[en] The objective of the project is to examine the chemical speciation of plutonium in UREX+ (uranium/tributylphosphate) extraction processes for advanced fuel technology. Researchers will analyze the change in speciation using existing thermodynamics and kinetic computer codes to examine the speciation of plutonium in aqueous and organic phases. They will examine the different oxidation states of plutonium to find the relative distribution between the aqueous and organic phases under various conditions such as different concentrations of nitric acid, total nitrates, or actinide ions. They will also utilize techniques such as X-ray absorbance spectroscopy and small-angle neutron scattering for determining plutonium and uranium speciation in all separation stages. The project started in April 2005 and is scheduled for completion in March 2008.

[en] Reductive nitrosylation and complexation of ammonium pertechnetate by acetohydroxamic acid has been achieved in aqueous nitric and perchloric acid solutions. The kinetics of the reaction depend on the relative concentrations of the reaction components and are accelerated at higher temperatures. The reaction does not occur unless conditions are acidic. Analysis of the x-ray absorption fine structure spectroscopic data is consistent with a pseudo-octahedral geometry with the linear Tc-N-O bond typical of technetium nitrosyl compounds, and electron spin resonance spectroscopy is consistent with a the d⁵ Tc(II) nitrosyl complex. The nitrosyl source is generally AHA, but may be augmented by products of reaction with nitric acid. The resulting low-valency trans-aquonitrosyl(diacetohydroxamic)-technetium(II) complex (1) is highly soluble in water, extremely hydrophilic, and is not extracted by tri-n-butylphosphate in a dodecane diluent. Its extraction properties are not pH-dependent; titration studies indicate a single species from pH 4.5 down to -0.6 (calculated). This molecule is resistant to oxidation by H₂O₂, even at high pH, and can undergo substitution to form other technetium nitrosyl complexes. The formation of 1 may strongly impact the fate of technetium in the nuclear fuel cycle

[en] The use of diketone ligands, such as hexafluoroacetylacetone (Hhfac) and tetramethylheptandione (Htmhd), can react in solvent-free and solvent-minimal routes to effect separation fission products from uranium matrices without any pre-preparation of the isotopes in question, and provides a path to drastically simplify the separations process and limits the potential for contamination and waste formation due to extensive handling, while increasing the rapidity of separation and isolation of target elements. In particular, the reactivity cesium, barium, strontium, silver, and lead with Hhfac can allow for rapid separations from uranium matrices such as oxide mixtures, in some cases in a matter of minutes. (author)

[en] Nuclear reactions are routinely used to produce a host of isotopes for medical, industrial, and fuel cycle applications. The separation of isotopes from an irradiated matrix commonly involves target dissolution after irradiation. The produced isotopes are then separated from the dissolved matrix. In irradiations the vast majority of the target is unreacted. The separations after dissolutions means the minuscule amount of product isotope must be removed from a large amount of target. Furthermore the target must be reformed for further isotope production, often with losses and waste generation. Targetry coupled separations exploit techniques to acquire the reaction product radionuclide without target destruction. This method utilizes advances in material synthesis with simplified separations that do not require target dissolution. A separation of products can ideally be achieved by washing a designed target with a solvent that will not destroy the matrix but will remove the fission products. The target can be reformed and irradiated to produce more product isotopes. The fundamentals of this concept have been demonstrated and will be presented

[en] The reduction of ammonium pertechnetate by sodium borohydride in 0.1 M NaOH/glacial acetic acid has been studied. The reduction products (solids and solutions) have been characterized by UV-Visible spectroscopy, Scanning Electron Microscopy/Energy-dispersive X-ray emission spectroscopy (SEM/EDS), and X-ray absorption fine structure (XAFS) spectroscopy. UV-Visible spectra of the solution, after reduction, exhibit bands at 350 and 500 nm that have been attributed to the formation of polymeric Tc(IV) species. SEM/EDS on the solid (X-ray amorphous) indicates the absence of metallic Tc and the presence of oxygen. EXAFS measurements further indicate that the precipitate exhibits a (Tc(μ-O)₂Tc) core structure. XANES is consistent with the formation of Tc(III) and/or Tc(IV). Results infer that reduction of aqueous Tc(VII) by borohydride in the presence of acetic acid does not produce metallic Tc, but a mixture of various oxidation states of Tc near Tc(III) and Tc(IV).

[en] The speciation of Tc(VII) in 12 M sulfuric acid was studied by NMR, UV-visible and XAFS spectroscopy, experimental results were supported by DFT calculation and were in agreement with the formation of TcO₃OH(H₂O)₂. In summary, the speciation of heptvalent technetium has been investigated in sulfuric acid. In 12 M H₂SO₄, a yellow solution is observed, and its ⁹⁹Tc NMR spectrum is consistent with a heptavalent complex. The yellow solution was further characterized by EXAFS spectroscopy, and results are consistent with the formation of TcO₃(OH)(H₂O)₂. No technetium heptoxide or sulfato- complexes were detected in these conditions. The molecular structure of TcO₃(OH)(H₂O)₂ has been optimized by DFT techniques, and the structural parameters are well in accordance with those found by XAFS spectroscopy. The experimental electronic spectra exhibit ligand-to-metal charge transfer transitions that have been assigned using TDDFT methods. Calculations demonstrate the theoretical electronic spectrum of TcO₃(OH)(H₂O)₂ to be in very good agreement with the experimental one. Recent experiments in 12 M H₂SO₄ show the yellow solution to be very reactive in presence of reducing agents presumably forming low valent Tc species. Current spectroscopic works focus on the speciation of these species.

[en] Full text of publication follows: The isotope ⁹⁹Tc is a problematic fission product due to its high fission yield, long half-life, and complex solution chemistry. In the proposed UREX process, ⁹⁹Tc is separated from other elements, converted to the metal, and included in a waste form. Waste forms containing Tc metal are convenient for disposal or eventual transmutation into stable ¹⁰⁰Ru. However, little data on the behavior of Tc metal in aqueous media exist. In order to better understand its corrosion mechanisms, a Tc metal electrode was constructed. The electrochemical studies were performed in Yucca Mountain water (pH = 7.65) under aerobic conditions at room temperature. The electrode was immersed for various time intervals and potentiodynamic measurements (log(i) = f(E)) were performed. Results indicate that the corrosion potentials (Ecor) shift to more positive values as the immersion time increases, e.g., Ecor = -54 mV/SCE for t = 2 min to Ecor = +46 mV/SCE for t = 15 h. Further analysis of the potentiodynamic curve after 15 hours of immersion indicate a trans-passivation behavior beginning at E = +150 mV/SCE, which is characteristic of oxide formation at the surface of the metal. In view of these results, a mechanism of corrosion was proposed for Tc metal that involves the formation of TcO₂. The behavior of Tc metal in acidic conditions (nitric, chloride and acetic) is under investigation. Speciation and kinetic data for the dissolution of Tc metal will be presented. (authors)

[en] The isotope ⁹⁹Tc (β_max: 250 keV, half-life: 2 x 10⁵ year) is an abundant product of uranium-235 fission in nuclear reactors and is present throughout the radioactive waste stored in underground tanks at Hanford and Savannah River. Understanding and controlling the extensive redox chemistry of ⁹⁹Tc is important to identify tunable strategies to separate ⁹⁹Tc from spent fuel and from waste tanks and once separated, to identify and develop an appropriately stable waste-form for ⁹⁹Tc. Polyoxometalates (POMs), nanometer sized models for metal oxide solid-state materials, are used in this study to provide a molecular level understanding of the speciation and redox chemistry of incorporated ⁹⁹Tc. In this study, ⁹⁹Tc complexes of the (α₂-P₂W₁₇O₆₁)^10- and (α₁-P₂W₁₇O₆₁)^10- isomers were prepared. Ethylene glycol was used as a 'transfer ligand' to minimize the formation of TcO₂ · xH₂O. The solution structures, formulations, and purity of Tc^VO(α₁/α₂-P₂W₁₇O₆₁)^7- were determined by multinuclear NMR. X-ray Absorption Spectroscopy of the complexes are in agreement with the formulation and structures determined from ³¹P and ¹⁸³W NMR. Preliminary electrochemistry results are consistent with the EXAFS results, showing a facile reduction of the Tc^VO(α₁-P₂W₁₇O₆₁)^7- species compared to the Tc^VO(α₂-P₂W₁₇O₆₁)^7- analog. The α₁-defect is unique in that a basic oxygen atom is positioned toward the α₁-site and the Tc^VO center appears to form a dative metal-metal bond with a framework W site. These attributes may lead to the assistance of protonation events that facilitate reduction. Electrochemistry comparison shows that the Re^V analogs are about 200 mV more difficult to reduce in accordance with periodic trends.

[en] Separation of lanthanides from actinides is a subject of intense interest for the nuclear fuel cycle. The need for this separation is primarily based on the neutronic properties of the lanthanides, limiting nuclear fuel behavior with their inclusion. The separation is challenging for trivalent actinides from lanthanides due to the similar oxidation state. For the actinides, separation of trivalent americium from trivalent curium is also complex. Because of their similarities, one of the few ways to separate americium is to oxidize it to a higher state. In this work characterization of americium higher oxidation states has been performed using spectroscopy methods. Americium in the formal pentavalent oxidation state has been prepared by oxidation of Am (III) to Am(VI) in acid solution using sodium bismuthate (NaBiO_3). The oxidation state and stereochemical arrangements of atoms in close proximity to americium have been determined by X-ray absorption fine structure spectroscopy (XAFS), consisting of EXAFS (extended X-ray absorption fine structure spectroscopy) and XANES (X-ray absorption near-edge spectroscopy) analysis. Density Functional Theory (DFT) was used to create a model of Am(V) and Am(VI) using data from EXAFS

[en] We report here an initial isolation study based upon the use of small uranium oxide particles dispersed in a soluble salt matrix to evaluate the relative recovery of fission products into acidic media. We further show that the macrostructures of the uranium microparticles are largely preserved, such that the bulk target material could be retained for additional irradiations or characterizations. Through this approach, fission products can be separated from the actinide-based target using low molarities of acid without the need to dissolve the actinide itself, reducing the amount of acidic waste. Extraction yields using two molarities of HCl and HNO₃ are compared.