Preparation of UC-rich solid solutions in the UC-US system

A two-stage procedure for the preparation of UC-US solid solutions rich in UC is described. In the first stage, UC is reacted with insufficient carbon to obtain complete conversion of the UO₂ to UC, thus yielding a UC-U cermet. In the second stage, the finally ground cermet is reacted with the stoichiometric amount of zinc sulphide, ZnS, to convert the uranium metal present to US. By this means, UC-US solid solutions have been prepared containing between 3 and 20 mol % US. By the use of uranium disulphide, US₂, in place of ZnS, this maximum figure can be increased to about 33 mol %. Results of chemical analysis and examinations by X-ray diffraction and metallurgical techniques are reported. The method is also applicable to the synthesis of other UC-UX materials where X may be phosphorus or chalcogens other than sulphur.     

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.     

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.     

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     

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     

Kinetic Studies of Fluorination of Uranium Carbides by Fluorine, (II)

The fluorination reaction of uranium dicarbide with fluorine to form UF₆ has been studied in the temperature range between 220° and 300°C using a thermobalance. The overall mechanism of reaction is similar to the case of uranium monocarbide, i.e. in the first step, UC₂ is rapidly converted to UF₄, polymeric fluorocarbon and gaseous fluorocarbon, while in the second step, UF₄ and polymeric fluorocarbon are converted rather slowly to UF6 and gaseous fluorocarbon. The amount of polymer is much larger than the case of uranium monocarbide, its weight ratio to UF₄ being 0.2 to 0.25 in the early stages of the reaction. The fluorination of ₄ to UF₆ always follows the linear law derived from the diminishing sphere model. The apparent activation energy was determined to be 19.5 kcal/mole. Differences in the fluorination between UC and UC₂ are discussed, with the effect of polymer taken into consideration.     

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.     

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.     

The deposition of uranium-sesquicarbide

Deposition of U₂C₃ from an aic-melted mixture of UC + UC₂ ($\text{7.0 ± 0.1}$ wt % carbon) was examined primarily by the change of electrical resistivity as a function of time at fixed temperatures: 1370, 1440, 1575 and 1650°C. The rate constant of the reaction UC + UC₂ → U₂C₃ was investigated in detail. Growth with one constant dimension, which has been named layer growth, was predominate at 1370°C and during the early stages of growth at higher temperatures. Various growth schemes, which are generally involved, yield various activation energies. The rateconstant of thereaction was obtained as $\text{k}{}_{\mathrm{n}}\text{= 1.7 × 10}{}^{\mathrm{?2}}\text{exp(?n × 24.67/RT)}$ sec^?1, where $\text{n}$ depends on growth scheme and is usually a number less than 3. The activation energy of 24.67 kcal/mol in the above equation was obtained from rate constants with similar $\text{n}$ values ($\text{n ≈ 3}$) at 1440 and 1575°C.     

Developing uranium dicarbide-graphite porous materials for the SPES project

Uranium carbide dispersed in graphite was produced under vacuum by means of carbothermic reduction of different uranium oxides (UO₂, U₃O₈ and UO₃), using graphite as the source of carbon. The thermal process was monitored by mass spectrometry and the gas evolution confirmed the reduction of the U₃O₈ and UO₃ oxides to UO₂ before the carbothermic reaction, that started to occur at T > 1000 °C. XRD analysis confirmed the formation of α-UC₂ and of a minor amount of UC. The morphology of the produced uranium carbide was not affected by the oxides employed as the source of uranium.     

Mechanistic study of oxygen thermal diffusion in hyperstoichiometric urania and urania-plutonia solid solutions

Sintered specimens of hyperstoichiometric urania ($\text{O/U = 2.035 to 2.045}$) and urania-plutonia solid solutions (15 mol% Pu, $\text{O/M = 2.003 to 2.061}$) were subjected to longitudinal thermal gradients between 750 and 1580°C in sealed noble metal capsules. Annular UO_2+x samples were equilibrated in radial thermal gradients of 1000 to 2025°C in localized inert gas/CO-CO₂ atm. The results show when impurity carbon is present at levels as low as 3 ppm, a stoichiometry gradient is produced in the solid by oxygen migrating from cool to hot regions. Below 1600°C, oxygen migration occurs almost exclusively through the CO + CO₂ gas mixture generated by reaction between UO_2+x and the carbon impurity. Above 1600°C, a UO₃ distillation path contributes significantly to the overall thermal diffusion process. The experimental data are compared with predicted heats of transport. Some special features of the CO/CO₂ oxygen distribution mechanism are discussed, and some new oxygen potential data are presented for hyperstoichiometric mixed oxide between 1350 and 1500°C.     

Oxidation of uranium carbides in sodium with a small oxygen content

Oxidation of uranium carbide in sodium containing oxygen (5–10 ppm) was studied by means of metallography, X-ray analysis and electron probe microanalysis for various exposure times between 25 and 1240 h at 700 °C. UO₂ and UC₂ phases were found to be formed in the oxidized layer of uranium monocarbide. Furthermore, carbon transfer occurred from carbide to stainless steel through sodium during oxidation.     

Oxidation and Gas Release Behavior of UC,U(C,N) and UN at High Temperature

A study was made of the oxidizing behavior at high temperature (800°–1,800°C) in vacuum of UC, UN and U(C,N) samples containing added oxygen in excess amounts, through observations of gas release, X-ray diffraction analysis, and microphotography.

The oxidation in vacuum of UC and U(C,N) was found to proceed above 1,200°C by stepwise reactions from one temperature interval to the next, the process differing however according to the chemical state of the oxygen present in the samples. In the temperature range below 1,200° C, the UC and U(C,N) samples reacted violently with the free oxygen present in dissolved state, to form UO₂. Between 1,200° and 1,400° C the UO₂ thus produced reacted with the UC or U(C,N), forming solid solutions of U(C,O) and U(C,N,O) respectively: Above 1,600°C, these solid solutions gradually decomposed back into UC and U(C,N), and U. In all stages of oxidation, large amounts of CO—and N₂ in the case of U(C,N)—evolved from the samples as reaction products. In the case of UN, no reaction was observed below 1,200°C, and only oxidized above that temperature to form UO₂ and N₂ by the action of the dissolved oxygen present.

These results indicate that in the case of UC and U(C,N), the quantity of gases evolving from the oxidation is dictated by the total amount of oxygen contained in the samples, while that from UN is dependent on the amount of molecular oxygen alone.     

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.     

Observations on the low-temperature creep properties of uranium carbide under irradiation

The creep behaviour of 97% dense hyperstoichiometric UC has been examined during irradiation in three-point bend tests carried out at 450°C up to a dose of $\text{1.65 × 10}{}^{26}\text{fissions/m}{}^3$. A rapid decrease in measured strain rate with dose was observed at each stress level, nominally steady-state creep being established above $\text{≈ 1 × 10}{}^{26}\text{fissions/m}{}^3$ when the creep rate was a factor of 8 lower than that observed in UO₂ irradiated under identical conditions. Creep rates were found to be directly proportional to stress at high doses. Comparison of results from this test with data from other experiments up to $\text{2 × 10}{}^{25}\text{fissions/m}{}^3$ in compression and tension indicates little variation in the radiation-creep constant between 450°C and 800°C. The creep rate for UC, much lower than that observed in UO₂, is consistent with recently reported determinations of the effective uranium self-diffusion coefficients under irradiation in those materials.     

Formation of uranium mononitride by the reaction of uranium dioxide with carbon in ammonia and a mixture of hydrogen and nitrogen— I synthesis of high purity UN

Formation of uranium mononitride by the reaction between UO₂ and C in an ammonia stream, a mixed 75%H₂-25%N₂ stream and a mixed 8%H₂-92%N₂ stream has been studied at 1400–1600°C. For the formation of high purity UN, there exists a minimum mixing ratio ($\text{C}\text{UO}{}_2$). It largely depends on the reaction atmosphere and temperature. The mixing ratio in the three atmospheres decreases in the following order: ammonia >75%H₂ + 25%N₂ > 8%H₂ + 92%N₂, and it decreases with increasing temperature. The total amount of impurities (carbon + oxygen) in the UN obtained is in the range of 500–1000 ppm. The time necessary for completion of the reaction in ammonia is shorter than in a mixed hydrogennitrogen.     

Retention and release of water by sintered uranium dioxide

The release of water and hydrogen upon heating sintered $\text{UO}{}_2$ pellets was measured by a direct mass spectrometric method of vacuum outgassing. The technique avoids water loss by sample transfer and measures, rather than the cumulative release, the rate of release with a sensitivity of 1 μg of water (as D₂O) per hour. Exposure of high-density $\text{UO}{}_2$ pellets to water (liquid D₂O) results in negligible water adsorption. Water in pellets fabricated with especially high open porosity (5%) was driven off by a linear temperature ramp below 200°C. A drying model for this process was developed and applied to the data. Strongly bound water was introduced into high-density UO₂by sintering in an atmosphere of D₂O and D₂. Release of the water or hydrogen began at ~500°C and was complete only at the melting point of UO₂(2800°C). The release kinetics are not diffusion-controlled; rather the process is governed by the rates of desorption of bound hydrogen-bearing species from at least three binding sites in the solid characterized by interaction energies between 20 and 50 kcal/mol. The D₂O/D₂ ratio of the desorbed gas was > 1 and did not correspond to thermodynamic equilibrium with stoichiometric urania. Hydrogen and water release kinetics are comparable below 2̃000°C, suggesting a common bound precursor. The total hydrogen (as D₂O or D₂) absorbed in the specimens was between 2 and 4 μg/g $\text{UO}{}_2$.     

Interaction between uranium sulphides and oxygen

Study of the oxidation of uranium monosulphide shows that certain phenomena occur at three temperatures: 350–380, 480, and 720°C. In the temperature range 350–380°C, an intensive incorporation of oxygen begins, accompanied by loss of SO₂ and S. Simultaneously UO_2+0.45 and UO₂SO₄ are formed. As the temperature increases, the amount of sulphur remains constant and only oxygen is incorporated. At 540°C the X-ray pattern of the product corresponded to that of U₃O₈, but the composition was UO_3.50 + UO₂SO₄. At higher temperatures the remaining sulphur was burnt and U₃O₈ was obtained. The reaction between uranium disulphide and oxygen proceeds in a similar way, except that at 345°C preferential oxidation of sulphur occurs. Investigation of the isothermal oxidation of US and US₂ at temperatures 250–305° C and under an oxygen pressure of 400 Torr showed that the rate law was initially exponential (lateral growth of oxide film), and that it later became parabolic (diffusion of oxygen through the oxide layer).     

Graphitization of free carbon precipitating through the reaction of UC or UC2 with N2

It has been found by X-ray diffraction that free carbon produced from reaction of UC or UC₂ with nitrogen gas is not graphite but an amorphous carbon. The degree of graphitization increases with increasing temperature and with decreasing pressure of nitrogen gas. Thermodynamic treatment of these amorphous carbons is discussed in terms of the degree of graphitization. The free energies of the amorphous carbons are calculated to be higher by about 500 cal/mol than that of graphite. Furthermore, the influence of the crystal structure of UC or UC₂ on the degree of graphitization is considered qualitatively.     

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.