[en] The tracer oxygen diffusivity in UO/sub 2-x/ has been measured along the lower two phase boundary. The diffusion couple consisted of two matched hypostoichiometric uranium dioxide wafers, one enriched with ¹⁸O and the other normal. Results showed much higher diffusion coefficients than those of stoichiometric UO₂. This directly proved that the major defect species in UO/sub 2-x/ is the anion vacancy. Activation energy of anion vacancy migration was measured to be 11.7 +- 3.0 kcal/mole. A diffusion model established for UO₂ and UO/sub 2+-x/ showed that in stoichiometric UO₂ both interstitials and vacancies contribute significantly to oxygen diffusion and neither can be neglected; at 1400⁰C their contributions are about equal. This model was extended to nearly stoichiometric UO/sub 2+-x/ to predict oxygen diffusion coefficients in these stoichiometry ranges. Also deduced from the model were the Frenkel defect energy and entropy of 85.6 +- 9.2 kcal/mole and 18.2 +- 7.3 eu, respectively. The contribution of Frenkel disorder to the excess enthalpy of UO₂ was evaluated. Calculation showed that Frenkel disorder accounts for 87% of the excess enthalpy at 3000⁰K. A simple two band model for electronic excitation, with a band gap of 2.0 ev and effective electron mass of 7.6 m/sub e/, accounted for the remainder of the excess enthalpy

[en] The tracer diffusion coefficient of oxygen in UO/sub 2-x/ was measured using a diffusion couple and ¹⁸O. At 1400⁰C, the contributions of vacancies and interstitials to the oxygen diffusion coefficient in stoichiometric UO₂ are about equal. The Frenkel energy and entropy agree with Szwarc's theory. A band gap of 2.0 eV and an effective mass of 7.6 m/sub e/ were obtained

[en] The tracer oxygen diffusivity in UO_2-x has been measured along the lower two phase boundary. The diffusion couple consisted of two matched hypostoichiometric UO² wafers, one enriched with ₁₈O and the other normal. These two were pressed together with a bond of liquid U in between. After a diffusion anneal the ₁₈O concentration profile was determined by ion microprobe mass analysis, from which diffusion coefficients were obtained. The results showed much higher diffusion coefficients than those of stoichiometric UO². This directly proved that the major defect species in UO_2-x is the anion vacancy. Activation energy of anion vacancy migration was measured to be 11.7 +- 3.0 kcal/mole. A diffusion model established for UO² and UO_2+-x showed that in stoichiometric UO² both interstitials and vacancies contribute significantly to oxygen diffusion and neither can be neglected; at 1400₀C their contributions are about equal. This model was extended to nearly stoichiometric UO_2+-x to predict oxygen diffusion coefficients in these stoichiometry ranges. Also deduced from the model were the Frenkel defect energy and entropy of 85.6 +- 9.2 kcal/mole and 18.2 +- 7.3 eu, respectively. Using these values, the contribution of Frenkel disorder to the excess enthalpy of UO² was evaluated. Calculation showed that Frenkel disorder accounts for 87% of the excess enthalpy at 3000₀K. A simple two band model for electronic excitation, with a band gap of 2.0 eV and effective electron mass of 7.6 m_e, accounted for the remainder of the excess enthalpy

[en] The first examples of polyoxometalates incorporating uranyl hetero groups have been synthesized by reaction of Na₉[A-PW₉O₃₄] with UO₂(NO₃)₂. Sodium, ammonium, and potassium salts of [M₂(UP₂)2(PW₉O₃₄)₂]^12- (M = Na, NH₄, K) were characterized by P and W NMR spectroscopy and single-crystal structural analysis. The sodium salt (1) contains an anion of C_i symmetry in which two PW₉O₃₄ groups sandwich two UO₂²⁺ and two Na⁺ and two NA⁺ cations. The uranium atoms have pentagonal-bipyramidal coordination, achieved by three equatorial bonds to one PW₉O₃₄ and two bonds to the other. The sodium cations have approximate tetrahedral coordination geometry. Single-line P and five-line P and five-line W NMR spectra confirm that the anion structure is maintained in solutions containing excess Na⁺. The ammonium and potassium salts contain anions in which the UO₂²⁺ cations (pentagonal bipyramids) are unsymmetrically sandwiched between the PW₉₃₄ groups, as are two closely associated cations, one internal and one external. In solution both, salts give two-line P and nine-line W NMR spectra that are consistent with the solid-state structures. Solution equilibria are rapidly established between the two structure types and are dependent upon the concentrations and identifies of the cations. Based on integrated P NMR spectra, the equilibrium constants for [Na₂(UO₂)₂(PW₉O₃₄)₂]^12- <-> [M₂(UO₂)₂(PW₉O₃₄)₂]^12- + 2Na⁺ are 128 ± 12 (M = K⁺) and 1.8 ± 0.8 (M = NH₄⁺). The requirement of the second cation in order to satisfy the mass-action expression for K_eq, coupled with a W NMR spectrum that implies C_s symmetry, demonstrates that the external ion-paired cation undergoes rapid exchange between several surface sites of the heteropolyanion. Solutions of 1 are stable at pH 7 in the presence of excess sodium cations, and the anion is transferable into toluene by phase-transfer techniques. Addition of calcium ions to solutions of 1 generates a new one-line P NMR spectrum of a Ca derivative which is assumed to be isostructural with 1

[en] A class of oxygen fluoride compounds and krypton difluoride show great promise in recovering and purifying plutonium and other actinides from nuclear waste and residues. Since 1983, a significant effort has been expended in three main areas of research; one area is the characterization of the reactive species and the fluorination reaction chemistry with various actinides. The second area is to develop efficient production methods for the active chemicals. The third area is actually in demonstrating application of these fluorinating agents to actinides recovery and purification. Substantial progress has been made in all three areas and some of the highlights in their research effort is discussed. 11 refs., 2 figs., 2 tabs

[en] The absorption measurements in an ordinary cell may require typically a few torr pressure of sample gas. At these pressures the absorption lines are usually pressure-broadened and, therefore, closely spaced transitions are poorly resolved even at diode-laser resolution. This situation is greatly improved in Doppler-limited spectroscopy at extremely low sample pressures. Two very long-pathlength absorption cells were developed to be used in conjunction with diode lasers. They were designed to operate at controlled temperatures with the optical pathlength variable up to approx. 1.5 km. Not only very low sample pressures are used for studies with such cells but also the spectroscopic sensitivity is enhanced over conventional methods by a factor of 10³ to 10⁴, improving the analytical capability of measuring particle densities to the order of 1 x 10'' molecules/cm³. This paper presents some analytical aspects of the diode laser spectroscopy using the long-pathlength absorption cells in the areas of absorption line widths, pressure broadening coefficients, isotope composition measurements and trace impurity analysis

No abstract available

[en] Presentations at this conference covered the topics of materials science/nuclear fuels, condensed matter physics, actinides in the environment/separation and analysis, actinides/processing, actinides/TRU wastes, materials science, TRU waste forms, nuclear fuels/isotopes, separations and process chemistry, actinides in the environment, detection and analysis, Pu and Pu compounds, actinide compounds and complexes

[en] A method of producing controlled homogenous condensation of a molecular feed gas containing a plurality of isotopes is described which comprises (a) supersonically flowing the feed gas through an expansion nozzle under flow conditions at which the expansion of the feed gas produces condensation when the gas is not vibrationally excited, and (b) irradiating the supersonically flowing gas with laser radiation of a wavelength which selectively excites those molecules in the feed gas containing a particular isotope

[en] The invention provides a method for producing controlled homogeneous condensation of a molecular feed gas containing several isotopes. The feed gas flows at supersonic rates through an expansion nozzle under conditions at which the gas would normally condense. The gas is irradiated with laser radiation of a wavelength that selectively excites those molecules in the feed gas that contain a particular isotope, thus preventing their condensation. Condensate particles may be aerodynamically separated from the flowing gas stream