B.S. Tani

Uranium release and secondary phase formation during unsaturated testing of UO2 at 90°C

Abstract Experimental results indicate that UO2 will readily react after being exposed to dripping oxygenated ground water at 90°C. A pulse of rapid U release, combined with the formation of dehydrated schoepite characterizes reactions between one to two years. Rapid dissolution of intergrain boundaries and spallation of UO2 granules appears to be responsible for the rapid U release. Less than 5% of the U is released in a soluble or suspended form. After two years, U release rates decline and a more stable assemblage of uranyl silicate phases form by incorporating cations from the leachant. Uranophane, boltwoodite, and sklodowskite are the final solubility-limiting phases for U in these tests. This observed paragenetic sequence (from uraninite to schoepite to uranyl silicates) is identical to those observed in weathered uraninite deposits. Dispersion of particulate matter may be an important release mechanism for U and other radionuclides in spent nuclear fuel.

Kinetics and mechanism of the (B1,Pb)2Sr2Ca2Cu3O10 formation reaction in silver-sheathed pires

Abstract A detailed kinetic and mechanistic analysis of the growth of the (Bi 2− x Pb x )Sr 2 Ca 2 Cu 3 O 10 phase in silver-sheathed wires has been performed by an isothermal equilibration method. Silver tubes loaded with precursor powders were processed into wires using established metallurgical techniques. The wire specimens were immersed in a preheated equilibration apparatus, heat-treated at the desired temperature in 7.5% O 2 , for varying periods of time, then quenched in a room-temperature silicone oil bath. The results indicated that the kinetic data followed a nucleation growth mode1 derived for a reaction at the interface between thin sheets and a fine powder or a fluid. Transmission electron microscopy confirmed the two-dimensional reaction geometry and revealed the presence of an amorphous phase at grain boundaries, where rapid transport diffusion appears to occur due to the absence of the stabilizing influence of the regular lattice. A reduction in activation energy was observed at temperatures 2819°C which is tentatively attributed to the onset of a liquid-phase-controlled reaction (i.e. a phase boundary crossing). The effects of powder processing parameters and precursor particle size on the kinetic behavior and the growth rate of the (Bi 2− Pb x )Sr 2 Ca 2 Cu 3 O 10 phase are also discussed.

120 K superconductivity in the (Bi, Pb)-Sr-Ca-Cu-O system

Abstract Superconductivity with onset at 120 K and zero electrical resistance at 105 K is observed in the system (Bi, Pb)-Sr-Ca-Cu-O with the composition Bi 0.7 Pb 0.3 Sr 1 Ca 1 Cu 1.8 Oδ. The transition temperature and room temperature resistivity of the samples depend on the rate of cooling from the sintering temperature. We report the synthesis, superconducting temperature, and X-ray diffraction measurements of the above compound.

Thermostability and decomposition of the (Bi,Pb)2Sr2Ca2Cu3O10 phase in silver‐clad tapes

The stability of the Bi2−xPbxSr2Ca2Cu3O10 (Pb‐2223) phase contained in silver‐sheathed oxide‐powder‐in‐tube specimens has been investigated by x‐ray diffraction, transmission electron microscopy, and energy dispersive x‐ray analysis. Silver tubes loaded with Pb‐2223 precursor powders were processed into tapes using established metallurgical techniques. The tapes were heat‐treated in a specially designed equilibration apparatus at selected temperatures (800–845 °C) for a range of times (10–5500 min) and quenched in liquid gallium held at ∼40 °C. The results showed that the Pb‐2223 phase is stable in a limited temperature interval between 810 and 830 °C in 7.5% oxygen. At 800 °C, this phase decomposes to Bi2Sr2CaCu2O8 (2212), Ca2PbO4, and CuO; while at temperatures ≥840 °C it partially melts with precipitation of Bi2Sr2CuO6 (2201) and Ca2CuO3. The effects of the silver cladding on the Pb‐2223 phase stability and microstructure are also discussed.

The crystal structure of beta-platinum dioxide

Abstract β-PtO 2 crystallizes as the orthorhombic CaCl 2 -type structure with a = 4·488(3) A , b = 4·533(3) A , and c = 3·138(2) A . The oxygen coordinates are found to be: x = 0·281 and y = 0·348. This leads to two long PtO bonds of 2·02 A, four short bonds of 1·98 A, and a mininum oxygen-oxygen approach of 2·40 A. The Pt(IV) radius is 0·63 A. A comparison of the PtO 2 structure with the rutile form of TiO 2 is presented.

Experimental evidence of a miscibility gap in the YBa2Cu3Ox system

Abstract Based on the results of EMF measurements, plots of the logarithm of the oxygen fugacities versus stoichiometry in YBa 2 Cu 3 O x show an inflection around x ≅ 6.65 at temperatures between 400–500°C. There is no apparent inflection at higher temperatures (550–750°C). These new low-temperature data are consistent with the presence of a miscibility gap at lower temperatures. The presence of this gap is evidenced directly by the intersection of the values of RT ln p (O 2 )extrapolated to lower temperatures.

Phase chemistry and microstructure evolution in silver-clad (Bi/sub 2-x/Pb/sub x/)Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ wires

The reaction kinetics and mechanism that control the conversion of (Bi,Pb)/sub 2/Sr/sub 2/CaCu/sub 2/O/sub z/ (Bi-2212)+alkaline earth cuprates to (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (Bi-2223) in silver-clad wires were investigated as a function of equilibration temperature and time at a fixed oxygen partial pressure (7.5% O/sub 2/). Measured values for the fractional conversion of Bi-2223 versus time were evaluated based on the Avrami equation. SEM studies of partially and fully converted wires suggest: (1) the transformation to Bi-2223 is two-dimensional and controlled by a diffusion process; (2) liquid phases are present during part of the Bi-2212 to Bi-2223 conversion; and (3) growth of the (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/ phase accompanies Bi-2223 formation.<<ETX>>

Formation of Perovskite Surface Layers by Oxidation of Cu‐La‐Sr Alloys

The recent discovery that perovskite-type crystal structures composed of La-Cu oxides substituted with small amounts of alkaline earth cations have exceptionally high superconducting critical temperatures has stimulated much interest in these potentially important materials. The most promising material reported thus far in the literature is La/sub 2-x/Sr/sub x/CuO/sub 4/. The substitution of an alkaline earth for La appears to stabilize a tetragonal structure as apposed to the orthorhombic structure of the parent compound, La/sub 2/CuO/sub 4/. In the work reported to date, the materials were prepared either by sintering mechanical mixtures of the oxides or carbonates or by co-precipitating the oxalates or carbonates with subsequent calcining and sintering steps.

Oxygen stoichiometry, phase stability, and thermodynamic behavior of the lead-doped Bi-2223 and Ag/Bi-2223 systems

Abstract Electromotive-force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made in the lead-doped Bi-2223 superconducting system in the temperature range 700–815°C by means of an oxygen titration technique that employs an yttria-stabilized zirconia electrolyte. The results of our studies indicate that processing or annealing lead-doped Bi-2223 at temperatures ranging from 750 to 815°C and at oxygen partial pressures ranging from ∼ 0.02 to 0.2 atm should preserve Bi-2223 as essentially single-phase material. Thermodynamic assessments of the partial molar quantities Δ S ( O 2 ) and Δ H ( O 2 ) indicate that the plateau regions in the plot of oxygen partial pressure versus oxygen stoichiometry (x) can be represented by the diphasic CuOCu2O system. In accord with the EMF measurements, it was found that lead-doped Bi-2223 in a silver sheath is stable at 815°C for oxygen partial pressures between 0.02 and 0.13 atm.

Effects of radiation exposure on glass alteration in a steam environment

Several Savannah River Plant (SRL) glass compositions were reacted in steam at temperatures of 150 to 200{degrees}C. Half of the tests utilized actinide-doped monoliths and were exposed to an external ionizing gamma source, while the remainder were doped only with U and reacted without gamma exposure. All glass samples readily reacted to form secondary mineral phases within the first week of testing. An in situ layer of smectite initially developed on nonirradiated SRL 202 glass test samples. After 21 days, a thin layer of illite was precipitated from solution onto the smectite layer. A number of alteration products including zeolite, Casilicate, and alkali or alkaline earth uranyl silicate phases were also distributed over most sample surfaces. In the irradiated SRL 202 glass tests, up to three layers enveloped rounded, and sometimes fractured, glass cores. After 35 to 56 days these remnant cores were replaced by a mottled or banded Fe- and Si-rich material. The formation of some secondary mineral phases also has been accelerated in the irradiated tests, and in some instances, the irradiated environment may have led to the precipitation of a different suite of minerals. The alteration layer(s) developed at rates of 2.3 and 32 {mu}m/day for the nonirradiated and irradiated SRL 202 glasses, respectively, indicating that layer development is accelerated by a factor of {approximately} 10 to 15X due to radiation exposure under the test conditions.