Oxygen Chemical Diffusion Research Articles

Studies on the oxidation of uranium-plutonium oxides by non-isothermal kinetics

Kinetic data on the oxidation and oxygen chemical diffusion of uranium-plutonium oxides is important in assessing the chemical behaviour of the oxide. The oxidation of uranium-plutonium oxides, both hypostoichiometric [(U,Pu)O2−x] and hyperstoichiometric [(U,Pu)O2+x], with varying plutonium content, was studied by non-isothermal methods of thermogravimetry. The kinetics of the oxidation were found to be controlled by oxygen chemical diffusion, with activation energies in the range 7–12 kcal mole−1 for (U, Pu)O2−x, and 15–19 kcal mole−1 for (U, Pu)O2+x. The effect of variation of plutonium content on the kinetics was negligible.

The chemical diffusivity of oxygen in UO 2 + x

The chemical diffusivity of oxygen in UO 2 + x

Oxygen self and chemical diffusion coefficients in [formula omitted

The experimental data available on oxygen self-diffusion coefficients D ∗ in UO 2±x have been summarized. Their dependence on temperature and stoichiometry was described by a simple theoretical approach. The oxygen self-diffusion coefficients D ∗ (x, T) in UO 2+x were converted into oxygen chemical diffusion coefficients D(x, T) , using a thermodynamic relation which was derived from Darken's equation. The oxygen potentials of UO 2+x appearing in the thermodynamic relation between D ∗ and D have been calculated from existing Δ G O 2 - models . The calculated D values show good agreement with the few measured oxygen chemical diffusion coefficients in UO 2+x .

Oxygen diffusion in β-Zircaloy

The diffusion of oxygen in β-Zircaloy-4 has been studied from 900 to 1500°C with survey measurements for β-zirconium and β-Zircaloy-2. The tracer diffusivity was measured over the entire temperature range and the chemical diffusivity from 1100 to 1450°C. The experiments were performed by using oxygen-18 as the tracer and activating it by proton bombardment. Some complementary measurements were made using Auger Electron Spectroscopy. The results indicated that the tracer and chemical diffusivity of oxygen in β-Zircaloy-4 are statistically identical, and that there is no oxygen concentration dependence over the oxygen concentration range studied, 0.1 to 0.6 wt.%. The temperature dependence of the diffusivity of 18O from 1000 to 1500°C is given by D = 2.48 × 10 −2 exp(−28200/ RT) cm 2/sec. The results for the β-zirconium and β-Zircaloy-2 indicated that the compositional differences between the three host materials exert no influence upon the oxygen diffusivity. An examination of the activation entropy, calculated assuming that Snoek's model describes the diffusion process, indicates that this model probably is not appropriate.

The chemical diffusivity of oxygen in oxygen deficient rutile for compositions close to TiO 2

The chemical diffusivity coefficient of oxygen has been measured from 1180 to 1520 K in oxygen deficient rutile TiO 2− x , in following the electrical conductivity change after a step in the chemical potential of oxygen. Comparison of the calculated values of titanium self diffusion and published experimental values in air proves that the behaviour of the system near the stoichiometric ratio may be formulated within the framework of a point defect model.

Electrically renewable and controllable oxygen getter

A brief review of the electric properties of stabilized zirconia indicates that it is an interesting candidate as a basic material for a renewable oxygen getter. Experimental results have confirmed this statement. The material can be electrically reduced simply by passing a dc current through it. Black zirconia thus obtained has an affinity for oxygen close to that of titanium and a high oxygen chemical diffusion coefficient. Under moderate current densities the electrical reduction can be performed in a monophasic region. Thus the material simply reoxidizes by dissolving oxygen, no other compound is formed and the process can be cycled without breaking the sample. Such a getter also presents the advantage of having an electric resistance which sharply varies near saturation. This can provide an easy test for controlling the efficiency of the getter. It was also shown that the electrochemical pump and gauge can be coupled in a very convenient gas circuit to test an oxygen getter.

Oxygen Chemical Diffusion Coefficient of Uranium Dioxide

The diffusion of oxygen in UO2+α was measured between 600° and 1100°C. Samples with initial O/U values 2.08 and 2.04 were reduced in hydrogen; reduction was monitored continuously by electrical conductivity measurements. The relation between total conductivity and average O/U ratio was derived, and the diffusion coefficients were obtained. The validity of the electrical conductivity vs O/U relation was checked by measuring weight loss during reduction. Diffusion‐controlled kinetics were confirmed by oxygen gradient measurements. The oxygen chemical diffusion coefficient is given by D= 0.5 exp ‐ This equation agrees with previously determined values of the oxygen chemical diffusion coefficient of UO2+α.