from PuO₂ surfaces are consistent with desorption enthalpies of water from a low surface area UO₂ that has been measured using ultra-high vacuum thermal desorption mass spectroscopy to be 42.2 kJ mol⁻¹.     

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 variation of the optical absorption of UO2: determination of the small polaron self-energy

The temperature variation of UV-VIS-NIR optical spectra of UO₂ have been investigated from room temperature up to 1173 K with careful in situ oxygen partial pressure control. The deduced optical absorption edge exhibits a strong temperature dependence. Its value decreases from ∼2 eV at room temperature to ∼0.8 eV at 1173 K. Such thermal behaviour is interpreted as the consequence of the existence of a strong electron-phonon coupling (small polaron). In the temperature range 300-1173 K, the model yields a hopping radius of ∼2 Å and a polaron self-energy of E_p=−0.38 eV.     

Vapour pressure and standard enthalpy of sublimation of H3BO3

A horizontal thermal analysis instrument was adapted as a transpiration apparatus for the measurement of vapour pressure of solid boric acid, H₃BO₃. The experimental parameters necessary for establishing a dynamic isothermal congruent vapourisation equilibrium of H₃BO₃ were identified. Using these optimized transpiration experiments, the vapour pressures were measured in the temperature range 326-363 K. The temperature dependence of the measured values of vapour pressures could be expressed using the expression, log(p/Pa) = 26.83(±0.09) − 9094(±246)/T (K). The standard enthalpy of sublimation, , of H₃BO₃ was estimated to be 174.1 ± 4.7 kJ mol⁻¹ at the mean temperature of the present measurements, viz., 345 K.     

Leaching behaviour of unirradiated high temperature reactor (HTR) UO2-ThO2 mixed oxides fuel particles

The dissolution of different mixed oxide (U, Th)O₂ particles under reducing conditions has been studied using a continuous flow-through reactor. The U/Th ratio seems to have no or little influence on the normalised leaching rate of thorium or uranium, The release rate of uranium from the outer surface of a Th rich matrix seems to follow the behaviour of pure UO₂ even though U is a minor component in these phases and the dissolution rate of Th is much lower. After long time U concentrations will become depleted at the solids surface and it will be expected that U release rates will become controlled by the release rates of thorium (rates at neutral pH < 10⁻⁶ g m⁻² d⁻¹). Under reducing conditions, the matrix of HTR fuel particles presents significant intrinsic radionuclide confinement properties.