S. Sampath

Oxidation state of uranium: an XPS study of alkali and alkaline earth uranates

X-ray photoelectron spectroscopy (XPS) studies are performed on some mixed oxides of uranium with alkali and alkaline earth metals (Na, Li, Rb, Tl, Sr, Ba) to investigate the presence of multiple valence states of uranium in these compounds. Photoelectron peak positions, peak widths and satellite positions were measured to identify the chemical states. The analysis of the shake up satellites and the deconvolution of the principal peaks are used to quantify the chemical states. The domination of U(V) chemical state is inferred in Na2U3O9 and Sr2U3O10.

Preparation and characterisation of Pu-pyrochlore: [La1−xPux]2 Zr2O7 (x=0–1)

Abstract Thermal and X-ray powder diffraction studies were carried out on mixtures of PuO2, ZrO2 and graphite and La2O3, PuO2, ZrO2 and graphite. Pu2Zr2O7, a new cubic phase with pyrochlore structure (a=1.0592 nm), was identified and characterised. The studies also established the existence of a continuous range of solid solution, [La1−xPux]2Zr2O7 with x=0 to 1.

Studies on stabilised zirconia as host phase for the fixation of actinides, rare-earths and sodium

Titania, zirconia and alumina based ceramic materials are potential solid waste forms being assessed to determine their suitability for long term immobilisation of radionuclides. The preparation, characterisation, solid solubilities and leachability studies on stabilised zirconia containing actinides, rare earths and sodium are reported. The range of solid solubilities of cerium, uranium, neodymium, plutonium and sodium was established in stabilised zirconia. Leaching studies in water and 0.1 M HCl at room temperature and 70°C showed very low leachabilities of uranium, plutonium and rare earths.

Thermal studies on alkaline earth uranates

Abstract The thermal stabilities of various alkaline earth uranates were investigated by thermogravimetry and X-ray powder diffraction. New uranates with the formal composition A 2 U 3 O 9 (A=Ca, Sr and Ba) having hexagonal unit cell were obtained and characterised. Oxidation of these uranates led to the formation of two new intermediate compounds of composition A 2 U 3 O 10 (A=Ca and Sr). Many of the calcium and strontium uranates were found to be structurally related to fluorite as long as the substitution of uranium by calcium or strontium was limited to 0.33 and the ratio of oxygen to metal was close to 2. Kinetic studies on the oxidation and the reduction of uranates showed that the oxidation kinetics of A 2 U 3 O 9 to A 2 U 3 O 10 (A=Ca, Sr) or A 2 U 3 O 11 (A=Ba) was controlled by nucleation and growth whereas the reduction kinetics of A 2 U 3 O 11 to A 2 U 3 O 9 (A=Ca, Sr and Ba), generally occurring in single step, was controlled by phase boundaries.

Existence of fluorite-type solid solutions in alkali metal-uranium-oxygen systems

Abstract The solubilities of Li2O and Na2O in UO2 + x have been established for the first time. They form cubic solid solutions, LiyU1 − yO2 + x and NayU1 −yO2 + x (y = 0–0.2 and x ⪋ 0). The lattice parameters of these solid solutions can be expressed, respectively, as a function of x and y: a0(nm) = 0.54704 − 0.0104x − 0.0495y and a0(nm) = 0.54704 − 0.0102x − 0.0345y. Oxidation of these solid solutions in air up to 773 K gave cubic MO2 + x and the kinetics of oxidation was found to be controlled by oxygen chemical diffusion similar to oxidation of UO2.

Solid state reactions of uranium oxide and alkali metal chromates: Characterisation of new uranates

Abstract New uranate phases, Rb 2 U 4 O 11 and Rb 2 U 3 O 8.5 in the Rb-U-O system and Na 2 U 3 O 9 and Na y U 1− y O 2− x ( y = 0–0.2) in the Na-U-O system were prepared and characterised by X-ray, thermal and chemical methods. Na y U 1− y O 2− x was found to be a solid solution of Na 2 O in UO 2 with fluorite structure with a limited solid solubility of 10 mole% Na 2 O. These new phases reported could lead, in addition to the modification of the phase diagrams, to a better understanding of the fuel-fission product-coolant-clad chemical interactions. The new phases were identified during studies on solid state reactions of chromates of caesium, rubidium, potassium and sodium with uranium oxides upto 1200°C in inert atmosphere. These reactions have shown that different uranates are obtained depending upon the O/U of the starting uranium oxide, with chromium separating as chromium oxide (Cr 2 O 3 ).

Thermochemical studies in the sodium-chromium-oxygen system

Abstract The compound NaCrO 2 in the sodium-chromium-oxygen system is important from the point of view of corrosion of stainless steel by oxygen-contaminated sodium in fast reactors. Thermogravimetry, differential thermal analysis in different atmospheres, and X-ray powder diffraction studies on the products showed NaCrO 2 to be stable in argon up to 1200° C. The compound was stable in a CO 2 atmosphere up to at least 900 °C, but in oxygen it was oxidised above 350 °C to a mixture of Na 2 CrO 4 and Cr 2 O 3 . NaCrO 2 was found to be the only phase of chromium(III) which could be obtained from solid-solid reactions of Na 2 CO 3 and Cr 2 O 3 . Na 2 CrO 4 showed a phase transition at 410 ± 5° C with a heat of transition of 5.9 kJ mol −1 .

Influence of selected metal oxides on the oxidation kinetics of graphite

Abstract The thermal decomposition of graphite in air was studied in detail by thermogravimetry in the presence of varying concentrations of U3O8, CeO2 and ThO2. The decomposition was found to follow first-order kinetics with a significant decrease in decomposition temperatures and activation energies, E, due to presence of the oxides. There was a decrease in log A, associated with a decrease in E, indicating the kinetic compensation effect. One of the possible mechanisms for catalysis, namely localized reduction of the oxide by graphite followed by reoxidation of the lower oxide by air, is not supported by the results obtained.

Studies on the kinetics of oxidation of PuyTh1-yO2-x (y = 0.2, 0.3 and 0.7) in air

Abstract Oxidation of sintered thorium-plutonium mixed oxide pellets was studied by thermogravimetry in the temperature range 400–1200 K. The kinetics of oxidation was found to be controlled by oxygen chemical diffusion with activation energies in the range 25–50 kJ mol−1. Oxygen chemical diffusion coefficients are reported for Pu0.3Th0.7O1.94.

THALLIUM URANATES AND OTHER (TL,U,O) COMPOUNDS: A STRUCTURAL AND THERMODYNAMIC STUDY

Abstract Tl 2 U 4 O 11 (s) and Tl 2 U 3 O 9 (s) were prepared and characterised in the Tl-U-O ternary system for the first time. Tl 2 U 4 O 11 was found to be isostructural with Rb 2 U 4 O 11 . The partial phase diagram of the Tl-U-O system was derived from equilibration experiments with several binary and ternary oxides in the system. The oxygen pressure over the phase fields Tl 2 U 4 O 13 (s) + Tl 2 U 4 O 12 (s) and Tl 2 U 4 O 12 (s) + Tl 2 U 4 O 11 (s) was determined by measuring the e.m.f.s in the temperature ranges 805 to 1179 K and 984 to 1141 K respectively. The oxygen pressure can be given by ln P (O 2 )(kPa) ± 0.01 = 31.03 − 32924/ T (K) and ln P (O 2 )(kPa) ± 0.01 = 53.23 − 93358/ T (K) respectively. Using the molar Gibbs free energy of formation of Tl 2 U 4 O 13 (s), the molar Gibbs free energies of formation of Tl 2 U 4 O 12 and Tl 2 U 4 O 11 were obtained. The kinetics of oxidation of Tl 2 U 4 O 12 and Tl 2 U 4 O 11 in air were also studied using a thermal analyser.