On the formation of UC-UN solid solutions

In order to elucidate the mechanisms of the formation of the solid solution between UC and UN, some experiments were carried out using several kinds of combination as the starting materials; (i) UC, (ii) UC₂, (iii) UC + UN and (iv) UN+graphite. The conclusions drawn from these experiments are:

1.

(1) Free carbon precipitating through the reaction of UC with nitrogen gas is not graphite but amorphous carbon with more or less incomplete graphitization. The degree of graphitization is a function of temperature. These amorphous carbons may have been a frequent cause of the scattering results obtained by many workers up to the present time.     

Potential nuclear fuels in the US-UC-UC2 system

A solid-solution study of the US-UC-UC₂ system at 1760 and 2170 K has revealed the range of compositions yielding the mandatory single-phase solid solutions for consideration as potential nuclear fuels. Two such compositions containing 20 and 40 mol%, respectively, of dissolved carbides have been selected, and convenient methods are described for their preparation from stoichiometric UC, ZnS and U₃O₈. Many of the physical and chemical properties of these compositions either as powders or as sintered pellets are reported. Compatibility with stainless steels under the test conditions is extremely good. However, sodium bonding is precluded through chemical reactions leading to the formation of Na₂S, and ultimately free uranium. Creep strengths are greater than for hyperstoichiometric UC. It is concluded that the 80 mol% US composition with less than 10 mol% UC₂is promising as a potential nuclear fuel, the 60 mol% US material being less so.     

The mechanism and kinetics of U2C3 formation by the synthetic reaction

In fast breeder reactors it is planned to use the fuel in the form of (U, Pu)C with a slight carbon hyperstoichiometry. It is therefore important to know under what conditions the synthetic reaction $\text{UC + UC}{}_2\text{U}{}_2\text{C}{}_3$ occurs, since the hyperstoichiometric carbon, which exists as a uranium dicarbide phase (UC₂) after manufacture, is to be converted to U₂C₃. The literature concerning the reaction conditions is partly contradictory. In this paper, the influence of thermal or mechanical pretreatments on the formation of U₂C₃ was investigated experimentally and is discussed in connection with other published data. It was found that the relative increase of the U₂C₃ nuclei density caused by grinding corresponds to the increase in surface of the ground material. A quantitative kinetic examination of the U₂C₃ formation was made and the activation energy for the synthetic reaction in powder was found to be $\text{394 ± 30}$ kJ/mol.     

Formation of U2C3 in the reaction of UC2 with UO2

The formation of U₂C₃ by the reaction of UC₂ with UO₂ has been studied by chemical and X-ray analyses at temperatures between 1400 and 1700 °C in vacuo. The reaction is represented by 7 UC₂ + UO₂ → 4 U₂C₃ + 2 CO.     

Etude thermodynamique du systeme U-O-C par mesure des pressions d'oxyde de carbone a l'equilibre

Reaction in UO₂ /graphite mixtures, leading to carbon monoxide evolution, was investigated. Isotherms were established in the range 1720–1870°C, representing CO pressure as a function of O/U and C/U ratios. Mono- and bivariant equilibrium pressures and phase diagrams are established. This has made it possible to deduce the composition of the solid phases at high temperatures. The ‘UO₂’ phase contains a few vacancies while ‘UC₂’' (in equilibrium with ‘UO₂’ and C) does not, but contains 0.06 at % of oxygen substituted for carbon. ‘UC₂’ in equilibrium with UO_1,97 and U(C,O) is strongly hypostoichiometric and contains little oxygen (UC_1.63O_0.03); the monoxycarbide, hyperstoichiometric, has the formula UC_0.95O_0.10. A maximum oxygen solubility of 25% in UC is observed. From these results, the free energies of formation, ΔG_f^O, were calculated for UO_1.97, UC_1.94O_0.06, UC_1.92O_0.06, UC_1.91, UC_1.63O_0.03, UC_1.66, UC_0.95O_0.01, UC and UC_0.75O_0.25     

Contribution a l'etablissement du diagramme de phases du systeme U-C-O a l'aide de la diffractometrie de rayons X a haute temperature sous pression controlee

During the progressive reduction of uranium dioxide by graphite under known pressures of carbon monoxide, we have measured the lattice parameters of the compounds “UO₂” (fcc), α-“UC₂” (tetragonal) and “UC” (fcc), and have evaluated the compositions of these non-stoichiometric phases near 1770°C. By this means, and for the first time thanks to these direct measurements at such high temperatures, it has been possible to establish with precision the range of the main univariant phase fields [“UO₂”, C, α-“UC₂”, CO] and [“UO₂”, α-“UC₂”, “UC”, CO]. The part of the U-C-O diagram thus mapped out completes the earlier isotherm for 1700°C determined by Henry et al.     

Electrical resistivity and lattice parameter of uranium monocarbide

The resistivity dependence of as-cast and annealed UC on temperature (77–300 K) as well as the $\text{C}\text{U}$ ratio have been investigated experimentally. Additionally, lattice constants of UC have been measured in its nonstoichiometric regions. Estimated values of the electrical resistivity of stoichiometric UC (annealed at 1500°C for 3 h) were (10 ± 2) μΩ · cm at liquid-nitrogen temperature and (34 ± 3) μΩ · cm at room temperature, and the value of the lattice constant was (4.958 ± 0.001) Å at room temperature. It was also estimated that 1 at% of carbon vacancies in UC_1?x and oversaturated carbon interstitials in UC_1+x result in resistivity increases of (12 ± 2) μΩ · cm and 6 μΩ · cm, respectively. A very narrow nonstoichiometric region was observed in UC at 1500°C. It might lie between UC_0.98 and UC_1.01     

Change of Composition of UC1-xNx in Nitrogen Gas

The UC-UN solid solution, UC_1-xN_x, exists stably in monophase within a certain range of nitrogen pressure at constant temperature. At the upper limit of this range, UC_1-xN_x, coexists with UC₂, or with carbon, and it precipitates metallic uranium at the lower limit which corresponds to the decomposition pressure of UC_1-xN_x.

In order to measure the decomposition temperature of UC_1-xN_x with given x under constant nitrogen pressure, it is necessary to heat UC_1-xN_x without changing its composition, at a constant pressure.

In the present work, preliminary considerations have been given to change in composition. Experiments were also performed in which the solid solution was heated at a rate of 200°C/min under different nitrogen pressures.

From the results, it is concluded that there exists a temperature range within which the value of x in UC_1-xN_x, is maintained constant when the nitrogen pressure is fixed.     

Nuclear carbonization and gasification of biomass for effective removal of atmospheric CO2

By the process of carbonization of biomass a portion of carbon element in biomass is stabilized as solid carbon, and the remaining portion of carbon, which is the volatile product from carbonization, is converted by the subsequent gasification and conversion process to carbon-neutral synthetic fuels, which can replace the fossil derived fuels currently used.In these processes, nuclear energy can effectively be utilized for supplying energy, thus avoiding the CO₂ emission from any biomass or fossil combustion. By utilizing nuclear energy, most of the carbons in biomass are converted to either stabilized solid carbon or carbon-neutral fuels.Thus, significant amount of CO₂ can efficiently be removed from the atmosphere by processing a part of annual growth of biomass, which leads to the decrease of atmospheric CO₂ concentration.     

Laser melting of uranium carbides

In the context of the material research aimed at supporting the development of nuclear plants of the fourth Generation, renewed interest has recently arisen in carbide fuels. A profound understanding of the behaviour of nuclear materials in extreme conditions is of prime importance for the analysis of the operation limits of nuclear fuels, and prediction of possible nuclear reactor accidents. In this context, the main goal of the present paper is to demonstrate the feasibility of laser induced melting experiments on stoichiometric uranium carbides; UC, UC_1.5 and UC₂. Measurements were performed, at temperatures around 3000 K, under a few bars of inert gas in order to minimise vaporisation and oxidation effects, which may occur at these temperatures. Moreover, a recently developed investigation method has been employed, based on in situ analysis of the sample surface reflectivity evolution during melting. Current results, 2781 K for the melting point of UC, 2665 K for the solidus and 2681 K for the liquidus of U₂C₃, 2754 K for the solidus and 2770 K for the liquidus of UC₂, are in fair agreement with early publications where the melting behaviour of uranium carbides was investigated by traditional furnace melting methods. Further information has been obtained in the current research about the non-congruent (solidus-liquidus) melting of certain carbides, which suggest that a solidus-liquidus scheme is followed by higher ratio carbides, possibly even for UC₂.     

Laser-joined Al2O3 and ZrO2 ceramics for high-temperature applications

A laser process is presented that has been specially developed for joining oxide ceramics such as zirconium oxide (ZrO₂) and aluminium oxide (Al₂O₃). It details, by way of example, the design of the laser process applied for to producing both Al₂O₃-Al₂O₃ and ZrO₂-ZrO₂ joints using siliceous glasses as fillers.The heat source used was a continuous wave diode laser with a wavelength range of 808-1010 nm. Glasses of the SiO₂-Al₂O₃-B₂O₃-MeO system were developed as high-temperature resistant brazing fillers whose expansion coefficients, in particular, were optimally adapted to those of the ceramics to be joined. Specially designed measuring devices help to determine both the temperature-dependent emission coefficients and the synchronously determined proportions of reflection and transmission.The glass-ceramic joints produced are free from gas inclusions and macroscopic defects and exhibit a homogenous structure. The average strength values achieved were 158 MPa for the Al₂O₃ system and 190 MPa for the ZrO₂ system, respectively.     

X-ray diffraction of UC-UC2 and UC-UN alloys at elevated temperatures

Unit-cell dimensions of U-C-N alloys were measured at temperatures of 760 to 2250 °C with an X-ray diffractometer. Lattice dimensions of the face-centered cubic phase in both the UC-UC₂ and UC-UN solutions are found to be linear functions of the UC mole fraction. In carbon saturated body-centered tetragonal α-UC₂ between 1000 and 1400 °C, a small but distinct decrease in rate of volume change (slope) occurs with increasing temperature. The data suggest that the C/U ratio in α-UC₂ at 1000 °C and below is 1.97± 0.03, a value in excess of that prevailing in carbon-saturated α-UC₂ at higher temperatures.     

The effect of powder-particle size on the compatibility behaviour of stainless steels with uranium carbide powder

Vacuum-bonded compatibility tests between slightly hyperstoichiometric UC powders (with a wide range of BET specific surface areas) and stainless steels 304, 304L and 316, were carried out at 600°C and 700°C. It was found that UC powders with BET specific surface areas below 200 m²/kg do not cause any measurable embrittlement, whereas finer powders with BET specific surface areas up to about 1000 m²/kg cause rather substantial embrittlement of the steels. It was further established that the transfer of carbon from the fuel to the cladding occurs largely through the gas phase in this type of test. It is also suggested that in the very fine powders which caused carburisation of the steels, (a) the surface oxidation of these powders was partly indirectly responsible for the carburisation of the steels, but (b) that further carbon transfer from the UC₂, phase in the fuel occurs more rapidly with particles with a very high BET specific surface area than with coarser particles.     

The mechanisms of in-pile fission gas release from UO2

In-pile release mechanisms of fission gas from UO₂ at low temperatures were studied. The release of ¹³³Xe, ¹³⁵Xe, ¹³⁸Xe, ^85mKr, ⁸⁸Kr and ⁸⁷Kr from a sintered UO₂ pellet was measured at temperatures ranging from 250 to 930°C using a graphite specimen holder. The release from the holder, in which a fraction of fission gas was recoil-implanted, was subtracted to obtain the net release from the UO₂ pellet. Knock-out release from the UO₂ was measured directly, and it was found that it was not the main release mechanism, at least not for short-lived nuclides. A ‘pseudo-recoil’ release model is proposed to explain the low temperature release under irradiation. In the model, some of the defects produced by fission fragments act as short-lived carriers for fission gas.     

Etude aux rayons X a haute temperature du systeme U-C-N en presence d'un exces de graphite et sous pression controlee d'azote

Pressed samples of initial compositions $\text{“UC}{}_2\text{” + 1 C}$ and $\text{“UC}{}_2\text{” + 2 C}$ were made to undergo progressive reaction under a gradually increasing pressure of nitrogen () in a high-temperature X-ray diffraction apparatus which operated in the range 800–2000°C. In this way, the various equilibrium domains [ α or β “UC₂”, C, N₂], [α or β “UC₂”, UC_yN_x, C, N₂] and [ UC_yN_x, C, N₂], were successively made manifest; this permitted the lattice parameters and equilibrium pressures of the carbonitrides to be determined, and also their standard free enthalpies of formation and compositions to be evaluated. It was established, in particular, that the “dicaibide” and the “monocarbonitride” in equilibrium on the monovariant “plateau” are hypostoichiometric and hyperstoichiometric, respectively; the stability of these compounds is enhanced by their large nitrogen contents, which increase with the temperature. It was also shown that above 1410°C, a temperature rise leads to a substantial drop in the nitrogen content of the virtually stoichiometric monocarbonitride which is in equilibrium with graphite under Torr. Between 800°C and 1410°C and Torr, an excess of carbon coexists with α “U₂N₃” and β “U₂N₃”, and the lattice parameters of these phases were likewise determined.     

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.     

First-principles study of UC2 and U2C3

The electronic structure and mechanical properties of UC₂ and U₂C₃ have been systematically investigated using first-principles calculations by the projector-augmented-wave (PAW) method. Furthermore, in order to describe precisely the strong on-site Coulomb repulsion among the localized U 5f electrons, we adopt the generalized gradient approximation +U formalisms for the exchange-correlation term. We show that our calculated structural parameters and electronic properties for UC₂ and U₂C₃ are in good agreement with the experimental data by choosing an appropriate Hubbard U = 3 eV. As for the chemical bonding nature, the contour plot of charge density and total density of states suggest that UC₂ and U₂C₃ are metallic mainly contributed by the 5f electrons, mixed with significant covalent component resulted from the strong CC bonds. The present results also illustrate that the metal–carbon (UC) bonding and the carbon–carbon covalent bonding in U₂C₃ are somewhat weaker than those in UC₂, leading to the weaker thermodynamic stability at high temperature as observed by experiments.     

Vaporization and Surface Ionization of LaC2, CeC2, PrC2, NdC2, ThC2 and UC2

The Langmuir vaporization and the surface ionization of LaC₂, CeC₂, PrC₂, NdC₂, ThC₂ and UC₂ from a heated graphite filament have been studied mass spectrometrically. It was found that there were present small amounts of neutral and ionic metal dicarbide molecules in addition to neutral and ionic metal atoms in the LaC₂, CeC₂, PrC₂, ThC₂ and UC₂-C systems with the exception of NdC₂-C, where neither neutral nor ionic metal dicarbide molecules were observed. The reason for this exceptional behavior of the NdC₂-C system is explained by the very small vaporization coefficients, as estimated from the measurements of neutral MC₂/M ratios and ionic MC₂/M ratios.

From the measurements of the heats of vaporization, it was surmised that the ionization potential of Th measured by the surface ionization comparison technique might be too high.     

Review of A2B2O7 pyrochlore response to irradiation and pressure

This article reviews recent research on swift heavy-ion irradiations and high-pressure studies on pyrochlores of the Gd₂Zr_2−xTi_xO₇ binary [1], [2], [3] and [4]. Applying three complementary analytical techniques (synchrotron X-ray diffraction, Raman spectroscopy and transmission electron microscopy) allowed for the investigation of the response of pyrochlore to irradiation and/or pressure. The chemical composition of pyrochlore has a strong effect on the character and energetics of the type of structural modifications that can be obtained under pressure or irradiation: For Ti-rich pyrochlores, the crystalline-to-amorphous transition is the dominant process. When Zr is substituted for Ti, an order-disorder transformation to the defect-fluorite structure becomes the increasingly dominant process. Except for Gd₂Zr₂O₇, single ion tracks in pyrochlore consist of an amorphous core, surrounded by a crystalline, but disordered, defect-fluorite shell. This shell is surrounded by a defect-rich pyrochlore region. In contrast to similar effects observed when pressure or irradiation are applied separately, the response of the pyrochlore structure is significantly different when it is exposed simultaneously to pressure and irradiation. The combination of relativistic heavy ions with high pressure results in the formation of a new metastable pyrochlore phase. TEM and quantum-mechanical calculations suggest that these novel structural modifications are caused by the formation of nanocrystals and the modified energetics of nanomaterials.     

News item

Stoichiometric UC can be quantitatively reacted with the stoichiometric amount of ZnS to yield a mixture of US and C by heating at 800–1000°C in vacuum or inert atmosphere. During the reaction the Zn is liberated as vapour. Intimately blending the US+C mixture with the requisite amount of stoichiometric UO₂ and heating at 1700°C under vacuum results in a reaction between the C and UO₂, $\text{2}\text{3}$ of the C being evolved as CO, the US under these conditions being inert. The UC formed dissolves in the US to give a solid solution of composition 75 mol % US-25 mol % UC. If desired, UC₂ can be used in place of UC, the initial reaction temperature being somewhat lower. UO₂ can be replaced by another uranium oxide such as U₃O₈. Solid solutions of reduced US content can be prepared by decreasing the amount of ZnS and uranium oxide employed, but attempts to increase the US mol % to over 90 by repeating the process on a 75 mol % US-25 mol % UC solid solution have not been entirely successful. The products obtained were probably solid solutions of US with nearly 10 mol % UC₂ and a small amount of a UOSUO₂ phase. The results of chemical analysis and examinationsby X-ray diffraction and metallurgical techniques are reported.