Paul A. Tempest

Oxidation of UO2 fuel pellets in air at 503 and 543 K studied using X-ray photoelectron spectroscopy and X-ray diffraction

Abstract An understanding of the low temperature oxidation behaviour of UO2 pellets in air is important in the unlikely event of gas ingress to a fuel can during handling or storage. The main parameter of concern is the production time of U3O8 particulate as a function of temperature. Factors which affect the UO2 → U3O8 transformation have been investigated by sequentially oxidising UO2 fuel pellets in air at 503 and 543 K and monitoring the growth of U3O and U3O7 using X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy. Initially oxidation proceeded at a linear rate by the inward diffusion of oxygen to form a complete layer of substoichiometric U3O7. This phase was tetragonal with a c a ratio of 1.015, significantly less than the value of 1.03 measured on UO2 powder when oxidised under identical conditions. This difference and the preferred orientation exhibited by surface grains were caused by growth stresses induced in the pellet surface. Both intergranular and transgranular cracking occurred and became nucleation sites for the growth of U3O8. The linear oxidation period associated with U3O7 growth was much shorter at 543 than at 503 K and U3O8 nucleated earlier. Spallation and the production of particulate were only observed during the formation of U3O8 when a 30% increase in volume arose from the U3O7 → U3O8 phase change.

A new method for the determination of x in UO2 + x: Optical absorption spectroscopy measurements

Optical absorption spectra of single crystal UO2 + x have been measured between 0.5 and 2.2 eV, for various values of x between 0.0 and 0.06, and previous spectra up to 6 eV have been reassessed. A steady increase in absorbance at all wavelengths was observed and was linear with x increase. Thus from absorption spectra the value of x in a single crystal of UO2 + x may be determined. The sensitivity of optical absorption spectroscopy permits changes in x as small as 0.0002 to be reliably determined. As excess oxygen entered the lattice a small peak appeared at 0.78 eV, and is identified with the formation of U5+. The oxidation processes in single crystal UO2 + x may therefore be investigated dynamically in-situ and in considerable detail by optical absorption spectroscopy.

Characterization of nickel-chromium-iron spinel-type oxides

Abstract A range of spinels containing nickel, chromium and iron have been prepared by forming solid solutions of the appropriate single oxides at 950°C, in atmospheres of CO 2 or CO 2 /2% CO. For each spinel series prepared a calibration curve has been generated which relates lattice parameter, as determined by X-ray diffraction, to spinel composition. These calibration curves facilitate the identification of the spinels in oxide layers formed on stainless steels. Energy dispersive X-ray analysis has been carried out on each standard spinel prepared to determine elemental composition. It was found that variations in the oxidizing or reducing nature of the reaction atmosphere affected the stability of spinels containing Fe(II) or Fe(III). Small quantities of nickel metal were identified in nickel rich spinels formed in CO 2 /CO atmospheres. This observation could be an important factor in the deposition behaviour of such surfaces under reactor conditions.

Oxidation of crystalline UO2 studied using X-ray photoelectron spectroscopy and X-ray diffraction

The oxidation of the (111) surface of crystalline UO2via U3O7 to U3O8 has been studied by X-ray photoelectron spectroscopy and X-ray diffraction. Oxidation was carried out at 463 and 573 K in 1 Torr† of oxygen. The formation of UO2+x and U4O9 was not observed under these conditions. Anisotropic growth of U3O8 revealed an epitaxial relationship between the cubic close-packed (111) plane of fluorite UO2 and the (001) basal plane of orthorhombic U3O8. It is proposed that the clustering of oxygen interstitial atoms in the fluorite lattice is instrumental in its ultimate breakdown and may be identified within the network of oxygen atoms present in U3O8.

The formation of U3O8 on crystalline UO2

Abstract The oxidation of the (111) surface of crystalline UO2 through U3O7 to U3O8 has been studied by X-ray photoelectron spectroscopy and X-ray diffraction. Anisotropic growth of U3O8 revealed an epitaxial relationship between the cubic close-packed (111) plane of fluorite UO2 and the (001) basal plane of orthorhombic U3O8. It is proposed that the clustering of oxygen interstitial atoms in the fluorite lattice is instrumental in its ultimate breakdown and may be identified within the network of oxygen atoms present in U3O8.

The nature of the oxide formed on 20%Cr/25%Ni/Nb steel oxidised at 1123k in co2

Abstract The oxide formed on 20%Cr/25%Ni/Nb stabilised steel at 1123 K in one atmosphere of CO 2 has been characterised using X-ray photoelectron spectroscopy and X-ray diffractometry. A duplex oxide was identified, the outer layer being a Mn(Fe,Cr) 2 O 4 spinel and the inner oxide rhombohedral Cr 2 O 3 . The presence of carbon was monitored through the oxide and its chemical state determined. The outer spinel contained a significant quantity of carbon in the form of a carbide while the inner Cr 2 O 3 layer was essentially carbon-free.

Preferred orientation and its effect on bulk physical properties of hexagonal polycrystalline materials

Abstract Averaging methods used to derive preferred orientation parameters of polycrystalline materials are examined with particular reference to those parameters suitable for predicting bulk physical properties from the properties of individual crystallites. Attempts to correlate experimental values of thermal expansion and irradiation growth coefficients of polycrystalline graphite and zirconium alloys with the commonly used preferred orientation f-parameter are reviewed. The physical basis behind the derivation of the f-parameter is examined in some detail and the consequent restrictions on its general applicability pointed out.

Formation and growth of spinel and Cr2O3 oxides on 20% Cr-25% Ni-Nb stainless steel in CO2 environments

The nature of the first-formed oxide on 20% Cr/25% Ni/Nb stabilized steel during exposure to CO2 at high and low temperatures has been determined by surface analytical techniques. These results together with a consideration of gas/solid interactions show that the oxide produced may be determined by kinetics or thermodynamic factors, and a diagram is presented to show that rhombohedral Cr2O3 or spinel may be the oxide first formed. Under most standard conditions, a mixed spinel oxide is formed initially, and the subsequent growth of a duplex oxide is analyzed in terms of a solid-state reaction in which the spinel oxide is reduced to Cr2O3 at the metal/oxide interface. Diffusion control of growth by either spinel or Cr2O3 is incorporated in new equations describing the kinetics of oxidation, and weight-gain predictions are tested against experimental observations.

The structural determination of fluorite‐type oxygen excess uranium oxides using EXAFS spectroscopy

Extended x‐ray absorption fine structure (EXAFS) spectroscopy has been carried out at 77 K at the uranium LIII edge for UO2, β‐U3O7, and U4O9 with the aim of determining the structure of these highly defective (oxygen excess) uranium oxide phases, which are of industrial importance. Use has been made of a difference Fourier technique for U3O7, in which the EXAFS of a perfect lattice model is subtracted. U–O bond lengths calculated from the remaining EXAFS signal, assumed to result only from interstitial oxygens, have been used to determine the atomic coordinates of these interstitials. The analysis of EXAFS data in terms of coordination number has allowed an insight into the defect aggregate arrangement of oxygens in U3O7 and U4O9. Furthermore, EXAFS data indicate that the uranium sublattice is perturbed by the incorporation of additional oxygen atoms.Extended x‐ray absorption fine structure (EXAFS) spectroscopy has been carried out at 77 K at the uranium LIII edge for UO2, β‐U3O7, and U4O9 with the aim of determining the structure of these highly defective (oxygen excess) uranium oxide phases, which are of industrial importance. Use has been made of a difference Fourier technique for U3O7, in which the EXAFS of a perfect lattice model is subtracted. U–O bond lengths calculated from the remaining EXAFS signal, assumed to result only from interstitial oxygens, have been used to determine the atomic coordinates of these interstitials. The analysis of EXAFS data in terms of coordination number has allowed an insight into the defect aggregate arrangement of oxygens in U3O7 and U4O9. Furthermore, EXAFS data indicate that the uranium sublattice is perturbed by the incorporation of additional oxygen atoms.

Oxidation pretreatment to reduce corrosion of 20%Cr-25%Ni-Nb stainless steel. I. Weight gain and oxide thickness measurements

The improvement in corrosion resistance afforded by a low-pressure selective oxidation pretreatment on 20%Cr–25%Ni-Nb steel is assessed in terms of weight gain and oxide thickness measurements. Both can and sheet specimens were oxidized in a simulated CAGR CO2 environment at 823, 923, and 1073 K, and gravimetric gross weight-gain measurements were supplemented by spinel and Cr2O3 oxide thickness measurements determined by X-ray diffractometry (XRD). The increased protection provided by the pretreatment resulted in a reduction in gross weight gain of 3–4 times at 823 K, two and three times at 923 K, and a somewhat smaller improvement at higher temperatures. The improvement stemmed from the high proportion of Cr2O3 selectively formed in the preoxide layer itself. Thermally induced lattice strains in the oxide scale have been assessed from measurements of lattice expansion by XRD.