The reactions of sodium with urania,plutonia and their solid solutions

The thermodynamic conditions for the reactions of sodium with urania, plutonia and solid solutions of urania-plutonia have been determined using an electrochemical method. The thermodynamic oxygen potentials for the phase fields containing liquid sodium, the oxide and the compound $\text{Na}{}_3\text{MO}{}_4$ ($\text{M = U}$, Pu or U_1-X Pu_X) have been measured in an EMF cell with a thoria-yttria electrolyte. Measurements were made in the temperature range 825–1000K and for all these systems studied the values of the equilibrium oxygen potential were similar. Some measurements of oxygen potential were also made for the reaction of sodium with urania-plutonia containing cations which simulate the presence of dissolved fission products. The values for the oxygen potential were higher initially than those obtained for the systems without the addition of cations. However, there was a tendency for the EMF values to drift with time towards those for the pure mixed oxide. The derivation of the valency of the plutonium cations for plutonia and urania-plutonia solid solutions from the equilibrium oxygen potentials is also discussed in terms of the phase relationships and thermodynamic properties of the Pu-O and U-PuO systems. These threshold values of the oxygen potentials are also compared to those determined from a study of the products and the kinetics of the reactions of sodium with pellets of urania-plutonia.     

Plutonium diffusion in hyperstoichiometric mixed uranium-plutonium dioxides

The diffusion behaviour of plutonium in hyperstoichiometric mixed uranium (15%) plutonium dioxides was investigated as a function of temperature and oxygen/ metal ratio. At O/M = 2.10 an activation energy of 86 kcal/mole is found in good agreement with recent results on uranium self-diffusion in UO_{2 + x}. At constant temperature (1400 °C) and varying O/M ratios the diffusion coefficients are found to increase proportional to xⁿ, with n being equal to 2.77. The results are discussed in the light of recent theoretical models and experimental results on cation diffusion in uranium dioxide. Comparison is made to earlier results obtained on nearly stoichiometric mixed oxide.     

A theoretical analysis of the electronic specific heat of solid urania

The electronic activity in condensed urania (UO_2±x) is analysed by modelling the metal ions U⁰, U¹⁺,…,U⁶⁺ as small polarons. In the solid phase this extends previous work treating U³⁺, U⁴⁺ and U⁵⁺ only with the finding that the introduction of higher order polarons has the potential to account for a large proportion of the anomalous rise in the excess specific heat.     

Electrical resistivity of β-zirconium-oxygen solid solutions as a function of oxygen concentration and temperature

The electrical resistivity of recrystallized, pure β-zirconium at temperatures between 900 and 1700°C as well as the resistivity of β-zirconium-oxygen solid solutions at temperatures between 1100 and 1700°C have been measured. It has been found that the resistivity increases linearly with temperature at a constant oxygen concentration and increases linearly with the oxygen concentration at a constant temperature. The supplementary resistivity $\text{Δρ/Δ?}$ caused by the dissolution of oxygen decreases linearly with increasing temperature, i.e. Matthiessen's rule is not obeyed. The temperature (T) and oxygen concentration (?) dependence of the electrical resistivity ρ of β-zirconium-oxygen solid solutions at temperatures between 1100 and 1700°C can be represented by the relation ρ = 91.9 + 2.30 × 10^?2T + 10²f(3.75 ? 1.03 × 10^?3T) with ρ in μΩ · cm, ? in atomic ratio n_O/n_Zr and T in °C. Taking these results and literature data into consideration, a survey of the electrical resistivity of zirconium-oxygen solid solutions, including the α-range, is given.     

The behaviour of single atoms of molybdenum in urania

Computer simulation techniques are used to investigate the behaviour of single atoms of Mo in UO_2±x. In UO_2−x, Mo is calculated to be present as neutral atoms in bound Schottky trio sites. In UO_2+x, most of the Mo is calculated to be in isolated uranium vacancy sites with Mo ionisation increasing with the O/U ratio. The behaviour near stoichiometric composition is more complex and is found to be very sensitive to changes in O/U ratio. An approximation to the free energy change associated with Mo incorporation in urania is plotted as a function of O/U ratio and Mo concentration. Although this plot is found to be in agreement with the observed insoluble character of Mo in urania, at high O/U ratios and very low Mo concentrations, Mo in solution may be preferred over Mo in the gaseous state.     

离子束混合中的扩散、热扩散与化学反应效应

离子束混合的理论研究目前集中在混合机制。已提出的机制主要有碰撞级联混合与辐射增强扩散混合。在文献[5、6]中我们给出了这两种机制统一处理的模型与数学表述。     

Lattice diffusion coefficient of oxygen in lithium oxide

Lattice diffusion coefficients of the oxygen ion in antifluorite-cubic Li₂O were determined employing polycrystalline samples, in the temperature range of 920 ~ 1130°C, by means of the gas-solid isotope exchange and solid-phase analysis technique; use was made of the relationship between the particle-size and grain-size dependences of the grain-boundary enhanced diffusion. The results were described by D = 1.52 × 10³ exp$\text{(?83.3 × 10}{}^3\text{/RT)}\text{cm}{}^2\text{s}$ and showed a good agreement with the previously reported results determined by the gas-phase analysis technique. Diffusion characteristics of the constituent ions in the antifluorite-cubic structure were discussed in comparison with those in fluorite-cubic crystals.     

First-principles investigation on dissolution and diffusion of oxygen in tungsten

Using a first-principles method, we have investigated dissolution and diffusion properties of oxygen (O) in tungsten (W). Single O atom prefers to occupy the tetrahedral interstitial site (TIS). Two interstitial O atoms are attractive and tend to be paired up at two neighboring TIS with a distance of 0.228 nm and a large binding energy of 1.60 eV, which indicates a strong tendency of O clustering in W. O is preferred to diffuse between the most nearest neighboring TIS with a diffusion barrier of 0.17 eV. By the estimation of pre-exponential factor according to an empirical theory, the diffusion coefficient as a function of temperature has been determined, which is 1.50 × 10⁻⁹ m²/s at a typical temperature of 500 K. The results provide a good reference to understand the behavior of O in intrinsic W.     

Assessment of the models of urania fuel oxidation in steam-rich atmosphere

The urania fuel oxidation model for a steam-rich atmosphere developed by Cox et al. using the extensive experimental database has been subsequently used widely but with some inconsistencies in implementations. They are listed and evaluated in this work to help improve the existing models as well as for future model development. The comparison of the equilibrium stoichiometry deviation, calculated using various models, is also given. Small differences between the equilibrium constants used for steam dissociation are found. In one application, the original model for steam dissociation was rewritten in terms of component partial pressures for simplification. It is shown that the modified model differs from the original model by a multiplicative factor (1 ? p _O2 /P _tot). A more complex equation that preserves total pressure is derived. However, the effect on the calculated fuel oxidation is much smaller than the effect of the uncertainty of the equilibrium stoichiometry deviation. The discrepancies in stoichiometry deviations and the estimate of the surface area of a pellet may lead to a double underprediction of fuel oxidation rate.     

The thermal neutron diffusion cooling coefficient in polyethylene

The diffusion cooling coefficient C for thermal neutrons in polyethylene at 20°C has been determined theoretically. Granada's Synthetic Model of the scattering law has been applied to describe the interaction of neutrons with polyethylene. Two approximations of the neutron energy distribution in finite homogeneous systems have been used. The result of the calculation using a rough approximation is C_B=2160 cm⁴ s⁻¹. According to a more advanced formalism which follows Nelkin's analysis of the neutron pulse decay in a finite medium, applying the diffusion theory with transport correction, the value obtained is C=2916 cm⁴ s⁻¹.