11 research outputs found
Spectroelectrochemical study of neptunium in molten LiCl-KCl eutectic
Neptunium behaviour in an LiCl-KCl eutectic melt at 723 K was studied using spectroelectro-chemistry. Cathodic reduction of neptunium(IV)-containing melts led to the formation of Np(III) ions and then neptunium metal. Electronic absorption spectra of Np(IV) and Np(III) chloro species in LiCl-KCl melt were recorded and resolved into individual Gaussian bands. The nature of neptunium complex ions in the melt is discussed. © 2007 Verlag der Zeitschrift für Naturforschung.I. B. P. thanks INTAS (Grant No. 03-55-1453) for financial support
Recommended from our members
Investigations of two chemical systems relevant to pyroprocessing used nuclear fuel
Pyroprocessing is an advanced technology for recycling used nuclear fuel. Pyrochemical processes encompass a wide range of chemical, physical, and electrochemical methods to partition fission products and other components from used nuclear fuel, which allows for the reuse of the actinides in nuclear fuel. This dissertation investigates two chemical systems relevant to pyroprocessing. The first investigation explores the possibility of using molybdate melts containing sodium molybdate (Na₂MoO₄) and molybdenum trioxide (MoO₃) to partition fission products from used nuclear fuel by crystallization. The difference in solubility of the fission product metal oxides compared to the uranium oxide or molybdate in the molybdate melt allows for these separations to occur. Uranium dioxide dissolves in the molybdate at high temperatures, and upon cooling, the uranium precipitates as uranium dioxide or molybdate, whereas the fission product metals remain soluble in the melt. The feasibility of UO₂ purification from the fission products was studied using small-scale experiments with gram quantities of uranium dioxide. The composition of the uranium precipitate as a function of molybdate melt composition was determined through a series of tests. The effectiveness of the partitioning of several fission product surrogates between the uranium precipitate and molybdate melt for various parameters in the process was also studied. A melt consisting of 20 wt% MoO₃- 50 wt% Na₂MoO₄-30 wt% UO₂ heated to 1313 K and cooled to 1123 K for the physical separation of the UO₂ product from the melt, and washed once with Na₂MoO₄ resulted in excellent separation of the UO2 from the surrogate fission products. The second investigation explored the phase equilibria of UCl₃ and NpCl₃ in the LiCl-KCl molten salt electrolyte used in electrorefining used nuclear fuel. The re-evaluation of the LiCl-UCl₃, KCl-UCl₃ and the LiCl-KCl-UCl₃ phase diagrams and the first known evaluation of the KCl-NpCl₃ system were performed. Samples of varying compositions within each of these systems were thermally analyzed by DSC to determine the temperature and types of the phase transitions. Samples were then analyzed by XRD to determine the identity of the phases formed, and ICP-OES or ICP-MS to establish the cation ratio. The LiCl-UCl₃ system displayed a simple eutectic system. The KCl-UCl₃ system displayed two eutectics and the K₂UCl₅ phase, which was identified by DSC and XRD. There was no evidence of a K₃UCl₆ phase. These LiCl-UCl₃ and KCl-UCl₃ phase diagrams were used to produce a portion of the LiCl-KCl-UCl₃ phase diagram relevant to electrorefining. The LiCl-KCl-UCl₃ system displayed two ternary eutectics and was consistent with literature data. The KCl-NpCl₃ system displayed two eutectics and the K2NpCl5 and K₃NpCl₆ phases, which were identified by DSC and XRD. The evaluation of these phase diagrams allows for an improved understanding of the LiCl-KCl-UCl₃ and KCl-NpCl₃ systems and their application to pyroprocessing.Keywords: Pyroprocessing, Used Nuclear Fuel, Phase Diagram
Boron-Doped Diamond as a Resilient Electrode Material in Molten Salts
Molten salt chemistry has a range of applications within nuclear technology, including for the Molten Salt Reactors (MSRs) and pyroprocessing to recover valuable actinides for energy and national security needs. However, the high-temperature, corrosive nature of molten salts makes them particularly challenging to deploy on an industrial scale and study in benchtop measurements. Material accountability and corrosion monitoring of MSR fuels are essential components to the successful deployment of MSRs, and electroanalytical techniques like cyclic voltammetry (CV) and spectroelectrochemistry (SEC) can provide a wealth of information to describe salt systems in situ. To perform such measurements, it is imperative to have materials that can withstand the harsh environment of molten chloride and fluoride salts. It is hypothesized that boron-doped diamond (BDD), a well-studied, ‘designer’ electrode material, can serve as a resilient working electrode in molten salt systems given its previous applications to other harsh environments.
To better understand the use of BDD in molten salts, it was first studied in aqueous systems using CV to determine the impact of crystal structure, morphology, and carbon hybridization (e.g., sp2 or sp3) on the electrochemical response. Potassium ferricyanide, hexaammineruthenium(III) chloride, europium trichloride, and uranyl nitrate were all evaluated using CV on the two distinct sides of free-standing BDD to find that there is, in fact, a difference between large and small-grain structures of polycrystalline diamond. Values for formal potential (Eo), peak separation (DEp), diffusion coefficients (D), and heterogeneous electron transfer rate constants (k) were compared for each side of the BDD to quantify the quality of each response. Then, a novel optically transparent electrode (OTE) design from free-standing BDD was applied to SEC measurements of aqueous potassium ferricyanide to determine Eo and D. Following successful measurements, this OTE could be a resilient design applicable for actinide species in molten salt systems to help characterize MSR fuels.
Then, BDD was exposed to chloride (NaCl-KC) and fluoride (FLiNaK) molten salts under various conditions for extended periods to determine if it would survive under the most extreme environments (i.e., exposed to air, in inert atmosphere, using non-purified salts, using thermally dried salts, etc.). The material was then evaluated using scanning electron microscopy (SEM), profilometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) to determine changes to the surface structure and chemistry after exposure. There was virtually no change to the material after exposure except in the case of chloride salt that was not dried and was exposed to air where etching was seen in some regions of the crystal structures. However, when aqueous CV was performed on the samples to determine changes to the electrochemical performance, there was very little change, including for the corroded sample.
Finally, BDD was used to perform CV on Eu3+/2+ and U4+/3+ redox couples in chloride salts using various BDD electrode geometries. Successful measurements of Eu were performed using free-standing BDD and thin-film BDD in LiCl-KCl, and U was evaluated with thin-film BDD in LiCl-KCl and MgCl2-NaCl electrolytes. The impact of BDD crystal structure with the differing electrode geometries for Eu and changing electrolyte cation side for U were significant and produced an interesting set of results that opened doors for a wide range of future studies. Ultimately, BDD successfully performed measurements of f-block species relevant to MSR fuels and should be further explored under various conditions and optimized electrode designs
From test tube to pilot plant, a 50 year history of the Chemical Technology Division at Argonne National Laboratory.
Recommended from our members
Chemical Technology Division. Annual technical report, 1995
Highlights of the Chemical Technology (CMT) Division`s activities during 1995 are presented. In this period, CMT conducted research and development in the following areas: (1) electrochemical technology, including advanced batteries and fuel cells; (2) methods for treatment of hazardous waste and mixed hazardous/radioactive waste; (3) the reaction of nuclear waste glass and spent fuel under conditions expected for an unsaturated repository; (4) processes for separating and recovering selected elements from waste streams, concentrating low-level radioactive waste streams with advanced evaporator technology, and producing {sup 99}Mo from low-enriched uranium; (5) electrometallurgical treatment of different types of spent nuclear fuel in storage at Department of Energy sites; and (6) physical chemistry of selected materials in environments simulating those of fission and fusion energy systems
Recommended from our members
Program Pu Futures 2006
The coordination chemistry of plutonium remains relatively unexplored. Thus, the fundamental coordination chemistry of plutonium is being studied using simple multi-dentate ligands with the intention that the information gleaned from these studies may be used in the future to develop plutonium-specific sequestering agents. Towards this goal, hard Lewis-base donors are used as model ligands. Maltol, an inexpensive natural product used in the commercial food industry, is an ideal ligand because it is an all-oxygen bidentate donor, has a rigid structure, and is of small enough size to impose little steric strain, allowing the coordination preferences of plutonium to be the deciding geometric factor. Additionally, maltol is the synthetic precursor of 3,4-HOPO, a siderophore-inspired bidentate moiety tested by us previously as a possible sequestering agent for plutonium under acidic conditions. As comparisons to the plutonium structure, Ce(IV) complexes of the same and related ligands were examined as well. Cerium(IV) complexes serve as good models for plutonium(IV) structures because Ce(IV) has the same ionic radius as Pu(IV) (0.94 {angstrom}). Plutonium(IV) maltol crystals were grown out of a methanol/water solution by slow evaporation to afford red crystals that were evaluated at the Advanced Light Source at Lawrence Berkeley National Laboratory using single crystal X-ray diffraction. Cerium(IV) complexes with maltol and bromomaltol were crystallized via slow evaporation of the mother liquor to afford tetragonal, black crystals. All three complexes crystallize in space group I4{sub 1}/a. The Ce(IV) complex is isostructural with the Pu(IV) complex, in which donating oxygens adopt a trigonal dodecahedral geometry around the metal with the maltol rings parallel to the crystallographic S{sub 4} axis and lying in a non-crystallographic mirror plane of D{sub 2d} molecular symmetry (Fig 1). The metal-oxygen bonds in both maltol complexes are equal to within 0.04 {angstrom} for each oxygen type. In contrast to the maltol structures, the cerium(IV) bromomaltol complex arranges the maltol rings in a drastically different manner while maintaining the S{sub 4} crystallographic symmetry (Fig 2). The coordination geometry around the cerium remains a trigonal dodecahedron, but the chelating ligands span a different set of edges as in the maltol structures; the two-fold related bromomaltol ligands twist away from planarity, breaking the D{sub 2d} molecular symmetry. It is unlikely that steric interaction with a bromine on the same molecule would have caused the observed rearrangement, as there would be sufficient separation between them to accommodate their bulk in the geometry of the plutonium and cerium maltol complexes. The extended packing in the unit cell of both the plutonium and cerium maltol crystals indicates that pi stacking occurs throughout the lattice via the maltol rings with close contacts between rings of approximately 3.6 {angstrom}. Introduction of the bromine to this structure would disrupt the packing that would allow these interactions, causing the molecule to adopt the geometry present in the bromomaltol structure. In this unexpected arrangement the complex is still able to maintain some pi stacking with the maltol rings of adjacent molecules with a close contact of approximately 3.3 {angstrom}. Additionally, the bromine on each ligand is arranged such that its next closest contact is with a bromine 3.64 {angstrom} away on another molecule. Despite the different ligand geometry, the bromomaltol structure exhibits metal-oxygen bond distances that are within 0.06 {angstrom} of those in the maltol complexes
Characterization of structural properties of U and Pu in model systems by advanced synchrotron based X-ray spectroscopy
This dissertation presents the investigations of different U and Pu model systems relevant for safety assessment studies of nuclear waste repositories using the X-ray based synchrotron techniques: U and Pu L3/M4,5 edges HR-XANES, L3 edge EXAFS and 3d4f RIXS as well as other complementary techniques, including XPS, XRD, SEM, TEM and UV-Vis-NIR techniques
Spectroelectrochemical study of neptunium in molten LiCl-KCl eutectic
Neptunium behaviour in an LiCl-KCl eutectic melt at 723 K was studied using spectroelectro-chemistry. Cathodic reduction of neptunium(IV)-containing melts led to the formation of Np(III) ions and then neptunium metal. Electronic absorption spectra of Np(IV) and Np(III) chloro species in LiCl-KCl melt were recorded and resolved into individual Gaussian bands. The nature of neptunium complex ions in the melt is discussed. © 2007 Verlag der Zeitschrift für Naturforschung.I. B. P. thanks INTAS (Grant No. 03-55-1453) for financial support