Uranium monocarbide

Uranium monocarbide is of interest as a possible nuclear fuel and material for nuclear thermoelectrical transformations. In order to accurately define the effect of the conditions for preparing uranium monocarbide (UC) on its composition, studies were made which established the optimum regime for preparing UC with a stoichiometric composition.Studies were made of the conditions for sintering and hot pressing of UC powder and also conditions for sintering UC + U alloys, giving specimens with a porosity of about 5%. The specific gravity of UC powder determined by a pycnometer is 12.97 ± 0.09 g/cm³, microhardness of the phase-923 ± 56 kg/mm².The thermal conductivity of UC in the range 100–700 °C varies from 0.028 to 0.04 cal/cm. sec.deg; the mean thermal coefficient of linear expansion in the range 20–1500 ° is 11.6 · 10^–6. Specimens of UC subjected to cyclic heat-treatment in the range 200–1000 ° withstood 500 cycles without failure. Specimens of UC + U withstood more than 1000 cycles.     

Properties of uranium vacancies in uranium monocarbide

From the excess resistivity obtained by quenching from temperatures between 1300 and 1600 °C a formation energy of 1.7 eV is determined. The quenchedin resistivity recovers with an activation energy of 2.2 eV in the temperature region between 400 and 600 °C. These two values are attributed to the formation and migration activation energies of uranium vacancies, respectively. It is further shown that uranium monocarbide remains single-phase, if the deviation from stoichiometry does not exceed more than a few tenths of one percent and if gases (mainly oxygen) are in solid solution.     

Adiabatic elastic constants of uranium monocarbide

Using the phase-comparison technique, the room temperature adiabatic elastic constants of UC single crystals grown by the radio-frequency, induction heated floating-zone technique have been measured as a function of carbon composition (4.52–4.96 wt% C). The temperature dependence of the moduli was measured from 80 to 900 °K in a hypostoichiometric sample. There is a systematic increase of c₄₄ with increasing carbon concentration. The elastic-moduli values are compared with published values for some alkali halides and refractory metal monocarbides. An increase in temperature results in a linear decrease in c₁₁ and c, but c₄₄ increases with temperature to a maximum at 80°C. The temperature coefficients of the moduli at T ? 80 °C are comparable to those observed in the alkali halides. These results indicate that the ionic crystal character of the interatomic bonds increases with temperature and that the carbon concentration in the UC phase can deviate from stoichiometric composition.     

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     

Redetermination of the carbon-rich phase boundary of uranium monocarbide

The phase boundary between UC_1+x and β UC₂ in the temperature range of 1900–2100°C and for C/U ranging from the stoichiometric up to the congruently vaporizing composition (1.00–1.08) was redetermined. Samples containing originally 4.75–4.80 wt % C were heated at constant temperatures under vacuum in a high-temperature thermobalance. Due to preferential vaporization of U the C/U ratio continuously increased under these conditions until precipitation of the second phase took place which could be deduced from the continuously recorded weight-change curves. The respective equilibrium phase boundary thus derived is markedly different from that obtained by earlier investigators on the basis of quenching experiments. Precipitation of β UC₂ occurs at much lower C/U ratios than assumed up to now.     

The effect of Fe,Ni and W impurities on uranium diffusion in uranium monocarbide

The self-diffusion of uranium with U-233 as tracer was measured in stoichiometric UC which was doped with Fe, Ni or W impurities. The impurities were added by flash evaporation on the UC and subsequent diffusion into the UC. Uranium diffusion was increased in the doped samples as compared to undoped UC at all temperatures studied (1380 to 2200°C). The increase was most pronounced (more than a factor of 100) at low temperatures. Simultaneously, pronounced grain-boundary penetration was observed at low temperatures, possibly due to eutectic formation at the grain boundaries. The ratio of grain boundary to lattice diffusion coefficients at 1380°C was of the order of 3 × 10⁴. The present data serve to explain some of the scatter of the literature data and correlate well with the known increased rate of sintering of Ni-doped UC.     

An atomistic approach to self-diffusion in uranium dioxide

The formation and mobility of point defects in UO₂ have been studied within the framework of the Density Functional Theory. The ab initio Projector Augmented Wave method is used to determine the formation and migration energies of defects. The results relative to intrinsic point defect formation energies using the Generalized Gradient Approximation (GGA) and GGA+U approximations for the exchange-correlation interactions are reported and compared to experimental data. The GGA and GGA+U approximations yield different formation energies for both Frenkel pairs and Schottky trios, showing that the 5f electron correlations have a strong influence on the defect formation energies. Using GGA, various migration mechanisms were investigated for oxygen and uranium defects. For oxygen defects, the calculations show that both a vacancy and an indirect interstitial mechanism have the lowest associated migration energies, 1.2 and 1.1 eV respectively. As regards uranium defects, a vacancy mechanism appears energetically more favourable with a migration energy of 4.4 eV, confirming that oxygen atoms are much more mobile in UO₂ than uranium atoms. Those results are discussed in the light of experimentally determined activation energies for diffusion.     

Kinetics and mechanisms of carbon self-diffusion in uranium carbides

Depending upon the temperature, uranium carbide, $\text{UC}{}_{\mathrm{x}}$, has the B1 (NaCl type) crystal structure and is characterized by a wide range of the carbon-to-uranium ratio ($\text{0.95 < x < 2.0}$). In this report, the kinetics of carbon diffusion in uranium carbides are examined and mechanisms are proposed which explain the diffusivities determined by numerous investigators for compositions in the range $\text{0.95 < x < 2.0}$. When the composition is hypostoichiometric, the defect structure in the carbon sub-lattice consists of constitutional vacancies; the concentration of these vacancies is fixed by the carbon-to-uranium ratio and equals ($\text{1?x}$). Carbon diffusion occurs by the random migration of carbon atoms from one octahedral site to an adjacent vacant octahedral site. The defect structure in the carbon sub-lattice of stoichiometric and hyperstoichiometric uranium carbides consists of C₂ groups and single carbon atoms in the octahedral sites; the concentration of unoccupied octahedral sites (vacancies) is negligibly small. Diffusion occurs by the random migration of single carbon atoms from doubly occupied sites to adjacent singly occupied octahedral sites. Quantitative expressions of these mechanisms are developed which accurately describe the diffusion kinetics. The preponderant discordance in the experimental results of other investigators are reviewed and examined; it is concluded that the wide variability in these results was probably caused by microstructural effects,     

Calorimetric investigations on stoichiometric barium and uranium oxides

Heat capacities and enthalpy increments of barium uranates: BaU₂O₇(s), Ba₂U₃O₁₁(s), Ba_2.875UO_5.875(s) and Ba₃UO₆(s) were measured using a differential scanning calorimeter and a high-temperature Calvet calorimeter. The heat capacities and enthalpy increments were measured in the temperature range 126-304 K and 299-1011 K, respectively. A set of self consistent thermodynamic functions such as entropy, Gibbs energy function, heat capacity and Gibbs energy and enthalpy of formation values for BaU₂O₇(s), Ba₂U₃O₁₁(s), Ba_2.875UO_5.875(s) and Ba₃UO₆(s) have been computed for the first time using the data obtained in the present study and other available experimental data.     

Fission-enhanced self-diffusion of uranium in UO2 and UC

In-pile self-diffusion measurements in stoichiometric UO₂ sinters and single crystals and in arc-cast stoichiometric UC have been performed using the thin layer condition and ²³³U as tracer. The nominal irradiation temperature was 900°C. The resulting diffusion coefficients $\text{D1}$ of $\text{1.5 × 10}{}^{\mathrm{?16}}\text{cm}{}^2\text{· sec}{}^{\mathrm{?1}}$ for UO₂ and $\text{2.2 × 10}{}^{\mathrm{?17}}\text{cm}{}^2\text{· sec}{}^{\mathrm{?1}}$ for UC for a fission rate $\text{S}$ of 1 × 10¹³f/cm³ · sec represent radiation enhanced diffusion and are higher by factors of 10³ to 10⁴ than (extrapolated) coefficients of thermal diffusion. The data are of immediate relevance for understanding and predicting such important quantities as in-pile sintering and densification, diffusion controlled creep and fission gas behavior in the outer zones of the fuel. They are at the upper limit of expected values.     

MUA型微量铀分析仪测定801矿铀的不确定度评定

测量不确定度是一个合理表征测量结果分散性的参数,体现着测量质量的高低。分析了用MUA分析仪测量801矿铀含量过程中不确定度的来源并计算各不确定度分量,得出合成标准不确定度和扩展不确定度,结果表明:MUA分析仪测量801矿铀的测量不确定度小,可信度高,其测量不确定度来源的主要因素为重复性实验不确定度。