Plutonium Carbides Research Articles

Some Thermodynamic Features of Uranium–Plutonium Nitride Fuel in the Course of Burnup

Calculation studies on the effect of carbon and oxygen impurities on the chemical and phase compositions of nitride uranium–plutonium fuel in the course of burnup are performed using the IVTANTHERMO code. It is shown that the number of moles of UN decreases with increasing burnup level, whereas UN1.466, UN1.54, and UN1.73 exhibit a considerable increase. The presence of oxygen and carbon impurities causes an increase in the content of the UN1.466, UN1.54 and UN1.73 phases in the initial fuel by several orders of magnitude, in particular, at a relatively low temperature. At the same time, the presence of impurities abruptly reduces the content of free uranium in unburned fuel. Plutonium in the considered system is contained in form of Pu, PuC, PuC2, Pu2C3, and PuN. Plutonium carbides, as well as uranium carbides, are formed in small amounts. Most of the plutonium remains in the form of nitride PuN, whereas unbound Pu is present only in the areas with a low burnup level and high temperatures.

Thermal radiative and thermodynamic properties of solid and liquid uranium and plutonium carbides in the visible–near-infrared range

The knowledge of thermal radiative and thermodynamic properties of uranium and plutonium carbides under extreme conditions is essential for designing a new metallic fuel materials for next generation of a nuclear reactor. The present work is devoted to the study of the thermal radiative and thermodynamic properties of liquid and solid uranium and plutonium carbides at their melting/freezing temperatures. The Stefan-Boltzmann law, total energy density, number density of photons, Helmholtz free energy density, internal energy density, enthalpy density, entropy density, heat capacity at constant volume, pressure, and normal total emissivity are calculated using experimental data for the frequency dependence of the normal spectral emissivity of liquid and solid uranium and plutonium carbides in the visible-near infrared range. It is shown that the thermal radiative and thermodynamic functions of uranium carbide have a slight difference during liquid-to-solid transition. Unlike UC, such a difference between these functions have not been established for plutonium carbide. The calculated values for the normal total emissivity of uranium and plutonium carbides at their melting temperatures is in good agreement with experimental data. The obtained results allow to calculate the thermal radiative and thermodynamic properties of liquid and solid uranium and plutonium carbides for any size of samples. Based on the model of Hagen-Rubens and the Wiedemann-Franz law, a new method to determine the thermal conductivity of metals and carbides at the melting points is proposed.

Molecular Structure and Bonding in Plutonium Carbides: A Theoretical Study of PuC3.

The most relevant species of plutonium tricarbide were characterized using theoretical methods. The global minimum is predicted to be a fan structure where the plutonium atom is bonded to a quasi-linear C3 unit. A rhombic isomer, shown to be a bicyclic species with transannular C-C bonding, lies about 39 kJ/mol above the fan isomer. A linear PuCCC isomer and a three-membered ring CPuC2 isomer were found to be higher in energy (150 and 195 kJ/mol, respectively, above the predicted global minimum). The possible processes for the formation of these species are discussed, and the IR spectra were predicted to help in possible experimental detection. The nature of the Pu-C interaction has been analyzed in terms of a topological analysis of the electronic density, showing that Pu-C bonding is essentially ionic with a certain degree of covalent character.

Fabrication of mixed uranium–plutonium carbide fuel pellets with a low oxygen content and an open-pore microstructure

Mixed uranium–plutonium carbides are considered as potential fuels for Generation IV fast reactors. Compared to that of oxide fuels, their fabrication with regard to required specifications is more difficult. Carbides are obtained by a two-step procedure from the mixture of UO2, PuO2 and graphite powders. Due to their extreme reactivity with oxygen and moisture, fabrication and handling of carbide fuels are performed in glove boxes maintained under a dynamic flow of nitrogen providing operational safety. Nevertheless, oxygen pickup during processing of the material seems unavoidable and thus must be limited by suitable procedures. The oxygen content in the carbides can vary over a wide range of values, which can affect the green density and hence the sintered density. The addition of zinc stearate in a suitable amount to the fuel powder leads to an open-pore microstructure. A high open porosity contributes to minimizing the residual oxygen content in the sintered pellets.

Experimental study of UC polycrystals in the prospect of improving the as-fabricated sample purity

Uranium and plutonium carbides are candidate fuels for Generation IV nuclear reactors. This study is focused on the characterization of uranium monocarbide samples. The successive fabrication steps were carried out under atmospheres containing low oxygen and moisture concentrations (typically less than 100ppm) but sample transfers occurred in air. Six samples were sliced from four pellets elaborated by carbothermic reaction under vacuum. Little presence of UC2 is expected in these samples. The α-UC2 phase was indeed detected within one of these UC samples during an XRD experiment performed with synchrotron radiation. Moreover, oxygen content at the surface of these samples was depth profiled using a recently developed nuclear reaction analysis method. Large oxygen concentrations were measured in the first micron below the sample surface and particularly in the first 100–150nm. UC2 inclusions were found to be more oxidized than the surrounding matrix. This work points out to the fact that more care must be given at each step of UC fabrication since the material readily reacts with oxygen and moisture. A new glovebox facility using a highly purified atmosphere is currently being built in order to obtain single phase UC samples of better purity.

High temperature radiance spectroscopy measurements of solid and liquid uranium and plutonium carbides

In this work, an experimental study of the radiance of liquid and solid uranium and plutonium carbides at wavelengths 550nm⩽λ⩽920nm is reported. A fast multi-channel spectro-pyrometer has been employed for the radiance measurements of samples heated up to and beyond their melting point by laser irradiation. The melting temperature of uranium monocarbide, soundly established at 2780K, has been taken as a radiance reference. Based on it, a wavelength-dependence has been obtained for the high-temperature spectral emissivity of some uranium carbides (1⩽C/U⩽2). Similarly, the peritectic temperature of plutonium monocarbide (1900K) has been used as a reference for plutonium monocarbide and sesquicarbide. The present spectral emissivities of solid uranium and plutonium carbides are close to 0.5 at 650nm, in agreement with previous literature values. However, their high temperature behaviour, values in the liquid, and carbon-content and wavelength dependencies in the visible-near infrared range have been determined here for the first time. Liquid uranium carbide seems to interact with electromagnetic radiation in a more metallic way than does the solid, whereas a similar effect has not been observed for plutonium carbides.The current emissivity values have also been used to convert the measured radiance spectra into real temperature, and thus perform a thermal analysis of the laser heated samples. Some high-temperature phase boundaries in the systems U–C and Pu–C are shortly discussed on the basis of the current results.

History of fast reactor fuel development

The first fast breeder reactors, constructed in the 1945–1960 time period, used metallic fuels composed of uranium, plutonium, or their alloys. They were chosen because most existing reactor operating experience had been obtained on metallic fuels and because they provided the highest breeding ratios. Difficulties in obtaining adequate dimensional stability in metallic fuel elements under conditions of high fuel burnup led in the 1960s to the virtual worldwide choice of ceramic fuels. Although ceramic fuels provide lower breeding performance, this objective is no longer an important consideration in most national programs. Mixed uranium and plutonium dioxide became the ceramic fuel that has received the widest use. The more advanced ceramic fuels, mixed uranium and plutonium carbides and nitrides, continue under development. More recently, metal fuel elements of improved design have joined ceramic fuels in achieving goal burnups of 15 to 20 percent. Low-swelling fuel cladding alloys have also been continuously developed to deal with the unexpected problem of void formation in stainless steels subjected to fast neutron irradiation, a phenomenon first observed in the 1960s.

A thermoanalytical study of the ignition behaviour of uranium dicarbide

A knowledge of the ignition behaviour of uranium and plutonium carbides is necessary to understand their high reactivity, arrive at procedures for their proper handling in various reactor and laboratory applications, and also to examine the conditions for a controlled oxidation of these carbides. This paper presents a study of the ignition behaviour of uranium dicarbide employing a thermoanalytical method. The onset of ignition of the carbide under various heating rates was investigated and the influence of the experimental variables was also examined.

The chemical thermodynamics of nuclear materials: IV. The high-temperature enthalpies of plutonium monocarbide and plutonium sesquicarbide

The high-temperature enthalpies of plutonium monocarbide and plutonium sesquicarbide have been determined with a copper-block calorimeter of the isoperibol type. The experimental enthalpy data, which were measured relative to 298 K, covered the temperature range from 400 to 1500 K. The calculation of the temperature rise of the calorimeter takes into account the added heat evolution from the radioactive decay of the plutonium samples. These enthalpy results, combined with the heat capacity and entropy of the respective carbide at 298 K available from the literature, have made it possible to generate tables of thermodynamic functions for the plutonium carbides. The behaviour of the heat capacities of both of the plutonium carbides, i.e., a relatively steep increase in the heat capacity as the temperature increases, may be attributed to the formation of vacancies within the crystal lattice although a theoretical treatment of this phenomenon is not given.

The low temperature specific heats of three plutonium carbides

The low temperature specific heats of plutonium monocarbide and of two samples of plutonium sesquicarbide are reported. The monocarbide shows a small anomaly superimposed on the normal sigmoid form of the specific heat curve, associated with magnetic ordering at the Néel temperature of 45 K. The sesquicarbides do not show magnetic ordering. The specific heats and derived entropies are given for comparison with other published data and with the high temperature data in the following paper.

Automat zur kohlenstoffbestimmung in uran- und plutonium-carbiden

Instrumentation, consisting largely of commercial components, is described for the automatic determination of carbon in uranium and plutonium carbides. For the determination, the sample is burned in oxygen and the change in electric conductivity of a sodium hydroxide solution during the absorption of the formed carbon dioxide is measured. Voltage values corresponding to the initial and final conductivity, the weights and the sample identification are stored on paper tape for final computer evaluation. The standard deviation of the method is 10 μg of carbon, and the maximum capacity is 10 mg of carbon. The time demand for one determination is 11 min.

Thermal Expansion and Phase Equilibria of the Carbon-Saturated Plutonium Carbides

An x-ray diffractometer furnace has been developed for use in the study of the high-temperature properties of refractory compounds of plutonium. The furnace design is based on a resistance heated strip which serves as the sample support. The mechanics of the support system are such that the location of the scattering center of the sample is maintained throughout the temperature range of the instrument, i.e., room temperature to 2500°C. The thermal expansion characteristics of carbon saturated Pu2C3 and PuC2 were determined. A least-squares fit of the lattice dimension data for Pu2C3 in the temperature interval from room temperature to 1600°C yielded Δa/a0=(1.29±0.02×10−5)(T−25)+(2.07±0.14×10−9)(T−25)2,where a is the lattice dimension in Å, and T is the temperature in °C. A least-squares fit of the lattice dimension data for fcc PuC2 in the temperature interval from 1660°C to 2000°C yielded Δa/a0=(17.9±1.6×10−6)(T−1660).

Vaporisation des carbures de plutonium

We have measured the vapour pressure of the different plutonium carbides by the Knudsen effusion method. We have especially studied the domains PuC+Pu 2C 3, Pu 2C 3 + C, PuC 2 + C and Pu 2C 3+PuC 2. We have evidenced the relation between the PuC diagram and the vaporization curves on the one hand and the very large influence of oxygen in vaporization phenomena on the other hand. The following energies of formation were obtained: δ H 298( PuC 0·9) = −10·5 Kcal δ H 298( PuC 1·5) = −15·9 Kcal they are compared with the results obtained by other thermodynamic measurements (calorimetry f.e.m. methods, determination of equilibrium pressures in PuCO system).

A simple method for the determination of carbon in uranium and plutonium carbides

A simple method for the determination of combined and free carbon in the carbides of uranium and plutonium is based on the examination of the oxidation of carbide and free carbon thermogravimetrically. The combined carbon (carbide) oxidises to carbon dioxide in air below 500° while the free carbon oxidises above 600°. This allows the determination of both combined and free carbon in a single sample, using a furnace tube and a conventional method for carbon dioxide.

965. The preparation and properties of some plutonium compounds. Part VII. Plutonium carbides

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